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previous: Jay Manning Hi,I have a Showman Reverb that... -- 3/19/1999 6:11 PM View Thread

Re: Silver face Showman Reverb

3/21/1999 10:34 AM
Graywater
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Re: Silver face Showman Reverb
Jay,  
 
Not ignoring all of Richie's good advice, but are you familiar with systematic troubleshooting, the technique I was trained in when the dinosaurs roamed the earth? The idea is to feed in a singal and either follow it from the input to where it dissapears (or isn't strong enough, in your case) or start at the output and see where it appears.  
 
All you need is a signal source and a detection device. The signal source can be a audio generator (ideally) or just a low voltage generator. Take a filament tranny, hook a 100k potentiometer across it and adjust it for around 150 mv (0.15vac) using your DMM (even better would be to use a bride rectifier off the secondary and put a 0.1 uf cap in series between the rectifiers and the pot - this gives you a 120 hz source which is closer to the guitar frequency range). Now take this 150mv input and feed it into your amp using a 1/4 plug. To detect the signal, use your DMM to measure the AC signal voltage at each tube grid and plate, anywhere you don't get sufficient is an area to concentrate on (a scope is a better detector if you've got one). "Sufficient" gain is a little variable and I'm not at home with the Showman schematic in front of me right now. But, the first stages should be about 30-35 and the following stages should be about 20 (I think) and I believe that the Fender PI is about 15 for each 1/2 (Doc, Richie, Bruce, RG, et at, help! I'm running on faulty memory here).  
 
Anyway, I hope you get the idea. This way you aren't randomly checking voltages and changing parts but instead determining the weak stage that needs attention.  
 
GW

 
Replies:
Benjamin Fargen Graywater, Nice tech t... -- 3/21/1999 2:26 PM
Dave Stork I'm with GW. In an old tube amp, a ... -- 3/25/1999 12:31 AM