Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tbone55 on June 30, 2024, 10:05:28 am
-
Hello There,
I have a Blues Jr that I converted using the Hoffman conversion kit that seems to be extremely louder than it was. I did use a different OT than the stock one that was in the amp. The replacement OT was for another Fender amp and is a bit more powerful than the stock one.
The problem isn't so much that the amp is louder but that there doesn't seem to be any real range of volume adjustment. Its really loud with the volume and master volume at 2 and does get louder if you turn it up further but not by a lot. I have no idea what I should be checking to determine why this is happening and I come to humbly ask for help.
I would greatly appreciate any help on what I should do to diagnose this.
Thanks in advance. Cheers!
-
Check the whole circuit, voltages for all tube pins, that is a good start, some nice pictures will help !
-
Do this first
https://el34world.com/Forum/index.php?topic=17701.0
-
Did you use Audio Taper (Log) pots for the volume controls etc?
-
People are going to ask for high resolution photos, voltages on the tube pins etc. which is helpful to diagnose trouble in an amplifier.
-
Check the whole circuit, voltages for all tube pins, that is a good start, some nice pictures will help !
Weird thing. First it was too loud and now when I went try it again there was no sound coming out. I reposted again with a different thread because of this and it has the voltages I measured vs what the voltages were when I built it. I think I've got a bad resistor on B+ output of the 47uf500v filter capacitor that's causing the problem. Have a look at my other post if you can and if you need more voltages I'll get them.
Thanks kindly.
-
Did you use Audio Taper (Log) pots for the volume controls etc?
Ye, they are all audio taper pots. Ran into a different problem and did a other post. Have no sound coming out if the amp now. If you'd care to look at it I've provided voltage readings now and from when I built the amp. I think I have a defective resistor that's causing the problem.
Thanks.
-
People are going to ask for high resolution photos, voltages on the tube pins etc. which is helpful to diagnose trouble in an amplifier.
Thanks kindly. The amp stopped working. No output. I re-posted again and included pin voltages for the power tubes now and when I first built the amp. I think I have a bad resistor in the B+ circuit right after the first filter capacitor, 47uf500v. The voltage after the resistor drops off dramatically affecting the power tube plate voltage. If you care to look ar that post and tell me what you think that'd be great.
Thanks kindly.
-
Did you do what I said in reply #2?
-
Thanks Everyone.
The amp is now working again and just as loud. Guess I'll have to leave it as for now and just turn the volume on the guitar down. 🤣
-
Well, I'm back. The loudness problem is still there but today when I turned the volume knob it worked the way it's supposed to.....for a second. Then the volume jumped back up. I fiddled wiyh the volume knob a bit more and managed to get it to the same thing again. For a bit it actualy functioned the way it's suposed to.
Off hand I'd say there's something not right with the pot but I took a resistance measurement across it and it read 2.5ohms at max volume and 860kohms at the lowest setting. Is there anything else I should check or do before getting another pot? Maybe I need to just spray the pot with some contact cleaner?
Thanks.
-
Tbone55,
The first two replies suggested that you sit down and verify that every connection is correct. That was excellent advice. Were this my amp, I would immediately think the same thing: bad connection--a mis-wired terminal or a bad solder joint. No way anyone here could do that; only you can do it.
But the mistake could be anywhere, given the symptoms you report and the lack of any more information to go on. Verbal descriptions don't help much in diagnosis. Without giving any more data, you make it much harder to figure out your problem. If you provide the pics, schematic, layout, and voltage list, you probably will get more interest and suggestions.
Good luck.
-
Thanks for your reply. I've gone through all of the connections and measured the values of the components in that part of the circuit and they all seem to be ok. I've also re-checked the connections to make sure that they're wired to where they should be and that checks out. I'm going to try and resolder the connections and see if that makes a difference.
If I need to replace the pot is there any difference in which 1meg pot I replace it with? My closest supplier has CTS & Fender 1meg audio taper pots with different tapers of 10% and 30%. I think the 30% taper are what Fender calls their vintage pots.
Thanks for your help.
-
I'm just replying because I can't attach any of my images. They're larger than the site will accept and I can't change the resolution on my camera or tablet. If I can get this straightened out I'll send them along with my voltage readings.
-
Here's an image of my chassis.
-
Here are the schematic and layout for the board including the voltages I've measured.
-
Here's the other image with the voltages as well.
-
You said the audio taper volume pot measures 2.5ohm max and 860k min, close enough and the pot is 1M. Disconnect the leads to the pot, measure from two outer ends, should be 1M or close, 860K is fine. Measure from one outer leg to wiper, should go from 0 ohms to 1M slowly not a fast jump, do this on each other leg. If not a slow increase the pot is bad, replace.
-
You said the audio taper volume pot measures 2.5ohm max and 860k min, close enough and the pot is 1M. Disconnect the leads to the pot, measure from two outer ends, should be 1M or close, 860K is fine. Measure from one outer leg to wiper, should go from 0 ohms to 1M slowly not a fast jump, do this on each other leg. If not a slow increase the pot is bad, replace.
Hello Dude,
Sorry I couldn't get back to you sooner. Had some family things to do. Here are the results. I did as you said and unsoldered all of the conections going to the volume pot.
Resistance measured across outer terminals - 879kohm
Resistance Measured from wiper to each outer terminal. Readingscwere taken at each volume level number.
Volume Level Resistance Resistance
1 2.1ohms 885k
2 17.06k 860k
3 50.8k 825k
4 79.5k 799k
5 108.3k 772k
6 135.2k 745k
7 160.2k 716k
8 297k 529k
9 528k 295k
10 717k 21.1k
11 881k 2.2ohms
12 882k 2.0ohms
There sees to be a big jump in the second resistance column from vol level 9 to 12. What's your opinion?
Thanks.
-
That does seem a bit strange. If your overall resistance from leg to leg measures 879k, Your resistances from outer legs to wiper should all add up to 879k.
I'm looking at 8-10, and it doesn't add up.
Maybe it's a dead spot in the pot?
-
You could be right. All I know is that the volume jumps dramatically from 1 to 3 and then doesn't seem to change much until you get around the 8-9 mark. I'm of the opinion that theres something wrong with the pot. I'm going to get a new pot tomorrow and I guess I'll find out. There are two different pots I was looking at that have different audio response curves. One is based on vintage fender pot with a 10% curve and another one with a 30% curve which is more modern. Not sure which one would be preferred.
-
So I purchased a new CTS 1meg audio taper pot and installed it. These are the resistance readings at each volume level.
New Pot Old Pot
Vol Level Resistance Resistance
1 961k 885k
2 960k 860k
3 943k 825k
4 921k 799k
5 903k 772k
6 883k 745k
7 852k 716k
8 803k 529k
9 593k 295k
10 319k 21.1k
11 11.95k 2.2ohms
12 0.2ohms 2.0ohms
Theres an obvious difference in the taper between these two pots. With the new pot installed the volume doesn't change from 1 to 2 but then jumps at 3. From there forward the volume diferences are very small until you get to about 9 where you start to get breakup. Because of the higher resistance of the new pot the sound level change is not as drastic as it was between 1 and 3 with the old pot. There's about am 80k difference between the two overall which to me is pretty significant.
What says the forum.
-
Is the new pot Liner or Audio?
If a pot across the outer legs is 950k, then in a Liner pot the middle would be half. But Audio pots are use in most amps for vol, as the taper is more suited to one’s ears. Giving a graduated change to your ears as you turn up, so the middle won’t be half. The new pot seems to be fine, l don’t think it’s the pot. Check your wiring, did you wire the pot backwards, didn’t EL34 suggest to check all the wiring. I know going over connections can be frustrating but follow Doug’s suggestions, go slow, mark each connection on the layout when your sure it’s correct. When the layout is fully marked and you’ve done this a few times check the solder joints, anything under the bd loose.
A bad component.., best l can suggest, maybe more could chime in, good luck.
-
New pot is audio taper. Pot does seem to be working properly. I've gone over the wiring as suggested by Doug highlighting eac connection as I went along. Everything seems to be as it should be. The amp is better than before and I think that's due to the new pot having a higher resistance. I'm going to play around with it some more. Maybe someone else might chime in that's had a similar expereience.
Thanks kindly for your help. Cheers!
-
Newer CTS pots are somewhat known for having a weird taper with a dead spot at very low volume. I think Alpha pots have a more usable taper.
/Max
-
Newer CTS pots are somewhat known for having a weird taper with a dead spot at very low volume. I think Alpha pots have a more usable taper.
/Max
Thanks kindly Max. That's exactly what my CTS pot appears to be acting like. From 1 to 2 there's nothing then at 3 the volume kicks in. I'm going to have a look at a Fender Pure Vintage 1meg pot tomorrow and see what its taper looks like. My problem now is that I had to enlatge the mounting hole tp accomadate the CTS pot and if I go back to an Alpha pot the whole is now too big. Maybe I can glue a washer with a smaller hole in the chassis if I go back to an Alpha pot.
Cheers!
-
Doug sells small washers to fill the gap when going from 3/8” hole to alpha shaft size, next time you order with Doug get some. But you can center the alpha pot easy and tighten, if room use a small star washer on the inside.
-
Doug sells small washers to fill the gap when going from 3/8” hole to alpha shaft size, next time you order with Doug get some. But you can center the alpha pot easy and tighten, if room use a small star washer on the inside.
Thanks kindly dude for that info. I'll add that to my next parts order. As it so happens I just discovered something that might be the reason my volume is so loud at such low volume levels but I need some expert advice.
There's a 1meg resistor that goes across the tip and sleeve terminals of the input jack. I was starting to test some component values and figured I should begin where the signal comes in. This resistor is only measuring 0.1ohms. There's obviously something wrong with resistor but since I'm not experienced enough to understand what the purpose of this resistor is maybe you could explain if this could be the source of my problem? I think it has something to do with impedance but not sure.
Cheers!
-
This resistor is only measuring 0.1ohms
Plug a jack in and measure again, no jack resistor shorted to ground.
-
This resistor is only measuring 0.1ohms
Plug a jack in and measure again, no jack resistor shorted to ground.
Plugged in a jack and it measures 963k. Oh well. The color code of the resistor is brown, black, black, green. Am I wrong but wouldn't this indicate a 10meg resistor, not 1 meg? It's measuring correctly but shouldn't the multilier band be yellow?
-
Alpha makes pots for ⅜" holes as well. I think you may be over complicating things.
I usually use either Bourns or Alphas, depending on what local stores have in stock. Both have worked very well in terms of taper and quality. I do prefer the slightly higher torque of the Alphas for amps, though. And Bourns for guitar pots.
/Max
-
Alpha makes pots for ⅜" holes as well. I think you may be over complicating things.
I usually use either Bourns or Alphas, depending on what local stores have in stock. Both have worked very well in terms of taper and quality. I do prefer the slightly higher torque of the Alphas for amps, though. And Bourns for guitar pots.
/Max
I didn't know that. My local parts suppliers are hit and miss. I think you're right about me over complicating things. I have another amp that I built from scratch and the volume control works perfectly. The volume increases very gradually from 1 to about 6 and then starts to get much louder with some breakup. That's my comparison. Anyway, I'll just recheck the wiring and components one last time. Maybe I missed something.
Thanks kindly for your help.
-
Just as a side note, several of us have had issues specifically with 1M CTS pots, see THREAD (https://el34world.com/Forum/index.php?topic=29896.0)
I had a few failures myself with 1M version, typically a dead spot on the bottom of rotation & now actually avoid using CTS these days. The Alphas are cheaper, smoother & seem to be reliable.
I DO make a point to get the 3/8" versions but keep some of Doug's little adapter bushings around for the 8mm.
-
Just as a side note, several of us have had issues specifically with 1M CTS pots, see THREAD (https://el34world.com/Forum/index.php?topic=29896.0)
I had a few failures myself with 1M version, typically a dead spot on the bottom of rotation & now actually avoid using CTS these days. The Alphas are cheaper, smoother & seem to be reliable.
I DO make a point to get the 3/8" versions but keep some of Doug's little adapter bushings around for the 8mm.
Thanks for the onfo. It's hard for me to get Alpha pots where I am. The places that carry electronic components either have CTS or some really cheap non brand stuff. It's sad when you spend more money to get what's supposedly a better made product and it doesn't work like it should. I may have to order some Alpha pots and those adapters from Doug as well as a few other items. I'll check to see if he has them for 3/8ths" hole as well. The other amp I built doesn't have this problem with the volume knob but I built it back in 2018. Maybe components were better then like everything else.
Cheers!
-
From the Blues Jr's that I played, if I remember correctly, didn't have a gradual volume raise like the other amp you mentioned. The Blues jr doesn't have a lot of clean, breaks up fast to my ears. What you have maybe be normal for the that amp. From what others mentioned, go with the Alpha pot..? If no 8mm washer, just use a small star washer on the back side and tighten so it doesn't move, beware the threads can strip if overly tightened.
-
From the Blues Jr's that I played, if I remember correctly, didn't have a gradual volume raise like the other amp you mentioned. The Blues jr doesn't have a lot of clean, breaks up fast to my ears. What you have maybe be normal for the that amp. From what others mentioned, go with the Alpha pot..? If no 8mm washer, just use a small star washer on the back side and tighten so it doesn't move, beware the threads can strip if overly tightened.
Thanks. You're probably right. I honestly can't remember what the amp was like before I did the conversion so maybe what I'm hearing is normal. I wind up turning the guitar volume down to compensate. I ordered some Alpha pots today but not the ones for 3/8ths holes. I don't know where I could get those. Will do as you've suggested when installing it.
Cheers!
-
What is your location country etc Tbone?
-
What is your location country etc Tbone?
I'm in Canada. I did find a supplier for the Alpha pots in my province but not for the ones for 3/8" holes. I've usually ordered my stuff through Doug because it's actually been easier. We get nicked for some ridiculous delivery rates here. Costs me more for delivery than the actual parts.
I've been thinking about trying a different pre amp than the 12AX7. Maybe put a 12AT7 or 12AU7 in the V1 position to soften it a bit. It just breaks up a bit too soon and I really have lower the volume level on my guitar like down to 5 to keep it clean. Your thoughts?
-
Try a 5751. Really nice tubes 👍
/Max
-
Also you might want to try a 12AY7
There's this place in ON CA that has 3/8" Alpho pots. https://nextgenguitars.ca/products/alpha-standard-solid-shaft-pot-choose-taper-resistance.html
-
Thanks kindly for the tube info guys. I may have a 12AY7 kicking around.
Fuñny but I ordered the Aplha pots from Next Gen. I didn't see where they had the one's for 3/8" hole. No matter, I'll put the one I ordered in and just make sure there's a lock washer on it. I'm going to change the pot first and see how the volume control reacts before I do the tube swap. I'll give an update once I do.
Cheers!
-
I got my Aplha pots late yesterday. One measured 1025k the other 965k. I decided to install the 1025k pot. The amp still gets loud at 3 but there's some volume now between 1 and 3. Still have to turn my guitar volume down to about 5 in order to keep the sound clean.
My next step is to replace the 12AX7 in V1 with a 5751 to try and tame the amp down. My question is, which brand would you recommend? My current V1 tube is Electro Harmonics and V2 & V3 are JJ's. The cheapest available us from JJ and the most expensive is a Sovtek. There's a really big difference in price between these two. Which make is a decent price with decent quality that doesn't break the bank?
Thanks.
-
I use the old GE "5 stars" so I can't comment on new production 5751.
Btw, it might be worth a shot lowering the plate resistors of the first stage to cut the gain down.
/Max
-
I use the old GE "5 stars" so I can't comment on new production 5751.
Btw, it might be worth a shot lowering the plate resistors of the first stage to cut the gain down.
/Max
I never thought of that. Any idea what value I should use to do that? I have absolutely none.
Thanks.
-
Try lowering the plate resistor of the first stage to 68k or 47k just to see if it helps? Or try a splitload 47k/47k? That way you keep the gain but reduce the signal level going in to the second stage.
You could lower the second stage Ra value as well if you want to experiment.
Cheap enough to try before buying new tubes 👍
/Max
-
my guitar volume down to about 5 in order to keep the sound clean
as a non-guitarist tech I ran into this problem often.
The guitarist seem to think there's "magic" in the guitar volume, "I sound awesome with guitar on 6, amp on 8"
as a tech I laugh, tell them to spent 10 minutes tweaking between ALL the knobs til you sound awesome again. You see the light come on, then you spend the next 30 minutes waiting til they rif themselves back to conversation :icon_biggrin:
-
my guitar volume down to about 5 in order to keep the sound clean
as a non-guitarist tech I ran into this problem often.
The guitarist seem to think there's "magic" in the guitar volume, "I sound awesome with guitar on 6, amp on 8"
as a tech I laugh, tell them to spent 10 minutes tweaking between ALL the knobs til you sound awesome again. You see the light come on, then you spend the next 30 minutes waiting til they rif themselves back to conversation :icon_biggrin:
Well, as an amateur guitarist and amateur tech all I can tell you is if I can't put the volume knob on my amp past 3 with the guitar volume at max without the sound breaking up my only option is to turn the guitar volume down to clean it up. That's the way it was done by guitar players in the simpler days. I can spend 30 minutes tweaking the knobs on my amp and it wouldn't get me anywhere.
The other amp I built doesn't have the problem my Blues Jr does. I can turn the volume control up to about 7 with the guitar volume at max and there's almost no breakup. Past that point breakup really starts to come on. All I have to do at that point is turn my guitar volume down to clean up my sound. That amp also uses 12AX7 in the preamp section.
Obviously different amplifiers will have different characteristics.
-
So, you can't leave the preamp volume low, and raise the master volume to keep it clean?
There's some good mods that can be done.
I have a Blues Junior I never liked the sound of, so I kind of emulated a super reverb type build.
One great mod is to change the stock tonestack into more of a vintage TMB style by changing the Mid pot to a variable resistor, and changing the values in the tone stack. That gives you better control of what frequencies you'd like to push.
Also, I think cooling the bias from stock sounds great, and your tubes will last longer.
I did an awful lot to mine, but it is clean... It's still loud, but I have a lot more control on the front end, and can get a lot of juiciness if I want it. I can plug it into my deluxe reverb cab, and it would be hard to tell them apart.
-
So, you can't leave the preamp volume low, and raise the master volume to keep it clean?
There's some good mods that can be done.
I have a Blues Junior I never liked the sound of, so I kind of emulated a super reverb type build.
One great mod is to change the stock tonestack into more of a vintage TMB style by changing the Mid pot to a variable resistor, and changing the values in the tone stack. That gives you better control of what frequencies you'd like to push.
Also, I think cooling the bias from stock sounds great, and your tubes will last longer.
I did an awful lot to mine, but it is clean... It's still loud, but I have a lot more control on the front end, and can get a lot of juiciness if I want it. I can plug it into my deluxe reverb cab, and it would be hard to tell them apart.
I can turn the master up while keeping the volume low to keep it clean but I'm trying to play at low levels while in my house. Maybe comparing it to my other amp isn't a fair comparison and it's just the nature of the beast.
I apologize but I don't know what TMB stands for but it sounds like some of the mods you talked about would be worth looking into. You mentioned using a variable resistor for the mid tone pot. Is that like a trim pot? I'd certainly like to try some of the suggestions given.
Thanks kindly for your help. Cheers!
-
Treble
Mid
Bass
-
Treble
Mid
Bass
It was too obvious. 🤣
-
I did not read all the answer .
A very loud volume in new built could be due to erroneous values of voltage divider resistors at tubes grids
-
Treble
Mid
Bass
That was just to obvious. 😆
-
Thanks. That wasn't too obvious. 😂
-
The tonestack mod is easy to try, you can just put a jumper between the leg of the mid pot that connects to the bass pot and the wiper of the mid pot.
To me, it provides better definition between the 3 tone pots.
If you turn your whole tone stack to 0 you get no volume.
You can turn up any one of the pots and get volume in different frequencies.
-
When you have an amp where the volume goes from zero to really loud with a very little sweep in between, you could try a 12AY7 in V1 socket. Still a very musical tube, but a useable sweep on the vol pot.
-
From what I understand of his volume problem, and having used 12AY7s, I doubt it will make any difference.
In my opinion, there's a building error.
-
there's a building error.
i'm in the; there's way to much pre gain for el-84's camp
wouldn't be surprised if you scoped it n found 2X max drive signal, n el-84's don't stay clean long with that much drive signal.
-
To drop the volume , a smaller pot value may do the job.
-
Treble
Mid
Bass
Jeez, that was too obvious for me. 🤣
-
Treble
Mid
Bass
Geez, that wasn't too obvious at all. 🤣
-
I have three Blues Juniors, one regular black BJ II Mexican build circa I dunno, one BJ III tweed reissue circa 2017, and one I built originally from Hoffman's kit -- but has now been converted to a BJ4.
Each has its own flavor. The stock BJ II has lots of chime, breaks up early, and eats EL84s like candy.
The BJ III is very sweet with its improved Jensen speaker, and also ate EL84s until I lowered the bias with an add-on variable bias board. It sounds nice.
My favorite, however, is an interpretation of Hoffman's BJ but converting it to a BJ IV format -- eg, self biased power section as opposed to the fixed (and ridiculously high Fender) bias scheme in previous versions. To be clear, the variable fixed bias in Hoffman's design also fixes the problem; I wanted to see what the difference is with self biasing when I built it (frankly, not much).
My long winded point is that biasing plays a crucial role in these amps, and you can -- to a certain extent -- dial in how much early breakup your power section has by biasing cooler/hotter as you wish. Many builders just dial in the 70% Pdiss as a biasing solution. That's fine, and it works, but it's worthwhile to experiment with biasing cooler, or hotter, to see what works best for you. We do know what hotter sounds like (Fender stock BJ II, III) and of course that results in shorter tube life. Cooler can work well, and I'll often run my EL84s between 50-60% with lovely tone. It has been fun to see how low I can bias EL84s and still sound good.
I like the discussion regarding the use of a 5751 as V1 and/or reducing the plate resistor value -- that will also give you some clean "space" to play with.
-
Try a 5751. Really nice tubes 👍
/Max
Between 12AX7 at 100 % gain and 5751 at 70% gain you wont hear any difference .
-
You sure will.
-
I can see replies up up from the last few minutes
-
I've also found that there's a difference in the dynamic response between the two tube types.
/Max
-
I can see replies up up from the last few minutes
Thanks. I'm seeing them to now.
-
Thanks everyone for your suggestions. There seems to be a number of things I could do to help fix the issue. It seems these Blues Jrs were always running hot and were prone to breakup very early. My amp is a Blues Jr III Special Edition although I don't know what's so special about it. My Hoffman conversion has the biasing resistor in it and I'm currently running the EL84s around -23mv.
The suggested fixes, as I see them appear to be as follows.
1. Replace V1 with a lower value (5751, 12AYX)
2. Replace V1 grid resistors with a higher value (47k, 68k)
3. Replace V2 and V3 with higher grid resistor values as well.
4. Mid tone pot modification/re wire
5. Lower bias voltage for EL84s to 50% or 60%
6. Replace volume pot with lower value. 500k?
The easiest to try seems to be to lower the bias voltage first. I'll try this first and post back the result.
Thanks again for all your suggestions. Cheers!
-
Don't forget about the plate load resistors. To lower the gain, you decrease the values 👍
/Max
-
Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.
-
Don't forget about the plate load resistors. To lower the gain, you decrease the values 👍
/Max
Thank You for reminding me of that. I knew I'd forget something. 🤗
-
Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.
Thanks for the additional tip. How does that work to lower the gain from V1?
-
5 minutes, you'll understand simply by playing.
rif for 3-5 minutes, unsolder cap, repeat rif
-
Do you know what your bias is yet?
-
Do you know what your bias is yet?
Thanks for providing the video. I've seen some of Uncle videos but not that one. So my amp is cathode biased and I have the variable resistor that allows me to adjust the tubes bias. I've currently got it so my bias is around -23mv on one tube and -21.8mv on the other. I've got it set a little bit cooler than -25mv that seems to recommended to give 70% plate dissipation. Since the bias is the easiest thing to change I'm going to try and set it a bit colder and see how that works. If I'm getting anything wrong please let me know.
-
Could you just make a voltage chart of your power tubes at all the pins?
As well as all your power nodes.
And all the pins of your little tubes...?
And maybe an as built schematic.
-
Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.
Thanks for the additional tip. How does that work to lower the gain from V1?
The cathode bypass cap works to boost the gain of a cathode-biased tube by shunting to ground, any signal that otherwise would appear on the cathode if there was no bypass cap. The effect of the cap is to ‘fix’ the cathode at a steady voltage, thus effectively maximising the plate-to-cathode voltage under signal conditions. (More plate-to-cathode voltage = more gain; all other things being equal). If you remove the cathode bypass cap, you reintroduce the voltage swing at the cathode, which then introduces cathode current feedback back into the stage, thereby reducing the gain. An unbypassed cathode on a 12AX7 triode has about 1/2 of the gain compared to a fully bypassed stage.
-
Has anyone already suggested lifting one end of the V1 cathode bypass cap? It's C1, a 47uF.
Thanks for the additional tip. How does that work to lower the gain from V1?
The cathode bypass cap works to boost the gain of a cathode-biased tube by shunting to ground, any signal that otherwise would appear on the cathode if there was no bypass cap. The effect of the cap is to ‘fix’ the cathode at a steady voltage, thus effectively maximising the plate-to-cathode voltage under signal conditions. (More plate-to-cathode voltage = more gain; all other things being equal). If you remove the cathode bypass cap, you reintroduce the voltage swing at the cathode, which then introduces cathode current feedback back into the stage, thereby reducing the gain. An unbypassed cathode on a 12AX7 triode has about 1/2 of the gain compared to a fully bypassed stage.
Thank You for providing that explanation. I appreciate it. 🤗
-
Could you just make a voltage chart of your power tubes at all the pins?
As well as all your power nodes.
And all the pins of your little tubes...?
And maybe an as built schematic.
Yes, I can. I followed the layout provided by Doug to build the amp. Would that be good enough?
-
So my amp is cathode biased and I have the variable resistor that allows me to adjust the tubes bias. I've currently got it so my bias is around -23mv on one tube and -21.8mv on the other.
I think you've done well! And you're getting some great advice here.
Just to help you with terminology: A BJ III is "fixed" bias -- a confusing term that simply states that a fixed negative DC bias voltage is applied to the power tube grid (along with AC signal from your PI). Confusing because we vary the bias voltage to suit our desired power tube state.
A BJ IV, however, is "self biased" or "cathode biased." In this biasing scheme, the bias is achieved by varying the cathode resistor value to achieve the desired state. All pre-amp tubes are self biased. Dealer's choice on the power tubes. For a good discussion on this, on why one vs the other scheme, check out https://www.valvewizard.co.uk/pp.html (https://www.valvewizard.co.uk/pp.html).
Your reference to mV as biasing doesn't have meaning by itself. We all assume you mean plate current, but really what you want to specify is power (P=V*I).
Uncle Doug's early instructional videos, while maddening to some, are generally excellent sources on the basics.
And over and over again, read Merlin Blencowe. And then again. When you think you have it, then Rich Kuehnel.
-
Thanks kindly acheld,
I am getting a much better understanding of how the biasing and also how the pre-amp section works. Sometimes the terminology is a bit confusing to me about biasing.
I was just watching one of Uncle Doug's video where he shows how to measure and calculate the bias for a 6V6 tube as well as what you would do to change the bias if needed. What I found really interesting was the the bias was set to provide the maximum power dissipation of the tube, 12W, and this was necessarry in order to provide enough headroom to avoid distortion. In the bias is set too cold the amp will distort a lot easier.
The complaint I've always heard up to the Blues Jr III has been that the biasing is too hot which causes early tube failure. Would this then mean that the plate dissipation would be 100% or 12w It seems the recommended PD is around 70% or approx 8.4w. It seems to me that I need to make the bias voltage more negative to clean up the sound a bit.
Your thoughts?
-
I'm sorry, I didn't see your earlier posting with voltages and schematic.
I believe the Uncle Doug video I linked is a cathode biased amp, and yours is fixed bias. usually
The method of measuring via the transformer is still viable, but since you have 1R resistors on your cathode, that is just as good.
Cathode biased amps can be biased hotter than fixed bias sometimes to maximum wattage or even slightly higher.
Fixed bias amps should be biased cooler, the often recommended number is around 70%.
Yours will be be safe at where you are biased, but you probably wouldn't want to go higher than 70 or 80%.
In fact for me it seems biasing it cooler can actually improve headroom, I usually bias at around 55%.
It doesn't sound to me like the distortion you are getting is a power tube thing though. Tubeswell has a good suggestion of lifting the bypass cap and maybe that will be enough of a gain drop.
Anyhow, I apologize for causing any confusion, I just skipped to page 2 and missed the posted voltages.
-
I was thinking about the volume and how it reacts with the grid leak R6 directly following (in parallel) to it.
Really, it limits your volume pot resistance at 120k, meaning it could explain why your volume seems to max out at 2-3 at the volume dial.
Most of the old fenders have the volume right after the tone stack, in which case the tone stack would replace that resistor, (sort of)?
It's possible fender put that value resistor there to limit oscillations in what is a high gain pre amp.
I'm not sure of suggestions in how to fix it easily, besides perhaps trying a lower resistance pot for the volume control, like a 500k or something, and possibly increasing R6 to find the sweet spot. Another suggestion may be to change the placement of your volume and control signal on the other stage with it's own voltage dividers. Even a linear pot might work better in that situation, even though it seems counter intuitive.
Or, still thinking, maybe change the voltage divider going into the tube, IE use a 100k grid stopper and a 1M grid leak.
I'm sure if that could be an issue somebody else might have other suggestions.
-
Thanks kindly for you input and suggestion Al. I've been watching a few of Uncle Dougs other videos on biasing to get a better understanding. I'm a bit confused about my amp because I thought it was Cathode biased because of the resistors from the cathode to ground but what about the trimpot that's controlling the grid? Doesn't that make the tubes grid biased?
I'm in the process of taking new voltage readings from the tubes as I didn't measure some of the pins in my previous table. I was also wondering the same thing about that 120k resistor across the volume pot. Lots of things to think about.
-
I'm a bit confused about my amp because I thought it was Cathode biased because of the resistors from the cathode to ground but what about the trimpot that's controlling the grid? Doesn't that make the tubes grid biased?
Your Hoffman BJ conversion is a fixed bias amp. The telltales are:
1. The presence of the biasing voltage circuit. See: https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf (https://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf), scroll down three pages and look for the C- next to the bias circuit and where it connects to the grids of the EL84s.
2. The lack of "large" resistors from cathode to ground. Most cathode biased amps also use a cathode bypass capacitor. Your amp has neither. (The 1 ohm resistor from cathode to ground is present solely to allow you to estimate plate current.)
As mentioned before, a BJ IV is cathode biased, as opposed to earlier models (and your amp) which were all fixed bias. An early schematic and PCB layout of the BJ IV are located here: https://www.tdpri.com/threads/fender-blues-junior-iv-schematic.1104154/ (https://www.tdpri.com/threads/fender-blues-junior-iv-schematic.1104154/). You will find a 270 ohm cathode resistor and 22uF cathode bypass cap from the cathode to ground. For learning purposes, would be really worth your time to compare Hoffman's beautifully simple schematic with the (necessarily) more complex Fender schematic to see the difference in the power tube biasing scheme.
-
Thanks ac for the explanation. I see where the C- voltage is that feeds the grids of the ouput tubes which is adjusted by the trimpot. Appreciate the help.
Cheers!
-
I've completed going through and measuring the tube voltages at each pin and each node. V3 definitely has something funky going on.
-
V3 definitely has something funky going on.
Nope, looks good to me. It's all in the art of biasing . . . our theme o' the day.
The "funky" thing you're seeing is that pins 3 and 8 are at much higher voltages than your pre-amp tubes. This is expected.
Look at your schematic for V1a, and you'll see that the cathode resistor (Rk) to ground is 1.5k. Now look at V3 for your long tail pair phase inverter, and take a look at the funkiness in the "tail" -- in other words, trace the path to ground from your cathodes and note there is about 45k ohms in between both cathodes and ground (DC resistance only, keeping it simple, and also note that this is a shared resistance between both cathodes in the tube).
This resistance keeps the cathode at a higher voltage than a much smaller (say, 1.5k) resistor would do, and that's how biasing is done.
For a great discussion of your AC coupled long tail pair PI, see: https://www.valvewizard.co.uk/acltp.html (https://www.valvewizard.co.uk/acltp.html). I can't come close to Merlin's ability to understand and explain . . .
-
Thanks ac for explaining that. I see what you're saying about the resistors but am still puzzled about the voltage being that high. I'll check out the reference you provided and see if I can wrap my head around what's going on.
So it seems the amp is working as it should after all and I need to look at doing some of the things that were suggested now to tame this little beast. Thanks sincerely for your help.
-
For a great discussion of your AC coupled long tail pair PI, see: https://www.valvewizard.co.uk/acltp.html (https://www.valvewizard.co.uk/acltp.html). I can't come close to Merlin's ability to understand and explain . . .
[/quote]
This was a great article. I had no idea thats how V3 worked. While I do find it hard to understand just how everything works it does explain why the voltage on the cathode is so much higher than V1 & V2. There's quite a big signal being fed to the power tubes.
One thing that I also found interesting is the use of either a 12AT7 or 12AY7 in place of 12AX7 in order to increase the headroom, maybe even a 5751. It might be worth a try to use one of them and reduce the signal level a little to help provide more headroom and less pre amp distortion.
Any thoughts?
-
either a 12AT7 or 12AY7 in place of 12AX7 in order to increase the headroom, maybe even a 5751. It might be worth a try to use one of them and reduce the signal level a little to help provide more headroom and less pre amp distortion.
Always worth a try to sub in different tubes. My personal bias is that if one tames the pre-amp too much, the life goes out of the amp. I once concocted a Princeton that would deliver 24 watts of undistorted 1kHz with the volume set at 90% -- this was for a jazz guitarist who told me that he wanted NO distortion at all. However, the amp lacked life IMO, and the prospective customer was not impressed. When I replaced all the 5751s in the pre-amp with standard 12AX7s, it came back to life. Now it's a nice sounding amp with some soft distortion at noon.
If I was going to sub in some tubes -- you could try a 12AY7 in V1, or maybe a 12AT7 in the PI. I do like 5751s, but the difference (from a 12AX7) is subtle. Lots of different opinions to be had on what tube works best where. Easy enough to try it and see what happens.
You made a list of stuff to try earlier. Don't forget about that.
-
Thanks for your reply ac. Changing the tube is just one of the things on my list and it's easy enough to do. I'd really to be able to turn the volume up a bit more before getting breakup, maybe to the noon position. I'm going to work through my list one change at a time and see what happens. I'll post my findings as I go along.
Thanks again fir all your help and advice. Much appreciated.
Cheers!
-
One great mod is to change the stock tonestack into more of a vintage TMB style by changing the Mid pot to a variable resistor, and changing the values in the tone stack. That gives you better control of what frequencies you'd like to push.
Hi Al,
What's the difference between a tone pot and a variable resistor? What values should I look at for the resistor values? Is there a schematic I should follow?
Thanks.
-
You slide the metal band left or right to adjust.
-
In the context of a tone stack, they would be the the same thing.
A variable resistor would be set up by using the wiper contact and one of the fixed poles.
Most often (but not always!) in a tone stack, the "tone pots" are set up as dividers, where signal arrives at one pole, and then is divided into the wiper and the second fixed pole.
More generally, a designer does need consider power dissipation and many other factors (see https://www.bourns.com/docs/product-datasheets/82.pdf (https://www.bourns.com/docs/product-datasheets/82.pdf) as an example spec sheet) when employing any resistor. Many commonly used pots are not as robust as you might think -- when used outside of their design parameters.
Willabe, of course, has got it right. Check out https://www.bourns.com/docs/product-datasheets/PTA.pdf (https://www.bourns.com/docs/product-datasheets/PTA.pdf), one my personal favorites. But also note the low power rating and the low maximum operating voltage.
-
Thank you for the explanation and the references. Much appreciated.
Cheers!
-
You just have to put a jumper between 2 lugs of your mid pot.
-
Thanks for providing that info AI. Haven't tried that yet but I will.
Cheers!
-
I have a question about one of the Hoffman mods I did. It has to do with V2. In order to get a stronger signal push on V2 Doug suggests putting jumper wires from pin 1 to 6, pin 2 to 7, and pin 3 to 8. Could the issue I'm experiencing have anything to do with this mod? Maybe I'm getting a much stronger signal than what is needed?
-
2nd image for previous post.
-
Could the issue I'm experiencing have anything to do with this mod? Maybe I'm getting a much stronger signal than what is needed?
No. Frankly it doesn't change much, though in my own experience there might be less noise.
-
Maybe I'm getting a much stronger signal than what is needed
only the scope will tell
subjective guessing usually results in subjective outcomes
you're driving tubes that need about 1/5th the signal a 6v6-ish tube would need for the "same outcome"
your PA "limiting" factors in a nutshell is your bias VDC whether fixed or self, that is the "clean swing range" before the tubes hit clipping or cutoff
once you exceed that the un-expected happens, sometimes it "Oh ya baby" other times it's "WTF was that???"
$35 gets you the tool, 35 hours reading gets you the knowledge, together great things can happen
-
I would say "unparallel the tube" and then see if it helps.
-
You said you could get clean sounds with the master volume up...
So, with the master @ 12:00, does it still break up at 2 on the volume dial?
Or do you have to turn the volume higher to get the same amount of distortion?
-
Could someone explain to me the reason for R6, 120k resistor, that is connected to the volume pot wiper terminal and ground? Is it acting as a grid leak resistor? Curious minds would like to know.
Thanks.
-
No. Frankly it doesn't change much, though in my own experience there might be less noise.
[/quote]
Thanks kindly. I really didn't want try and unsolder the connections. 😁
-
only the scope will tell
subjective guessing usually results in subjective outcomes
you're driving tubes that need about 1/5th the signal a 6v6-ish tube would need for the "same outcome"
your PA "limiting" factors in a nutshell is your bias VDC whether fixed or self, that is the "clean swing range" before the tubes hit clipping or cutoff
once you exceed that the un-expected happens, sometimes it "Oh ya baby" other times it's "WTF was that???"
$35 gets you the tool, 35 hours reading gets you the knowledge, together great things can happen
[/quote]
Thanks for the reply. Nothing replaces knowledge and experience. I had an opportunity once to get a scope and let it get away. Sure wish I had it now. Pictures are worth a thousand words. I sure have been reading a lot lately 😂.
-
I would say "unparallel the tube" and then see if it helps.
Thanks. I might try it but not yet.
-
You said you could get clean sounds with the master volume up...
So, with the master @ 12:00, does it still break up at 2 on the volume dial?
Or do you have to turn the volume higher to get the same amount of distortion?
With the master around 12 the volume stays clean when it's at 2. It stays pretty clean up to around 6 where it starts to break up. The amp seems to jump in volume when I move the volume from 2 to 3 and stays around that level until it starts to breakup around 6-7.
I'm borrowing a 12AT7 from a friend to try in the V1 position. Once I've had a chance to install it and see what kind of a difference it makes I'll report back my findings. If that doesnt work I still have a few other recommendations I can try.
Thanks for your help.
-
With the master around 12 the volume stays clean when it's at 2. It stays pretty clean up to around 6 where it starts to break up. The amp seems to jump in volume when I move the volume from 2 to 3 and stays around that level until it starts to breakup around 6-7.
I'm borrowing a 12AT7 from a friend to try in the V1 position. Once I've had a chance to install it and see what kind of a difference it makes I'll report back my findings. If that doesnt work I still have a few other recommendations I can try.
Thanks for your help.
Personally, I would think that has something to do with the 10k/120k voltage divider off the volume control, as far as the weird pot reactance goes.
I don't have an answer for you, but things i would be trying is measuring resistances from your wiper to ground at the point when these changes take place. And then maybe do some math or experimentation.
It sounds like you can get all the in betweens at the right place on the knob, but it just isn't where you'd like it to be.
-
[quote author=Tbone55 link=topic=31828.msg351587#msg351587
Personally, I would think that has something to do with the 10k/120k voltage divider off the volume control, as far as the weird pot reactance goes.
I don't have an answer for you, but things i would be trying is measuring resistances from your wiper to ground at the point when these changes take place. And then maybe do some math or experimentation.
It sounds like you can get all the in betweens at the right place on the knob, but it just isn't where you'd like it to be.
Thanks Al. I've been thinking the same thing. I just don't understand what the purpose of the 120k resistor on the pot is for. It looks almost as if it would act as a grid leak resistor somehow but a much lower value than what you normally see. I'm still wrapping my head around all the circuits work and have done quite a bit of reading in the last while.
I will do as you've suggested and measure the resistance at the point where the volume changes. I've also gotten hold of a 12AT7 tube to try. Will definitely report back with my findings.
Cheers!
-
Yes, it's a grid leak. There's just lot of gain in the pre amp, so looks like they were dumping some signal. It wouldn't hurt anything to lift one leg of the 120k resistor and see how that changes the response on your volume dial. But you'll get to a point and everything will start oscillating.
It might be a way to see if you can increase that grid leak some. Just remember being it's parallel it works hand in hand with the setting on your pot. Once you find the magic ratio, (which could be 12:1), then you can increase both the grid stopper and grid leak at the same ratio to change the response on your pot.
-
Thanks AI for confirming what I thought. Since the volume pot is in parallel to the 120k resistor they act the same way as two resistors in parallel would and the overall resistance is always less than 120k?
-
Ok. I have some results to report.
1. 12AT7 in V1 position
- a bit less gain but not as much as I would have expected. The volume still gets loud quick at 3 without much change until around 6-7.
2. Lifted one end of C1, 47uf by-pass capacitor
- gain dropped significantly and the volume control had a much more gradual increase in volume with break up around the 7-8 mark. This is much more in line with what I want. It allows me to make more use out of my distortion pedals. It seems to have a bit more bass in it as well.
I need to decide if I'm going to use a much smaller value capacitor in place of the 47uf in order to get a bit more highs. Not sure what value I should be considering. If anyone has a suggestion let me know.
Cheers!
-
Use this calculator to determine the gain with different cap values:
https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/
Here is the Valve Wizard's graph showing the frequency cutoff for different cap values:
Starting in the middle of page 25 here: https://www.valvewizard.co.uk/Common_Gain_Stage.pdf
-
Thanks so much d. Those two references should really help me out a lot.
Cheers!
-
Those references were amazing d! I have to admit that I don't fully comprehend everything I read but based on that calculator and the valve wizards chart it would seem I need to use a capacitor value that's orders of magnitude smaller than 47uf. It looks to me that I might want to use a 1uf or maybe even a 470nf capacitor. Does this make sense?
-
Tbone55, I think this ultimately comes down to making a choice by listening. :icon_biggrin:
FWIW, some Marshall's used 0.68uF bypass caps. I used 1uF in the last amp I built.
-
Tbone55, I think this ultimately comes down to making a choice by listening. :icon_biggrin:
FWIW, some Marshall's used 0.68uF bypass caps. I used 1uF in the last amp I built.
You're absolutely right. My ear will be the best judge. I was leaning towards a 1uf cap myself so I'll try that first. Thanks for your help.
Cheers!
-
Another update. I bought a 1uf and 0.47uf cap to try in place of the 47uf bypass cap C1. They both worked quite well and made the volume work in a more gradual fashion. The amp starts to break up around 7-8 and the bass is a lot tighter. I settled on the 0.47uf cap as it was a Mallory.
The original 47uf cap is directional but the Mallory isn't. Also the Mallory is rated at 630vDC and the 47uf at 25v. That's all I was able to find. Is this okay?
-
Did some reading and found out you can use a non polarized capacitor in place of the polarized ones. Electrolytics are polarized and come in compact sizes for very high values. The 0.47uf mallory I used is huge compared to tge original 47uf cap.
Just ordering some parts from Doug as spares including a 0.68uf cap to try out. I have learnt quite a bit from this post and would like to thank everyone for their contributions and patience. This has been an invaluable learning experience from a great group of people.
My sincerest and utmost thanks to all. Cheers!