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Old 3rd June 2012, 03:28 PM   #1
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Default PP and PPP

Hey Everyone,

I read in Morgan Jones, (pg. 458 third. ed.) that parallel output valves reduce the HT needed, reducing the cost of the power supply. And reduces the output impedance reducing the needed turns ratio of the output transformer.

Unfortunately he devotes only one paragraph to this. I happen to have 17 6P15P output tubes. And three Magnavox inter stage transformers. Can anyone explain the change in output impedance by using push pull, push pull parallel or parallel SE configurations? Or that can point me toward a source that may?

Kevin
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Old 3rd June 2012, 06:02 PM   #2
7N7 is offline 7N7  United Kingdom
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Originally Posted by mr2racer View Post
Hey Everyone,

I read in Morgan Jones, (pg. 458 third. ed.) that parallel output valves reduce the HT needed, reducing the cost of the power supply. And reduces the output impedance reducing the needed turns ratio of the output transformer.

Unfortunately he devotes only one paragraph to this. I happen to have 17 6P15P output tubes. And three Magnavox inter stage transformers. Can anyone explain the change in output impedance by using push pull, push pull parallel or parallel SE configurations? Or that can point me toward a source that may?

Kevin
Paralleling output valves does not reduce the HT required; the idea is that it means that more power may be obtained by multiplying the current whilst using the same HT, and as you say, means that the output transformer may be specified to have a lower primary impedance.

I have never understood how the value of primary impedance is calculated for pentodes or beam tetrodes, nor can I recall ever having read anything on this. What I can say is that 6P15P looks to be very close to EL822: a 12W Pa pentode with good gm and a very high anode resistance (100k). This makes it ideal as a high-current constant current sink, and I used EL822 for this in one of my amplifiers. I keep meaning to buy some 6P15P for this reason as EL822 is getting rather pricey. Incidentally EL822 is vastly superior judging by the published curves to EL821; 6P15P's published curves are also excellent.

Whether this type is suitable as an output valve I leave it to others to comment!

Good luck

Paul
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Old 3rd June 2012, 11:20 PM   #3
AJT is offline AJT  Philippines
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paralleling output tubes is a lot of work....

while you can lower HT for the same output power, your filament current requirements go up.....plate load is half than when using just a pair....

i resist temptations to do it if i can...
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Old 4th June 2012, 01:35 AM   #4
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I have never understood how the value of primary impedance is calculated for pentodes or beam tetrodes, nor can I recall ever having read anything on this.
Same as any other hollow state design problem: loadlines. Did a write-up on this Here (different uname)
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Old 4th June 2012, 01:48 AM   #5
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So if a pair of EL84 have an output impedance of 8k four tubes in PPP would have an output impedance of 4k? Is it that easy?
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Old 4th June 2012, 02:02 AM   #6
TheGimp is offline TheGimp  United States
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Pretty much. Although drive requirements increase, and tube matching comes into play.
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Old 4th June 2012, 02:20 AM   #7
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So if a pair of EL84 have an output impedance of 8k four tubes in PPP would have an output impedance of 4k? Is it that easy?
Yes, it is. The same plate voltage will be used, but twice the current will be drawn from the power supply, and twice the power delivered to the output.

Leakage and Miller capacitances will also double, but drive voltages stay the same.

It's especially important to take grid stops seriously, use 'em on both G1 and G2.

All good fortune,
Chris
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Old 4th June 2012, 03:01 AM   #8
AJT is offline AJT  Philippines
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Yes, it is. The same plate voltage will be used, but twice the current will be drawn from the power supply, and twice the power delivered to the output.

Leakage and Miller capacitances will also double, but drive voltages stay the same.

It's especially important to take grid stops seriously, use 'em on both G1 and G2.

All good fortune,
Chris
i use carbon comps here exclusively....
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