Rocket build - Leaky Cap putting DC on V4 & V5 Grids

[sunnydaze]

Hello,

First, thanks to forum and the wealth of knowlegde contributed by it's regulars.

As my title suggest, seem to have a problem with a leaky cap that's allowing around 2.5 - 3 DCV to reach the grids of the first two powertubes. The amp sounds excellent and is extremely quiet except some noise when moving the treble pot. It had been that way since I built it about six months ago.

I decided to check my voltages againts the published specs, and am pretty much in line throughout except noticed a few volts DC on the grids of V4 & V5. it seems C10 (as per he the ampgarage layout) is a little leaky - has around 175 DC volts on the hotside and around 3 DCV on the other side. C11 seems to be blocking the DC fine.

I switch C10 with another, a Sozo with a DCmax of 400 volts. As soon as I put it in, fully blocked the DC (at least well below 1 DCV) and no more noise on the treble pot. Great! A few startups later, noise is back. Checking the new cap, found once again leaking aroud 3 DCV. Switched caps again, and at first fully blocking and no noise on the treble pot. But after a few startups, back to the same place. One thing that is a little confusing to me about the noise on the treble pot, is I can't seem to measure any DC on the pot itself, but the noise does go away for the short periods of time when C10 is not leaking DC.

I'm wondering if my problem is the 400 DCV max on the sozo's, or if something else could cause the caps to leak DC.

Any comments or suggestions would be appreciated.

thanks
mike

[JamesHealey]

sounds to me like something is spiking through that cap, either from something grounding out or the power tube is pulling too much current?

I dunno but if it's causing the cap to go bad there is an issue, the cap should not leak any DC maybe a little like 200mV or something nothing major at all 3V is a huge leak to be fair.. enough to cause problems.

[sunnydaze]

sounds to me like something is spiking through that cap, either from something grounding out or the power tube is pulling too much current?

I dunno but if it's causing the cap to go bad there is an issue, the cap should not leak any DC maybe a little like 200mV or something nothing major at all 3V is a huge leak to be fair.. enough to cause problems.

Thanks for the quick reply. Good call, after reading your note, I figured it out. As I said, all of my voltage readings are pretty close to the published numbers. What I didn't mention is mine is the Ceriatone version which has a so called half power switch on on the cathode resistors. Mine is using the Heyboer Dynaco variant OT and choke, both from RJ and a Hammond PT. I also have an extra 66uf cap on hotside of the standby, which kind of makes up for the diff in the Ceriatone power supply.

After writing my first note, I went through my spares and found a 630V cap for the c10 spot, and it seemed to be working. But your note made me wonder what could be causing a spike.

It appears the half power switch shouldn't be switched unless in standby mode. Before flipping the switch cap is fine, after switching cap is leaking.... Luckily, the tubes seem fine.

I never really use the switch, but I did flip when checking the tube bias and measuring some voltages.

I think I'll rewire so that the power tubes are on a single cathode resistors (as per ampgarage Rocket) and use the switch to switch a cathode bypass cap in or out on v1b.

thanks
mike