Replacing a 12AU7 in a cathode-follower.

[Cobra2]

http://search.ebay.com/search/search...satitle=IRF820

Easy to find...cheap.
Only trouble, most sellers do not ship overseas.
I would buy 20-30, if anybody bother taking a "lot"

Arne K

[billbo]

Technically its not "all valve" so it might not impress the purist at heart. Although it is the best of both worlds I have to say. You cannot beat the output "Z" of the mosfet. You still have the tube line stage which has an effective load impeadance of well into the hundreds of megaohms which will reduce distirtion dramatically.
Here is the mosfet follwer retrofit.

[beppe61]

Originally posted by dshortt9

Yes, the 12BH7A has a transconductance/plate resistance of 3300/5200 vs 2200/7700 for the 12AU7A. I find that it gives me more of everthing I want without the coloration of the 12AU7. The filament current is 0.6A vs 0.3A for the 12AU7 but this should cause no problem. The pinout is the same.
It is a drop-in replacement.
You would probably need to change a resistor or two to use the ECC99.

Yes, the 12BH7A ... gives me more of everthing I want without the coloration of the 12AU7

Thank you very much.
I will try one as soon as possible.
More than a problem of coloration I have a problem of "drive" in the bass.
I have been already suggested to use a ecc88 with sections paralleled as cathode-follower.
But I should built a new unit from the scratch.
Kind regards,

beppe

[beppe61]

Originally posted by billbo

Technically its not "all valve" so it might not impress the purist at heart.
Although it is the best of both worlds I have to say.
You cannot beat the output "Z" of the mosfet.
You still have the tube line stage which has an effective load impeadance of well into the hundreds of megaohms which will reduce distirtion dramatically.
Here is the mosfet follwer retrofit.

Thank you very much for the very kind and interesting advice.
I will look throught it.
I understand that the combination of a valve followed by a mosfet is used in even very high end units, like if I am not worng the VTL top of the line preamp.

Kind regards,

beppe

[beppe61]

Just one more general question about tubes used in a cathode follower configuration.
If I increase the bias current in the 12AU7 tube near to its maximum allowed (what is it I do not know) will I get a lower output impedance and more "driving abilities" from it (maybe shorten its lifetime as a side effect) ?

Thank you very much indeed.
Kind regards,

beppe

P.S. I am very interested in the cathode follower thing.
It is just so simple that it could not be that bad if correctly designed.
After all there are so many extremely good tube amps that have cathode followers in their signal paths.
I do not understand why buffers have such a bad reputation

[SY]

I do not understand why buffers have such a bad reputation

Because they're so often designed and/or used incorrectly. Logic, experience, and sound engineering do not intrude on the thought processes of audio legend.

To your other question, in general, yes, you can somewhat decrease source impedance by running the current up. Don't go beyond max ratings or the tube will quickly self-destruct, and probably take out nearby innocent bystanders.

[Cobra2]

What about paralelling?

Arne K

[beppe61]

Originally posted by SY

I do not understand why buffers have such a bad reputation

Because they're so often designed and/or used incorrectly.
Logic, experience, and sound engineering do not intrude on the thought processes of audio legend.

To your other question, in general, yes, you can somewhat decrease source impedance by running the current up.
Don't go beyond max ratings or the tube will quickly self-destruct, and probably take out nearby innocent bystanders.

I do not understand why buffers have such a bad reputation
Because they're so often designed and/or used incorrectly.
Logic, experience, and sound engineering do not intrude on the thought processes of audio legend
Thank you. Your words confirm a my suspect.
Of course I understand that every device has a limit in its performance.

To your other question, in general, yes, you can somewhat decrease source impedance by running the current up.
Don't go beyond max ratings or the tube will quickly self-destruct, and probably take out nearby innocent bystanders
Very interesting. I understand.
All considered I think I could bear some shortening in the tube life-time for some gain in performance, anyway within the tube max ratings of course.

Thanks again and kind regards,

beppe

[beppe61]

Originally posted by Cobra2
What about paralelling?
Arne K

Thank you for your kind advice.
Actually I have already been advised of :
1) going with a more robust tube
2) paralleling sections.
Option 2 is not feasible in my unit that is little and sports just one socket, of course without modifying completely it.
Option 1).
I will be experimenting a 12BH7 in the 12AU7 position as soon as I get this tube.
I see it widely used in buffer/driver positions in very well regarded tube amps.
The voltage to heater is regulated by a fixed voltage regulator with a maximum 1 ampere output.
This should bear the 600mA consumption of a 12BH7, instead of the 300 mA of a 12AU7.
With the help of a friend I will try to understand if it is possible to bias up the tube by the way.
Maybe a biased-up 12BH7 could be a nice improvement.

I want to strecht this unit up to its maximum capabilities .... before "self-destruction"

Kind regards,

beppe

[gorgon53]

hi
5kohm inputimpedans is really low.
If you have enough gain in your preamplifier you could try to put some resistor in serie to your poweramplifiers input, doesnt cost much and may work if you experiment a little