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Old 5th May 2003, 02:02 AM   #1
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Default Stereo SRPP - one dual triode tube per channel, or...?

Hi,

If I used one tube for the lower triode of both channels and the other tube for the upper triode, wouldn't that help with heater-cathode voltage difference? I could use a potential divider and raise the heater of the second triode up to it's cathode's voltage, because both triode sections would be at the same voltage. Using one tube per channel means that the two triodes in the tube are operating at different voltages, and the heater will have to be set to somewhere in the middle, which will probably not be as good for the tube, perhaps also for the sound.

Is there any downside to using one tube for the lower pair and the other tube for the upper pair?

Thanks,
Saurav
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Old 5th May 2003, 02:38 AM   #2
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The only disadvantage is that you'd have two heater supplies. As long as you don't exceed the cathode to heater voltage rating, I doubt that there is a sonic or tube life penalty.

That's one of the many advantages of indirectly heated tubes.


Sheldon
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Old 5th May 2003, 02:41 AM   #3
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Oh... the schematic had 2 separate transformers for the output triodes, but the drivers were off the same transformer. OK, thanks, that answers that question.
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Old 5th May 2003, 05:08 AM   #4
Morse is offline Morse  United States
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Hi Saurav;

Only downsides I see are the already noted cost of the separate filament supplies and the (possibly) longer signal paths.

OTOH, if you use completely separate filament supplies (eschewing the PS trafo's 6.3V secondary windings) you can go to filament trafos with enough voltage to use regulated 6.3VDC supplies (you usually want at least 5V over the regulated output voltage on the input to a SS regulator - check the regulator's datasheet for the exact details on appropriate supply voltages though). Also, you can run your PS trafo a bit cooler, all else being equal, since the 6.3V winding will not be drawing current.

If you've the $$ for it, I think it's an approach with some merits worthy of consideration.

Good luck and all the best,
Morse
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Old 5th May 2003, 10:09 PM   #5
Zen Mod is offline Zen Mod  Serbia
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in my opinion and xperience-some tubes are prone to sort of oscilation (whistling and pops) ,even if you elevate Uf to exact preferred point.
I do not prefer SRPP (even if few of my preamps were of same topology),but in case when I made some SRPP I always use different envelopes for upper and lower parts of SRPP.
cheers!
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Old 5th May 2003, 10:22 PM   #6
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Hi,

Quote:
If I used one tube for the lower triode of both channels and the other tube for the upper triode, wouldn't that help with heater-cathode voltage difference?
Yes, it would....of course it would....you won't even have to bias the heater.

Quote:
Using one tube per channel means that the two triodes in the tube are operating at different voltages, and the heater will have to be set to somewhere in the middle, which will probably not be as good for the tube, perhaps also for the sound.
There's a simple answer to that, you surpass the heater to cathode voltage...you need to bias, and no this won't degrade sonics.However, if you disrespect this you can expect the tube to squeel loudly...

Quote:
Is there any downside to using one tube for the lower pair and the other tube for the upper pair?
Not really...

Heater crosstalk anyone? ...for experts only!!!.

Cheers,
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Old 5th May 2003, 10:44 PM   #7
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Quote:
you won't even have to bias the heater.
So if I left the transformer for the upper tube's heater floating, then it would have no DC reference of it's own, and so wouldn't exceed the tube's cathode-heater maximum? Is that what you're saying?

I thought I would have to use a voltage divider to give that heater supply a DC reference, and I would have to pick a "correct" DC reference which is close to the cathode voltage of the upper triode.

Quote:
Heater crosstalk anyone?
What is the magnitude of this? And this would apply to any design that uses two triodes in a single envelope for two channels, right? So all those designs which have a pre/driver stage that uses two different tubes, and the amps have one of each tube and run both channels through the same tube, they should have the same degree of heater (or any other intra-envelope) crosstalk.
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Old 5th May 2003, 11:04 PM   #8
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Default HEATER TO CATHODE...HELLO???

Hi,

Quote:
So if I left the transformer for the upper tube's heater floating, then it would have no DC reference of it's own, and so wouldn't exceed the tube's cathode-heater maximum? Is that what you're saying?
Yes, and it's easy to see for yourself.
Unless I completely misread you...

Quote:
I thought I would have to use a voltage divider to give that heater supply a DC reference, and I would have to pick a "correct" DC reference which is close to the cathode voltage of the upper triode.
If the triodes halves are in the same envelope, then yes...if you use separate envelopes and heatersupplies than there's nothing to worry about.

Quote:
What is the magnitude of this?
Purist territory...
Not just stereo but also intertube related....you could tap the music from the heater and in most case it ain't pretty....
As said above...purist territory...

Cheers,
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Old 5th May 2003, 11:28 PM   #9
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Quote:
Yes, and it's easy to see for yourself.
Unless I completely misread you...
No, I understand. I thought a heater supply always had to have a DC reference.

Quote:
Purist territory...
Thought so, but it doesn't hurt to double check
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Old 5th May 2003, 11:51 PM   #10
EC8010 is offline EC8010  United Kingdom
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I thought your first post was entirely correct.

You do need to be careful about the voltage between the heater and cathode of a valve. There is always heater/cathode leakage resistance, so a higher heater/cathode voltage implies more leakage current (Ohm's law). If you short-circuit the heater to ground at AC, no leakage/audio/signal voltage can appear. If you leave it open-circuit, it can.

Of course, it's a royal pain needing a separate heater supply for the upper valve. One way around this can be to use TV valves (which usually have better insulation).
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