Tube Rectifier flash and Popped Fuse in new AC30 build

[Mason]

Hi all, this week I finished building an ac30/6 clone, following the layout/schematic for the Weber kit here, https://tedweber.com/6v30-c-kt/ click the layout/schematic section at the bottom of the page. I'm not actually using his parts, just the chassis. The build went well and I've been playing it all week and it sounds great. But today I've run into an issue with the rectifier and a blown fuse.

I was playing the amp for about 10 minutes then shut it off and plugged in a different cab and a few minutes later turned it back on. I heard some pops/thump and saw the pilot light go out. The fuse had blown. I replaced it and powered it up again and as soon as I took it off standby I saw the GZ34S rectifier flash. I turned it off immediately.

My first thought was maybe the wall voltage was a bit high, as I had been playing the amp mostly hooked up to a variac all week set to 120v but this time I wasn't. My wall voltage reads about 122-124. After plugging into the variac all seemed fine, I turned the variac down a bit to be careful and played for a few minutes before shutting it all off.

Is this a sign of a bad rectifier, or bad filter caps? I didn't find any shorts in the caps. My one modification to the circuit was changing the filter cap values to 40uf for the mains and 20uf for the rest. But I think that should still be low enough for the GZ34S to handle.

My only other thought is that my bias may be set too high. I've got 10.95v on the cathode resistor and my plate voltage is 327v. 4 EL84 tubes sharing one 50 ohm cathode resistor.

[sluckey]

Your bias is set for 17W per tube. Replace the 50Ω with a 100Ω and play with that a bit and recheck the bias. You can probably decrease the 100Ω some more if desired.

[Mason]

So you think the high bias is the culprit more than a bad rectifier? I do agree the bias is too high but didn't think it would cause this.

[sluckey]

So you think the high bias is the culprit more than a bad rectifier? I do agree the bias is too high but didn't think it would cause this.

I didn't think or say that. I just rolled the numbers you posted and you are cooking those tubes.

But, you are pulling 219mA just with the output tubes. Probably add 10 to 20 more mA for the little tubes. That's a lot for a GZ34 that's rated for 250mA max DC current. So, it's possible that is arcing the GZ34. Did you notice that Weber used his SS rectifier?

Here's a tip... Don't use that standby switch! That's really hard on a fully loaded GZ34. A GZ34 takes about 15 to 20 seconds to heat up and pass current. That's all you need.

[pdf64]

The standby is a hot switching type, so will be bad for real rectifier valves, rather than Weber’s copper cap.

[Mason]

Interesting. Just read up on hot switching. So if I did want to use the standby switch I should wire it after the first filter cap? I'll just leave it on but I would like it to be functional.

And so that also seems like it could be the culprit for the rectifier flash. Should I replace this valve now? Or will it be fine if I stop using the standby switch?

[tubeswell]

Interesting. Just read up on hot switching. So if I did want to use the standby switch I should wire it after the first filter cap? I'll just leave it on but I would like it to be functional.

And so that also seems like it could be the culprit for the rectifier flash. Should I replace this valve now? Or will it be fine if I stop using the standby switch?

It could be you have a bad/shorted rectifier tube. In which case, the reservoir cap is also probably shot. A shorted rectifier will dump VAC onto the reservoir cap. Electrolytic caps aren't designed to take high VAC, and so if the reservoir cap has been exposed to high VAC from the rectifier, it is also likely to have shorted (thus dumping high VAC from the PT's High Tension Winding straight to ground), which will blow your fuse. Replace the rectifier and probably the reservoir cap.

Also check out R.G. Keen's 2008 immortal amp mod article (in Premier Guitar Magazine) on using SS rectifier protection diodes. https://www.premierguitar.com/articles/the-immortal-amplifier-mod-1 (Like many other things, Rob Robinette copied this onto his website)