Odd 6G2 Bias Trem Problem

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sliberty
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Odd 6G2 Bias Trem Problem

Post by sliberty »

I built a Fender 6G2 copy some time ago, but never bothered hooking up the foot switch for the trem. This week, when I tried to do so, I discovered that closing the switch (grounding the oscillator RC network) resulted in a loud sputtery noise (the trem did stop of course). Removing the footswitch, and simulating the switching action with alligator clips from ground to that same spot resulted in the same noise. I replaced all three caps in the oscillator circuit as well as the 1M resistor at the point where the switch is supposed to tie in and the cathode cap for the oscillator, but these changes fixed nothing.

Any ideas on what might be causing this and how to resolve it?
tubeswell
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Re: Odd 6G2 Bias Trem Problem

Post by tubeswell »

This could be lead dress related, or it could be a problem with the triode in that LFO stage. Have you tried a tube swap (with a known good tube)?
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sliberty
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Re: Odd 6G2 Bias Trem Problem

Post by sliberty »

Thanks for the reply.

I have tried several known good tubes in both V2 (which includes the oscillator) and V1 (just because). No difference.

As for lead dress, I was afraid someone would say that :) Frankly, lead dress is black magic to me. I don't know any of the do's and don'ts that I should be watching out for. But I will say that it isn't likely to be related to the foot switch jack wire, because the same thing happens with an alligator clip that I have moved all around. So that basically leaves the wires that run over to the speed and intensity pots. I have them fairly short, so I can't move them around much. But whatever moving I can do didn't reveal anything either.

Having said that, I would expect lead dress to cause noise when the tremolo is engaged (oscillator active), not when it is shut off (oscillator stopped).

Weird. Any other thoughts are welcome.
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martin manning
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Re: Odd 6G2 Bias Trem Problem

Post by martin manning »

Did you try grounding out the oscillator in a different place, say at it's cathode resistor ground? I'd also try disconnecting the lead going to the top of the intensity pot to see if the noise is coming through that path. Is it possible that the noise is a result of an ultrasonic oscillation? You might see if disconnecting the feedback loop has any effect.
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sliberty
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Re: Odd 6G2 Bias Trem Problem

Post by sliberty »

There are three Caps in the feedback circuit. The normal place to ground is between the .01’s, and I get noise there. I also tried between the .01 and the .02, and got noise there as well. As for the cathode, I’ve lifted the cathode, and that stops the Trem, and emits no noise. As for grounding in different locations, yes, since I am now testing with alligator clips, I’ve conn cited to the chassis at various locations, each of which still resulted in the noise.

Keep the ideas coming guys. I really appreciate it.

This amp is for me, so I plan to install a worka round for now. I am putting a switch onto the back of the intensity pot, and when the pot turns fully counter clockwise, the switch will lift the cathode. I stole the idea from the power switch of an old Champ. So Trem won’t be foot switchable, but will will be defeatable. But this is only a temporary solution of course. Something is wrong and Ned’s to be fixed, especially in case I want to sell it at some point.
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martin manning
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Re: Odd 6G2 Bias Trem Problem

Post by martin manning »

Sorry, I meant disconnect the global FB loop from the speaker.
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sliberty
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Re: Odd 6G2 Bias Trem Problem

Post by sliberty »

I'll try the feedback loop suggestion next.

I am also going to amend the description of the noise.

It is a loud rhythmic popping sound. It pops at a speed that seems to be related to the phase shifts in the feedback loop of the oscillator. By that I mean that if I ground at the usual place between the two .01uF caps, it pops at one speed, but if I ground between the .01uF and the .02uF caps, the rhythmic popping is much faster. Also, I typically prefer my trem fairly slow, so I have paralleled one of the .01uF's with a .1uF to slow things down a lot. With that extra cap in place, the popping is slower, if I lift it, the popping is faster. However, the speed knob doesn't impact the speed of this noise.
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sliberty
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Re: Odd 6G2 Bias Trem Problem

Post by sliberty »

Lifting the global feedback did not eliminate the noise.
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Re: Odd 6G2 Bias Trem Problem

Post by sluckey »

That trem circuit has been used in several Fender amps. The circuit works well. I'm suspecting layout issues or maybe even a wiring error. One thing to try would be move the trem oscillator B+ supply from the plate node cap to the screen node cap to provide more filtering. That plate node is pretty dirty but the screen node would be much cleaner.

Could you post some hi rez pics of that area of the circuit?
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martin manning
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Re: Odd 6G2 Bias Trem Problem

Post by martin manning »

Ok, that helps to know what sort of noise you have. So the LFO is actually still oscillating, in a way?
Maybe you are aware, but the usual practice for reducing the LFO frequency is to replace one or both of the 0.01u with 0.02u, keeping the RC phase shifts approximately equal
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sliberty
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Re: Odd 6G2 Bias Trem Problem

Post by sliberty »

martin manning wrote: Fri Dec 22, 2017 8:19 pm Ok, that helps to know what sort of noise you have. So the LFO is actually still oscillating, in a way?
Maybe you are aware, but the usual practice for reducing the LFO frequency is to replace one or both of the 0.01u with 0.02u, keeping the RC phase shifts approximately equal
The suggestions I've read indicated paralleling a cap as viable, but I did go much farther than they used in their examples. They usually just added another .01uF in parallel, resulting in a .02uF value. Obviously my value change is more dramatic, but since the noise is still present even when I remove that extra cap, I figure the large cap is not the cause of the problem. But maybe I should change both of the .01uF's to .047uF's to keep the phase shifts roughly equal anyway. Next time I open her up.
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sliberty
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Re: Odd 6G2 Bias Trem Problem

Post by sliberty »

sluckey wrote: Fri Dec 22, 2017 7:36 pm That trem circuit has been used in several Fender amps. The circuit works well. I'm suspecting layout issues or maybe even a wiring error. One thing to try would be move the trem oscillator B+ supply from the plate node cap to the screen node cap to provide more filtering. That plate node is pretty dirty but the screen node would be much cleaner.

Could you post some hi rez pics of that area of the circuit?
Thanks. Next time I open up the amp, I'll take some photos, and try the screens node of the B+ supply.
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martin manning
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Re: Odd 6G2 Bias Trem Problem

Post by martin manning »

sliberty wrote: Fri Dec 22, 2017 9:29 pm...but since the noise is still present even when I remove that extra cap, I figure the large cap is not the cause of the problem.
I agree, and I wasn't suggesting otherwise. You might have some other issues, like slow starting and/or stalling out.
tubeswell
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Re: Odd 6G2 Bias Trem Problem

Post by tubeswell »

Which pre-amp tube envelope is your LFO stage in? (It helps to avoid parasitic coupling if you keep it in the same envelope as the cathodyne splitter)
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Re: Odd 6G2 Bias Trem Problem

Post by bmac »

This is a very old thread, so I assume you found and fixed the issue. If not, or just for general info on the 6G2 circuit, I just completed a 6G2 build and had the exact same problem. It sounded like a loss of filtering, so I tested clipping in an additional filter cap and realized that where I had connected the positive lead on the preamp filter cap was causing the problem. What was happening was that grounding the trem footswitch was interrupting the circuit to that cap. I moved the + connection to the proper place according the the schematic, and all was fine. Great sounding amp BTW.
Brian
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