Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AHeck on November 18, 2021, 10:27:18 pm
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Thanks for reading...
I've built this circuit on my prototype setup and I'm wondering if the B+ is too high for the EL84s for any extended period of time. It sounds very good, but that does no benefit if it's not going to keep sounding good. It's mostly A5h from the schematics on this site, but I lowered the resister coming back from the Volume Pot and the resister to ground after the second gain stage. Also found it necessary to cool off the bias considerably. The original schematic lists BT as 336, B+1 as 334 and B+2 as 333. Maybe I'm missing something. Thanks again.
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Answered earlier today?
https://el34world.com/Forum/index.php?topic=27263.msg310615#msg310615
"I'm not sure what the point is of trying to run EL84 above about 300v"
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So what are the HT voltages?
I suggest to aim for a HT of about 320V, so consider adding another zener.
What impedance is the OT?
… Also found it necessary to cool off the bias considerably…
Why?
The EL84 may be overbiased, too far from class A; what happens to their cathode voltage when it’s cranking out max power? Too much of a further increase in bias (bias shift) will cripple the tone. and output.
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So what are the HT voltages?
I suggest to aim for a HT of about 320V, so consider adding another zener.
What impedance is the OT?
… Also found it necessary to cool off the bias considerably…
Why?
The EL84 may be overbiased, too far from class A; what happens to their cathode voltage when it’s cranking out max power? Too much of a further increase in bias (bias shift) will cripple the tone. and output.
HT is 326V
Bias was 130%, I cooled it down to 90% dissipation
6.2K OT, it’s the Hammond 125E(?) so I can change that. Speakers are two 8s//
Getting killer tone and output, just don’t like running the power section at that high a voltage.
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I wouldn't fret the Voltage, the current is in the sweet spot for guitar self-biased
play it like you stole it!
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…
HT is 326V
Bias was 130%, I cooled it down to 90% dissipation
6.2K OT, it’s the Hammond 125E(?) …
Can you show your working?
Also HT voltage generally responds to the current being drawn, so cooling the bias may be counter acted by an increased HT.
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I have several EL84 amps. Most have a very similar power amp. My lowest B+ is 325V and my highest B+ is 350V. I've never been concerned about any of them. However, they are not used in a gigging situation.
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Apologies in advance for the off-topic question.
V1a and V1b feature dual cathode bypass capacitors, one electrolytic, and one non-polarized. What is the point of this design feature?
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Basically the small cap can respond to high frequencies better than the big electrolytic.