Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Porpoise Head on June 04, 2025, 07:01:02 am
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Hi all,
I'm in the process of building a mesa 290 power amp workalike using 2 el84s, 2 6v6 and 1 12ax7 for the phase inverter. I try to use spare parts wherever possible and have a nice trafo available with a 12.6-0-12.6 v secondary. Is the wiring scheme in the picture correct? And will noise induced by the heaters cancel out when wired this way? I depicted the phase relationships using + and - signs (hopefully correct). I also wonder whether the center tap arrangement is correct as drawn.
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For certainty I attached the mesa mark IV schematic to show the idea => 6v6 tubes operate in pentode mode, el84 can run in triode or in pentode mode. Cathodes can be disconnected to shut off one pair (so only run el84 or 6v6). I figured to run the 12ax7 PI off the 6v6 side since 6v6 heaters draw less (0.45 A) than the el84's (0.76).
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I'm starting to think I'm being extremely stupid. Isn't it as easy as this:
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Anyone? I'd rather not mess up my trafo or end up with a very noisy heater circuit :worthy1:
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I'm starting to think I'm being extremely stupid. Isn't it as easy as this:
That looks viable, whereas the schematic in the first post not so much.
In regard of hum cancellation it's probably not ideal, but my guess is that is pretty difficult to cause much heater noise this far down the signal chain.
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Hi pdf64, thanks a lot for the confirmation. I'll add a resistor in parallel with the 6v6 side to get side a and b exactly balanced in teams of current draw.
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I'm starting to think I'm being extremely stupid. Isn't it as easy as this:
Yes that's your best option. I'll add a resistor in parallel with the 6v6 side to get side a and b exactly balanced in teams of current draw.
There is no point adding a resistor just to consume extra current, it won't do anything useful. As for hum balancing, it's not ideal, so your best bet would be to add a humdinger across the 12AX7 heater.Cathodes can be disconnected to shut off one pair
I think a better way to do it would be to switch off the heaters to one pair. No reason to let the tubes burn away if they're not being used.
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... your best bet would be to add a humdinger across the 12AX7 heater. ...
I think doing that would also need the winding CT to be disconnected from ground?
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I think doing that would also need the winding CT to be disconnected from ground?
Yes, I missed that in his diagram
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Like so? I also added the pin numbers for good measure. Taking the 6aq5 as example, I plan on connecting from the winding to pin 4, and then from pin 3 to the other tube's pin 4. Reasoning was that both tubes (one on each side of the push-pull amp) swing positive (pin 4 w.r.t. pin 3) simultaneously and therefore I get some extra cancellation of hum. However I do not know whether the heaters have a 'direction' so this might be nonsense.