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Hello everyone,
It looks like I need some advice from the experts. After years of slowly collecting parts, working on other projects, and procrastinating--I finally built a Morrison Micro SE DC 2A3 stereo power amp. I followed the widely-available schematic very closely, with perhaps some more current available on the power transformer. Anyway, with all the tubes in, a load connected, and after settling down after turn on, I measure a B+ of about 436-438 volts. That is rather high compared to the published 400 volts. In any event, it does sound good.
I know that Morrison purposely runs the tubes hot and out of spec. I want to run it the way he planned rather than return everything to a "normal" operation. Do I need to make some adjustments to make it run according to his design? What should I do?
Any suggestions would be appreciated.
Best,
David
Follow Ups:
I know that Morrison purposely runs the tubes hot and out of spec.
JC didn't advocate for the abuse of all tubes, just specifically the bi-plate 2A3. He has always insisted that the 15W max plate dissipation for the bi-plate 2A3's was an extremely conservative rating. The original 2A3 was a single plate for which the 15W dissipation makes sense but when the later tubes were converted to the dual plate version the maximum specifications were never changed. It is also important to note that JC is also an advocate of "Low and Hot" operating points. He prefers lower operating voltages at higher currents and it wouldn't surprise me if the chosen operating point of the 2A3 in the micro were 250V @ 80ma.
The easiest solution for you to quickly lose 30V or so of B+ is to reduce the size of the 10µF first cap in the poser supply. I'd start by cutting it in half to 5µ (I'd strongly suggest an oil cap here) and see where that leaves you.
dave
I agree that a "low and hot" operating point is the key to understanding the Micro circuit. Running the 2A3 at 200v and 80mA (-30v bias) seems most likely; a paralleled 6SL7 in this circuit should have around 170 volts at the plate, so the voltages work out.
I calculate about 1.7 watts at clipping with a 3000-ohm load - not efficient, but very low distortion.
"I calculate about 1.7 watts at clipping with a 3000-ohm load - not efficient, but very low distortion. "
Maybe just the ticket for a horn speaker.
Yes indeed. Or anything that can be driven by a 45 amp. It would be interesting to compare with a 45, since they make about the same power.
Another intriguing comparison would be with a 2A3 at the conventional 250v/60mA but with a 5K transformer. Both sacrifice efficiency for low distortion.
"(I'd strongly suggest an oil cap here) "
Why?
I find they sound better in this application.
dave
Sorry Dave, I should have asked in the first one. When you say oil, do you have a specific capacitor type in mind ? I'm only familiar with oils from seeing them mentioned online and there seem to be many. Motor related, Russian, Brand name audio stuff like Mundorf, etc.
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: 12/09/22
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.
If memory serves me right, when I built this in the 90s I measured anywhere from 18> 20 watts plate dissipation from my Chinese and RCA bi-plate 2A3s. Plate dissipation = plate voltage (B+ - cathode voltage) x plate current (cathode voltage/2500 ohms). So if you're within that range then you've come close to the original design.FWIW, I revisited the circuit topology using a paralleled 6C8G input driver and chose a more conservative operating point for the 2A3. I still like the sound very much!
Edits: 12/08/22 12/08/22
If its within reason, the extra B+ should not matter.
Won't the extra B+ change the operating points of the tubes in the audio circuit?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yes, but going from 400VDC to 436VDC is a small change.
What power transformer did you use?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre'
Thanks for your reply. I used an Edcor XPWR010, with the HV secondary of 750 V, 175ma, and of course a center tap.
Best wishes,
David
Is there 750vac at the secondary?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi that is the spec provided by Edcor, though I can't check it again tonight. Is there a rectifier tube compatible with the 5V4G that has enough voltage drop? Would that work?
I was going to ask you about the DCR of the power supply choke and the DCR of the primary of the output transformer and also the value of the first cap in order to figure out why the voltage is high but I guess it doesn't matter.
I think the rectifier with the largest voltage drop is the 5r4.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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