Re: Fet source follower
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
billbo said:
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 said:
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
yes...
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.
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.
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
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?
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?
dshortt9 said:
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
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.
You would probably need to change a resistor or two to use the ECC99.
Mosfet source follower
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.
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.
Attachments
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..
http://search.ebay.com/search/search.dll?from=R40&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
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
Mosfet source follower
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.
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.
Attachments
dshortt9 said:
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
Re: Mosfet source follower
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
billbo said:
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
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
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
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.
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.
SY said:
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
Cobra2 said:
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
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