Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tbone55 on August 08, 2024, 06:20:10 pm
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I've been looking at doing some mods to my Blues Jr and was wondering if anyone here may gave done them already and did they make a difference. My amp has been converted using Doug's conversion kit including the tube board. References are based on his schematic which I'm attaching.
Location Description Current Value New Value
C2 Coupling Cap .0022uf .022uf
C6 Bass Pot .022uf 100uf
C7 Mid Pot .022uf .047uf
C25 B+ filter cap 47uf 100uf
C2, C6 and C7 changes are supposed to emulate the tone stack of a Princeton Reverb. 0 C25 is supposed to help provide a more filtered signal I believe.
Should I bother with any of these mods or stick to what I have?
Thanks.
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I wouldn't mess with C11. If you remove it, the amp won't work. If you bypass it, I doubt you would notice a difference.
Increasing C25 would definitely provide more power supply filtering, but shouldn't be necessary. Seems like way overkill.
The tone stack, you could play with if you don't like the way it currently behaves. Although, 100uF for the bass cap is insane.
Dave
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I wouldn't mess with C11. If you remove it, the amp won't work. If you bypass it, I doubt you would notice a difference.
Increasing C25 would definitely provide more power supply filtering, but shouldn't be necessary. Seems like way overkill.
The tone stack, you could play with if you don't like the way it currently behaves. Although, 100uF for the bass cap is insane.
Dave
Thanks kindly Dave. You're right about C11. That was an error on my part. The original Blues Jr schematic has a .033uf capacitor in front of the .01uf cap that was suggested to be removed, not the .01uf cap.
I kind of thought the 100uf cap for the bass was a bit too much myself but had to ask. Maybe a .01uf cap would be okay to try.
The other mod I think I'll try is on the mid pot. Run a lead from the center terminal to the outer lead connected to the bass pot.
I should really have looked at a schematic for a Princeton Reverb unit to see what value of caps are used for the coupling capacitors and the tone stack.
Cheers!
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You could go a lot higher on the bass cap. Maybe a .1 or really whatever you want, but 100uF is insane.
Dave
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What are you trying to get out of it? Why doesn't it work as is, (for you)?
I think maybe there was a misprint on your bass cap suggestion, a 0.1 uf, (100 nf), is fairly standard on a blackface tone stack.
The other values you listed should probably work without doing anything else.
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My favorite places to mod the BJr aren't conventional places. I like to play with the taper resistor on the volume pot. 220k instead of the 120k.
Fat cap to 1μF instead of 22μF.
Add a 10μF bypass cap to the 3rd stage.
680pF bypass cap on 3rd stage plate resistor and removal of cap to ground at master vol.
Change 430k that feeds master volume to 390k.
I'd put the 100μF in place of the 47μF reservoir cap, and move the 47μF to the screen node.
Increase screen resistor to 1k.
There are a few other tricks like making the 3rd stage cathode switchable between 1.5k, 1.5k & 10μF, and 10k that you can do. The 10k cold clipper gives you a really cool overdrive flavor at the expense of some volume. Making the changes to gain knob taper and master volume help combat these. As does engaging the fat switch.
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What are you trying to get out of it? Why doesn't it work as is, (for you)?
I think maybe there was a misprint on your bass cap suggestion, a 0.1 uf, (100 nf), is fairly standard on a blackface tone stack.
The other values you listed should probably work without doing anything else.
I changed the by-pass capacitor, C1 to .47uf to get a more gradual response from the volume knob as the gain and volume was very abrupt before. It feels like the amp now has a more Marshall type tone particularly when I use a distortion pedal. The tone controls don't seem to make very much of a change so I thought maybe changing the cap values and doing the mid pot wiring mod would give me a bit more response. I have a Celestion Vintage 30 in the amp so maybe that's part of the problem?
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0.047μF on a cathode is essentially a bright cap. The bulk of the guitar fundamental frequencies are recessed by about -6dB with that value. This is why your tone controls don't do much. You've left them very little to work with.
Bump that back up to at least 0.68μF (typical Marshall value) but preferably closer to 2.2μF.
Check the taper of your pot. A 10% audio taper is unlikely to behave as you've described. I'm guessing you have either a 30% or linear taper there.
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0.047μF on a cathode is essentially a bright cap. The bulk of the guitar fundamental frequencies are recessed by about -6dB with that value. This is why your tone controls don't do much. You've left them very little to work with.
Bump that back up to at least 0.68μF (typical Marshall value) but preferably closer to 2.2μF.
Check the taper of your pot. A 10% audio taper is unlikely to behave as you've described. I'm guessing you have either a 30% or linear taper there.
Thanks kindly for your input. The C1 by-pass cap value I used is actually 0.47uf, not .047uf as I had posted. I have a .68uf and a 1uf cap I'm thinking of trying. It's a long way from the 47uf cap originally used but it seems to have made the volume pot work more progressively.
I'm using an Audio taper Alpha volume pot but I don't know if the taper is 10% or 30%. It's definitely not a linear pot because the designation on it is A1m. I'll look up the specs for it.
Could you tell me what each of the mods you've done are meant to do? I don't want to do a bunch of mods without understanding them first plus I don't want to do a bunch of mods all at the same. I know there are some mods that need to be done together to work properly.
Thanks again for your input. 🤗
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My favorite places to mod the BJr aren't conventional places. I like to play with the taper resistor on the volume pot. 220k instead of the 120k.
Changes the taper of the pot and presents a higher resistance to ground for stronger signal and high end
Fat cap to 1μF instead of 22μF.
Partially bypasses the 2nd stage for a more focused FAT setting. Less low end mud in the tone
Add a 10μF bypass cap to the 3rd stage.
fully bypass the third gain stage for approximately 6dB boost
680pF bypass cap on 3rd stage plate resistor and removal of cap to ground at master vol.
Rolls off the high end above 3kHz and removes a passive low pass filter to ground
Change 430k that feeds master volume to 390k.
Changes the voltage divider ratio of the master volume for more available signal at higher settings
I'd put the 100μF in place of the 47μF reservoir cap, and move the 47μF to the screen node.
"Stiffens" the power supply for quicker transient behavior on low notes and reduces power supply ripple
Increase screen resistor to 1k.
Reduces voltage applied to the screens and limits screen current for better protection of the EL84. Has the effect of slightly reducing max power and changes perceived compression characteristics
None of these things are exclusive to the BJr. They are common areas to make adjustments on all tube amps. It would be good to learn what's happening and why at the electronics level. You'll soon be able to devise your own mods to get where you want to go tonally.
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Thank You for those explanations Stratomaster,
I would definitely like to learn more about what's happening at the component and electrical level. While I have basic understanding of what the components do and how one section of the amp feeds into the other I have trouble understanding how some things flow within certain parts of the sections.
A good example is, if V1 gets its plate voltage from B+, where does the cathode get its voltage? I know the filaments get heated in order to get electrons to boil off the cathode towards the grid and to the plate but the cathode is tied to ground which I thought would be 0 volts. This is just one of the things I can't seem to grasp. Please excuse my ignorance of the subject. It's like magic to me.
Cheers!
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Thank You for those explanations Stratomaster,
I would definitely like to learn more about what's happening at the component and electrical level. While I have basic understanding of what the components do and how one section of the amp feeds into the other I have trouble understanding how some things flow within certain parts of the sections.
A good example is, if V1 gets its plate voltage from B+, where does the cathode get its voltage? I know the filaments get heated in order to get electrons to boil off the cathode towards the grid and to the plate but the cathode is tied to ground which I thought would be 0 volts. This is just one of the things I can't seem to grasp. Please excuse my ignorance of the subject. It's like magic to me.
Cheers!
Circuits need to complete to function. Plate voltage is present at the plate. The grid is at 0V. The cathode is elevated above ground by the cathode resistor.
Election flow vs conventional current flow trips people up, but essentially the current at the plate needs to flow to ground to complete the circuit. It gets there through the cathode with the voltage difference between cathode and ground being set by the cathode resistor.
If you're willing to put the time in to learning this will get you a long way: https://www.scribd.com/document/492056850/AX84-Theory-ax84-m35
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If the tone stack isn't toning enough, I think probably the best mod for that amp is to jump the wiper of the mid pot to the top (bass) side. It can be tested with an alligator clip, or a piece of scrap wire. It really gives purpose to every pot in the tone stack. All of the vintage fenders are wired that way, as well as the reissues for that matter.
There was a fellow who passed a few years ago, Bill M, he spent his time modding the blues jr. You can do a search and go down his rabbit hole. He had a site, which is no longer available, but it is archived, and I believe still accessible. I wouldn't recommend all the mods, but some are definitely beneficial and easy, especially since you don't have the original pcb to deal with.
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https://web.archive.org/web/20190315234454/http://billmaudio.com/wp/
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Thanks Stratomaster. Sincerely appreciate your explanation and advice. I think the electron flow vs the conventional flow is tripping me up. I'll check out the link you provided as I do want to get a better understanding of this stuff.
On another note, I've been trying to find the specs for my Aplha pot to see what kind of a taper it has and haven't been able to come up with anything. Would you have any idea where I might find it?
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Hello Newman. Sorry, I couldn't resist that Seinfeld reference. 😁
I'm going to do that mid pot mod as you've suggested before I do anything else. Just haven't been able to yet. I have heard about Bill M but have never looked up the mods he did. Thanks for the link.
Cheers!
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While I loved Bill Machrone's writing in PC Mag back in the day, and the fact that I dropped down the amp rabbit hole assisted by his website, and some of his mods, I think Stratomaster's approach makes more sense and is applicable to many amps. The BillM mods were all about how to improve the stock BJ's of the time w/o replacing the circuit board.
That said, Bill Machrone was a professional writer as well extremely knowledgeable in all things BJ, and his notes are definitely worth reading. Glad to see that web archive is working -- the last time I looked, many links were broken.
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Looks like I'm in the rabbit hole of amp mods :laugh:
I've been reading the link Stratomaster provided and it's great. There's a lot to digest and I can't say it's solved the mystery for me but it is making things a bit clearer. I'll be reading it quite a few times, that's for sure.
Thanks to AI for providing the link for Bill M. I remember when I did my Blues Jr Hoffman conversion seeing something about him but I never looked up his stuff.
I've done the mid pot mod connection to the bass control. Seems there's a bit more response with the tone controls and there's no sound if all of them are turned off. I was looking at the Princeton Reverb schematic and saw that the tone stack comes first before the volume pot whereas in the Blues Jr it's the other way around. Just curious.
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Thanks Stratomaster. Sincerely appreciate your explanation and advice. I think the electron flow vs the conventional flow is tripping me up. I'll check out the link you provided as I do want to get a better understanding of this stuff.
On another note, I've been trying to find the specs for my Aplha pot to see what kind of a taper it has and haven't been able to come up with anything. Would you have any idea where I might find it?
When in doubt, measure.
A 10% taper means when the pot is set to its midpoint that it will read 10% of its value when resistance is measured between the wiper and one outer lug. And 90% to the other outer lug. Measure first across the two outer lugs to establish the true total resistance of that pot.
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Thanks stratmaster for that information. I didn't know that's how you could determine the taper.
I wanted to ask you a question about one of the mods you've done, replacing the 120k resistor on the volume pot with a 220k one. Does this mod help with making the volume pot respond in a more gradual way or is that dependent on the pots taper? You mentioned that using a 220k resistor helped provide a stronger signal. I'm trying to understand how the volume pot resistance being in parallel with the 120k resistor works.
Thanks.
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Thanks stratmaster for that information. I didn't know that's how you could determine the taper.
I wanted to ask you a question about one of the mods you've done, replacing the 120k resistor on the volume pot with a 220k one. Does this mod help with making the volume pot respond in a more gradual way or is that dependent on the pots taper? You mentioned that using a 220k resistor helped provide a stronger signal. I'm trying to understand how the volume pot resistance being in parallel with the 120k resistor works.
Thanks.
For questions like this it's best to just sketch what's happening to help in understanding.
A pot just a strip of material of a fixed resistance that the wiper rides on essentially acting as a tap. You can think of it as two resistors in series that vary in value relative to each other as the pot is turned but always add to the same value.
With this in mind you can draw this scenario as shown. Rp1 is the resistance between where the signal enters the pot and the wiper where the signal exits the pot. Rp2 is the resistance to ground from the wiper.
This is a resistive voltage divider. The signal is attenuated by an amount determined by the ratio of Rp1 and Rp2.
Now comes the 120k, which is labeled Rt. It is in parallel with the wiper to ground portion of the pot (Rp2 by our earlier designation). To understand that it's doing let's look at the pot at max volume.
Rp1 goes to zero and Rp2 becomes 1M, but Rt is in parallel with Rp2 , meaning the overall resistance to ground at that point is the parallel sum of Rp2 and Rt. It should now be pretty clear that increasing Rt will increase the resistance to ground at max volume.
Higher resistance to ground results in higher signal voltage presented to the grid of the following stage. The frequency content will also be affected as low resistance to ground will tend to darken the overall signal.
You can repeat this exercise at a few different rotations to see the effect of the parallel resistance on the shape of the pot taper.
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Thanks stratmaster for that information. I didn't know that's how you could determine the taper.
I wanted to ask you a question about one of the mods you've done, replacing the 120k resistor on the volume pot with a 220k one. Does this mod help with making the volume pot respond in a more gradual way or is that dependent on the pots taper? You mentioned that using a 220k resistor helped provide a stronger signal. I'm trying to understand how the volume pot resistance being in parallel with the 120k resistor works.
Thanks.
For questions like this it's best to just sketch that's happening to help in understanding.
A pot just a strip of material of a fixed resistance that the wiper rides on essentially acting as a tap. You can think of it as two resistors in series that vary in value relative to each other as the pot is turned but always add to the same value.
With this in mind you can draw this scenario as shown. Rp1 is the resistance between where the signal enters the pot and the wiper where the signal exits the pot. Rp2 is the resistance to ground from the wiper.
This is a resistive voltage divider. The signal is attenuated by an amount determined by the ratio of Rp1 and Rp2.
Now comes the 120k, which is labeled Rt. It is in parallel with the wiper to ground portion of the pot (Rp2 by our earlier designation). To understand that it's doing let's look at the pot at max volume.
Rp1 goes to zero and Rp2 becomes 1M, but Rt is in parallel with Rp2 , meaning the overall resistance to ground at that point is the parallel sum of Rp2 and Rt. It should now be pretty clear that increasing Rt will increase the resistance to ground at max volume.
Higher resistance to ground results in higher signal voltage presented to the grid of the following stage. The frequency content will also be affected as low resistance to ground will tend to darken the overall signal.
You can repeat this exercise at a few different rotations to see the effect of the parallel resistance on the shape of the pot taper.
Thanks kindly for explaining that. I kind of understood what was happening but your explanation has really made it much clearer.
I did as you suggested in order to determine the taper of my volume pot. At the mid position the resistance between the outer lug where the signal comes in, to the middle lug (Rp1) is 971kohms. From the center lug to the other outer lug to ground (Rp2) it's 58kohms. Outer lug to outer lug it's 1.025Mohms.
For a 10% taper then, the resistance should read 102.5kohms on one side and 922.5kohms on the other side. For a 30% taper, the readings would be 307.5kohms and 717.5kohms. Based on my readings then my taper appears to be around 5.5% (58/1025). If this is the case I would expect the volume to increase in a much more docile manner than it currently does. Have I done something wrong measuring or hooking the pot up maybe?
Thanks.
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Pots aren't precision devices unless you specifically order precision ones. Yours is near enough to 10%.
That's about a gradual as it gets. I'm not sure why the amp is coming on so suddenly.
Here's where I learned most everything I know about pots if you want to dive in. http://www.geofex.com/article_folders/potsecrets/potscret.htm
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Thanks stratomaster. I think I'll try using a 220k resistor and see how not only how the volume responds but if there's a bit more treble available. I'd like to get some more highs from the amp if that's possible especially with my humbuckers. The coils are split for single coil use as well and they could use a bit more highs also.
Cheers!
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Thanks stratomaster. I think I'll try using a 220k resistor and see how not only how the volume responds but if there's a bit more treble available. I'd like to get some more highs from the amp if that's possible especially with my humbuckers. The coils are split for single coil use as well and they could use a bit more highs also.
Cheers!
Lift C9 at the junction of the 430k resistor and master volume pot. That will definitely get you more highs. If it's too much you can add a resistor in series with that cap or do the plate bypass I suggested on the 3rd stage
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Thanks for the tip stratomaster. I'll try that out and let you know how it goes. There are so many different mods that can be done trying to determine which are suitable for what you want isn't easy. The more I read the deeper into the rabbit hole I go. As they say, in for a penny, in for a pound. 😁
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Hi stratomaster, just wanted to confirm that I've understood these mods you suggested.
1. Add a 10μF bypass cap to the 3rd stage.
10uf cap goes across R17 of V3.
2. 680pF bypass cap on 3rd stage plate resistor and removal of cap to ground at master vol.
680pF cap goes across R16/100k plate resistor of V2.
Remove C9, .001uf cap after R18/430k resistor going to Master Volume.
3. Increase screen resistor to 1k.
1k resistors replace R35 & R36, 100ohm at pin 9 of output tubes.
Have you changed the value of your bypass cap at V1 from 47uf to another value while doing these mods or did you keep the value the same? I was playing around with a program that shows how the frequency response changes when you change this capacitor. There is a very flat boost across the spectrum with the 47uf cap which to me explains the bassiness of the amp. I'm waiting for some parts from Doug to try a couple of different capacitors at C1 as well as maybe some resistors.
Is there an order you would suggest in doing the mods?
Thanks.
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I have played with partially bypassing the first stage. While it did remove some of the bassiness it also substantially increased noise. I'd recommend against it in this amp. I've even started avoiding this practice on Princetons and partially bypassing the 2nd stage instead.
There's a thread I started a few months back where we discussed this phenomenon if you're interested in the technical details. The Valve Wizard himself even weighed in. Based on the comments there I've intentionally increased first stage bypassing in designs where the first stage is intended to be fully bypassing to reduce the impedance to ground for noise. 100-220μF on first stage, partial bypass on 2nd stage.
I think you'll find changing the FAT cap to 1uF to be closer to what you're looking for.
Instead of partially bypassing the first stage you can reduce the coupling cap coming out of the first triode. Be judicious with the decrease in cap size though. It can easily go too far.
As far as order, take out C9 and see if you like the highs first. If removing this cap increased brightness too much, then move onto the plate bypass cap on R16. You may need to experiment here as 680pF will be a bit dark if you think the amp is lacking highs in its current state. 330-470pF might be a better starting point.
Then put that cap you removed in place of the .0022 coupler off the first stage (assuming it's a 400v or better cap). See if that tames the lows enough for you. If it's too much, then you can try the two values in between in the future (.0015, .0018)
Then the 1k screens. You may not notice a tone change with this.
Afterthought: if you like the 0.001μF in C2 but still think the amp is a bit bottom heavy, put the 0.0022μF cap you removed from C2 as the PI input cap (C11). Save the cap you removed from there for another mod to tighten it up to be discussed later if necessary.
Afterthought 2: You mentioned the volume control coming on too quick. I think if you add a 100k resistor in series with C2 (you can do this at the pot lug) the rapid volume transition will shift a bit later. You'll lose the last 9% or so of signal, but you wouldn't notice unless you're regularly diming the amp. And even then the difference would be minimal.
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Thank you so much stratomaster for your detailed reply and suggestions. So far I've tried using a .47uf bypass cap at C1 first but that didn't really help that much with the volume jump I was experiencing. I just tried using a 12AY7 in V1 position and that really tamed the pre-amp and also made the volume have a nicer transition but now the breakup occurs around 9.
I think I'll try putting the C1/47uf back in and see what happens then I'll try lifting C9/.001 cap and see how highs are. The .47uf cap I'm using in C1 isn't polarized and I'm wondering if that's making a difference as well. From what I understand the polarized cap cuts off a portion of the negative part of the input signal to ground. Is this correct?
Thanks Again very much. Cheers!
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Sounds like you have a good plan. I'd encourage putting C1 back to 47μF (or even increasing it for less noise) and instead cutting C2.
Polarized vs non polarized for the cathode bypass makes no difference. The cathode of the tube is positive relative to ground, so the directionality of the polarized cap must be correct for the cap to remain functional and intact. But once that's set, they'll operate identically. Read a bit on this subject as your understanding isn't correct. What you're describing sounds more like a hard clipping diode to ground.
You can try a 5751 in V1 to land in between the 12AX7 and 12AY7 in terms of gain. I think the 100k series resistor idea will probably get you there without having to mess with less popular tube types.
You can also try leaving C1 out entirely. But this would increase noise similar to the partial bypass.
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Thanks again stratomaster for the great advice. I'll have to read up about capacitors a bit more. Maybe I misunderstood the video I saw about the by-pass cap so I'll watch it again. Your help is greatly appreciated.
Cheers!
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Hello stratomaster,
I just watched a great video about the effect of cathode resistor values on gain and tone of the pre-amp stage. The video dealt with the second gain stage of a JCM 800 and used 3 different value resistors, 10k, 3.5k and 2.5k. The 10k resistor gave the cold clipping you were talking about and strangely only clipped the top part of the signal when the gain was turned up but it also gave the least amount of gain compared to the 3.5k and 2.5k. The lower values also clipped the top and bottom of the signal when the gain was turned up. The video also discussed the role of the by-pass capacitor and how it's value affects the frequency response in a 12AX7 tube. There were references to the Valve Wizard and his frequency response chart. Lots of good information for me to digest.
I've re-installed the 47uf by-pass cap in the amp but haven't had a chance to try it out with the 12AY7 I've put in V1. I'll try it that way before I lift the C9 cap to see how the highs are affected.
Cheers!
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Yes. Typically a cold clipper isn't bypassed. You take an instant ~6dB gain hit by foregoing the cathode broad cap. Plus it shifts the bias point of the triode cold enough to intentionally reach cutoff/clipping on one end of the swing. It's a terrible practice in theory, but it creates a nice asymmetric clipping effect. With clipping comes a decrease in voltage gain as well.
This is why I do this to the 3rd stage and engage the fat switch. This means I have 2 bypassed gain stages, plus an increase in voltage from changing the tapering resistor on the volume pot, driving the 3rd stage. This was something I took from the Trainwreck schematics. The JCM800 only has one stage before the cold clipper. My approach has 2 and plenty of gain to spare (plus the changes to the master volume voltage divider after the cold clipper).
It's easy enough to put on a switch or just install to experiment with. If you're more of a clean player, then stick with the standard cathode arrangement.
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Thanks for the feedback stratomaster. Interesting how the asymetrical clipping of the 10k resistor results in a different tone and how different values change the gain and tone. There's quite a difference in the voltage swing between those resistors that represents the gain. With the 10k resistor one side of the voltage swing is so close to the clipping point of the load line. Most of the stuff I play isn't absolutely clean and I use several distortion pedals so I want the amp to work well with them.
Cheers!
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Hello stratomaster. This is the current status of the amp.
1. 47uf by-pass cap on first gain stage of V1
2. 12AY7 used for V1
3. C9/.001uf cap lifted from ground.
My Observations:
1. Gain from volume control reduced greatly but volume increase response is more gradual with breakup ocurring around 9-10.
2. Significant increase in bass frequency response before lifting C9 cap.
3. Lifting C9 cap increased treble frequencies but still feel more required.
4. Amp feels a bit lifeless.
My feeling is that the 12AY7 is way to low in gain and that maybe a 5751 would be better. Before trying one I'm going put back the 12AX7 to see how the amp responds with C9 lifted. From what I've been reading another way to reduce the gain and still use the 12AX7 would be to reduce the plate resistor value from 100k to 68k or 47k. I don't know how this would compare to just using a 5751 tube. Not sure what to do about getting more treble response but I should probably wait until I'm satisfied with what changing the gain does first.
Any words of wisdom?
Thanks.
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1. 47uf by-pass cap on first gain stage of V1
2. 12AY7 used for V1
3. C9/.001uf cap lifted from ground.
My Observations:
1. Gain from volume control reduced greatly but volume increase response is more gradual with breakup ocurring around 9-10.
2. Significant increase in bass frequency response before lifting C9 cap.
3. Lifting C9 cap increased treble frequencies but still feel more required.
4. Amp feels a bit lifeless.
...
Any words of wisdom?
Before you change anything try swapping the V1 and V2. If this is an improvement, then read on.
I don't like 12AY7s and prefer to cut gain through strategic use of voltage dividers instead of reduced plate resistors or odd tube types.
My recommendation is as outlined in a previous post. Use 12AX7s in all 3 locations. Add a 100k to 220k resistor in series with the input side of the volume pot. This will cut gain without the need for oddball tubes. If you don't like the pot taper after this then you can increase the 120k tapering resistor to 180k or 220k.
Replace the 0.0022μF coupler coming out of V1a with the 0.001μF cap lifted from C9.
This should bring the amp back to life.
Note: some versions of the production BJr have a bright cap on the volume control. You can also try this, but I'd recommend doing the above first.
My hypothesis as to why your amp sounds lifeless now: it's because you've reduced the 1st and 2nd stage to 40% of the intended gain with the 12AY7. Gain is multiplicative, so the effect is compounded by the substitution of two lower gain triodes in series. Additionally the 2nd and 3rd stage are unbypassed unless the FAT switch is engaged. This presents a weak signal to the PI.
Regarding the effect of lifting C9 - this cap forms an aggressive low pass filter in the stock schematic--rolling off everything above about 370Hz. By lifting the cap you should be hearing a dramatic increase in high end. I think your low preamp gain is the reason you aren't hearing the full effect of this change.
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Thanks kindly stratomaster for the sage advice. I suspected that the 12AY7 was really killing the amp. I will do as you've suggested and swap V1 and V2 first to see what happens and take it from there. I agree about not playing with the plate resistors to alter the gain.
There's a program I was playing with that lets you change the values of caps and resistors as well tube type to see the changes in frequency and gain and I don't think changing the plate resistor value to reduce gain is that effective. Plus from what I've read, the 100k resistor seems to be the sweet spot for the 12AX7 because it's very close to the tubes own internal resistance.
Well, time for some more fiddling. Cheers!
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If you have a 12AT7, or a comparable tube (maybe 12AY7, or 12AU7), they often work really well in the PI position in a push pull fixed bias amp. It may require some adjustments as far as bias and how the cathode is elevated. If you're really interested you may have to go down another rabbit hole and draw out some load lines.
Other than that, I agree, changing tubes in the preamp positions are really more of a bandaid than a solution.
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Thank You kindly for your input Newman. It happens that I have a 12AT7 to try in the PI section. I've just finished testing swapping V1/12AY7 and V2/ 12AX7 and will be posting my observations. I will definitely try out the 12AT7 in the PI position as even some reading I've done suggests its good in that position because of its characteristics.
Thanks again.
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Hello stratomaster,
I've done the tube swap as suggested and the amp sounds a lot better. The highs are coming through a lot more, the tone controls are responding better and amp has life. Sounds like a completely different amp now. There was no volume jump either. Definitely going in the right direction.
Should I now put a 12AX7 in V2 and attach a 100k or 220k resistor at the input of the volume pot to see how that works to control the gain?
Thanks.
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That's what I would do, with one caveat below. Glad we're moving in the right direction.
I just noticed that the Hoffman schematic has both triodes of V2 wired in parallel, but with the stock plate and cathode resistor values for a single triode. This may be at the root of your volume jump observation, namely the plate resistor. This change from stock dials up the gain of that stage a good bit. If you're happy with the tone of the amp as it is now, then you can leave it as is. Otherwise try the following.
1) Put a 12AX7 in V2 and test with no further changes. This should bring back your volume jump.
2) Add a 100k in parallel with the plate resistor for V2. (I know I said I prefer not to mess with plate resistors earlier, but this is a parallel triode in a place I wasn't expecting it). This should result in a tone response similar to what you have now, but with a bit more gain and character (I'll have to do some math to confirm this statement, but I'm pretty confident on it based on intuition.)
3) If the abrupt volume jump returns after step 2, try the series resistor to the volume control.
4) if the bass response is still too much sub in the 0.001μF you removed from C9 into C2.
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Thanks stratomaster.
I had questioned the parallel connections of the plate and cathode possibly being the problem not because I understood what was happening but because it was suggested to do this to obtain more gain and it was the only change to the tube. It was an intuitive guess 😂.
I'll do as you've suggested and report back. Cheers!
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Reporting back stratomaster.
Switched in the 12AX7 in V2 in place of the 12AY7 and was pleasantly surprised. There was no jump in the volume. Increase was nice and progressive and the amp tone was the same. I was sure the amp volume jump was going to come back. Guess I won't have to put that resistor in series with the input of the volume pot after all to tame the gain, or change the value of the taper resistor on the volume pot either. Only thing left for me to do is take the C9/.001uf cap I removed and put it in place of the C2/.0022uf to see if I can get a bit more treble.
I don't know if this is pertinent or not but will ask anyway. I have two Electro Harmonix 12AX7's and one JJ 12AX7 in my amp. Originally I had the JJ in V1 but now it's in V3. What are the chances that the JJ tube was causing my volume jump problem to begin with? The reason I ask is that except for having removed the C9/.001cap everything is back as it was when I was having the volume jump problem to begin with. Could there be a problem with my JJ tube?
Thanks.
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Reporting back stratomaster.
Switched in the 12AX7 in V2 in place of the 12AY7 and was pleasantly surprised. There was no jump in the volume. Increase was nice and progressive and the amp tone was the same. I was sure the amp volume jump was going to come back. Guess I won't have to put that resistor in series with the input of the volume pot after all to tame the gain, or change the value of the taper resistor on the volume pot either. Only thing left for me to do is take the C9/.001uf cap I removed and put it in place of the C2/.0022uf to see if I can get a bit more treble.
I don't know if this is pertinent or not but will ask anyway. I have two Electro Harmonix 12AX7's and one JJ 12AX7 in my amp. Originally I had the JJ in V1 but now it's in V3. What are the chances that the JJ tube was causing my volume jump problem to begin with? The reason I ask is that except for having removed the C9/.001cap everything is back as it was when I was having the volume jump problem to begin with. Could there be a problem with my JJ tube?
Thanks.
Differences in amp tone with tubes in different positions are definitely real-- though I'm unaware of any failure mode that would manifest as a repeatable volume jump. The volume control isn't affecting the amplification factor of the triodes, just the voltage into the 2nd triode-- so I'm skeptical about the tube being the cause of what you observed. I'm willing to bet both tubes are just fine. And I'm glad that behavior is resolved.
There are a number of ways to get more treble. The coupling cap isn't one of them. It will roll off low end. This might give the effect of more treble, but it's roundabout.
To directly affect treble response here are a few options: You can try a bright cap on the volume pot. You can put the 0.47μF cap you used to have in C1 in parallel with the 1.5k cathode resistor for the 3rd stage. You can put a 330-470pF cap in the tonestack in place of the 250pF. You can even play games with the negative feedback to create a presence control that cancels fewer of the high frequencies. It all depends on how far you want to take it.
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My Thanks again stratomaster for your advice.
The amp is working quite well now but I could maybe use a bit more treble. I don't mind trying a few more things to see what happens. So changing the C2 coupling capacitor isn't really going to increase treble, just reduce bass and even though it might sound like I'm getting a bit more top end it's really not going to contribute that much.
I'll try putting the 0.47uf by-pass cap I had used in the first gain stage as the by-pass cap for the third gain stage across R17 cathode resistor as you've suggested and see how that goes. Now that I understand what the by-pass cap does better and it's effect on frequency response in the tube I see why you've suggested using it.
You've suggested changing the C5/250pf cap for the treble control but what do you think about changing the cap values for the bass and mid controls?
Thanks again and Cheers!
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You're more than welcome to experiment with the low and mid caps, but they'll be roundabout ways to more treble as well.
I personally like low mid heavy tones with a warmer high end, so I gravitate towards 250pF, 0.1 bass and 0.022 mid with a 75k slope resistor. A 0.047 mid cap will thin out the low mids. And dropping the bass cap will help with the sub-bass/boom.
Raising the slope resistor is another way to tilt the tone towards the treble. You can try 120k-150k.
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Thanks stratomaster.
I was thinking of using a .1uf cap for the bass and a .047uf cap for the mids but for now I'll leave everything as is until I see what effect putting the .47uf by-pass cap on the 3rd gain stage does.
I'm reading some information on low and high pass filters and how to calculate their cut-off frequencies to get a better idea of what value capacitors I might want to use in the tone stack.
I'll get back to you once I've tried the 0.47uf by-pass cap in V2.
Cheers!
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R11 is the slope resistor. It basically sets how much of the signal remains in the treble filter vs passes through to the bass and mid filters. The larger this resistor, the more high end in the signal. The smaller, the more bass and mid contribute to the tone.
It also sets works to set impedance for the previous triode, but that only becomes important with lower values as the effectiveness of your tonestack will diminish.
I'd recommend not increasing the bass cap from stock if you think your amp is currently too bassy. If you have another 0.022μF cap you can just put that in parallel with the mids cap--close enough to 0.047. This will greatly sculpt the low mids. And may be a better tweak for you than the C2 mod.
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Thank you again stratomaster.
I've been reading an article by Rob Robinette about how the TMB tone stack functions and there's a lot to take in. I didn't realize there was so much interaction ocurring betwen the tone controls themselves and that the slope resistor value affects what frequencies are blocked or passed.
My basic takeaway is that I should stick with the 0.022uf cap for the bass and maybe go with a 0.047uf cap for the mids. The .1uf cap for the bass seems to be more oriented around getting the Fender shimmer because of the way the treble and bass interact at their midpoints. It has something to do with phase shifting which is way above my pay grade.
Should I still try putting the 0.47uf cap across R17 or put a 0.047uf cap on the mid control in place of the 0.022uf already there? There are so many questions I have but I don't want to hog your time.
Thanks again. Cheers!
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My basic takeaway is that I should stick with the 0.022uf cap for the bass and maybe go with a 0.047uf cap for the mids.
I like this approach. However, it is going to reduce low mids and bass, not increase treble. It could be what you're after.
The .1uf cap for the bass seems to be more oriented around getting the Fender shimmer because of the way the treble and bass interact at their midpoints. It has something to do with phase shifting which is way above my pay grade.
I haven't gone down the rabbit hole of phase shift in the tmb tonestack and prefer to look instead at the frequency response curves generated by something like the Duncan Tonestack Calculator tool.
There may by something to the phase shift, but I prefer to get the frequency content I'm looking for over mythical swirl qualities.
Should I still try putting the 0.47uf cap across R17 or put a 0.047uf cap on the mid control in place of the 0.022uf already there? There are so many questions I have but I don't want to hog your time.
You can try either or both. The 0.47μF cap across R17 will be an aggressive treble/mid boost akin to a 6db high shelf on everything above around 500Hz. I plotted 3 tonestack curves below for reference and discussion.
The 3 tonestack curves below are with all controls at halfway with the tapers in the BJr taken into account.
Green: Your current tonestack
Orange: T-250pF. M-0.022μF. B-0.1μF
Blue: T-470pF. M-0.047μF. B-0.022μF
You can see from the orange curve why I don't recommend increasing the bass cap.
The blue curve has an appreciable decrease in lows and low mids (not aggressive though) courtesy of the 0.047μF mids cap. Additionally there's a drastic increase in upper mids and treble content courtesy of the 470pF treble cap.
The slope resistor is 100k in all 3 curves.
Don't worry about taking up my time asking questions. I enjoy the problem solving and working with someone who wants to learn and is open to my ideas.
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I'm starting to suspect there is a problem with the negative feedback in your build, namely the "Meat" control. Your amp should be almost painfully bright after removing C9.
Would you mind removing the connection at the speaker jack to the negative feedback circuit and seeing what effect this has on your sound? I'd expect the amp to get noisier, louder, and dirtier. If it also gets substantially brighter, then my suspicions about the NFB circuit are validated.
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That's fantastic stratomaster!
Thank you for sharing your knowledge and willingness to entertain what are probably some dumb questions. I honestly didnt think I'd get this deep into this stuff but the more I learn the more I want to learn. It's pretty obvious from the tone stack curves you plotted that I don't want to use the 0.1uf cap for the bass pot. Plotting these curves makes it so much easier to understand the effects of different tone caps and slope resistor values. The knock against the Blues Jr has been that it's boxy sounding because it was very mid rangey.
I'm going to try the 0.047uf cap for the mid pot and the 0.47uf cap for the by-pass mod first as I don't have a 450pf cap right now and see how it goes. I'll get back to you with my observations.
Cheers!
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I'm starting to suspect there is a problem with the negative feedback in your build, namely the "Meat" control. Your amp should be almost painfully bright after removing C9.
Would you mind removing the connection at the speaker jack to the negative feedback circuit and seeing what effect this has on your sound? I'd expect the amp to get noisier, louder, and dirtier. If it also gets substantially brighter, then my suspicions about the NFB circuit are validated.
No problem. I'll do that before I do anything else and get back to you.
Thanks.
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I disconnected the meat pot feedback line from the speaker input jack and am getting no sound from the guitar at all. There's sound from the amp when I turn both the volume and master volume up full that's a bit noisy but not that bad.
I re-checked my connections going to the speaker jack from the OT and they're fine. My feedback wire was connected to the positive terminal of the output jack where the green wire from the OT was connected to. I can't see why there'd be no sound just because I disconnected the NFB from the output jack. I'm puzzled.
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If you're getting amp noise then something before the volume control is the problem and it likely has nothing to do with cutting the NFB line.
Could be internal to the amp, could be input jack, could be the cable, guitar, etc.
Try plugging in directly. No guitar. Then touch the tip of the cable plug with your thumb. If you get buzzy thumps then suspect your guitar wiring. Which may have been at the root of your high end problems.
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Plugged a cable directly into the amp with no guitar. Touched the other end of the cable with my finger and no sound. It's acting exactly the same as when I had my guitar plugged in.
The amp was working perfectly fine the last time I turned it on. Should I try re-onnecting the NFB line back to output jack?
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There is no failure mode where removing negative feedback causes a problem with signal entering the amp before the first volume control. You said you could hear noise from the amp that corresponds to turning the knobs. This rules out a speaker jack issue.
Try different cables, check the solder joints on the input jack. Make sure the input jack is tight. Check if you accidentally bumped a wire, left a piece of solder or a lead or an alligator clip, desoldering braid (guilty myself of the last two). Make sure the loose wire from disconnecting the NFB is tucked away from making contact anywhere or taped off
Push on the jack tip itself with the amp on. You should be able to hear it short/unshort. Use a non conductive probe and be careful with the live amp (all the usual precautions).
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Problem found.
V1 was not functioning. Somehow, when I swapped tubes, Pin 1 got bent so I had no B+ voltage going to the plate. No wonder there wasn't any sound. I straightened the pin out, put the tube back in and sound was back.
The amps has a bit more noise but sounds much better. The notes have more clarity and the amp sounds even more alive and responsive. I wouldn't say it sounds shrill at all, just clearer. I have to play around with it a bit more but I like what I'm hearing. Is it the NFB then that's causing a problem? I don't know what it's supposed to do exactly as I haven't read up on it yet.
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Problem found.
V1 was not functioning. Somehow, when I swapped tubes, Pin 1 got bent so I had no B+ voltage going to the plate. No wonder there wasn't any sound. I straightened the pin out, put the tube back in and sound was back.
The amps has a bit more noise but sounds much better. The notes have more clarity and the amp sounds even more alive and responsive. I wouldn't say it sounds shrill at all, just clearer. I have to play around with it a bit more but I like what I'm hearing. Is it the NFB then that's causing a problem? I don't know what it's supposed to do exactly as I haven't read up on it yet.
The negative feedback is an out of phase lower amplitude signal that is applied at the 2nd input of the phase inverter. It is meant to add stability and reduce distortion and noise in the amp. It works by mixing this mirror (out of phase) signal with the signal from the preamp applied to the phase inverter--essentially subtracting from the signal to prevent it from getting too out of hand.
With the NFB disconnected you're hearing the raw sound of the amp. In amps not originally designed to forgo NFB this can result in objectionable noise levels and instability at higher volumes. But we can usually get away with it in my experience.
In theory with the "Meat" control set to max the negative feedback is essentially defeated and the response should be nearly identical to disconnecting the feedback. I have a suspicion that there's a wiring or component value error in your amp OR the fixed presence filter network (7.5k and 0.1μF) is insufficient for your needs, OR perhaps both.
To test this theory you can reconnect the NFB and max out the Meat control. This should sound nearly identical to what you hear with the NFB disconnected. As you reduce the Meat control, the effect of the NFB will become more apparent AND the fixed presence network will be more effective at affecting the high end.
I'd recommend playing around with this control and examining the components in the NFB loop and PI tail to make sure there isn't a value error. Particularly the 7.5kΩ resistor. If this is accidentally say a 75kΩ then the amp could feel muted in the high end. One of the Bill M mods was to replace this resistor with a 10k to 25k pot for more control over the Presence. This is an option for you as well.
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Thanks stratomaster.
I'll re-connect the NFB and do as you've suggested with the meat control as well as measuring the component values. I kind of understand your explanation of how the NFB works. It's basically feeding back part of the output signal to the PI cancelling out some of the original signal to keep it from getting out of hand and reducing noise. At least that's how I understand it in the simplest terms.
Will report back with my results as soon as I can. Cheers!
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Hello stratomaster.
With the NFB re-connected, when I turn the Meat control to it's minimum position (clockwise) the amp sounds pretty well the same as it did with the NFB disconnected. When I turn the Meat control towards its maximum position (counter clockwise) the highs start to go away and the amp becomes a lot more muted sounding. Not good.
I've checked the values of the components you suggested to and they all measured correctly. So right now only the C9/0.001uf cap is disconnected from the circuit and everything else is per the schematic. I'm curious as to why the amp doesn't sound as shrill as you expected with this cap removed. It's actually sounding pretty good as is.
What say yee?
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Could be a matter of preference: ie what's shrill to me is bright and clear to you.
Could be the speaker you're using is muted in the high end.
Could be a value error somewhere in the build causing additional unintended high frequency roll off.
Could be a build error bleeding off high end.
Without seeing and hearing it, I couldn't tell you for sure.
My guess was that there was a wiring or component value error in the fixed presence network that was muting your high end. If there is no error, you could just not like what that filter is doing. With the meat control set for minimum feedback the effect of that network is essentially zero. But as you add in the feedback, you're also increasing the signal into that presence filter.
With negative feedback the effect on tone is the opposite of the effect on the feedback signal. What I mean by that is if you filter the highs out of the NFB signal, then there are fewer highs being subtracted from the overall tone. The 7.5k resistor is decreasing the effect of the 0.1μF cap it is in series with. That cap is shunting high frequency NFB content to ground. If you decrease the resistor, then you should be shunting more high frequency NFB signal and therefore get brighter output at the speaker.
You can give this a try. Put a resistor in parallel with the 7.5k. Start with something between 2.2k and 10k--whatever you have on hand is fine. Hopefully this gets you closer to the high end you're looking for at higher NFB (meat) settings.
Note: This should not affect your tone at the minimum feedback setting on your Meat control because of reasons discussed earlier. So you won't lose that tone that you're liking. We're just trying now to get the amp to be more usable over the entire range of that control.
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Once again thanks stratomaster.
Tone is very subjective so yes, maybe what's shrill to you isn't to me. I am liking the sound I'm getting right now compared to how it sounded before but I certainly don't mind trying a few more things and seeing where that takes me so I'll try putting another resistor in parallel with the 7.5k one and see how that sounds.
I didn't mention in my earlier post that the volume control has a gradual increase in it and not that sudden jump I was getting between 2 and 3 previously but now with all 12AX7'S in the amp the volume is higher at lower volume levels and breakup is sooner. I might try the 12AY7 in V2 again just to see.
I have a Celestion Vintage 30 in my amp that might be contributing to that more muted high end I was experiencing. I had even installed a 0.068ufcap in my guitar to try and coax some highs out. Another factor may be the pickups in my guitar. My neck PU is around 10k and the neck around 12k. The hotter pickups get the more compressed the sound with less highs. Maybe a combination of things between the amp and the guitar contributed to what was going on.
I'll let you know how it goes with that resistor. Cheers!
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I find Vintage 30s spikey in the high mids and treble, so I don't think that's it. It could be the overwound pickups. For that you want to go higher in resistance on the pots, not lower in value of the tone pot. You're attempting to affect the resonant behavior of an RLC circuit, so R gives you the biggest bang for the buck. Plus the tone cap only really comes into play as you roll the pot down.
I think your best bet is to put another 100k in parallel with the 3rd stage plate and stick to the 12AX7, but if you're getting where you want to go with the 12AY7 then have at it. Ultimately it's your amp. I just want to help you get where you want to go with it.
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Thanks kindly stratomaster. I haven't tried anything yet as I've been busy with some guests from out of town. I will try putting another 100k resistor across the plate of V2 to cut down the gain of V2 and see how that goes. Is there a tonal difference I might notice?
I believe my tone pot is 500k but I'll have to check. I use a push pull pot so I can split my humbuckers. Unfortunately there's no way to use two different capacitor values for humbucking and single coil without getting into drilling holes and using toggle switches.
Should I still try putting the 0.47uf cap across the cathode of V2 as a by-pass cap to tame a bit of the low end? It's not a big deal to do and it would be interesting to see the results. Should I try hooking up that 0.001uf cap that I currently have disconnected? Am I wrong but do smaller cap values block high frequencies and let lower frequencies pass? I was under the impression that the 0.001uf cap would allow lower frequencies to pass to ground but obviously that's not the case if lifting it gives you more highs. I seem to have the wrong impression.
Thanks again. Cheers!
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The only mods I've recommended that will reduce lows are replacing C2 with 0.001μF and increasing the mids cap to 0.047μF.
The 0.47μF cathode bypass cap will actively boost upper mids and highs. It will not reduce the low end content you currently have.
Adding the 0.001μF back in will be counterproductive. It is forming an aggressive passive low pass filter with the 430k resistor that precedes it. I'm not sure why you'd want to put that in. The cap is passing high frequencies--but to ground and out of the signal path.
As far as the tonal changes with reducing the 3rd stage plate resistor you'll notice less compression, less gain, and more perceived headroom.
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Thanks for clarifying stratomaster.
I see how the 0.001uf cap forms a low pass filter with the 430k resistor I just didn't understand how it worked. The high pass low pass filter thing sometimes confuses me but your explanation has made it clear why I wouldn't want to re-connect it. Is the 430k resistor of any consequence because it still remains in the circuit?
The other mods you mentioned for increasing the highs are still doable and even putting the 0.47uf by-pass cap is on the table. Maybe boosting the mid-highs and highs will help. They're easy mods to do and interesting to see what happens.
What kind of speaker do you use in your amp? I can't say I've been impressed with my Vintage 30 and had been contemplating a speaker change but when I started looking I just became more confused. Any thoughts on what might work better with this amp?
Cheers!
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Is the 430k resistor of any consequence because it still remains in the circuit?
Yes, the 430k forms the upper portion of a voltage divider along with the input lug to wiper of the Master volume pot.
The wiper to ground is the lower leg. You can modify this resistor to change the maximum amount of signal at the input of the PI. If you go back to my original mod list I think I mentioned this. There's a practical lower limit on the resistor though and you'll not want to drop this too much, especially if you're trying to stay clean and less loud.
The other mods you mentioned for increasing the highs are still doable and even putting the 0.47uf by-pass cap is on the table. Maybe boosting the mid-highs and highs will help. They're easy mods to do and interesting to see what happens.
Yes, you should definitely tweak and experiment--especially if you aren't quite there with the amp. It's the best way to learn.
What kind of speaker do you use in your amp?
I haven't had a Blues Jr in over a decade, but when I did I put a Cannabis Rex in it. Remember I like warmer tones. After my mods I've heard a Texas Heat sound amazing in a customer amp.
I can't say I've been impressed with my Vintage 30 and had been contemplating a speaker change but when I started looking I just became more confused. Any thoughts on what might work better with this amp?
Vintage 30s have a strong following. I don't particularly like them, but they definitely do the bright and tight tones well.
If I were to recommend something similar but a bit more to my liking I'd pick the G12-H30, but speakers are very subjective and there are almost as many options as there are opinions on each option. I'm going to have to punt on a direct recommendation for your needs knowing that our preferences are so different.
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Thanks stratomaster.
I appreciate the feedback and candor. You're absolutely right about speaker choice. It really is subjective and dependent on the amp and sound you're trying to achieve so I can't blame you for not wanting to make a suggestion. I'll try the few remaining mods in my amp and see how that goes.
I do have a question for you. What are your thoughts on using a 12AT7 in the PI position? The amp is really loud even with master volume turned way down. Would this help reduce the loudness and would I have to change some of the values of the components of V3? I know you recommended me adding another plate resistor in parallel on V2 to reduce the gain. Would this be enough to have the same effect?
Thanks again. Cheers!
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The real fix is to replace the 50k linear Master Volume pot with an audio taper.
A reduced plate resistor on V2 will reduce gain. A 12AY7 in V2 will reduce gain. Separating the parallel triodes will reduce gain. All of these things will result in a quieter amp, but I think the taper of the MV pot will continue to be an issue even with these measures.
I'll have to draw some load lines, but I think you're ok 1 for 1 swapping a 12AT7 into the PI with this design. It's an easy thing to try and see if you like--especially if you already have the alternate tubes in your possession.
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Hey stratomaster,
I replaced V2 with a 12AY7 and the there's still plenty of volume available even with the volume control set low. The only time the gain really dropped was when I had the 12AX7 in V1. With the 12AY7 in the V2 position I don't feel like I lost too much gain.
The amp sounds pretty good to me the way it is now so I may not do any of the other mods discussed for now. I would like to try the 12AT7 in the PI position but will wait to hear back from you first. I've read about the load lines for different tubes but haven't tried actually trying to draw one.
Thanks.
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I think you're right about using a 50k audio pot for the master volume. Been looking for one and not having any luck. It seems the audio taper ones from Apha jump from 25k to 100k. I'll have to see if there's one from CTS I can source.
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Use a 100k pot and reduce the resistor. A 390k is the closest standard value.
As far as a 12AT7 in the PI, go ahead and try it. You'll get a slight drop in voltage gain which may not be enough to really notice. You can play games with the plate resistors and bias resistor to try and eek out another dB or two, or better balance the outputs--but that's gilding the lily stuff you can tweak once you're in the ballpark of what you want.
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Thanks stratomaster. I can get a 100k audio pot no problem and may already have a 390k resistor. I'll give the 12AT7 a try and what happens.
I was trying to understand more about the long tailed pair and the NFB portion of the circuit in the amp. I've read about what the long tailed pair does in the PI and how the NFB works but I'm having difficulty understanding this combo circuit in this amp. Could you give me a short description of how this section of the amp works? Everything I've looked at is completely different than this amp.
Thanks and Cheers!
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I was trying to understand more about the long tailed pair and the NFB portion of the circuit in the amp. I've read about what the long tailed pair does in the PI and how the NFB works but I'm having difficulty understanding this combo circuit in this amp. Could you give me a short description of how this section of the amp works? Everything I've looked at is completely different than this amp.
Looks like a normal long tail pair PI. Doug's has an added meat control.
Print out a copy of the Blues Jr. and Doug's Blues Jr. and compare the PI's/NFB loop/meat control. Look for any extra parts.
Link for Rob Robinette amp site, resonance/meat control;
https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control
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Randall Aiken at Aiken Amps has an excellent technical write up on the long tail pair. Just example circuit is nearly identical to the Hoffman BJr. https://www.aikenamps.com/index.php/the-long-tail-pair
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Thanks for the references stratomaster.
Cheers!
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Link for Rob Robinette amp site, resonance/meat control;
https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control
I'm pretty sure the Meat control isn't a Resonance control. The latter is just an adjustable passive high pass filter similar to what you'd find in a G&L guitar. The Meat control as implemented here looks like a basic pot as variable resistor in series with the 75k feedback resistor. It's just changing the ratio set by the feedback resistor and the 4.7kΩ tail. The 22μF is there to keep DC off the pot--and is intentionally made very large to not affect the guitar frequency range at all (this could easily be a 1μF cap and function exactly the same). There's a DC offset caused by the same 4.7kΩ resistor that would otherwise make the Meat control noisy when adjusting. One could easily leave that cap out and just deal with a bit of rustling when turning that pot, as I often do when adding an NFB pot to an amp.
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Link for Rob Robinette amp site, resonance/meat control;
https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control (https://robrobinette.com/Voicing_an_Amp.htm#Resonance_Control)
I'm pretty sure the Meat control isn't a Resonance control. The latter is just an adjustable passive high pass filter similar to what you'd find in a G&L guitar. The Meat control as implemented here looks like a basic pot as variable resistor in series with the 75k feedback resistor. It's just changing the ratio set by the feedback resistor and the 4.7kΩ tail. The 22μF is there to keep DC off the pot--and is intentionally made very large to not affect the guitar frequency range at all (this could easily be a 1μF cap and function exactly the same). There's a DC offset caused by the same 4.7kΩ resistor that would otherwise make the Meat control noisy when adjusting. One could easily leave that cap out and just deal with a bit of rustling when turning that pot, as I often do when adding an NFB pot to an amp.
Ok, when I 1st looked at Doug's schematic, thought that's what it did. So I had it right the 1st time, but took it down when I did a search and found this;
https://el34world.com/Forum/index.php?topic=24439.msg262696#msg262696
Then I saw meat aka 'resonance' and did a search for that and found the Robinette post on his site.
So, it looks like it's the cap values and using 1 cap in series or 2 caps at different uF values with the series -FB pot you get a meat or resonance control.
And you can use both a presence/highs control and resonance/lows control at the same time/in the same -FB loop.
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It's simpler than that. A single cap with signal though it will act as a high pass filter much like a coupling cap (value dependent). A resistor in parallel with that cap gives you the ability to bypass the effect of the cap by giving low frequencies a lower impedance path around the cap. The resistor attenuates those lower frequencies though--just less so than the cap. If you make this resistor a pot instead then you can adjust the effect of this high pass filter all the way down to nothing (shorting the cap).
Since it's in the NFB circuit the cut in lows results in an increase in lows to the speaker.
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Thanks for your input guys.
You're way ahead of me in understanding what's going on in the Hoffman circuit. Am I understanding this correctly that the Hoffman "Meat" control is acting like both a Presence and Resonance control then? The 22uf cap allows a broad range of frequencies to enter from the speaker but then the individual capacitors split the frequencies up that are fed back to the grid?
After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.
I'm sure I've probably got this all wrong. 😁
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Am I understanding this correctly that the Hoffman "Meat" control is acting like both a Presence and Resonance control then?
No, I don't think it is a Resonance. The capacitor is large enough to be considered a straight wire from an AC perspective.
After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.
The opposite. Resonance cuts lows (high pass) in the NFB signal. Presence cuts highs (low pass).
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After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.
The opposite. Resonance cuts lows (high pass) in the NFB signal. Presence cuts highs (low pass).
He didn't say in the NFB loop, he said at the speaker, so he's right.
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Am I understanding this correctly that the Hoffman "Meat" control is acting like both a Presence and Resonance control then?
No. I think strtomaster is right, the meat control just kills off the NFB signal going to the presence control.
The 22uf cap allows a broad range of frequencies to enter from the speaker but then the individual capacitors split the frequencies up that are fed back to the grid?
There's 2 different value caps around the pot in the resonance circuit.
After reading the Robinnete link I understand the Presence control is a high pass filter and the Resonance control is a low pass filter.....
No, backwards, presence control is a low pass and the resonance control is high pass.
....and each one blocks out respective frequencies so they don't get cancelled when fed back, resulting in either more lows (resonance control) or more highs (presence control) at the speaker.
Yes.
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Well I knew I had something wrong. 😂 Thanks for straightening me out.
So the meat pot is controlling the amount of NFB by passing or blocking different frequencies as you present either more or less resistance to the A/C signal coming back from the speaker acting as a variable NFB control. The 22uf cap is large enough so that that no frequencies passed by the meat control will be blocked and can make their way to the secondary grid of the PI.
So do the two capacitors, C12 and C13, comprise the presence control along with R25 and R26? I'm trying to wrap my head around the signal flow from where it enters into the first grid and the long tail pair as it passes through the circuit tied to the 2nd grid an NFB.
Thanks.
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The Meat control is independent of frequency. It's essentially a resistive voltage divider. It acts indiscriminately across all frequencies and only affects amplitude. It's a volume control for your NFB circuit.
C13 is the capacitor responsible for the frequency response of the Presence circuit. R25 and C13 alone.
There is some interaction between these parts of the circuit, but it's easiest to see them this way to get the concept.
C12 is the input capacitor for the 2nd input of the PI. If it's sufficiently small it can reduce low end. More info on this cap is at the Aiken link above.
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Thanks stratomaster for the clarification. So the meat control only controls the amplitude of the feedback signal either letting it aĺl pass through or reducing it by turning the meat controll. When it's fully open only the 75k feedback resistor is in play but when its fully closed the full 1meg plus the 74k resistor are in play.
Is the meat control interacting with the presence section then to determine which frequencies will be part of the NFB signal going back to the second grid then? How is the presence section determining what frequencies will pass to ground and which will be passed through with the NFB? I know C13 determines the frequencies because of its value but how does this value determine the cut off point to ground and the rest of the circuit?
Thanks.
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Thanks stratomaster for the clarification. So the meat control only controls the amplitude of the feedback signal either letting it aĺl pass through or reducing it by turning the meat controll. When it's fully open only the 75k feedback resistor is in play but when its fully closed the full 1meg plus the 74k resistor are in play.
Yes.
Is the meat control interacting with the presence section then to determine which frequencies will be part of the NFB signal going back to the second grid then?
No.
Meat control is independent of the presence control. It's been said already.
How is the presence section determining what frequencies will pass to ground and which will be passed through with the NFB? I know C13 determines the frequencies because of its value but how does this value determine the cut off point to ground and the rest of the circuit?
You need to go back and re-read about presence controls in NFB loops.
https://robrobinette.com/Voicing_an_Amp.htm#Presence_Control
https://www.aikenamps.com/index.php/what-is-negative-feedback
Uncle Doug;
i=2ubxWulfch_V0MDj
Do some searches here and on line for NFB loop and presence controls.
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Thanks Willabe.
The presence control in the Aiken model has a 5k pot and a capacitor connected to its wiper. If I understand its function, the 5k pot is a shunt for the high frequencies coming from the NFB circuit, passing them through the capacitor to ground.
The more high frequencies shunted to ground the less the negative feedback and less the effect on the high frequencies from the original input signal. This boosts higher frequencies at the speaker. The fewer high frequencies shunted to ground mean more negative feedback is applied against the original input signal with more high frequencies cancelled out. This reduces the amount of high frequencies at the speaker.
If I understand the Hoffman circuit correctly, the meat control is varying the strength/amplitude of the return signal which affects the amount of negative feedback. The C22uf cap allows all the frequencies to pass but high frequencies are shunted through the R25 C13 presence circuit. Since the majority of high frequencies are shunted to ground, the NFB consists of all the other frequencies. The strength of this NFB is dependent upon the amplitude of the signal determined by the meat control. This signal is applied against the original input signal being fed into the 2nd grid and cancels out a portion of the same original input frequencies thus boosting the highs at the speaker.
Have I got it right?
Thanks.
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Uncle Doug explained all this in a very simple and clear way.
If I understand the Hoffman circuit correctly, the meat control is varying the strength/amplitude of the return signal which affects the amount of negative feedback. The C22uf cap allows all the frequencies to pass.......
Yes.
...... The C22uf cap allows all the frequencies to pass but high frequencies are shunted through the R25 C13 presence circuit. Since the majority of high frequencies are shunted to ground, the NFB consists of all the other frequencies. The strength of this NFB is dependent upon the amplitude of the signal determined by the meat control. This signal is applied against the original input signal being fed into the 2nd grid and cancels out a portion of the same original input frequencies thus boosting the highs at the speaker.
This is where your tripping up. Your making this over complicated. Uncle Doug went through all this, go back and watch it again.
The meat control has nothing to do with the presence control except that their both in the NFB loop. They function independently of each other.
You can have an amp with just a presence, or just a meat control, or just a resonance control, or an amp could have any 2 of these or all 3 of these.
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Thanks Willabe. I do tend to over complicate stuff.
Is this the way the two circuits function?
1. NFB controls only the amplitude of the signal coming back.
2. Presence circuit allows high frequencies to pass to ground.
3. Remaining frequencies travel through to the grid and combine with the same input frequencies to cancel them because they are out of phase.
4. Result is higher frequencies from the input signal are boosted at the speaker because lower fequencies were cancelled.
5. The meat control is there to determine how strong of a signal is sent back to mix with the input signal. This determines how much frequency cancellation will occur and therefore the boost of the higher frequencies to the speaker.
In the Uncle Doug video the NFB was returned to the cathode of the second half of the 12AX7 Champ but in the Deluxe Reverb it went back to the cathode of the first half of the 12Ax7. Is this because one amp is single ended and the other push pull or does this have to do with the presence control in the Deluxe?
Thanks again.
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Your not getting this.
Uncle Doug went all through this stuff. Watch it again until you get it.
And the way your phrasing things is hard to understand what you mean. And your still mixing up parts of presence and meat controls do.
1. NFB controls only the amplitude of the signal coming back.
2. Presence circuit allows high frequencies to pass to ground.
3. Remaining frequencies travel through to the grid and combine with the same input frequencies to cancel them because they are out of phase.
4. Result is higher frequencies from the input signal are boosted at the speaker because lower fequencies were cancelled.
5. The meat control is there to determine how strong of a signal is sent back to mix with the input signal. This determines how much frequency cancellation will occur and therefore the boost of the higher frequencies to the speaker.
1. No.
2. Yes, but your saying it backwards. A presence circuit set to 0 has no effect on the NFB loop, as you turn up the presence knob it sends the highs to ground removing them from the NFB loop. So those highs are not in the NFB loop anymore. So because their not canceled by the NFB loop the sound from the speaker has more treble.
3. Yes, their still in the NFB loop.
4. Nope. Nothing was boosted, hi's were just not cancled in the FB loop, lows are still in the loop. Re-read #2.
5. Nope. That would be a presence circuit. Meat control just takes out the amount of NFB fed back to the PI from full FB all the way down to 0 FB.
In the Uncle Doug video the NFB was returned to the cathode of the second half of the 12AX7 Champ but in the Deluxe Reverb it went back to the cathode of the first half of the 12Ax7. Is this because one amp is single ended and the other push pull or does this have to do with the presence control in the Deluxe?
No and no.
Has to do with the PI. SE or PP doesn't mater, it just has to go back to at least the tube before the power tubes, doesn't mater if their SE or PP.
On the Deluxe Reverb amp, the PI is a 12AT7, and the NFB signal gets injected into the 2nd have of the 12AT7, Vb.
You need to buy some books on tube amps.
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At this point I'm totally confused. Time to get some books or just leave my amp as is and forget about understanding any of this stuff. 🤣
Thanks.
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Merlin's books and Kevin O'Connor's TUT books are very good, you could look for a used book.
And there are many other good books on tube amps now a days.
Not everybody can understand everything, we all have different gifts of areas of understanding for different things. Don't be too hard on your self.
Find what gift's God gave to you and work with them. :icon_biggrin:
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At this point I'm totally confused. Time to get some books or just leave my amp as is and forget about understanding any of this stuff. 🤣
Thanks.
For what it's worth, your summaries of your understanding were improving and pointing towards a grasp of the concepts that were missing earlier in the thread. I think you've learned a lot and show the capacity and desire to want to know-- just not yet the initiative to seek out the information on your own. That's the missing ingredient.
As far as giving up on the amp, that's a baby--bathwater scenario. While you may not yet have a deep understanding of the why, the what has been thoroughly documented on this thread. There are plenty of recommendations and explanations of what these recommendations will accomplish. I'm sure you'll find the ingredients for what modifications take your build closer to your ideal within the posts here.
I think you should set aside some time to try them and experiment. So long as you're following best safety practices no harm can come to you or the amp by following these recommended component value swaps.
You're probably not going to like the effect of every one of the recommendations, but you'll learn what they do, and that could be useful on a different amp in the future. Learning what voicing changes are suitable to you is a valuable piece of info. And so long as you're willing to put the time in, you can always go back to stock. Once you experiment for a while, come back with your observations and learnings.
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Thanks Willabe and stratomaster for your advice and words of encouragement.
After having read the Robinette and Aiken articles plus watching the Uncle Doug video I thought I had a better grasp of NFB and presence circuits. Seemed simple enough what they're both doing but obviously it's not that simple in my case. I really wanted to understand how they're working in the Hoffman amp. Guess it's time to get some books like the one by Merlin Blenclowe that's been recommended.
As far as my amp is concerned there are a few things I'm going to try.
1. Replace my master volume 50k linear pot with a 100k audio pot to see if I get a more gradual increase in the overall volume. I'll be replacing the 430k resistor that's in series with the pot with a 390k one.
2. Maybe try a 12AT7 in place of the 12AX7 PI tube. Will probably do this before I try replacing the master volume pot.
3. Replace the .022uf cap at the mid pot with a .047uf cap.
4. Place a 0.47uf by-pass cap across the cathode resistor of V2 to reduce some low end.
5. Replace C2/.0022ufcap with C9/.001uf cap to allow a bit more highs through.
The amp is sounding pretty good to my ears right now but maybe these few extra tweaks will improve it a bit more. I'm not going to attempt anything as far as the presence circuit or NFB goes since I haven't got it right in my head yet. Your feedback on these changes would be appreciated.
I do have a question for you about capacitors and their frequency response. How does its value affect what frequencies will pass? My understanding is that a smaller value, say .001uf vs .1uf , the smaller value allows highs through and the larger value blocks highs. A capacitor can't just give you more highs or more lows, it can only reduce one or the other from getting through. Is this correct?
Thanks and Cheers!
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Reporting back stratomaster.
Only thing left for me to do is take the C9/.001uf cap I removed and put it in place of the C2/.0022uf to see if I can get a bit more treble.
There are a number of ways to get more treble. The coupling cap isn't one of them. It will roll off low end. This might give the effect of more treble, but it's roundabout.
To directly affect treble response here are a few options: You can try a bright cap on the volume pot. You can put the 0.47μF cap you used to have in C1 in parallel with the 1.5k cathode resistor for the 3rd stage. You can put a 330-470pF cap in the tonestack in place of the 250pF.
From way back around post 41. We already covered that you will not get more high end by changing that coupling cap.
You can absolutely find the answer for your question about how cap values affect frequency response for yourself. This is the missing ingredient I referenced earlier.
I think you have everything you need in this thread to figure your amp out and tons of additional resources to help with learning the technical aspects. Put in the time for yourself. It is rewarding.
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Merlin's books for guitar/bass amps are divided into; preamp design and power supply design. Get the 2nd edition if you get 1. I both these books, but 1st edition. I have not held either 2nd edition in my hands. I don't know what he might have changed or added. I did see where Merlin said he got a lot wrong in the power supply 1st edition book.
Kevin O'Connors TUT 1 covers preamps, power amps and power supplies and more. They are expensive. You can find them used. I have this book, I think it's it's a great book to start with.
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I agree with stratomaster.
But here's 1 more last reply for now. (You can still ask all the questions you want here. I'm just out until you do some more on your own.)
I do have a question for you about capacitors and their frequency response. How does its value affect what frequencies will pass? My understanding is that a smaller value, say .001uf vs .1uf , the smaller value allows highs through and the larger value blocks highs. A capacitor can't just give you more highs or more lows, it can only reduce one or the other from getting through.
No, as the cap gets smaller in value, it keeps shaving off bottom end low frequency. .01 > .02 > .047 > .1, starting with the .01 lets the least bass through, then each next cap lets more low end frequency through. It doesn't change the highs.
But if you add bass it can sound the same as removing treble. Think of a stereo or guitar amp. Put the treble and bass knobs both at 12:00. Turn the treble full up, the high end now swamps out the bass end, with out having turn the bass down.. Do the same with the bass, turn it up full, it swamps out the treble, without having turned the treble down.
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Thank You stratomaster.
I should have looked back through the post about swapping C1 and C13. I'm waiting for a 100k audio taper pot to try on the master volume control so I won't be trying that yet. No one around where I am had one.
I'll probably try putting the .47uf cap across the cathode resistor of V2 first since this seems the more logical way of fine tuning the sound. Will let you know as soon as I can the result. I'd also better look into getting some books. Turns out I know a lot less than I thought.
Cheers!
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Thank You Willabe for your book suggestions. I'm going to see if I can get a hold of them and improve my poor understanding of this stuff.
Thanks also for clarifying how the cap value affects frequency. Don't know why I got that one wrong. Caps are passive components and can only filter out frequencies, not add them. They can can only affect frequencies that are already present.
Thanks again for all your help. Cheers!
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Thanks also for clarifying how the cap value affects frequency. Don't know why I got that one wrong. Caps are passive components and can only filter out frequencies, not add them. They can can only affect frequencies that are already present.
Yes. :icon_biggrin:
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Hello stratomaster,
Today I placed the .47uf cap across the cathode resistor of V2. As far as I could tell there was a very small boost of the highs but nothing earth shattering. I'm going to leave the cap in place. I'm currently using the 12AY7 in V2. I can't hear much of a difference between it and the 12AX7. Turning up the volume/gain control even to 3 is pretty loud but at least it's more gradual now rather than the volume jump I was getting before.
I want to try the 12AY7 in V1 and put the 12AX7 back in V2 again before deciding to play with the plate resistor values of V1 to reduce the gain. I've been playing with the amp books frequency response calculator and there's about a 1db difference between using a 12AX7 with 68k or 47k plate resistors vs a 12AY7. The 12AX7 might just be easier so I don't have to have different tubes. I wonder if there's any tonal difference? Is it necessary to create load lines in either case or are the component values already there ok to use?
Also, I got a book, Vacuum Tube Amplifier Basics by EJ Jurich today. I wanted to get the Merlin Blencow book but I thnk this will do for now. It starts off with the very basics of electronics, what resistors and capacitors are, how to read their values, what they do etc, difference between AC and DC, Ohms Law formula, calculating power, transformers, eventually getting into vacuum tubes.
I'm waiting for some components I ordered to do a bit more work on the amp. I ordered a 100uf F&T cap to relace the 47uf filter cap, a 100k audio taper pot and some 330k resistors to change out the current 100k linear master volume pot and 430k voltage divider resistor.
I really do want to learn and understand more about how the circuits in the amp work and tweaks to improve them. If I've caused you some frustration in this process I apologize. Your advice, help and patience is greatly appreciated and I hope we can continue to have conversations.
Regards and Cheers!
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Keep experimenting. You won't harm the amp with preamp tube substitutions. No load lines necessary.
I'm surprised that the 0.47μF cathode cap didn't make much of a difference to you. Between that and lifting the 0.001μF cap you should be getting another 9-12dB of treble content into the PI. This reinforces my opinion that there's something else wrong with your amp.
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You said/did nothing wrong, you have nothing to apologize for. :icon_biggrin:
Glad you found a book. I probably have over 50 different tube amp books I've bought over the last 20+ years. :laugh:
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I'm surprised that the 0.47μF cathode cap didn't make much of a difference to you. Between that and lifting the 0.001μF cap you should be getting another 9-12dB of treble content into the PI. This reinforces my opinion that there's something else wrong with your amp.
Beats me too. I have no idea what it could be or where to look. I'll keep experimenting with the tube in V1. I guess this would be a great time to get the amp on a scope to see what's going on. If I can find someone that can do that I will. I'm going to check the values of each resistor and capacitor first then go over and trace my layout again to make sure everything is connected where it should be. Will keep you posted.
Cheers!
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You said/did nothing wrong, you have nothing to apologize for. :icon_biggrin:
Glad you found a book. I probably have over 50 different tube amp books I've bought over the last 20+ years. :laugh:
Thanks Willabe. Hopefully what I'm reading will actually sink in. 🤣
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I've been going over my amp to confirm the component values (only resistors and pots) as well as re-trace the wiring from the layout (see attachments). I did find that I had reversed the inputs from the PI going to V4 and V5 but I'm pretty sure that's not causing a problem. Other than that all the voltages seem to be good.
Tried putting a 100k resistor to the input of the volume pot but it didnt have much of an effect on how quickly the volume/gain came. There's no jump in volume but the amp still gets loud fast. I would expect the volume to come on more gradually since it's a 10% taper pot. Think I'll remove the 120k resistor attached to the wiper tab and to ground.
I've put the amp back to the original configuration before I started trying the mods. I felt I had l had to in order to hear what the amp was like again and be able to confirm my original observations. The amp does sound boxy and in need of some low end tuning especially with humbuckers. Even with the Meat control dimed the humbuckers felt too bassy. The single coils were a bit better.
After reviewing all of the comments again from the post I've decided to try a few other mods you suggested and a couple you're not so keen on.
1. Change the fat cap from 22uf to 1uf.
2. Put a bright cap across the volume put.
3. Put a 330pf, 470pf or 680pf across the plate resistor of V2 .
4. Change the by-pass cap on V1-A from 47uf to 2.2uf. This will depend on what happens when I do #1.
That's it for now. I'll let you know tge results. Cheers!
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Good. Glad you're experimenting. I think you misunderstood the effects of some of the mods you're considering auditioning, but that's ok. You may stumble across something you didn't know you liked.
1. Change the fat cap from 22uf to 1uf.
This will change the frequencies boosted by the Fat switch. The result will be a more high mid/treble focus boost vs stock.
But only in Fat mode. This would be a good mod to try if you find you always use Fat mode, or if you find FAT move too loose. It wouldn't benefit you at all if you don't regularly use that mode.
2. Put a bright cap across the volume put.
You should absolutely try this. Especially if you've confirmed there are no build errors bleeding off high end.
3. Put a 330pf, 470pf or 680pf across the plate resistor of V2 .
This essentially is the opposite of the bright cap. If you're lacking in high end this will only make it worse. However, if you add a bright cap and it moves your sound in the right direction but is a bit too aggressive this mod can help tame it to an acceptable level. This is where experimentation will come in handy.
4. Change the by-pass cap on V1-A from 47uf to 2.2uf. This will depend on what happens when I do #1.
As explained above, mod 1 affects only the Fat mode. The results will be a tighter amp with a but more background noise. If this is an acceptable trade off to you, then it could work. However I'm skeptical since you previously installed a 0.47μF cap here and still found the amp to be bass heavy and lacking in highs. Please do try it though. I have a 2.2μF in V2a on my Super Reverb and love the clarity I get vs a full bypassed stage. I still think the greatest benefit to your amp would be to leave C1 as is and change C2 to 0.001μF which you've yet to try.
On reversing PI leads: this is a big deal from the perspective of the negative feedback loop that we've discussed as an area of potential problems. The fact that you preferred the tone with NFB disconnected also points to a problem. If the grid wires are reversed, but the transformer is wired as shown in the diagram you will get positive feedback and oscillation issues. Sometimes these issues are obvious with squeals and howls--other times it's more sneaky with phase cancellations and parasitic oscillations. I'd recommend wiring it per the diagram, or at least swapping the primaries of the transformer to see if that improves the sound.
Regarding taper resistor - Removing the 120k will increase the signal out of that volume pot and make your volume problem worse. Draw out the voltage divider as I've shown previously and you'll see this. But try it anyway. The best way to learn is to do.
Regarding the resistor into the volume pot - try a larger value, especially if it was moving you in the right direction. Don't change tactics entirely (erroneously looking at the taper resistor) if the first attempt doesn't achieve the desired results. I believe I suggested a range, try the you're end, 220k before moving on. You can even experiment with higher values if you want the amp to operate more in the clean region and are ok with limiting gain/overdrive.
On Meat control - depending on how it's wired "dimed" could mean stock NFB or next to no NFB. With no NFB the sound is going to be bassy and loose. To expect otherwise is to not understand what it is doing.
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Good. Glad you're experimenting. I think you misunderstood the effects of some of the mods you're considering auditioning, but that's ok. You may stumble across something you didn't know you
Ain't that the truth 😂.
1. Change the fat cap from 22uf to 1uf.
This will change the frequencies boosted by the Fat switch.
Since I don't normally use the Fat switch I'll forego this mod.
3. Put a 330pf, 470pf or 680pf across the plate resistor of V2 .
This essentially is the opposite of the bright cap.
Of course. It's going to bleed away the higher frequencies from the signal going to the coupling cap.
4. Change the by-pass cap on V1-A from 47uf to 2.2uf. This will depend on what happens when I do #1.
I still think the greatest benefit to your amp would be to leave C1 as is and change C2 to 0.001μF which you've yet to try.
I'll take your advice and do this as soon as I can get a .001uf cap, probably to tomorrow.
On reversing PI leads: this is a big deal from the perspective of the negative feedback loop that we've discussed as an area of potential problems. The fact that you preferred the tone with NFB disconnected also points to a problem. If the grid wires are reversed, but the transformer is wired as shown in the diagram you will get positive feedback and oscillation issues. Sometimes these issues are obvious with squeals and howls--other times it's more sneaky with phase cancellations and parasitic oscillations. I'd recommend wiring it per the diagram, or at least swapping the primaries of the transformer to see if that improves the sound.
I wired up the power tubes per the diagram and the amp was feeding back and I had to shut it off. Should I reverse the OT leads and see what happens? I'm kind of surprised the amp wasn't howling in the first since I had the PI signal leads reversed. Maybe this is the source of the problem?
On Meat control - depending on how it's wired "dimed" could mean stock NFB or next to no NFB. With no NFB the sound is going to be bassy and loose. To expect otherwise is to not understand what it is doing.
Maximum NFB at 12 and minimum at 1. Only the signal strength (amplitude) is affected by the meat pot. The stronger the signal it lets through the more effect on cancelling lower frequencies and boosting highs. The weaker the signal that gets through the less of an effect on cancelling lower frequencies and the bassier the sound.
I still have to understand more about the signal path for the NFB signal when it returns to the grid of the 2nd half of the PI as well as how the presence section of the PI tube functions. I thought I had tough a time learning how to use CAD a few years back so I could draw up some guitars and build them on a CNC machine. It seems like childs play next to this. 😁
Thanks again. Cheers!
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Maximum NFB at 12 and minimum at 1. Only the signal strength (amplitude) is affected by the meat pot. The stronger the signal it lets through the more effect on cancelling lower frequencies and boosting highs. The weaker the signal that gets through the less of an effect on cancelling lower frequencies and the bassier the sound.
This isn't right, and I'm having a hard time expressing why. There are a number of erroneous assumptions you're making that are implied by your phrasing but not explicitly stated.
So I'll start here. The NFB is just a resistive voltage divider formed by R27+R60 on the upper leg and R26 on the lower leg. If you don't know what a resistive voltage driver is, stop and go research. Meat control is just a variable resistor on the upper leg of the divider. Where this control is set determines the ratio of the negative feedback into the phase inverter.
At minimum pot resistance you have a 75k:4.7k divider for a ratio of 0.059. If you stopped and reviewed voltage dividers earlier then you'll already know how this was calculated. If you skipped reviewing dividers and are lost on the calculation, go and read up on dividers.
At maximum pot resistance you have a 1M+75k:4.7k divider for a ratio of 0.004. This is basically zero and should produce a sound similar to disconnecting NFB altogether.
Minimum pot resistance should be at full counter clockwise on the knob if you wired it as in the Hoffman documentation. This is MAXIMUM NFB as the ratio of the divider is highest.
Therefore Maximum pot rotation results in minimum NFB.
The fixed presence network (R25 & C13) is just a low pass filter that shunts highs in the NFB signal to ground and prevents them from reaching the PI input. The effect is more highs through the speaker. However, this only works when NFB is present. The higher the Meat control, the less the effect of the network since NFB is reduced--to the point where it's doing nothing at higher Meat control settings.
So with the Meat control "dimed" there's is (almost) no NFB. The presence network is doing nothing, so all frequencies from the preamp pass through the PI to the speaker unaffected by NFB.
This results in a looser sound, rawer highs, and more distortion in the signal. There's also more background noise and could introduce instability.
Minimum Meat control is stock Blues Jr NFB arrangement. This keeps the amp tight, and the fixed presence network brings in an element brightness to the sound. So if you want a tighter sound, turn the Meat control down, not up.
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He's mixing up the meat control with the presence control again.
Keeps adding frequency to the meat control function description. :think1:
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He's mixing up the meat control with the presence control again.
Keeps adding frequency to the meat control function description. :think1:
To be fair, there is an element of frequency in the sense that the fixed presence network works most higher NFB. Perhaps he understands this but articulates it unclearly? Or perhaps my description of this is adding to the confusion?
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To be fair, there is an element of frequency in the sense that the fixed presence network works most higher NFB. Perhaps he understands this but articulates it unclearly? Or perhaps my description of this is adding to the confusion?
I think it is adding to the mix up.
He needs to see and understand the 2 circuits totaly separately. He see's them together.
The Uncle Doug clip shows them separately.
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Thank you stratomaster for that clear and concise explanation of the NFB circuit and presence network. I can't express my appreciation for your patience with me. I did read about voltage dividers, I just didn't clue in that's how the NFB was affecting the return signal.
In summary, the voltage divider determines the ratio of signal being returned to the grid of the PI, thereby determining the amount of NFB.
Wth the meat pot turned fully clockwise only the 75k and 4.7k make up of the voltage divider network. The amount of signal being fed back then is:
4.7k/(4.7k + 75k) = 0.059 or 5.9% Maximum NFB
Maximum NFB means more lows are cancelled by the NFB signal and the sound is more trebly.
When the meat control is turned fully counter clockwise so it's at it's maximum value (1meg), the ratio becomes :
4.7k/(1meg + 75k)= 0.004 or .4%. Minimum NFB
Minimum NFB means fewer lows are cancelled by the NFB signal and the sound is more bassy.
My meat pot is wired per the diagram but is acting the reverse of your response. When turned fully clockwise (12) I get maximum NFB and the pot acts like when I disconnect the NFB line. When turned fully counter clockwise (1) I get minimum NFB the bass becomes more pronounced and flabby.
The right outer terminal of the meat pot is connected to the speaker + side of the speaker output jack. With the meat pot turned fully clockwise the signal sees R27/75k and R26/4.7k that makes up a voltage divider circuit. This provides max NFB signal.
With meat pot turned fully counter clockwise the signal now sees the full 1meg resistance of the meat pot + 75k NFB resistor plus + 4.7k voltage divider circuit and the signal is greatly reduced by the time it passes through the 1meg + 75k and is at the R26/4.7k resistor point. This signal is almost neglibile and will barely have any affect on reducing low frequencies and the sound will be a lot bassier.
I'm hoping I've got this right now and just want to confirm with you before discussing the presence circuit. I've attached images of the information about voltage dividers I read out of the book I got.
A sincere thanks for your help. By the way, I bought some .001uf caps to try in place of C2 and some 220pf and 470pf caps to try on the volume pots as bright caps. Both places I went to didn't have 330pf caps but I might have some values I can put in parallel to get close.
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Willabe and stratomaster,
I appreciate both of your inputs. I was getting frequency and signal mixed up. When I think of the signal coming back from the speaker I see it in terms of all the frequencies coming back and the strength/amplitude of the frequencies. I didn't see how each of them were being handled differently between the NFB and the presence section but contributing to the overall NFB circuit function.
The circuit in Uncle Doug's video is different than the Hoffman but are doing the same thing. After reviewing Uncle Doug's video again I understand how that circuit works now and with stratomasters explanation I have a much better understanding of how the Hoffman circuit works (well almost).
Part of my problem is determining how the signal is travelling within the circuits. Especially in this part of the circuit with the long term pair, presence and NFB. I'm probably guilty of over complicating things and making it harder on my self. I'm an overthinker. 😁
I hope I'm headed in the right direction.
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Sorry, I forgot to add the images from my amp book about voltage dividers.
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Hello stratomaster,
I've been trying to understand better how the volume pot functions and went back and looked at the circuit example you drew out. I've drawn out 3 scenarios of the circuit, pot full off, pot part way up, pot at max that I'd like you to look at and tell me if I've got them right. I think it's easier for you to see if I'm grasping the concept rather for me to try and explain in words what I think is happening. A picture is worth a thousand words is it not?
I've been spending a lot more time reading and researching and haven't tried any of the mods yet. The Robinette article on amp voicing is deep. Most of it is way over my head but he sure has some different takes on shaping the tone.
I didn't want to try anything with reversing the output leads of the PI to the output tubes or switching around the OT B+ leads going to the power tubes until I heard back from you. I'm baffled as to why the amp functions properly with the PI output leads going to the wrong tubes but the OT B+ leads hooked up correctly. As I said, reversing the PI output leads to the PT's caused the amp to howl.
Thanks and Cheers!
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Your understanding of the pot is exactly backwards. This is why I'm having a hard time taking your word for it when you say the Meat control gives a tighter sound when dimed.
Just look at your last sketch for example: The wiper is grounded. You even sketched it this way. Where would the signal some from if the wiper is at 0v?
Resistors elevate voltage above ground. In a voltage divider the further from ground you are the closer to the original signal level you'll get out.
I think you'll see this if you draw an arrow on your sketches showing the direction of the signal path.
Re explanation of your part way on scenario: the signal "sees" all of the resistance simultaneously, not stepwise. So the input sees 785.7k to ground in this scenario. This is the principle behind a Thevenin equivalent resistance. It's also why the wiper to ground resistor affects the taper. It is always in parallel with the lower leg of the voltage divider. Total resistance to ground as seen by the input signal (Thevenin eq resistance) is less important than the divider ratio in practice.
Also your voltage divider ratio isn't just determined by the pot, but also the tapering resistor in parallel with the wiper to pot resistance.
Redraw this as two series resistors with Rp1 as the top leg and a new R for the bottom leg. This bottom resistance will be the parallel sum of 120k and 300k, so ~ 85.7k.
I'm going to say this in the most constructive way I know how: your understanding of electronics is profoundly incomplete. I think it's best if you start at the beginning with a Fundamentals of Electronics type of text or course before diving into the tube amp specific texts. Otherwise you'll continue to misunderstand and will be dealing with expansions of topics that rely on a strong grasp of fundamentals.
I'm not insulting your intelligence or ability. I'm just making an observation that you dove in the deep end without learning to swim first.
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I appreciate your honest assessment. No offence taken.
I'd appreciate you reviewing the revised volume circuits I've made based on different pot settings. In my mind I understood what was happening but I just didn't draw it correctly.
I'd also appreciate your feedback on my previous post about the NFB and presence circuit for my own peace of mind and learning. I think I've got a much better grasp of them bit would like some confirmation.
The one last thing I would appreciate your help with is the issue with the PI output that's reversed to the power tubes. The OT is wired as per the schematic, blue to V4, brown to V5. I tried to reverse the PI output leads to V4 and V5 but the amp just howled.
I'll understand if you don't want to continue providing me help. I'll just try a few simple mods that I haven't tried before and see how it goes.
Thanks for all your advice and patience.
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See the attachment. Your understanding is still backwards but somehow also muddled.
It's not that I don't want to help you (see the last 3 pages of posts with suggestions and detailed explanations) it's that the help I can provide is most useful with a strong grasp of the fundamentals. Until you put in the time to learn these things well enough to catch your mistakes on the basic building blocks of analog circuits we'll continue to talk past each other.
You need to be able to look at your diagram and intuit things like 'the max and min pot diagrams couldn't possibly be identical'. By not catching that you went on to try and solve an impossible voltage divider and further clouded your understanding in the process. Once you get the fundamentals down come back and reread this thread. I assure you it will make more sense.
As far as the PI. The correct orientation due OT primaries is the one that doesn't howl. There's a 50/50 chance of getting it right with the fix being to just swap the wires if it's wrong. Seems like my guess was wrong that you had some phase cancellation occurring that was reducing high frequency response.
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Hi stratomaster,
I will make one last effort to redeem myself. I've gone back to basics with just a simple volume pot connection for a guitar and have attached images of the sketches I've made.
With respect to the PI issue, while it's been some time since I did the conversion of my Blues Jr (2019), I do remember that I originally had the PI outputs connected as per the schematic/layout and the amp howled. Someone on the forum told me to reverse the PI output leads and tte howl would go away, and it did. I'll probably reverse the OT leads and the PI ouput leads so the layout is correct.
While it seems I don't have a grasp of the fundamentals, for myself, I feel that I have learned quite a lot. Before this I had no idea what a NFB circuit was, what a by-pass cap does, what load lines were and why they're important, what a long tailed pair phase inverter was or how the presence circuit worked and so much more. Most of this is thanks to you.
Cheers!
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Did you consider using that unused triode on V2 for a cathode follower?
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Did you consider using that unused triode on V2 for a cathode follower?
There is no unused triode on the Hoffman version
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I've tried several mods on the amp.
1. Replace C2 with a .001uf cap.
- provided a slight increase of highs
2. Lifted C9 from ground
- realy increased the highs, maybe too much
- Meat control has more useable NFB range than before. I only have to keep at just over the halfway point.
3. Put a 330k resistor on imput of volume pot.
- volume doesn't ramp up as quickly as before
4. Replaced R19 Master Vol pot with a 100k audio taper pot and R18 with a 330k resistor.
- master volume now has a much more gradual increase than before.
This is probably a dumb question but why would you use C9 to bleed off so much of the high frequency to ground?
Thanks.
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I've tried several mods on the amp.
1. Replace C2 with a .001uf cap.
- provided a slight increase of highs
It did no such thing. It limited low frequency content (as has been discussed several times before)
2. Lifted C9 from ground
- realy increased the highs, maybe too much
- Meat control has more useable NFB range than before. I only have to keep at just over the halfway point.
Yes, when I first mentioned this mod I said it was a defeating an aggressive low pass filter. That's why I was surprised at your reports that you needed more highs than this. It's also why I recommended a plate bypass cap to coincide with it.
3. Put a 330k resistor on imput of volume pot.
- volume doesn't ramp up as quickly as before
Good. This is exactly the intended effect. You are losing a good bit of gain at max volume with that value. Not a problem if you like it. Just be aware.
4. Replaced R19 Master Vol pot with a 100k audio taper pot and R18 with a 330k resistor.
- master volume now has a much more gradual increase than before.
Yes the linear stock pot is a poor choice for this application. The 100k buys you a bit more signal at the upper end, but nothing earth shattering.
This is probably a dumb question but why would you use C9 to bleed off so much of the high frequency to ground?
I think it's a carryover from the Fender design. Some versions of the Blues Jr have both a bright cap at the volume pot and this cap to ground. I think they were intended to balance each other out. The Hoffman version deleted the bright cap, but kept this cap. A popular Blues Jr mod was to clip the bright cap and leave the cap to ground in place. This could be why Hoffman chose to keep this cap.
Thanks.
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Thanks stratomaster.
1 What I meant about the C2 cap was that I'm hearing more highs because it's limiting more lower frequencies. My bad for phrasing it the way I did.
2. I now understand why you recommended a plate by-pass cap to go with removing the C9 cap. I had reading about this recently. It'll tame some of the highs. I'll check your post to see what cap value you recommended.
3. I'm not seeing a huge drop in max volume with the 330k resistor. I'm having to raise the volume/gain a bit more for breakup but I prefer having more room at the front end to use my pedals.
Thank you kindly for your other reply and for answering my question about the C9 cap. I've been spending a lot of time reading about basic electronics to get my understanding and knowledge level up. It'll require me reading the material over quite a few times.
In the meantime I'll put that by-pass cap around the plate resistor and see how it goes.
Cheers!
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I didn't havea 680pf cap to bypass the plate resistor so I put a 220pf and 470pf cap in parallel to give me 690uf. Close enough for rock'n'roll. 😁
It did as advertised and removed some of the highs. The Meat control still has a good working range and it really helps especially when switching between humbuckers and single coils. I may play around with some cap values for the treble and mid controls. Overall, the amp is sounding much better.
I'm not sure if there's anything else I should try at this point but I'd be happy to hear your thoughts.
Thanks.
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You can try just the 220 or 470pF. With the parallel triode it would make sense to use lower values across that plate resistor. My original recommendation was for a single triode.
If the amp is doing what you like then there's no need for tweaks unless you just want to experiment and learn.
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Thanks stratomaster. I'll give those cap values a try on their own. I'd certainly be willing to experiment some more with your guidance and I certainly want learn some more. Maybe something with the presence section? Whatever you think you would be the most logical thing to try next. I'm game. 😁
Would it make any difference at this point if I changed out C8/.0022uf for the C9/.001uf I removed?
Cheers!
Cheers!
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Everything makes a difference. Either try it and see what difference it makes or look into passive filters in your electronics studies. Or both.
These are questions you can answer for yourself. You don't need me for that.
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One of the biggest improvements you can make on a Blues Junior is a new cabinet. With the exception maybe of the Mk4 they have a tendency to sound boxy. You’re fighting against that inherent trait with all the mods. Plug it into a different cabinet and see what you think. Different speakers help too but the cabinet is the main culprit holding back what the electronics can do.
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One of the biggest improvements you can make on a Blues Junior is a new cabinet. With the exception maybe of the Mk4 they have a tendency to sound boxy. You’re fighting against that inherent trait with all the mods. Plug it into a different cabinet and see what you think. Different speakers help too but the cabinet is the main culprit holding back what the electronics can do.
It's definitely worth a try. Thanks.
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This is the final tally of the mods I wound up doing if anyone's interested.
1. Changed C1 by-pass cap from 47uf to 22uf.
2. Changed fat cap from 22uf to 2.2uf.
3. Removed low pass filter before master volume. (R18 & C9)
4. Replaced master volume with 1M audio pot
5. Put a 330k resistor on the input terminal of the volume/gain pot, R5.
6. Replaced R17 cathode resistor with 10k resistor cold clipper.
7. Replaced V1 with a 12YA7. Waiting to try a 5751.
8. Replaced V3 with a 12AT7.
9. Replaced B+ 47uf cap with a 100uf cap.
10. Replaced screen node 22uf cap with 47uf cap.
Thanks to all that chimed in, particularly stratomaster. Cheers!
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This is the final tally of the mods I wound up doing if anyone's interested.
1. Changed C1 by-pass cap from 47uf to 22uf.
This does nothing in the guitar range. The way to tighten the signal from this stage is putting a 0.001 in C2 as I tried to advise numerous times.
2. Changed fat cap from 22uf to 2.2uf.
I like this change. 1 to 2.2μF are great cathode values for boost functions.
3. Removed low pass filter before master volume. (R18 & C9)
4. Replaced master volume with 1M audio pot
This drastically alters the voltage divider in the design, and can slam the PI input provided the preamp is putting out adequate gain. I'd recommend going back to your 100k log pot with a 330k or 390k at the input.
5. Put a 330k resistor on the input terminal of the volume/gain pot, R5.
We've already discussed the significant cut in gain from this value.
6. Replaced R17 cathode resistor with 10k resistor cold clipper.
This is a parallel triode. That 10k is meant for a single triode. You'll need a 5.1k resistor there to get the intended effect.
Also it will cut a lot of gain and is meant for adding asymmetric clipping to preamp distortion. If you're after clean then it's counterproductive and can sound thin.
7. Replaced V1 with a 12YA7. Waiting to try a 5751.
Neither of these are good choices for taking advantage of the cold clipper. If you're after clean, then keep these tubes and revert the 3rd stage to stock
8. Replaced V3 with a 12AT7.
9. Replaced B+ 47uf cap with a 100uf cap.
10. Replaced screen node 22uf cap with 47uf cap.
Thanks to all that chimed in, particularly stratomaster. Cheers!
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Thank You Kindly Stratomaster.
1. I did put a .001uf cap in as you suggested but I forgot to mention it. The 22uf cap is a BillM mod. The frequency response
calculator shows a small db drop across the frequencies. I thought it would help tame the front end a bit.
2. The fat cap change to 2.2uf works well as a boost for a lead. Helps cut out some of the bottom end. Another BillM mod.
3&4. Changing the master volume pot is entirely on me (as is everything else :icon_biggrin:). I did it based on the some of the
Fender schematics I saw and some stuff I read. Of course I had no idea of the consequences. Should have asked here before
trying it. will correct that.
5. Yes, we did discuss the gain drop of putting a 330k resistor on the volume pot. I honestly can't hear any significant gain drop
but will try a 220k anyway.
6. Really screwed up with the cold clipper not thinking about it being a parallel triode that would affect the value of the cathode
resistor. I'm pissed at myself because I'm pretty sure I was aware that I had to undo half of the triode in order to use the 10k
resistor. I know I don't have a 5.1k resistor so maybe I will put another 10k resistor in parallel. Apparently this was another
BillM mod. I'd like to hear how it sounds when it's set up properly because it doesn't sound good right now.
7. I was experimenting with the 12AY7 in V1 but of course had no idea that it would affect how the cold clipper would work. That
and the fact that I had the wrong resistor for the cold clipper really made things worse. Will be putting the 12AX7 back in to try
with the cold clipper. If it's a bit too much I'll revert back to the original 1.5k resistor and try with the 12AY7.
I've tried to order the Blencowe books but Amazon US won't ship one of them to Canada for some reason. Amazon CA does carry them. Can't understand why they would ship one of the books no problem but not the other one. Ordered them from a different source but will be some time before they get here.
Thanks again for your advice. Cheers!
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Wow, lot's of info here. Thanks all. I have a new to me 2010 cream board BJ and want to mod it. I have done the BillM mods to many BJs and will do to this one too, but the Fat cap reduction to 2.2uF and 3rd stage coupling cap change to .022uF interest me as well. I found a IV version schematic and it appears Fender has made these mods to the amp as well. Interesting. They made it a cathode bias amp surprisingly, a radical change. The IV is being applauded so maybe it will be an amp that is acceptable right out of the box. A pine cabinet will hopefully finish my project, if they ever go on sale.
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Here a better link for the same doc on reply #11
http://ax84.rru.com/media/ax84_m35.pdf
great thread btw!