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In Reply to: RE: Full-wave C- Supply posted by Lee of Omaha on July 03, 2016 at 16:48:19
I have done this on several Citation 2s and I like the sound very much but in the case of the Mc240 and other Mac amps,we have a different situation.Mcintosh uses a bias tracking system and it works in conjunction with the power transformer primary and secondary meaning,when the voltage goes up or down,the proportionality of the bias goes up or down to maintain the correct bias.You always set the bias on a Mac amp with a variac and a voltmeter.On an Mc240,you set the variac to run the B+ 440vdc on the plate and minus 47 on G1. If you don't have minus 47,you adjust the bias resistor up or down until you get minus 47 at 440v.This way when you plug it back into a normal AC wall voltage,the amp will track perfectly regardless of where the voltage moves.
What I do on Mc240s is exactly what McShane did on the Citation 1 and the Citation 5..You put in the 820uf doublers and than dual 100ufs down stream,along with a pair of dual 50uf paralled in for added decoupling capacitance..Then add a bussbar and bolt right on to the choke bolts..I also use Schottky diodes but the GI diodes are fine. The other thing I do is add a hash filter that Eli came up with and that gives you an even quieter background.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/03/16 07/03/16Follow Ups:
I don't understand. I'm not talking about regulating the C- supply, just using a bridge to rectify (and a resistive divider to establish bias). We're not going to regulate C- or B+, just filter the crap out of both.
I understand the inadvisability of regulating C- without regulating B+. Did you misunderstand our intent or do I misunderstand what you are saying?
Lee
I guess I misunderstood because when we do a full wave bridge on a Cit 2 bias,we end up using a torroid with dual secondaries that we series connect and then use a 4 diode bridge.
To be honest,doublers in the Mac and Cit 2 amps regulate very well by virtue of their low resistance secondary windings especially when you use it with the new doubler caps they have today.If you use the 820uf panasonic or Nichicon 105 degree caps along with a good buss bar and an added hash filter,you aren't going to do any better than that for filtering.I suppose there are other ways to do it,but Jim McShane has tried many different setups before he came up with the one for the Citations and we just implemented with the Macs because the power supplies mirrored each other. The added decoupling also helps with stability.I don't know if you have seen some of my posts but I have always put up the end results with measurements and waveforms and that kind of gives you an idea of where you are at..You can try a different way if you keep the same taps..I thought you were using a different bias transformer and with the Macs you can't if you want it to track correctly.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/03/16
Ah, we are on the same page.
I don't think "regulate" is the word you're after in "...doublers in the Mac and Cit 2 amps regulate very well..." since the voltage will still scale with line voltage. I recognize the quality added by low resistance windings.
I was speaking hypothetically when I said regulate but what I really mean is stability..Today's Mac tube amps like the reissue Mc275 doesn't use any form of regulation that we use in conventional circuitry.It essentially mirrors the original circuit. The reason behind that I would assume is the fact that the unity coupled circuit doesn't operate independently of the other elements in the circuit.It moves proportionally up or down with signal. In a conventional pentode connected output circuit, we can regulate G2 without regulating the plate voltage.
I am curious tho. I would love to see what would happen in a Mac amp if we did regulate the plate and or G2 just to see what would happen to operating conditions.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I think you'd be ok if you regulated B+ AND C-, but not just one. Since G2 and the plates are connected, if you regulate one of them I think you'd drastic change the operation of the amp.
I believe you are right.I may give it a try as well.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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