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Old 4th December 2011, 05:00 PM   #1
GloBug is offline GloBug  Canada
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Default Tame the beast.

Well it's been a little over a year since I built this PPP with a couple old Magnavox. I have my UFO/RH84 running, so I can bench the big amp for a while.

For the most part it is an "Amp142or132", paraphase inverter.
Some of the changes I made was switching the inverter to 6CG7, an added 6CG7 input, choke/150u on the screens. The power supply is SS with lot's of capacitance. My OPT's are 3.9K with 3ohm and 1.5ohm taps. It has a few other values changed as well.

I liked it before I added the extra 6CG7, I still love the sound now, it's just TOO LOUD. I suppose I could bypass the extra tube, but that would leave me with an unused socket.

It's just stupid loud, I have my computer set on "1" AND I have to turn my computers preamp down -15db, just to get it to behave like a normal amp would at "1".
It is also too sensitive to noise, my sound card is next to my video card-I can hear it processing. It has guitar amp type gain.

Don't get me wrong, it's an amazing amp I'm sure lot's would like to have the power. A freight train sounds like a freight train, and transient explosions in movies will turn your pants brown if your not expecting it. I've never experienced anything like it, albiet my experience is somewhat limited compared to those here.

What I'm thinking is a complete redo.
I would love to try this amp in class A triode mode.
I understand a 6V6 in triode mode does not have much power, but considering I have four 6V6 per channel I think I can live with that.

I have two PT's for it, one is 280v, the other is 290v (132/142). I forget which I have in there.

I uploaded the standard schematic of a Magnavox 132 if you want to get a general idea of the layout before I made the mods.

Generally speaking, can I apply any regular class A PP schematic to my PPP?
I'm open to suggestions and advice.
Attached Images
File Type: jpg 6V6PPP.JPG (178.0 KB, 512 views)
File Type: jpg 132pic.JPG (340.6 KB, 498 views)

Last edited by GloBug; 4th December 2011 at 05:03 PM.
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Old 4th December 2011, 06:05 PM   #2
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To avoid current hogging, set up each tube of a PPP group with its own RC bias network and an individual idle current set pot. connected to a negative rail. Close matching of gm remains important, but reasonably small variations in cathode current are easily dealt with. Combination bias, as described, has LOTS going for it.

Having large reserves of power is a good thing. The only change I'd make to what you already have is the installation of voltage dividers at the I/P of each channel, to reduce sensitivity and eliminate the "hair trigger" volume control.
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Old 4th December 2011, 07:37 PM   #3
GloBug is offline GloBug  Canada
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Thanks.

So I guess something around a 1k pot would be good for bias pot's so long as they can handle a couple watts? Some of the bias trimpot's I googled looked underrated. I will look some more.

So basically just hard-wire a fixed volume control at the input and be done with it.
I do have an extra hole where the second switch is located, I suppose I could stick a pot in there. (I installed the second switch in case I wanted a standby switch, kinda came to the conclusion that I don't need it. I'm on the fence.)
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Old 4th December 2011, 07:48 PM   #4
GloBug is offline GloBug  Canada
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I guess I could get away with a small trim pot if it was in series with a larger wattage resistor.
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Old 5th December 2011, 12:09 AM   #5
GloBug is offline GloBug  Canada
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Any other opinions?

Beuller? lol
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Old 5th December 2011, 10:33 PM   #6
GloBug is offline GloBug  Canada
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Nobody can answer the question?
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Old 5th December 2011, 10:39 PM   #7
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Have you used both sections of it i.e. one section per channel?
Trim pots / dividers will do nothing for the noise. Also , get yourself a better soundcard (preferably external)

Another idea, by glancing at your schematic: Have the 12AX7 as 1st stage, and use the 6CG7 as a nice cathode follower to drive the 6V6s. Needs some redesigning, but a cathode follower never hurt anybody.

Last edited by costis_n; 5th December 2011 at 10:42 PM.
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Old 5th December 2011, 10:45 PM   #8
GloBug is offline GloBug  Canada
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Thanks, yes I don't mind redesigning it, I was mainly wondering if, generally speaking I could apply most PP schematics to a PPP amp, then I can pick something out from there.
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Old 5th December 2011, 10:46 PM   #9
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Sure you can!
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Old 5th December 2011, 11:27 PM   #10
GloBug is offline GloBug  Canada
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OK Cool, I know it seems like an odd question to most, just wanted to make sure. Thanks!
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