New Build Issues

[wpaulvogel]

I think you’re problem is lead dress creating an oscillation but running a 6V6 like a 6L6 or EL34 doesn’t make sense to me. Put EL34’s in it unless you built an amp around a power transformer that can’t support the heater current but then I’m confused with the transformer design. Usually high voltage transformers have enough heater winding to support the tubes that are designed for the high voltage.

[sluckey]

Probably some hi-freq oscillation going on only when you touch your meter probe to the plate pin. The oscillation fools the meter. When this happens just measure B+ at the center tap and use that number for plate voltages.

I wanted to repeat this but also add... This does not indicate a problem with the amp. I have several perfectly working amps that exhibit this behavior. Usually see it with my amps that use high B+ voltages, like my Twin Reverb or Sunn Sceptre. Sometimes I can even hear a hi-freq squeal. I'm not trying to correlate this to high voltage B+ amps, but I can't recall seeing this when I probe the plates of my amps that operate below 400V.

[ViperDoc]

I noticed an imbalance in power tube plate voltage when I changed the location of my filter cap ground. I recently built a true point-to-point Express using a floating buss wire that grounds the entire amp to the input jack. I first placed my filter cap ground at the power amp end of the wire and my plate voltages were balanced. Later, having misinterpreted a recommendation in Blencoe about grounding part of the power amp at your GNFB, I tried moving the filter cap ground to where I had my GNFB, which if I recall, essentially placed my filter cap and PI supply cap grounds in the same place on the wire. When I fired up the amp, my plate voltages were severely imbalanced, one high, one low. I returned the FCG to the end of the wire, separating the ground positions and the voltages returned to normal. Mind you, this is all grounded on the same twisted wire, but when placed in different locations on the wire, the voltages changed. Maybe look for NFB that affects your output section, as well as ground point locations. Maybe there's a ground current issue to be worked out.

[R.G.]

OK, with grid stoppers in, my next go-to's would be lead dress and then grounding issues.
Lead dress is first because it's easier to poke around, moving wires with a wooden stick. Chopsticks are preferable to pencils because the lead inside conducts. Ouch. Ask me know I know this...

There is a form of high frequency oscillation that happens when an earlier stages is grounded through the current path wires from the output stage. The output stage's ground current wobbles the previous stage's "ground" up and down, and is effectively input through the cathode (especially with a cathode cap). If the phase and frequency is just right(wrong...) it sings. Also depends on the wire gauges, goodness of solder joints, and phase of the moon.

[sluckey]

How does the amp sound?

[dorrisant]

The amp sounds very good...all but that little distortion that I mentioned. If I had it here I would post an audio clip and show the lead dress. All of the filter caps are grounded at a copper buss that is in turn grounded at the input jack. The only exception is the filter cap for the FX loop that is grounded at the send and return jacks on the back panel. Power tubes are grounded through 1Ohm resistors right between the power tube sockets. Maybe that one needs to be moved, idk...

Sorry Roberto, I did not mean to offend anyone at all. I don't quite understand what the principal is behind what you advised but would love to know more.

[dorrisant]

After further review... it is not anything to do with anything discussed above. It was the speaker cab. Too much bottom end for a crappy sealed cab. I sat on top of it (275lb) and it all went away. Ha!