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Hi all,

I pulled my Triwatt out for the first time in a while and it sounds great, however pulling the OD switch out kills the signal. I haven't had a chance to look inside yet, and I assume I miswired the switch somehow. Anything else I should look for that might cause that?

Thanks!

So I opened it up and everything looks OK visually, the pot and switch seem to be wired correctly. I noticed the bias switch was in the KT-66 position and I have 6V6GTA's in the amp, so I put it back to the 6V6 position and tried to rebias it. I don't have enough voltage to get into the correct range. I put the switch back in the KT66 position and I was able to bias the tubes to 18.5 and 20.5 mv respectively. I put it all back together and the overdrive still doesn't work - when I pull out the switch it kills the signal regardless of where the pot is. It's not a tragedy, I have pedals, but I'd like to figure out why it doesn't work.

Did it work in the past?

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I really don't remember - I built it, tried it once or twice and went on the road for two years. I'm going to look carefully at the bias circuit today and see if I made a mistake there - it's obviously not working correctly, though it is working. I'll inspect the OD section carefully as well.

I'm looking through the amp and there's two issues I see, and I suspect they're related. The output of my power transformer is 370 volts on both sides, not 345, and my b+ is 475 volts. So that's within +- 20 % voltage spec for a Fender but is it ok for the triwatt? Also, would that cause my bias voltage to be too low? My AC is typically around 126.

I'm taking a close look at the od section now.

Edit: OK, so an hour later I think I found the problem with the OD - one of the shielded cables was shorted internally.

The voltage thing concerns me, though. Some representative voltages: V6 pin 3 494V, pin 4 461V pin 5 -47V. V5 pin 3 492V, pin 4 462V, pin 5 -47V. V4 pin 6 293V, pin 1 298V, pin 2 78V. V3 pin 6 207, pin 1 302. V2 pin 1 205V, pin 6 128V. V1 pin 1 165V pin 6 146. The maximum bias voltage I can get in the 6V6 position with a 'matched' pair of NOS Sylvania 6V6 GTA's is 9mV on one tube and 12.3 on the other. Does that sound right?

I checked over the parts in the power supply section and they seem to be ok and installed correctly.

Thanks!

My ability to confuse myself is remarkable. I had mislabeled the KT66/6V6 switch, and the voltages were never correct in what seemed like the correct position. Now it's fine, the OD was fixed by replacing the shielded cable. There were a few less than ideal solder joints I fixed, and the amp works great. Thanks!