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In Reply to: RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum) posted by dave slagle on December 09, 2022 at 05:39:32
Thank you everyone for all your advice. I measured and calculated the plate dissipation as approx. 18.52 watts. Does that mean everything is OK? Would it be better to get a 4.7uf initial power supply cap from Parts Connexion (the power supply caps are Obbligato poly and oil) and/or try a 5R4--in order to lower the B+? Cheers, David
Edits: 12/09/22Follow Ups:
I would first determine the needed cap value and then you can experiment with different types. The last thing you want to be doing is spending $$$ on fancy parts that may or may not be the right value.
dave
Hello again,
I just found an old 5R4 in the basement. Since it is directly heated, should I put one or more thermistors somewhere in the power supply? Would an alternative rectifier harm the sound in some other way?
By the way, I have pairs of both bi-plate Chinese and old single-plate Sovtek 2A3's. Would they work with 18 or 19 watt plate dissipation on the triodes?
Thanks,
David
Well.... since JC had a hand in the design of the Sovtek 2A3 I kinda think you will be OK. There was a conversation with him it seems like 20 years ago on the Joenet where he explained that even though the 2A3 has the same filament power and plates as the 300B, it was the cathode coating that limited the emission to a max of around 24W max by his estimation.
dave
Hello,
Evidently, I accidentally asked this question in another thread. Will a JJ 2A3 work in the Micro circuit?
Thanks,
David
it should...
unlike the sovtek 2A3 which according to JC has a different cathode composition than the 300B, the JJ 2A3 lists the same 40W of plate dissipation as the 300B.
dave
Where I mentioned that some had claimed the Sovtek/EH were but 300B with a 2.5V filament.
No, he said, if you run them a 300B operating points the plate will be fine but cathode coating will not last.
"Reality cannot exist because it cannot keep up with the lies on the Internet."
If my surmise(*) is correct, that means it's not suitable for use in the Micro, at 80mA current. I have not found an official maximum current for standard biplate 2A3s.
Just install the 5R4 directly, no other parts required. You should end up with about 400 volts DC.
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