6H30P based cathode-follower schema kindly requested

[beppe61]

Dear Sirs,


I would like to build a nice cathode-follower to buffer a passive pre using the very powerful 6H30P.
I intend to use it to drive a power amp with a low input impedance (about 10 kohm).
Unfortunately I have not been able to find a suitable schematic.
So I am here to ask for your very valuable help.
Are there any free schema in the web that you are aware of?

Thank you so much for your attention.

Kind regards,

beppe61

[Bandersnatch]

Hey-Hey!!!,
I would suggest that a pentode would make a better cathode follower than the 6H30. The parameter of use for a cathode follower is transconductance, and in second place, current capacity. The 6H30 is good in both. It is moderately expensive though, and 15 mA/V gm is easy to get for less than 3 USD per...

The critical parameter for good CF circuit performance is the load. This is the cathode load in parallel with the circuit you're plannig on driving. You said that this is 10k in the form of a SS amp. A good current source looks like the best idea from the chair I'm sitting in.

The circuit will depend on the means you choose todo it with. A negative supply would probably be a good idea, but may not be needed if the grid reference is at high voltage...
cheers,
Douglas

[SY]

You don't need anything "powerful" to drive a 10K load. Anything that can swing half a milliamp AC will be more than adequate. My own preamp uses an ECC88 type tube to drive the 10K input Z of my power amp and it seems to work beautifully.

If you're just in love with the idea of using that tube (and that's OK, I've been there!), you might adapt the Heretical circuit. There's no reason that it won't work part-for-part with the 6H30 as long as you adjust the series resistor in the heater circuit.

[beppe61]

Originally posted by SY

Dear Sir,

thank you very much for your extremely kind and valuable reply.
Please let me quote you and answer hereafter:

> You don't need anything "powerful" to drive a 10K load. Anything that can swing half a milliamp AC will be more than adequate.

I am very glad to hear that. Being quite ignorant on the topic I was overestimating the needs in term of current sourced by the tube (for this I thought about the 6h30p).

> My own preamp uses an ECC88 type tube to drive the 10K input Z of my power amp and it seems to work beautifully.

Could you tell me a little more about it?
The ECc88 is indeed another very interesting tube to build a very simple but nice CF to drive my-amp.
I do not need voltage gain in my application so a nice CF would suffice.

> If you're just in love with the idea of using that tube (and that's OK, I've been there!), you might adapt the Heretical circuit. There's no reason that it won't work part-for-part with the 6H30 as long as you adjust the series resistor in the heater circuit.

Not at all. I have no prejudice against any tube.
But having read someone call it "the supertube" and being it not overtly expensive ....
I am open to wahtever tube could be a nice CF.
I read with great interested the thread about the tube buffered gain clone.
I am beginning to appreciated the "flavour" a tube can give to digital sources.
Musical Fidelity with their X-10 give this option.

Thank you very much again for you kind and valuable reply.
Kind regards,

beppe61
ITALY

[beppe61]

Originally posted by Bandersnatch
Hey-Hey!!!,
I would suggest that a pentode would make a better cathode follower than the 6H30. The parameter of use for a cathode follower is transconductance, and in second place, current capacity. The 6H30 is good in both. It is moderately expensive though, and 15 mA/V gm is easy to get for less than 3 USD per...
The critical parameter for good CF circuit performance is the load. This is the cathode load in parallel with the circuit you're plannig on driving. You said that this is 10k in the form of a SS amp. A good current source looks like the best idea from the chair I'm sitting in.
The circuit will depend on the means you choose todo it with. A negative supply would probably be a good idea, but may not be needed if the grid reference is at high voltage...
cheers,
Douglas

Dear Mr. Douglas,

thank you so much for your kind and valuable reply.
You have givem me a lot of material to study.
Kind regards,

beppe61

[reinhard]

Have a look at Allen Wright´s web site
www.vacuumstate.com
In the schematics section you find his Super Cathode Follower,
clever constant current, constant voltage design, near zero
distortion
You can build that with 6H30 or better use 6H6.

Reinhard

[beppe61]

Originally posted by reinhard
Have a look at Allen Wright´s web site
www.vacuumstate.com
In the schematics section you find his Super Cathode Follower,
clever constant current, constant voltage design, near zero
distortion
You can build that with 6H30 or better use 6H6.
Reinhard

Dear Mr. Reinhard,

thank you very much for the valuable link.
Actually I was thinking about something more basic, the school text-book CF, with just a resistor from the cathode to ground.
I have no claim of high-end performances.
I have on noticed that with digital source the simple introduction of a tube in the audio chain gives a "nice" flavour to the sound.
Maybe it indeed adds distorsion, but of a nice kind.
It makes the digital more human ?!
I would like very much to know your opinion on this.

Kind regards,

beppe61

[SY]

There are some real advantages to using a current source rather than the canonical resistor. You can get away with the resistor alone if either the tube is biased up with a high positive voltage on the grid (and a concommitantly large cathode resistor) or a large cathode resistor is returned to a negative supply. The simple resistor-to-ground is almost certain to give you poor performance.

Current sources are cheap and easy to build.

[Joel]

A cathode follower is not going to like a 10k load, although a pentode can be made to like it more than a triode.
For best operation, a cathode follower should have the same AC load that it would use if it were being run plate-loaded.

If it were me, I would keep the 6H30Pi, and use a step down transformer, like the Lundahl LL1660S. That setup will give you a consistantly low output impedance that will laugh at a 10k load. You'll also have the added benefit isolated outputs breaking any ground current issues between the pre and power amps.

Joel

[Brett]

Originally posted by Joel
A cathode follower is not going to like a 10k load, although a pentode can be made to like it more than a triode.
For best operation, a cathode follower should have the same AC load that it would use if it were being run plate-loaded.

A 6H30 has no particular problem with a 10k load. I've used them that way before and they sound fine and have a low THD with a predominantly low order profile, especially only swinging a volt or two.

If it were me, I would keep the 6H30Pi, and use a step down transformer, like the Lundahl LL1660S. That setup will give you a consistantly low output impedance that will laugh at a 10k load. You'll also have the added benefit isolated outputs breaking any ground current issues between the pre and power amps.

Then the circuit is running at under unity gain and I see no benefit at all to that over an anode loaded 6H30 in this instance.
If you're going to go to that sort of expense and effort (Tx's over a cathode resistor) anode load it and get a Tx with a TR of 10+. With any decently configured 6H30 that's going to give a low output Z, roughly unity gain (depends on TR chosen), low distortion and will happily drive 10k. If you want to reduce the distortion more use an active load (CCS and parafeed) and use the 'mu' output to drive the Tx for lower output Z still though now you have an extra cap in the circuit.

I agree about the advantage of galvanic isolation.