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Can anybody please tell me why my tremelo channel has decided to vacate my amp. It has a footswitch and when i put it on it tremelo,s for a split second and fades away, I have checked my valves and all seem ok so could it be a capacitor and if so what is the cap called , it must be a special one . as you have probabley noticed i am no amp gurru so any info would be of great help , thankyou.
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First try switching a different 12ax7 into the socket (I'm assuming 12ax7 oscillator, I don't recall a watkins that used anything else, but the following only applies if it is indeed a 12ax7, 12at7 or 12au7).
Unplug the amp from the wall while it is on. This will help drain the filter caps. Wait a while, and short the caps to chassis with a screwdriver. Plug in the footswitch and set the controls as though the amp were on and you were playing with tremolo. There are dangerous voltages in tube amps and if you are not experienced with these please look up tube amp safety in google. 400v shocks are no laughing matter and can kill you outright or injure you with reflexive movements etc. Very bad!
The oscillator in your amp most likely takes a copy it's own output and feeds it into it's own input through a network of say 3 capacitors. One of these will be a 200-600v rated part attached to pin 1 or pin 6 of the oscillator tube (don't have schematic nearby, so I'm not sure).
Make sure each of these three or four caps (they'll lead back to say pin 2 if they're from pin 1, or pin 7 if they're from pin 6) reads infinity between leads on your meter (set to megohms).
These oscillators depend on the tube to amplify at highest possible gain. Another possibility is that there is a failed cathode bypass cap (which boosts gain). This sounds likely, so check the electrolytic caps (if any) attached to pin 3 and pin 8 of this tube. These pins should read somewhere between 600 ohms to 5k ohms to ground in typical circuits. If they read 0 to ground, you need to replace these caps.
Good luck, vacuum tube oscillators LOOK confusing until you stare at them for a while. They're essentially a loop of capacitors from anode to grid, with resistors inbetween each cap to ground. The value of these caps and resistors sets the speed. The tube itself tries to amplify it's own output through this "filter", resulting in a pulsing signal. If the filter is broken, the tube is not amplifying well, or the signals are not coming back to the tube correctly, it will not have tremolo.
Valve wizard is a great site to learn about this stuff, here's the link for tremolo oscilators.
http://www.freewebs.com/valvewizard1/trem1.html
Schematic heaven may have a complete schematic of your amp:
http://www.schematicheaven.com
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