recommendation for OT ax84 HO/Champ mixed breed

[tubesornothing]

Gutted a 5E1 champ clone for its transformers and build an AX84 HO.  Sounds anemic.  I have 400V on the 6V6 plate with 18V at the cathode with a 220R/100uf.  The ax84 calls for a 5K OT, the Champ is 7K - any suggestions for an OT for this thing that'll make it perform better?

[PRR]

> 400V on the 6V6 plate with 18V at the cathode with a 220R

What's the cathode current? 18V/220=......??

What's cathode current in many Champs?

What's the 6V6 dissipation?

You want to idle a SE amp at "middle" current. I think you have this jammed at maximum current.

[Fresh_Start]

To elaborate a little, the HO schematic I've got on file shows 261 VDC on the plate and 11.6 VDC on the cathode.

11.6 VDC/220 ohms = 52.7ma

52.7ma * (261 - 11) = 13.8 watts plate dissipation = happy place for single ended 6V6

Your amp:

18VDC/220 ohms = 81.8ma

81.8 * (400 - 18) = 31.2 watts plate dissipation = burning up single ended 6V6

5E1 Champ cathode resistor is 470 ohms.  Try that.  Or get a different power transformer.  The OT isn't the problem; it's your power tube bias.  BTW that 6V6 may very well be toasted.

Cheers,

Chip

[tubesornothing]

Thanks guys - you are right - I am being LAZY.  I will do the math and put in the right cathode resistor.  See how it sounds then.  Maybe even make it fixed bias.

[PRR]

> Maybe even make it fixed bias.

Why?

The self-bias SE amp is a known-good stable safe plan.

_IF_ you pick an appropriate cathode resistor FOR TUBE USED (6V6 is not EL84).

You need it to be stable because for best power/tube you want to aim at 90%-101% rated plate dissipation.... you can't have 20% slop.

Fixed-bias won't take-up tolerances between "same type" tubes, you must manually re-bias each tube replacement, on installation and through burn-in.

(Not so critical in push-pull because you can bias at 50% or 80% and let it wander +/-20% safely.)

And fixed-bias means more tricky-stuff to fail. Bias rectifier, trim-pot.

_AND_ in this case what you really want is LESS plate-cathode voltage. Cathode resistor drop is fine. For 7K load in 12W-14W plate you want about 280V plate-cathode. Add self-bias and OT DC drop, you may want 330V B+. Not 400V.

That EL84 was not a lot happier, except it did have the "right" cathode bias resistor for an EL84 and was not running 80+mA.

Alternatively: insert 6L6, self-bias to 50mA, 7K load, look for 20W plate dissipation and maybe 7 Watts output. Though the cheap OT found in most EL84 champs may sound strained.

[tubesornothing]

You are right, too much B+.  I'll see if I can find another PT (I dont want a sag resistor for a little rock amp).

[moody]

You are right, too much B+.  I'll see if I can find another PT (I dont want a sag resistor for a little rock amp).

Where did you get this? And what is wrong with using an odd resistor here or there.  Re-read what is said above.  Change the cathode resistor, play around with how it sounds.  If you're going to change the iron then you might as well start a whole new amp.  Get the iron you have working right (and it will).