Low Heater Voltage on Herzog Clone

[Firestorm]

Thanks for the help. I wonder why this thing needs such big caps. I wonder if those big bypass caps explain anything.

KOC tends to use 22uF caps almost everywhere because they're a standard value, readily available. His Herzog schematic has 22uF for the plate supply; you have 16uF. There's very little difference. Assuming 400VDC, 16uF stores 1.2 joules; 22uF stores 1.76 joules. Should be plenty, either way, for an SE 6V6.

[bcmatt]

Even Fender bumped the Champ caps up to 20uF (3 of them) from the previous 8uF ones by the time the Vibro Champ came out (AA764). There must have been some good reason for them to justify the extra few cents it probably cost them.

[The New Steve H]

Mine are 8, not 16.

[bcmatt]

Mine are 8, not 16.

Still?!??!

Oh wait, you said the first two are paralleled, so it is like a 16uF and then 2 8uFs. Both Gar Gillies and KOC have 4 sections of caps though. Perhaps you just need to add another resister and cap to the chain at the other end.

I heard somewhere that adding another resister and cap is generally a better solution than increasing cap values...

[The New Steve H]

I'm just glad I happened to look at Merlin Blencowe's book today. I would not have seen this coming.

[Firestorm]

It's good to read Merlin's books. But don't get your hopes up. The filters you have now are adequate for the job. Odds are, its something else.

[The New Steve H]

In that case I wasted 11 bucks.

What else causes motorboating?

[The New Steve H]

I'm looking into motorboating causes. Apparently big coupling caps can cause it. I have a .047uF cap between the 12AX7 stages, but the Fender Champ and the TUT Herzog use .022uF. Worth changing?

It only happens when the main volume (1M pot on V1a plate) is high.

Also wondering if I screwed up the filter caps. When I first installed them, I put two in backward for extremely complicated reasons no one else here is sophisticated enough to understand. They test out ok, but I'm wondering if they're secretly unhappy about having 400 volts run through them backwards.

[Firestorm]

What else causes motorboating?

"Motorboating" proper, really does sound like a motorboat, or a bad-running motorcycle: a real "put-put-put" sound. I can't say I've ever heard it in a single-ended amp. There are other low-frequency oscillations, though. Usually these are the result of positive feedback, which can come from lead dress. Sometimes high frequencies will modulate the 120 Hz hum and produce sums and differences at low frequencies. You said the amp seemed to lack high frequencies, which it shouldn't, so I wonder if something isn't quite connected right and you have your high frequencies going somewhere they shouldn't go.

If your signal is strong enough to drive the 6V6 to cutoff, that will distort and could cause low frequency problems. Yould have to measure.

You can solve a multitude of problems with global negative feedback. I know the Herzog doesn't have it, but its sister amp, the Banshee, does. Maybe try adding the feedback circuit from the Champ and tweaking the values to taste. You'll lose some output, but you will also lose hum and distortion. Don't use too much or you might create another, similar problem.

[Firestorm]

I have a .047uF cap between the 12AX7 stages, but the Fender Champ and the TUT Herzog use .022uF. Worth changing?

Couldn't hurt.

They test out ok, but I'm wondering if they're secretly unhappy about having 400 volts run through them backwards.

Hmmm. I'd think that if they didn't explode, they'd probably heal. But maybe.

[The New Steve H]

I guess I could try to record the noise and upload a file. Of course, it will quit as soon as I push "record."

[The New Steve H]

Paralleled a 20uF cap with the parallel 8uF caps on the B+. I have not noticed any difference. I still get weird noises at top volume.

I put an LED in the jewel lamp, wired to the heater supply with a 100 ohm resistor to keep it from being too bright.

I don't know what to think of this thing. It's definitely useful as a distortion pedal, but I wouldn't say the tone is all that thrilling. Maybe I should tear it down and make a TUT version.

I've noticed it sounds better on the 4 ohm tap.

[bcmatt]

I've noticed it sounds better on the 4 ohm tap.

Hmm, I suppose that would make sense if Gar used trannies with just 4ohm secondary. You have a 10 ohm power resister there, right?

[The New Steve H]

Yes I do.

I still don't understand why there are three schematics. Some say the first one is the original Herzog, and the second one is the Seventies version. But what's the TUT version for?

Maybe it's the only one that's worth a crap.

[Firestorm]

But what's the TUT version for?

Maybe it's the only one that's worth a crap.

That's certainly what Kevin would say