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Not DIY

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Pelliott321

Post subject: Not DIY

Posted: September 19th, 2021, 11:00 am

Joined: January 15th, 2015, 7:19 am
Posts: 1706
Location: Baltimore MD

New amp
Modded McGary SA-1 brought to the "E" level.
New caps, better biasing on power tubes, and upgraded power trans

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SoundMods

Post subject: Re: Not DIY

Posted: September 19th, 2021, 11:19 am

Joined: July 24th, 2015, 4:17 pm
Posts: 1747
Location: Parkville, Maryland

Pelliott321 wrote:

New amp
Modded McGary SA-1 brought to the "E" level.
New caps, better biasing on power tubes, and upgraded power trans

Attachment:

mcgary2.jpg

Attachment:

mcgary1.jpg

So -- you went Ultralinear in lieu of SET. Does the McGary enable triode operation as well? I ask this because I found that KT-88s (I expect the beefier KT-90 as well) and EL-34s don't seem to do well running Faux triode where the KT-66s seem to prefer that operation.

BTW -- despite what some designers are doing with EL-34s, Mullard originally advised against that operation of EL-34s being that they are pentodes and not beam tetrodes. Their comments included the fact that the suppressor grids would overheat.

And how does the "self-bias" work when they advertise compatibility with a long list of tubes (6L6GC, KT66, KT77, EL34, E34L, 6CA7, 6550, KT88 and KT90)? Is there a means to adjust the bias? Some early designs had adjustable cathode bias. Is that what McGary did?

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Pelliott321

Post subject: Re: Not DIY

Posted: September 19th, 2021, 11:30 am

Joined: January 15th, 2015, 7:19 am
Posts: 1706
Location: Baltimore MD

The SET is still there.
Evaluation of the McGary will take awhile.
Just because I own it doesn’t mean it’s the best.
It sound good, very musical.
It cam with 6550
I will be trying KT66
Plenty of power for either the 97db Razz or the 85db Quads.
The biasing in the McGary is cathode bias with a twist that is proprietary and I can’t talk about.
All I can say it’s not microprocessor controlled like PrimaLuna or VAC.

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SoundMods

Post subject: Re: Not DIY

Posted: September 19th, 2021, 12:07 pm

Joined: July 24th, 2015, 4:17 pm
Posts: 1747
Location: Parkville, Maryland

Pelliott321 wrote:

The biasing in the McGary is cathode bias with a twist that is proprietary and I can’t talk about.

I am almost certain that what they have done with "a twist" is a matter of public record established almost 60 years ago -- so why the mystery?

The versions I have seen include adjustable cathode bias on each tube together with balance between pairs as the most complex -- to just simple tandem-connected cathode pairs with a common bias resistor with bypass.

Then there is the Zener diode "school" that try to regulate cathode bias although that kind of defeats the purpose of cathode bias. Or 3-pin regulators with adjustment -- I've seen that scheme as well. Then there is the ancient G.E. scheme copied by Jadis where you have a certain amount of fixed bias by setting a negative voltage on the control grid as a fixed bias starting point with cathode-bias taking care of "the heavy lifting."

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