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Post subject: Twin Reverb Master Volume 135w dist. / early break up
Posted: Wed Mar 04, 2020 9:30 am
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Twin Reverb. Not sure the year, but is the 135w master volume with pull switch.

Vibrato channel breaks up / distorts ( mostly asymmetrically clipping with the top half of the waveform being the most affected)

I have replaced ALL electrolytics, power and bypass. Swapped / subbed in new tubes. Replaced 100k load resistors in that circuit.

Data I'm working with:

1. The reverb tank is missing, seems like from the schemo this shouldn't cause any issues???
a. Maybe a should stop right here, cause it would cause this funky clipping somehow?
2. The vibro channel can only take about 300mV before it starts to clip (Attached is a JPG of the schematic with waveforms and voltages)
https://imageshack.com/i/plF4Q81vj


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Post subject: Re: Twin Reverb Master Volume 135w dist. / early break up
Posted: Sun Mar 08, 2020 10:48 am
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20v P-P is probably OK. Check it against the clean channel.
5.5 VDC on the grid of V3 is a problem. If you're not using the reverb, pull V3.
The 3.3Meg resistor is used to pull down the vibrato output to mix equally with the Reverb output.
Again, if you're not using the reverb, you can tack another resistor across the 3.3M to increase the clean gain of that channel. Start with a 1M.


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Post subject: Re: Twin Reverb Master Volume 135w dist. / early break up
Posted: Mon Mar 09, 2020 1:42 am
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Here is something Ive always did to mine is put a 120k R across the in and outs on the inside where the reverb tank plugs in which makes the reverb circuit distortion and it cranks some say other wise I disagree with these opinions. I just this week repaired one that was in my first band 1980 this amp was under water for three days and when I found it it was in the bed of the owners truck swimming in garbage juice. I bought it some how Its back to life. any way see if that the distortion you see on the scope, .. ill send ya some vids if they will send .[youtube][/youtube][url][youtube][youtube][/youtube][/youtube][/url]https://www.youtube.com/watch?v=WfNIfQ7BpvA[youtube][youtube][youtube][/youtube][/youtube][/youtube]


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Post subject: Re: Twin Reverb Master Volume 135w dist. / early break up
Posted: Mon Mar 09, 2020 11:25 am
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Both the clean and the vibro channel put out 20v P-P on their plates before break up. It's the GRID and PLATE of V4B (vibro channel) that has the issue.

I understand the purpose of the 3.3M. Is it waiting for a "pass-through" signal from the reverb circuit, even if the REVERB control is all the way down?

I have pulled V3, same issue.


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Post subject: Re: Twin Reverb Master Volume 135w dist. / early break up
Posted: Mon Mar 09, 2020 9:07 pm
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Hmmm. The reverb signal doesn't go through the 3.3M. The vibrato signal does. It is lowered so drastically that its signal could be affected by the loading of the Reverb control.
Set the amp up with just enough gain to see the clipping. Then raise the Reverb control from zero and see if the clipping goes away.
Yes, I know it doesn't get rid of it, but you may find the source.
If you don't use the reverb, consider removing the wire to the control to remove that load.


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Post subject: Re: Twin Reverb Master Volume 135w dist. / early break up
Posted: Tue Mar 10, 2020 5:13 am
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TimsAudio wrote:
20v P-P is probably OK. Check it against the clean channel.
5.5 VDC on the grid of V3 is a problem. If you're not using the reverb, pull V3.
The 3.3Meg resistor is used to pull down the vibrato output to mix equally with the Reverb output.
Again, if you're not using the reverb, you can tack another resistor across the 3.3M to increase the clean gain of that channel. Start with a 1M.


Sound like maybe the 0.022mfd coupling cap, between V2B and V3, is leaking VDC.

Image


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