6L6GC PP Problem

[PassFan]

Well I just finished a 30 watt pp design by David Hafler and have run into something odd. Both channels check out fine and match up pretty well voltage wise but, I have run into what appears to be frequency dependant distortion. At barely discernable volumes everything sounds great but when I increase input I get distortion in the lows and highs with a clean, clear midrange. Could this be from using cheap coupling caps between the stages or could a faulty bias setting be responsible. Hopefully someone has run into this before.

[Sch3mat1c]

No. Only the OPT comes to mind. It must have very low inductance and high shunt capacitance for this to happen; are you using it at a much higher impedance than designed?
What's the bias current in the output stage?

Might also be parasitics, what's the scope show?

Tim

[SY]

A faulty bias setting will show up in the output stage cathode current; if the voltage across the cathode resistors are in spec, that's not the problem. It may be oscillation, it may be a wiring error (are you sure you're feeding signal to BOTH output grids?).

Measure some of the DC voltages under no-signal conditions in the output and driver stages and post them- maybe there's a clue there.

[dhaen]

Hi,

Sounds like badly out of balance PP to me.
Check that the current going through each output valve is the same.

Cheers,

[PassFan]

Found at last. All measurements on both channels matched up pretty close and one channel worked so it gave me a place to start. It wound up being a loose joint or a bad coupling cap (I replaced all on the bad side and installed a 47k resistor on the input just in case I was overdriving the sensitivity), and a problem with one of my drivers on that side ( a loose grill was buzzing on a speaker). I appreciate all the good comments and ideas from you guys and I am still tweaking to see what she will do. I still have to change those crappy coupling caps out on the other channel and buy another set of matched output tubes (dropped one ). When I open it up I'll probably take out the input resistors just to take it back to the original design. The circuit is at Tubecad under classic magazine articles. It's an Ultralinear push pull Williamson style circuit. I posted a pic a couple of months ago in a thread titled Running Tube Amp Construction here for those that wish to peek. Thanks