Tremolo Pot Affects Bias When Off

[Ed_Chambley]

Clone amp similar to a AA964 circuit.  Replacing tubes and rebias I ran into a problem where trem pot will reduce the negative bias with the tremolo off.  I assume a wiring problem, but I thought I would ask if anyone has seen a similar problem.

[kagliostro]

In the Princeton you must set the bias (voltage/current) with the tremolo circuit off

K

[Ed_Chambley]

In the Princeton you must set the bias (voltage/current) with the tremolo circuit off

K

Tremolo is off.  That is my concern.  Tremolo works with footswitch, off and on.  When off the more you increase the intensity the less negative bias voltage.  It will actually go positive at full turn.  With the tubes out and the trem pot closed I get the proper bias voltage.  Not sure if it is a grounding issue. The owner uses the tremolo a lot and he said the intensity pot is always sitting just over half turn.  In this position with tubes in I get a reading of -4ma when checking with a 1 ohm resistor.  Both tubes.

[sluckey]

I assume you're referring to the Princeton and not the Vibrolux? If wired like the Princeton the Intensity pot will not change the bias. Look for a wiring error. Is the bias adjustable?

[Ed_Chambley]

I assume you're referring to the Princeton and not the Vibrolux? If wired like the Princeton the Intensity pot will not change the bias. Look for a wiring error. Is the bias adjustable?

Bias is adjustable.  It is a custom build, but the circuit is basically a Princeton with some different values at v1 for running a 12Ay7.   Non-reverb amp.  I was thinking wiring problem but the owner said he never noticed any change in tone or volume when turning the intensity pot down with the tremolo off.  I have not tried it myself, but I would think there has to be.

[sluckey]

Can you post a schematic? Maybe it's not as close to a Princeton as we think? Regardless though, if the Intensity pot changes the bias voltage on the grids of the output tubes, that's a bad thing. Should be fairly easy to correct.

[sluckey]

It will actually go positive at full turn.

Somehow I overlooked this statement. About the only way for bias voltage to go positive would be a leaky coupling cap between the PI plates and grids OR a leaky cap between the trem tube and Intensity pot. Disconnect these three caps and see if the voltage on pin 5 of the 6V6s now remains constant as the Intensity pot is turned. I'd start with the trem coupling cap.

[floyd]

In his PR based Sweet Spot , David Allen uses a 47@100V cap on the intensity pot, ( left leg , looking at the back of the pot ) to ground ,for this very reason... ground the positive end.

[Ed_Chambley]

Can you post a schematic? Maybe it's not as close to a Princeton as we think? Regardless though, if the Intensity pot changes the bias voltage on the grids of the output tubes, that's a bad thing. Should be fairly easy to correct.

Should be easy.  I do not have the schematic as I did not know who built the amp and neither does the owner.  He did mention using yellow jackets to try EL84's and one of the tubes had an internal short, well he said it crackled and flashed.  Did not think much about it till now.  I will check the caps.  The amp bias is perfectly adjustable with the intensity zero so I would think it has to be in the tremolo circuit.  Schematic attached is what the amp basically is.  There is a 1 meg pot wired where I have circled green instead of a resistor.  Whoever built it called this a depth control and is beside a dwell on the back.  I guess it works, I don't know, but I do not want to test it until I find the problem.

The .1uf cap in red is the one you are speaking of?  I really want to get it corrected before he is setup with some RCA NOS tubes.

Yes there is a dwell pot that goes to nothing.  I guess the intention was to add reverb at some point.

[sluckey]

The .1uf cap in red is the one you are speaking of?

Yes, that's a prime suspect. Just lift one end of the cap. Doesn't matter which one. If the Intensity pot no longer changes the bias voltage (especially to a positive voltage!) then the cap is probably defective.

All that is assuming the amp is correctly wired like a Princeton. If not, well, all bets are off.

[Ed_Chambley]

The .1uf cap in red is the one you are speaking of?

Yes, that's a prime suspect. Just lift one end of the cap. Doesn't matter which one. If the Intensity pot no longer changes the bias voltage (especially to a positive voltage!) then the cap is probably defective.

All that is assuming the amp is correctly wired like a Princeton. If not, well, all bets are off.

Thanks Steve. Great call.  Working now.

The depth knob works as well, but it seems to just blend the tremolo.  Tremolo is extremely strong and sounds nice.

I have never seen a weber kit, but I am thinking this started as one.  It is rather noisy.  It needs about 10 feet of wire removed.

The PT has a bias tap that was not used.  Anyway, I will know a little more later.  Gotta talk to the owner about his budget for the amp.  The amp chassis was designed for a reverb, but it is not as deep as a Fender chassis.  I did not create the mess, but I can offer to fix it.  Voltages are a lot higher so I changed the rectifier tube as well.  A 5v4 worked better and got the plates of the 6v6 just under 450v.

I am not dogging the person who built it tho.  It looks better than my first 2 builds, well better than they used to.

[Ed_Chambley]

In his PR based Sweet Spot , David Allen uses a 47@100V cap on the intensity pot, ( left leg , looking at the back of the pot ) to ground ,for this very reason... ground the positive end.

Floyd, thanks for the  photo.  I guess it is a little added insurance.

[sluckey]

In his PR based Sweet Spot , David Allen uses a 47@100V cap on the intensity pot, ( left leg , looking at the back of the pot ) to ground ,for this very reason... ground the positive end.

That cap is just a second filter cap for the bias voltage. It won't prevent the kind of trouble Ed had.

[Willabe]

Glad you found it.     

A 5v4 worked better and got the plates of the 6v6 just under 450v.

That's still high for a NOS RCA set of 6V6's. A few of the newer 6V6's will probably handle it.


              Brad     

[floyd]

In his PR based Sweet Spot , David Allen uses a 47@100V cap on the intensity pot, ( left leg , looking at the back of the pot ) to ground ,for this very reason... ground the positive end.

That cap is just a second filter cap for the bias voltage. It won't prevent the kind of trouble Ed had.

David Allen states that he uses that cap to prevent the trem from " bleeding through " when not in use.

[sluckey]

David Allen states that he uses that cap to prevent the trem from " bleeding through " when not in use.

That cap is tied to the wiper of the bias pot as well as the end of the INT pot. It provides extra filtering for the bias supply which also provides an AC ground at that side of the INT pot. It's that AC ground that prevents trem bleed thru ONLY when the INT pot is turned to zero. So, David's statement is true. However, if you use the footswitch to turn the trem off, the trem oscillator is shut down and there won't be any trem signal to bleed thru.

But that has nothing to do with the problem that Ed had. Nor would it have prevented his problem. Ed had a leaky coupling cap from the plate of the trem oscillator tube that was putting a positive voltage on one side of the INT pot. The other side of the INT pot had negative bias voltage on it. Therefore, as the INT pot was rotated, it changed the bias voltage being fed to the grids of the output tubes. He could vary the voltage from some negative value all the way to some positive value and that's bad for the output tubes. In a properly operating bias vary trem circuit, changing the INT pot will have no effect on the negative bias voltage. Changing the INT pot should only change the amplitude of the trem signal that rides on the bias voltage.

[Ed_Chambley]

Glad you found it.     

A 5v4 worked better and got the plates of the 6v6 just under 450v.

That's still high for a NOS RCA set of 6V6's. A few of the newer 6V6's will probably handle it.


              Brad     

You are correct sir, but in actual use they handle it quite well.  Been running a pair in a deluxe reverb ab763 with the plates at 460 v bias at -21 for about 5 years and still running strong.  I do not know who much punishment they can take, but I have found the old 6v6gt will handle voltages over the data of a 6l6gb.  I may just be lucky, but I think they were underrated.  I have ran some coin base 6v6gta sylvanias at 490 and they ran fine.

I am a little careless with 6v6 tho, but I have quite a few.  Normally I would use the new Tungsol, but they sound a little harsh and that is the main reason the guy brought me the amp.  He wanted RCA power tubes and a smooth plate telefunken in v1 so I need to change the 12ay7 and will have to change the bypass value.

I may end up installing a zener on the CT.  BTW, the owner said make it right and can you add reverb?  I said I can put windshield wipers in a jackrabbit's butt if he wanted me to.  Here we go.

[Ed_Chambley]

David Allen states that he uses that cap to prevent the trem from " bleeding through " when not in use.

That cap is tied to the wiper of the bias pot as well as the end of the INT pot. It provides extra filtering for the bias supply which also provides an AC ground at that side of the INT pot. It's that AC ground that prevents trem bleed thru ONLY when the INT pot is turned to zero. So, David's statement is true. However, if you use the footswitch to turn the trem off, the trem oscillator is shut down and there won't be any trem signal to bleed thru.

But that has nothing to do with the problem that Ed had. Nor would it have prevented his problem. Ed had a leaky coupling cap from the plate of the trem oscillator tube that was putting a positive voltage on one side of the INT pot. The other side of the INT pot had negative bias voltage on it. Therefore, as the INT pot was rotated, it changed the bias voltage being fed to the grids of the output tubes. He could vary the voltage from some negative value all the way to some positive value and that's bad for the output tubes. In a properly operating bias vary trem circuit, changing the INT pot will have no effect on the negative bias voltage. Changing the INT pot should only change the amplitude of the trem signal that rides on the bias voltage.

Understanding the reason when positive is fed to the first lug, why did it not have any AC on the pot?  Is it the .022 to ground stopping it?  Although I did not check for ac, I did not notice any scratchy sound.

[sluckey]

Understanding the reason when positive is fed to the first lug, why did it not have any AC on the pot?  Is it the .022 to ground stopping it?  Although I did not check for ac, I did not notice any scratchy sound.

Because you had the trem turned off. The trem signal is the only AC that could be on the pot. You noticed the dc bias voltage changing when you had the trem off. This was due to the leaky cap. That cap was also leaking positive dc voltage when the trem was on but since the idea of bias vary trem is to put the very low frequency LFO signal on top og the bias voltage (causing the bias voltage to change at the trem frequency) you would probably not notice that the dc bias level had shifted because of the leaky cap.

So, what symptoms initially led you to discover that the INT pot would reduce the negative bias with the trem off? Or did you just happen to realize this while you were checking the bias current thru the 1Ω resistor?

[Willabe]

I have ran some coin base 6v6gta sylvanias and they ran fine.

Now you just jogged my memory with that. Jim Kelly amps run Sylvania 6V6's at 490.


                  Brad      

[Ed_Chambley]

Understanding the reason when positive is fed to the first lug, why did it not have any AC on the pot?  Is it the .022 to ground stopping it?  Although I did not check for ac, I did not notice any scratchy sound.

Because you had the trem turned off. The trem signal is the only AC that could be on the pot. You noticed the dc bias voltage changing when you had the trem off. This was due to the leaky cap. That cap was also leaking positive dc voltage when the trem was on but since the idea of bias vary trem is to put the very low frequency LFO signal on top og the bias voltage (causing the bias voltage to change at the trem frequency) you would probably not notice that the dc bias level had shifted because of the leaky cap.

So, what symptoms initially led you to discover that the INT pot would reduce the negative bias with the trem off? Or did you just happen to realize this while you were checking the bias current thru the 1Ω resistor?

No symptom.  The amp lead dress made suspicious, but the event that brought it to my attention was simply viewing the meter after I had set the bias.  I gave it a test run with the meter still connected.  Turned on the tremolo and turned up the intensity and noticed a wicked bias swing.  Switched off the tremolo and the bias was sitting at -4ma.  Turned it up more and it went positive and I immediately shut it down.  Sure sign of trouble in paradise.

[Ed_Chambley]

I have ran some coin base 6v6gta sylvanias and they ran fine.

Now you just jogged my memory with that. Jim Kelly amps run Sylvania 6V6's at 490.


                  Brad      

Never looked a Jim Kelley amps.  It all has to do with what we like.  When I lower the voltage on a fixed bias amp I am usually thinking distortion and actually prefer Old Tung Sol 5881 where you would normally see a 6V6.  The only 6v6 I have ever had a problem with are Ruby's.  I pulled a pair out of a Egnater Tweaker and put them in my high voltage 763 deluxe and they looked like the 4th of July.  They hit the garbage can.
What I like about running high voltages on 6v6's is the bass response is much better for me.