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Hi B+ on Morrison Micro (moved from Tube DIY Asylum)

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Posted on December 8, 2022 at 13:12:13
David Lawrence
Audiophile

Posts: 313
Location: North Dakota
Joined: April 23, 2007
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

 

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RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 8, 2022 at 13:30:39
Tre'
Industry Professional

Posts: 17133
Location: So. Cal.
Joined: February 9, 2002



What power transformer did you use?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 8, 2022 at 13:56:47
David Lawrence
Audiophile

Posts: 313
Location: North Dakota
Joined: April 23, 2007
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

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 8, 2022 at 14:31:24
Tre'
Industry Professional

Posts: 17133
Location: So. Cal.
Joined: February 9, 2002
Is there 750vac at the secondary?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 8, 2022 at 15:05:02
David Lawrence
Audiophile

Posts: 313
Location: North Dakota
Joined: April 23, 2007
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?

 

Measure the filament voltage, posted on December 8, 2022 at 16:29:46
Alpha Al
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Posts: 2948
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since:
December 3, 2015
If its within reason, the extra B+ should not matter.

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 8, 2022 at 16:42:17
Tre'
Industry Professional

Posts: 17133
Location: So. Cal.
Joined: February 9, 2002
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"

 

RE: Measure the filament voltage, posted on December 8, 2022 at 16:43:12
Tre'
Industry Professional

Posts: 17133
Location: So. Cal.
Joined: February 9, 2002
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"

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 8, 2022 at 17:38:06
je2a3
Audiophile

Posts: 308
Location: PA
Joined: July 15, 2003
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!

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 05:39:32
dave slagle
Manufacturer

Posts: 5406
Location: NYC
Joined: April 27, 2001
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 10F first cap in the poser supply. I'd start by cutting it in half to 5 (I'd strongly suggest an oil cap here) and see where that leaves you.

dave

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 09:54:07
David Lawrence
Audiophile

Posts: 313
Location: North Dakota
Joined: April 23, 2007
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

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 12:44:33
David Lawrence
Audiophile

Posts: 313
Location: North Dakota
Joined: April 23, 2007
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

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 13:28:04
Ian L
Audiophile

Posts: 1343
Joined: July 5, 2002
"(I'd strongly suggest an oil cap here) "

Why?

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 16:53:12
Paul Joppa
Industry Professional

Posts: 7274
Location: Seattle, WA
Joined: April 23, 2001
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.

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 18:17:56
dave slagle
Manufacturer

Posts: 5406
Location: NYC
Joined: April 27, 2001
I find they sound better in this application.

dave

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 18:20:37
dave slagle
Manufacturer

Posts: 5406
Location: NYC
Joined: April 27, 2001
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

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 18:25:01
dave slagle
Manufacturer

Posts: 5406
Location: NYC
Joined: April 27, 2001
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

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 9, 2022 at 19:01:08
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
Just install the 5R4 directly, no other parts required. You should end up with about 400 volts DC.

 

Recall a conversation I had with him at an audio show..., posted on December 10, 2022 at 06:33:08
Ivan303
Audiophile

Posts: 48527
Location: Cadiere d'azur FRANCE - Santa Fe, NM
Joined: February 26, 2001
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.




First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: Recall a conversation I had with him at an audio show..., posted on December 10, 2022 at 09:04:27
Paul Joppa
Industry Professional

Posts: 7274
Location: Seattle, WA
Joined: April 23, 2001
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.

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 10, 2022 at 11:31:02
Ian L
Audiophile

Posts: 1343
Joined: July 5, 2002
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.

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 10, 2022 at 11:49:15
Ian L
Audiophile

Posts: 1343
Joined: July 5, 2002
"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.

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 10, 2022 at 12:45:48
Paul Joppa
Industry Professional

Posts: 7274
Location: Seattle, WA
Joined: April 23, 2001
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.

 

RE: Measure the filament voltage, posted on December 11, 2022 at 05:59:08
Alpha Al
Industry Professional

Posts: 2948
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since:
December 3, 2015
Yes, but going from 400VDC to 436VDC is a small change.

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 12, 2022 at 10:40:33
David Lawrence
Audiophile

Posts: 313
Location: North Dakota
Joined: April 23, 2007
Hello,

Evidently, I accidentally asked this question in another thread. Will a JJ 2A3 work in the Micro circuit?

Thanks,
David

 

RE: Hi B+ on Morrison Micro (moved from Tube DIY Asylum), posted on December 12, 2022 at 12:35:19
dave slagle
Manufacturer

Posts: 5406
Location: NYC
Joined: April 27, 2001
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

 

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