Replacing a 12AU7 in a cathode-follower.

[beppe61]

Originally posted by billbo
If you want very low O/P Z, Try an IRF710/IRF720.
It is good for up to 100Khz bandwidth and you can use the same power supply (up to 300 volts) with little mods. O/P Z about 25 ohms.
Fets are the next best thing to tubes.
...
Regards
Billy...

Very interesting Sir, thank you.
I am afraid I have no idea of a working schematic by the way.
Is the IRF710 to be used as a buffer ?
Could you kind redirect me to any web pages ?

Thank you very much again.
Kind regards,

beppe

[Tony]

please try also SY's favorite, IRF820, over here they cost less than U$1 each...

[beppe61]

Originally posted by Tony
please try also SY's favorite, IRF820, over here they cost less than U$1 each...

Please excuse me Mr Tony but I am lost.
Could you kindly give me some more details/informations ?
Are you talking about a single IRF820 base buffer ?
Is the IRF820 a mosfet ?
Where I can find a suitable schematic ?
Maybe with a lower Voltage supply ?
I am open to any suggestion/advice of course.
I am just a very unexperienced learner.
My knowledge in elelctronics is almost useless.

Thank you very much indeed.
Kind regards,

beppe

[Tony]

Is the IRF820 a mosfet ?

yes...

Where I can find a suitable schematic ?

the web is full of it, if you are familiar with the foreplay preamps using 12AU7's, using an IRF820 as fource follower, you can make a 1 tube preamp.

Maybe with a lower Voltage supply ?

yes, you can also use bigger current in mosfet, from 2mA in the foreplay to about 10 in the IRF820 if you wish, but at that current you will need a small heatsink on the mosfet.

[beppe61]

Originally posted by Tony

yes...

Thanks again Sir. Just a few more kind questions ....

the web is full of it, if you are familiar with the foreplay preamps using 12AU7's, using an IRF820 as fource follower, you can make a 1 tube preamp
To be minimalist at a maximum degree, an IRF820 buffer should suffice in my case, as I do not need voltage gain.
After all I had a cathode-follower in my mind originally.
Nevertheless I could not find yet a complete schematic for a IRF820 buffer.

yes, you can also use bigger current in mosfet, from 2mA in the foreplay to about 10 in the IRF820 if you wish, but at that current you will need a small heatsink on the mosfet
A small heatsink is not a problem I think.
I will try to find a suitable schematic then.

Thank you very much again and kind regards,

beppe

[dshortt9]

I have found that the problem may partially be the sound quality of the 12au7. They just seem to sound a bit less dynamic and more colored than I want. I have very sucessfully cured this by simply replacing it with a 12BH7. This tube is almost the same but has a little higher transconductance. It is almost like finding the best 12AU7. I have simply plugged this tube in wherever a 12AU7 is used. Recently, I had a Transcendent preamp, and a Transcendent T-4 amplifier where they did just fine.
The nex step up from the 12BH7 is to use an ECC99 the same way, just plug it in. I would need the schematic of your present preamp to know if the ECC99 is OK to use or not. It may not be suitable. This tube has 4 times the trasconductance of the 12AU7.
What is the value of the pot that you are using?

[beppe61]

Originally posted by dshortt9

I have found that the problem may partially be the sound quality of the 12au7.
They just seem to sound a bit less dynamic and more colored than I want. I have very sucessfully cured this by simply replacing it with a 12BH7.
This tube is almost the same but has a little higher transconductance.
It is almost like finding the best 12AU7.
I have simply plugged this tube in wherever a 12AU7 is used. Recently, I had a Transcendent preamp, and a Transcendent T-4 amplifier where they did just fine.
The nex step up from the 12BH7 is to use an ECC99 the same way, just plug it in. I would need the schematic of your present preamp to know if the ECC99 is OK to use or not. It may not be suitable. This tube has 4 times the trasconductance of the 12AU7.
What is the value of the pot that you are using?

Thanks a lot Sir.
If I understand correctly the 12BH7 is a direct replacement for the
12au7, while the ECC99 needs a further check.
I have no service manual/schematic of this preamp unfortunately.
Inside it is a little crowded (p2p wired).
not very easy to write down the circuit.
I will try with the help of a friend anyway.

Thanks and regards,

beppe

[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.

[billbo]

The IRF820 will work great. Its just that if you look at the specs for the IRF710 and IRF 720 the inter electrode capacitance is much less giving you more bandwidth. But I will not object to the 820 at all. I have no web pages to produce bit I have schematics which I have drawn years ago. Here is the line preamp which I have built. I will post the source follower retrofit for it in another post. Notice it is directly coupled the the cathode follwer eliminates one coupling capacitor. This is one of many circuits that I have whipped up in the past.
Regards
BIlly D.

[kevinkr]

The IRF820 is considered obsolete, and I have not been able to source it locally. (I've tried) I think the same is true for both the 710 and 720..