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cathode bypass in PP output stage?

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PRR

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Joined 2003
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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.
 
diyAudio Senior Member
Joined 2002
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,;)
 
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....
 
UncleNed said:


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!:)

Boa noite...vou a tomar uma cerveja....

OK!..bebamos à nossa saúde!:drink:
 
diyAudio Senior Member
Joined 2002
Hi,

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).

as for distortion point of vue the shared un bypassed resistor is best!

Separate cathode resistors can be better still...sonically.

OK!..bebamos à nossa saúde!

No argument from me there...saudade!

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

Sorry!!...thanks for the correction...:)


Separate cathode resistors can be better still...sonically

That's to subjectif for me...what is good for one people can be the worst for other people...

Knowing that...i never enter in subjectif discussions about sound quality....sorry,one more time...
No argument from me there...saudade!

Saudade ...du chocolat de la Grand Place de Bruxelles!!!;)
A bientot!
 
RichJones said:
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???

CCS. Based not on theorising but having done it with both DHT's (47, GM70, 813) and IHT's (6V6, EL84, EL34, KT88). But this of course limits you to class A.

Rather than repeat all the arguments again here, go to Audio Asylum Tube DIY, and read the last two or three weeks of threads, it's been a hot topic.

As for which one sounds the best, well that's up to you. I've tried them all and know what I prefer.
 
diyAudio Senior Member
Joined 2002
Hi,

Not always, but there are a number of performance advantages with it here and it's far, far, far from a subtle strain-like-you're-constipated-trying-to-hear-it difference.

Sure enough, but, as usual, it takes some intelligence to use the darn things in the appropriate places...

You just can lift a newbie from zero to a hundred...it takes some steps IME.

Sayonara,;)
 
RichJones said:
Best option:

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

E) fixed bias :)
or separate catode resistors bypassed


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

398V are plate-to-catode or plate-to-ground?
i.e. 23.88W or 20.16W on plate?
Sovtek 6B4G are 15W max, as far as I remember :confused:
 
From Ned's site

6B4-G Sovtek This new monoplate 6B4 looks like a baby 300B! Biases just a hair less than the old US ones, which should make it a bit easier to drive. Looks like just the ticket for people who'd like to experiment with using 6B4 in amps that have 6L6, etc, but don't want to try higher plate voltages with NOS tubes... these have a plate voltage rating of 450V and can be used at a plate dissipation of up to 30W.

I have run them at around 24W with no problems. They sound pretty good at these higher levels too;)

Tom
 
Konnichiwa,

RichJones said:
Best option:

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


I know plenty of people like the CCS, but it is not always the best solution. Worth trying though. one of the biggest problems with the CCS will be the impedance to ground which may track hum into the circuit. Also, you need a way of balancing the Valves current with ANY form of common cathode ballast. The way I do this is shown in the attached Circuits one for semi-fixed bias (also easily adapted for fixed bias) and the other for self bias.

To measure the current balance I simply make sure that the two halves of the transformer have the same DCR (if not simply add a suitable resistor to balance the DCR) and measure the voltage on the two anodes of the Valves via a pair of 1M "safety" resistors. If there is no voltage difference, then the two valves draw identical current.

I personally would likely tend most towards a choke BTW and seperate, capacitive bypassed cathode resistors is definitly the worst sounding solution....

Sayonara

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