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Old 14th November 2013, 09:28 PM   #1
mskl99 is offline mskl99  United States
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Default More questions on parallel push-pull

I want to design a monoblock ultralinear (40% taps) power stage with 2 pairs of 6L6GC tubes. The image below is taken from the RCA datasheet. It appears that the best plate-plate resistance for power vs distortion is around 6-8K for a single pair of 6L6's in non-UL mode. I have a couple of questions.
1. How does UL operation change this graph? Does it shift both curves down (less power, less distortion), but keep them looking basically the same in relation to plate-plate resistance? Or are the curves changed significantly.
2. If I use parallel tubes, should I cut the plate-plate resistance in half when I choose an output transformer, since the effective gm of the pair is doubled and the effective ra is halved. It seems many people don't do this.
3. If I am going to run the amplifier barely into AB1, how does this affect the choice of transformer. Do I want a less steep load line (higher plate-plate resistance.)
Hammond actually makes an output transformer (1650T) designed for parallel push-pull operation with 6L6GC tube, and it only has a 1.9K impedance, which I don't understand. Hammond Mfg. - "Classic" Push-Pull - Tube Output Transformers - (1608 - 1620, 1645 & 1650 Series)

I know this is a lot to answer, but there seems to be a real paucity of info on parallel push-pull operation. If somebody could direct me to a reference I'd greatly appreciate it.
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Old 14th November 2013, 09:37 PM   #2
DF96 is online now DF96  England
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Parallel push-pull just means twice the current at the same voltage, so half the impedance. It also means a greater load on the previous stage; mainly Miller capacitance for UL and triode-mode. Apart from that there is not much to say, which may be why not much is said.
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Old 15th November 2013, 12:41 AM   #3
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Current hogging can be a problem, when PPP is employed. It is a given that all the tubes in a channel's set will be closely matched. Good matching is a starting point. "Fixed" bias, with an individual trim pot. for each tube, is in order.

The issue of stress on the driving circuitry DF96 mentioned is exacerbated by the comparatively low grid to ground resistance allowed, when "fixed" bias is employed. I strongly suggest either Mullard or Williamson style small signal circuitry, with source follower buffers DC coupled to the tubed stuff, be employed.
Eli D.
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Old 15th November 2013, 09:54 AM   #4
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Erm, up to this day I've thought that PPP is synonymous to Circlotrons and similar designs?!?

Best regards!
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Old 15th November 2013, 01:52 PM   #5
wicked1 is offline wicked1  United States
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Originally Posted by Eli Duttman View Post
Current hogging can be a problem, when PPP is employed.
How important is this in the real world? I've reviewed every PPP schematic I can find, and most don't use separate bias adjustment per tube. Some use a 20-50ish ohm resistor at the cathodes or/and anodes. Most don't even bother w/ that.

I built basically Pete Milletts parallel pp 807 amp, but regulated all the supplies and bias, and am using even harder to match 829b's for the outputs. So far, no issues. But I am nervous about it. (I spent 2 years building the thing, I'd hate to start developing issues).
Pete took no extra measures to deal w/ the parallel tubes. I guess it wouldn't be too difficult to add another couple caps and pots for the bias adjustment, if needed, though..

Anyway.. some of the threads I've read about parallel tubes make it seem like a bad idea that will never work, and others make it seem like a non issue.
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Old 15th November 2013, 05:19 PM   #6
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Originally Posted by wicked1 View Post
How important is this in the real world?
I'm with Eli on this one unless you plan to use only very well matched tubes or some sort of auto biasing scheme. To claim it can't work is foolish considering all of the prosperous and revered manufactures that build PPP amplifiers. Audio Research, Conrad Johnson, McIntosh, Quicksilver and dozens more. There are trade offs in both designs that must be addressed. Some feel that multiple coupling capacitors, even well matched, smear the sound. (ala Jadis...but they screwed it up) However it really all comes down to keeping the output stage as well balanced as possible by whatever means within your design philosophy.
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Old 15th November 2013, 07:19 PM   #7
mskl99 is offline mskl99  United States
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Putting in 4 trimpots for the monoblock doesn't sound that bad. A few more traces on a PCB.
Back to my original question (one of them at least); what should I use for the impedance of the transformer. Should I find a good tradeoff between distortion and power for push-pull, then divide that number by 2 for PPP? Is it that simple? Or should I just find a design from a great company like AR, Conrad Johnson, et al, and use the anode-anode impedance that they use?
I'd really like to give this a try.
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Old 15th November 2013, 07:55 PM   #8
wicked1 is offline wicked1  United States
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Yes, work it up for a regular PP and then use 1/2 the impedance.
And, you are correct about your #3 question in your first post. Less steep generally = more class a.

Or do what most people do and use what another successful design uses.
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Old 15th November 2013, 08:13 PM   #9
multi is offline multi  Australia
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Built a lot of 4 x 6550 amps always used a cap for each tube and bias pot, even made a 12 x 807. Have repaired multiple commercial 4-8 x EL34's that did not use seperate caps and all the tubes had blown up only needs one tube to go and it takes out the rest. Changed the commercial amps to separate caps and no more trouble. Have tried direct coupling with 6Bl7's gets very complicated with 4 tubes, Best to stick to two tubes. never tried it with Mosfets.
You get very good damping with multiple tubes.
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Old 15th November 2013, 08:43 PM   #10
tinitus is offline tinitus  Europe
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how about seperate CCS on each cathode ?
sometimes we know very little, and sometimes we know too much
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