Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubesornothing on October 26, 2010, 12:11:48 am
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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?
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> 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.
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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
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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.
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> 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.
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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).
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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).