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Post subject: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Sun Mar 29, 2015 8:49 am
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Location: Nanterre, France
Hello,

I finish to clean and refurbish my Twin Reverb. I changed the Power Supply capacitor in the doghouse, clean the pot track....
Now it is time to test.
The Normal channel sound very well. Warm, clean and the tone stack is fully working.
The Vibrato channel is not so nice. It doesn't sound like the Clean one but more muddy with distortion (both volume pots being at the same level for the test). In addition when I switch On the vibrato, I get a motor boat sound increasing when I move up the intensity pot.
The reverb is working perfectly.
I didn't change yet any tube, so could be a preamp tube can be the root cause but I'm unsure.

Any advice to fix that ?

Thx in advance
Pascal

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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Sun Mar 29, 2015 9:30 am
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I'd guess that you have a microphonic component buried somewhere in the vibrato channel, possibly in the tremolo circuit. It could be a tube but I'm thinking it might be a coupling cap. Try some all-new tubes in that channel (V2 through V5) and note any improvement. If none is observed, you'll need to dig deeper.

Arjay

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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Sun Mar 29, 2015 9:56 am
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You didn't have this "motorboating" issue before changing the caps under the doghouse? It is possible that the new 20mfd/500VDC in the third position --- next to the two 35mfd/350VDC, or whatever big caps you used --- is bad. It has happened. What brand caps did you use?

Also, did you replace the bypass caps? Maybe wrong mfd or reversed polarity?

Coupling caps would be the next to look at.


Good luck! Keep us posted.


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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Mon Mar 30, 2015 3:25 am
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If the Normal channel sounds fine, I would think that the power supply caps were fine (doghouse). :idea:

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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Mon Mar 30, 2015 4:52 am
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Location: Nanterre, France
Guys,

Thx for tour advices.
All filter caps are new. I used TAD, Mallory and Weber brands.

I wasn't able to test it before to change the caps and to do some adjustments. The previous owner told me that it was not working but after to investigate, the failure came from the power switch.

I swapped the tubes but the issue still persist. I even used new tubes like 12ax7 EH and 5751 Sovtek but only the sound is different and that's normal.
I have some 25mf and 5 mf, so next step would be to change the coupling cap because the originals are always in place (around 37 years old...not good :? )

The sound from the Normal channel is very good, warm and well define and clean.

I'll keep you posted.

Pascal

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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Mon Mar 30, 2015 5:05 am
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BMW2002Ti wrote:
...did you replace the bypass caps?...


This is where I would look next. Electrolytic caps are more likely to fail than coupling caps.

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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Mon Mar 30, 2015 6:45 am
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"I have some 25mf and 5 mf... "


These are the cathode bypass caps, off most of the 12AX7 and 12AT7 cathodes-to-ground. They are electrolytic and have polarity. You may want to measure the resistance of the cathode-to-ground resistor paralleling each bypass cap. Be sure the tubes are pulled before measuring the resistance. If the resistance is off by more than 15-20% --- replace.

Coupling caps are non-polar and usually range from 0.01mfd to 0.47mfd. Measure the DC voltage downstream of the coupling cap. This is usually on the grid of the next stage tube. Part of the function of a coupling cap is to isolate DC voltage applied to previous anode from the grid of the next stage. Should not be more than few ten's of mV DC.


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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Mon Mar 30, 2015 9:08 am
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Guys,

I made a mistake. I was speaking about by-pass cap :roll: not coupling. My fault....
I'm French, so my English is sometimes not good enough.

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Post subject: Re: Twin Reverb Silver Face 78' with Motor boat sound
Posted: Mon Mar 30, 2015 9:59 am
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Vous n'a pas causé de problème. La bonne chance avec l'amplificateur!


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