Blues Jr Mods

[Tbone55]

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.

[Dave]

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

[Tbone55]

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!

[Dave]

You could go a lot higher on the bass cap. Maybe a .1 or really whatever you want, but 100uF is insane.


Dave

[AlNewman]

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.

[stratomaster]

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.

[Tbone55]

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?

[stratomaster]

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.

[Tbone55]

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. 🤗

[stratomaster]

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.

[Tbone55]

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!

[stratomaster]

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

[AlNewman]

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.

[AlNewman]

https://web.archive.org/web/20190315234454/http://billmaudio.com/wp/

[Tbone55]

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?

[Tbone55]

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!

[acheld]

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. 

[Tbone55]

Looks like I'm in the rabbit hole of amp mods 

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.

[stratomaster]

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.

[Tbone55]

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.

[stratomaster]

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.

[Tbone55]

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.

[stratomaster]

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

[Tbone55]

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!

[stratomaster]

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

[Tbone55]

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. 😁

[Tbone55]

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.

[stratomaster]

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.

[Tbone55]

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!

[stratomaster]

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.

[Tbone55]

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!

[Tbone55]

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!

[stratomaster]

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.

[Tbone55]

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!

[Tbone55]

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.

[stratomaster]

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.

[Tbone55]

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!

[AlNewman]

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.

[Tbone55]

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.

[Tbone55]

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.

[stratomaster]

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.

 

[Tbone55]

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!

[Tbone55]

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.

[stratomaster]

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.

[Tbone55]

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!

[stratomaster]

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.

[Tbone55]

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!

[stratomaster]

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.

[Tbone55]

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!

[stratomaster]

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.