Replacing a 12AU7 in a cathode-follower.

[beppe61]

Dear Sirs,

I am using a line stage consisting in a pot followed by a 12AU7(ECC82) in a cathode-follower configuration.
I have been told that substituting my present Philips Miniwatt for a mil spec tube like the 5814A, others things remaining unchanged, I will get a more powerful and defined bass.
I like very much resolution and detail in thelower part of the audio bandwidth.

Thank you very much indeed.
Kind regards,

beppe

[SY]

Going to a mil-spec tube is unlikely to make your bass more powerful. That said, the 12AU7 is not a particularly great choice in a cathode follower- the transconductance is pretty meagre. You might consider a higher gm tube like the classic ECC88 family or one of the Russian high-slope triodes.

[beppe61]

[QUOTE]Originally posted by SY
Thank you very much for your kind and helpful reply.

Going to a mil-spec tube is unlikely to make your bass more powerful. That said, the 12AU7 is not a particularly great choice in a cathode follower- the transconductance is pretty meagre
Unfortunately this unit is built around a 12AU7.
I am trying to get the best bass out of it.
Nevertheless I woudl like to understand if it would be better to ditch it and going with another solution.

You might consider a higher gm tube like the classic ECC88 family or one of the Russian high-slope triodes
Actually looking at commercially available units most of them use ECC88s as buffers.
I have to say that about two years ago I made an extremely simple low voltage (70-80 V) ecc88 buffer and I liked its sound.
Stupidly I put aside that simple prototype and lost the schema.
Are you aware of any kit of buffer using a ecc88 ?
The prototype I made was without a pcb but I would like to get a complete kit in order to get a nice result.
I already have a nice attenuator to use with this buffer to drive a good pair of mono amps I already have.


Thank you very much indeed.
Kind regards,

beppe

[SY]

It will fit pin-for-pin in your current preamp. You'll need to change the heater voltage from 12V to 6V, which isn't too hard. Depending on how the CF is biased, you might need to change a resistor value, but maybe not- do you have a schematic of your current unit?

[beppe61]

Originally posted by SY
It will fit pin-for-pin in your current preamp.
You'll need to change the heater voltage from 12V to 6V, which isn't too hard.
Depending on how the CF is biased, you might need to change a resistor value, but maybe not-
do you have a schematic of your current unit?

Thank you so much again Sir.
First of all I have to state that I am a very unexperienced DIYer.
And I am a little scared about working with the unit on inside to check values and voltages.
What interests me very much is the possibility of powering the ecc88 with low voltages.

do you have a schematic of your current unit?
Unfortunately not Sir.
It is an old British Micro Croft preamp 1st series.
Established that a ecc88 buffer would be much better than a ecc82 buffer I would like take that route.
My amps are a though load, at about 5 kohm of input impedance.
But with a 0,5V signal I can get max power.
So gain in the buffer stage is not needed.
So to sum up all my ramblings, the question is which could be the best tube for a basic cathode-follower design.
If a kit of a very basic buffer based on a single ecc88 (or other better tube for buffer) is available I would prefer to buy it of course and not proceeding with mods on my current preamp-
Starting from a kit more chances to build it right I believe.

Thank you very much indeed.
Kind regards,

beppe

[kevinkr]

Wow, no wonder you don't have much bass! Are those amplifiers really 5K input impedance?

I think the ECC88 is going to be a little hard pressed to drive such a low load impedance, but it will do a much better job at it than the 12AU7A.

One wonders at how much additional current the power supply in the croft can provide. I'd be careful about increasing current in the CF by more than a few mA..

You will also need to increase the value of the output coupling caps or if possible increase the input impedance of your amplifier to at least 20K if not higher.

[beppe61]

[QUOTE]Originally posted by kevinkr

Wow, no wonder you don't have much bass! Are those amplifiers really 5K input impedance?
the amps are old Albarry M408.
This 5K is the value I found in a old magazine.

I think the ECC88 is going to be a little hard pressed to drive such a low load impedance, but it will do a much better job at it than the 12AU7A
so even the ECC88 could be not up to the task.

One wonders at how much additional current the power supply in the croft can provide. I'd be careful about increasing current in the CF by more than a few mA
Thank you for the advice.

You will also need to increase the value of the output coupling caps
I have already done this. I put a 10uF film cap at the outputs.

or if possible increase the input impedance of your amplifier to at least 20K if not higher
Unfortunately I do not have the schematics.
That would be a very interesting option of course.
As I listen only to digital I like the flavour tubes impart to the sound. It is more listenable.
Maybe a fet buffer would be better all considered ?
I am determined to follow the route of a buffered passive pre as a line stage.
I like its minimalism extremely.
Any suggestion ?

Thanks a lot and kind regards,

beppe

[cerrem]

I have to disagree...
The 12AU7 in follower configuration is roughly 400 ohms output impedance.. give or take depending on operating conditions of course... I have designed many 12AU7 followers in Hi-End applications with good sound and good measured results....
Yes, the ECC88 makes a mucho better follower in the 80 ohms range... But the 12AU7 still can get the job done...

Chris

[beppe61]

Originally posted by cerrem
I have to disagree...
The 12AU7 in follower configuration is roughly 400 ohms output impedance.. give or take depending on operating conditions of course...
I have designed many 12AU7 followers in Hi-End applications with good sound and good measured results....
Yes, the ECC88 makes a mucho better follower in the 80 ohms range...
But the 12AU7 still can get the job done...
Chris

Thank you very much indeed Chris for the valuable advice.
Do you think that all 12AU7s are equal or for instance the mil spec 5814A can give a more robust output ?

Anyway your following words Yes, the ECC88 makes a mucho better follower in the 80 ohms range make me think that a ECC88 would be a wiser choice considering that this buffer must drive a 5K load (!).

Thank you very much again and kind regards,

beppe

[cerrem]

The 5814A and the 12AU7A are the same tube...they come off the same factory run .....The difference is that the 5814A tubes usually measure stronger than 12AU7A spec and are seperated and sorted.... They are the cream of the crop 12AU7A tubes... SO this means the gm will be higher and the plate resistance a bit lower which makes for better low frequency reponse compared to a plain 12AU7A... The spec sheet for the 5814A read exactly like a 12AU7A but that is just for satisfying MIL requirements...in reality they measured better than the data sheet...

Chris