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Replacing a 12AU7 in a cathode-follower.

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#41
gorgon53 said:
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
Click to expand...

Thank you very much for your kind and valuable reply.
Unfortunately the peamp line stage is nothing more than a ecc82 buffer, one tube for both channels.
I am using it now with the 5K input amps and the overall sound is nice. I like the timbral accuracy.
Nevertheless it fails to give an adequate "punch" with modern music.
Next experiment would be to swap the 12AU7 for a 12BH7.
If also the latter would be not enough I will change the preamp.
What is your opinion on the 12BH7 ?

Kind regards,

beppe
 
#43
gorgon53 said:
Hi
I find it a good idea and it is worth trying.
Good luck
Click to expand...


Thank you very much.
On March 11 I will be at a vintage audio fair here in Italy in Montichiari
In that occasion I hope to find a tasteful NOS 12BH7
I will report about the result on this thread.
I hope everything will work.

Thanks again and regards,

beppe
 
#44
Well, I have found a NOS RCA 12bh7 and I am using it in place of the original ecc82.
My amps are a tough load having 5K of input impedance.
Bass is still a little weak but mids and highs are clearly better, more transparent.
Overall I prefer the new tube and I will leave it in.

Thanks to everybody and kind regards,

beppe
 
#46
salas said:
12BH7 needs 10mA to play as it can.
What kind of bias does it develop now?
Click to expand...


Really I do not know how to check.
Problem is I do not have the schematic of this preamp.
How can I measure the bias current ? Is it easy to do ?
By the way the sound is better than with the previous ecc82.
Thanks and regards,

beppe
 
#47
You will get the bass you want by using an ECC99, however that may or may not work as a drop in replacement. It is pin for pin compatible but may not work with your part values. You could try one for a minute and see if it gets too hot. I have used it sucessfully in some places and not in others.
 
#48
beppe61 said:


Really I do not know how to check.
Problem is I do not have the schematic of this preamp.
How can I measure the bias current ? Is it easy to do ?
By the way the sound is better than with the previous ecc82.
Thanks and regards,

beppe
Click to expand...

There is a resistor in series with the cathode of the tube. See its value. With a voltmeter you will check the voltage across this resistor. The current is going to be I=V/R. It passes through the tube.
 
#49
Originally posted by dshortt9
You will get the bass you want by using an ECC99, however that may or may not work as a drop in replacement.
It is pin for pin compatible but may not work with your part values.
You could try one for a minute and see if it gets too hot.
I have used it sucessfully in some places and not in others.

Thank you very much Sir for your kind and interesting advice.
In the next days with the help of a friend I will try to draw out the buffer schematic in order to evaluate the possibility of using a ECC99.
There was a time when the units were provided with the full electric schema 😱
Now it is a top secret 😱
Thanks again and kind regards,

beppe
 
#50
Originally posted by salas [/i]
There is a resistor in series with the cathode of the tube.
See its value.
With a voltmeter you will check the voltage across this resistor.
The current is going to be I=V/R.
It passes through the tube.

Thank you very much Sir.
I will try to check in the weekend with a tester.
If I understand well I=V/R=bias current, that should be 10 mA at least for a 12BH7 to work fine.
Am I right ?
Thanks again and Kind regards,

beppe
 
#52
Originally posted by salas

Yes, 10 mA for 12BH7 and ECC99 is enough to enter their comfort sonic zone.
In 12AU7 cathode followers, its more common to see 5mA

Thank you very much for the helpful advice.
I will try to calculate/measure the present bias current.
Kind regards,

beppe
 
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