200s popping/clicking

[truth_hz]

Hey all, I've been a big Sunn fanatic for a long time. I've fixed up 4 or 5 over the last few years, and seen quite a few in varying condition. The 200s I'm working on now has an issue that I haven't run into before...

Here's the skinny. Grabbed this amp a a local music shop, and it had been 'serviced' supposedly. It was less than a great job unfortunately. I won't go into how bad it was...

So, I was able to locate a correct power switch, get it installed. I also added a rotary impedance selector on the OT for access to 4-8-16 taps(handy). I also replaced the can cap with a CE 40-40-20-20.

This is where stuff started to go wrong. The first CE cap had a bad section right out of the box, and quickly gave up the ghost! Enter 2nd CE cap... As a precaution I built up the first filter section with discrete parts mounted on terminal strips. 2x100uf in series with 220k 2w balancing resistors, then wired the rest of the new can up as 40-40-40 with the last 2 20uf sections in parallel for a little extra preamp filtering. Replaced the 15k and 33k droppers with MF 2w resistors as well.

So the problem I'm having is this: hit the power, let it warm up, and when I throw the standby into play mode, I get some rapid clicking/popping sounds.

This is with NO tubes except the rectifier installed. I am a little wary of arcing, because I don't want to buy ANOTHER can cap if something shorts badly. So I just kill the power. Haven't left it on long enough to really narrow down where the problem is. I'm confident about my power supply wiring.

I have heard some evidence that it could be related to the feedback network. Going to take a look at that and the 390pf cap there on the power tube socket that returns to the PI. I'm only reading 47R across the 1K in there right now, but I haven't removed anything for proper measurement.

The 47K PI resistors are still the old CC type, that I think I'll replace.

But I cant really wrap my head around what might be causing it because I'm testing it with no tubes(and with a 1A fuse, just in case), so there shouldn't be and real current draw anywhere.

I've heard of this clicking/popping problem from reading some posts here, but never a definitive answer as to what it is. In my case, all electros are new, so it -shouldn't- be that...

Any help or suggestions would be much appreciated!

[loudthud]

Powering up the amp with only the rectifier tube installed is a bad idea because the full B+ voltage will get to all the caps and it will be 30 to 50V higher than normal.

Check the power tube sockets for any signs of arching between pins 2 and 3. Sometimes you'll see a crack or a black carbon trail.

[truth_hz]

Powering up the amp with only the rectifier tube installed is a bad idea because the full B+ voltage will get to all the caps and it will be 30 to 50V higher than normal.

Check the power tube sockets for any signs of arching between pins 2 and 3. Sometimes you'll see a crack or a black carbon trail.

Excellent point, thanks for the response.

The first filter section is rated at 700v, though the second and beyond are 525v from the can cap. From memory, I was observing ~560v-ish at the first node, with the second only marginally lower(no tubes).

I did look for arcing on the sockets, but they don't seem to show any signs.

I'll check it out with tubes in and report back.