distortion on leading edge of note

[Structo]

Anybody else think that the added cap on the bias pot could be doing something to the phasing (ripple) of the bias voltage?

[pula58]

Have you tried replacing or re-tensioning the output tube sockets? I am wondering if perhaps one of the tubes is not operating because of an open connection in the socket. If this were the case, you'd lose the benefit of the self-canceling push-pull topology.

HI Lou,
I am pretty sure both output tubes are operating because I get full power output as expected (except that I didn't expect the sidebands!).
ALso, checked both tubes with a bias meter. I'll check this though.

[pula58]

Anybody else think that the added cap on the bias pot could be doing something to the phasing (ripple) of the bias voltage?

Hi Tom,
Thanks for your post.
I added that 2nd cap on the bias network thinking that perhaps ripple on the bias was causing the trouble. It made no difference, and the problem existed before I added that cap. Plus, as pother posters have said, the bias supply is 60Hz, no t120Hz.

Back to the lab..somethings gotta give!

P.

[martin manning]

What about a bad filter cap? I believe you've added filtering (paralleled another one in) but have you swapped them out?

[Structo]

Yeah, sorry I am shooting in the dark at this point.

[pula58]

Yeah, sorry I am shooting in the dark at this point.

me too !!
I do appreciate your posts!

[pula58]

What about a bad filter cap? I believe you've added filtering (paralleled another one in) but have you swapped them out?

I have swapped the 1st and 2nd filter caps out with new ones - Unfortunately it made no difference. This seems consistent with the ripple looking about right at the B+ and screen supply.

[Firestorm]

Have you completely ruled out crossover distortion on strong transients? Maybe you could run an O-scope trace of both output tube plates to see where the zero-crossing point lies when the grids begin to conduct. Also shooting in the dark.

[pdf64]

Just noticed that the last pic shows 2 separate star grounds.
Did you try that idea of taking the B+ winding CT directly to the reservoir negative terminal, then grounding to the main star from there?
Pete

[pula58]

Firestorm and PDf, I shall look into the things you mention.

As an Aside, I just played the amp for a while. It measure-in at 42W into an 8ohm load (before clipping occurs) I have another amp (Fargen Blackbird) that clocks-in similarly at 41W. The fargen has so much more oomph..like...a lot. My build is noticebly weaker in feel compared to the Fargen even though the output power (measured at 300HZ) is practically the same.

Todays comparison (my build vs. Fargen) jives with how I felt the first time I powered-up the amp (my build) and played it - it is somehow anemic, even though I measure 42W max output power.

Something is definitely wrong....if I could only find out what.
The basic tone of the amp is good, but, anemic, and with this side-band problem.

[surfsup]

Do you have a newer oscope with a filter? Is the filter "on"? This might be hiding an oscillation on the trace.

[pdf64]

Does the Fargen have a sag resistor on the B+?

[martin manning]

Something is definitely wrong....if I could only find out what. The basic tone of the amp is good, but, anemic, and with this side-band problem.

You really are running out of things to try here. Do you have another OT you can swap in? Once in a while I hear about a bad OT being the cause of weak-sounding and distorted output.

[pula58]

Does the Fargen have a sag resistor on the B+?

No sag res in the Fargen, but the sidebands, and generally the amp feeling weak to me, pre-dates the installation of the sag resistor.

[Structo]

When you tried both amps, were you using the same speaker for both?