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Old 29th October 2003, 09:37 PM   #1
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Default cathode bypass in PP output stage?

Best option:

A) common cathode resistor bypassed
B) common cathode resistor unbypassed
C) choke
D) solid state CCS

This is for 2 6B4G's (Sovtek) running at 398v 60mA each 62v bias through a common 470 ohm resistor.

Thoughts???
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Old 29th October 2003, 10:25 PM   #2
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Quote:
Best option:
Hi Rich!

For me the best option is not in your list...for me the best option is two cathode resistors bypassed...(one for each tube)...


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Old 29th October 2003, 11:05 PM   #3
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> Best option

Depends on if you work Class A or ever get up into Class B.

In Class A, if your tubes are balanced, it hardly matters.

If your tubes are not balanced, get better tubes.

If you can't balance better, you probably get lower distortion with a bypass. If they are horribly unbalanced, Jorge's technique of two resistors and two bypasses may be the best of a bad deal. My impression is that this usually isn't the "best" plan, but sometimes it is.

In Class AB, music/speech distortion is lower with a bypass, and the constant current source really defeats Class AB action.

But hell. Do what works for you, your amp, and your music. It would only cost a few bucks to try all five plans. You could even rig a switch for instant comparison.
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Old 29th October 2003, 11:35 PM   #4
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Hi,

In real life no tubes are ever perfectly balanced so I'd go for Jorge's solution.

I run a PP 2A3 amp for fun and don't have the cathodes bypassed
but have the Zprim at a high value too. (6K6)

This amp is a measle 12Ws driving a two way 80dB/W speaker at best but rocks the building nonetheless...

Ah, I think the PSU has got a major say in that too...

Theory's one thing, hands on experience is quite another...

Cheers,
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Old 29th October 2003, 11:46 PM   #5
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Quote:
In real life no tubes are ever perfectly balanced
Hi Frank!

Yes ...and in the case of one of tubes dies ( with the shared cathode resistor) ,the other one will be ,with over bias, and can destroy it self...
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Old 29th October 2003, 11:55 PM   #6
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Quote:
For me the best option is not in your list...for me the best option is two cathode resistors bypassed...(one for each tube)...
Only problerm with this idea is that he's using 6B4-G, which has a filament-cathode. So he would have to have separate filament windings on the power transformer or a second filament transformer to do this.

Sovtek 6B4's are current production, so getting matched pairs isn't a problem. If he just has to have NOS 6B4, then the suggestion makes more sense.

Boa noite...vou a tomar uma cerveja....
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Old 29th October 2003, 11:57 PM   #7
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Hi,

Quote:
Yes ...and in the case of one of tubes dies ( with the shared cathode resistor) ,the other one will be ,with over bias, and can destroy it self...
Persackly...

Too much theory can be a dangerous thing....

I always, always use separate cathode resistors....bypassed only by the best caps or not at all.

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Old 30th October 2003, 12:01 AM   #8
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Hi,

Quote:
So he would have to have separate filament windings on the power transformer or a second filament transformer to do this.
Ah...of course, that's mandatory...

Quote:
Boa noite...vou a tomar uma cerveja....
Enjoy the Bud...

Cheers,
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Old 30th October 2003, 12:10 AM   #9
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Quote:
Originally posted by UncleNed


Only problerm with this idea is that he's using 6B4-G, which has a filament-cathode. So he would have to have separate filament windings on the power transformer or a second filament transformer to do this.

Yes...in this particular case ,that must need a little more complexity...

that must not be needed...(Ned)

My only preference for the double resistor...is concerning fiability...as for distortion point of vue the shared un bypassed resistor is best!

Quote:
Boa noite...vou a tomar uma cerveja....
OK!..bebamos nossa sade!
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Old 30th October 2003, 12:18 AM   #10
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Hi,

Quote:
My only preference for the double resistor...is concerning fiability...
If you don't mind:

You mean to say reliability...(fiablite being a french word).

Quote:
as for distortion point of vue the shared un bypassed resistor is best!
Separate cathode resistors can be better still...sonically.

Quote:
OK!..bebamos nossa sade!
No argument from me there...saudade!

Cheers,
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