vintage tube I/V from the year 1960

[Bernhard]

Today it came to my posession a shure stereo conversion preamplifier model M65E.
2 x ECC83 TFK <> tubes, gain 63dB in microphone mode.
My first idea was to change the circuit to a gain of 2 for use as a playstation buffer.
Second idea was to use it as I/V with a 0,68ohm I/V resistor to amplify to 2V.

Do I need the input coupling caps ?
Can I replace the ECC83 with any of E88CC or CCa ?
If it is a 4 x os DAC, does it make sense to use a transformer after 0,68 ohm ?
Any suggestions to improve the circuit ? Bigger output caps ?

Another thing that came to my mind, I read that opamp works best as buffer with no gain. What if the resistor of a standard op amp I/V is changed so that output will only be 2mV instead of 2V ?
Would that be more like a buffer operation ?

[Jocko Homo]

Another thing that came to my mind, I read that opamp works best as buffer with no gain.

Where did you read that? I can't think of many things that sound worse than an op-amp operated as a buffer.

Jocko

[Guido Tent]

Hi

I'd leave the unit as is, don't know about the sound but it looks cute.

For IV conversion other tubes suit better, and you coulod consider using a cathode as input.

cheers

[Franz G]

For IV conversion other tubes suit better, and you coulod consider using a cathode as input.

I attach here a scratch. Something like this idea?

Franz

[Guido Tent]

Originally posted by Franz G


I attach here a scratch. Something like this idea?

Franz

Hi Franz,

Looks good !

I have some suggestions to improve the circuit though:

You may use the same (10k) load on the cathode of the second stage, and tie the supplies of both stages together. Ignoring external load, the power supply now sees "no" current swing.

Some people at the forum may object to the use of the cathode follower at all. Taking the signal from first stage anode will result in some k-ohms output impedance. If that is too high, we still need the CF. Improving the CF includes the use of a current source in the tail, which does away the advantage I describe above.....

Once we are on the subject of current source loading cathodes, also the first stage may be equiped with a CS (instead of the 68 ohm): This greatly reduces distortion

succes

[X.G.]

Originally posted by Franz G


I attach here a scratch. Something like this idea?

Franz

your I/V resistor is too higher.....the output volts of yours is very stronger

[Audiofanatic]

Originally posted by Guido Tent



Hi Franz,

Looks good !

I have some suggestions to improve the circuit though:

You may use the same (10k) load on the cathode of the second stage, and tie the supplies of both stages together. Ignoring external load, the power supply now sees "no" current swing.

Some people at the forum may object to the use of the cathode follower at all. Taking the signal from first stage anode will result in some k-ohms output impedance. If that is too high, we still need the CF. Improving the CF includes the use of a current source in the tail, which does away the advantage I describe above.....

Once we are on the subject of current source loading cathodes, also the first stage may be equiped with a CS (instead of the 68 ohm): This greatly reduces distortion

succes

I agree with Guido for at least 100%.

Best regards,


Audiofanatic

[Bernhard]

Thanks for replies.
Still the question: Can I replace the ECC83 with ECC88 or 81 or 85 or CCa or do they need other anode voltages ?
I would like to just the box to test tubes go - no go ...

Is the 86ohm in Franz's schematic the I/V resistor ?

Why is it bad to use the circuit as is ?
A 0,68 ohm from Iout to gnd would give very little distortion of a 1541.
The box would act like a preamp.
I have completely no knowledge of tube design, only see that three coupling caps at input, between stages and on output.

[Franz G]

Still the question: Can I replace the ECC83 with ECC88 or 81 or 85 or CCa or do they need other anode voltages ?

No, as the heather is different: with ECC83 there is 12.6V at pin 4 and 5, at ECC88 it is 6.3V between this pins.

You can find tube datasheets here: http://frank.pocnet.net/sheetsE.html

Some people at the forum may object to the use of the cathode follower at all. Taking the signal from first stage anode will result in some k-ohms output impedance. If that is too high, we still need the CF. Improving the CF includes the use of a current source in the tail, which does away the advantage I describe above.....

Yes, the idea of the CF is keeping the output Z low.

Is the conclusion from your statement: don't use current source for the CF?

Is the 86ohm in Franz's schematic the I/V resistor ?

Exactly, this is at least the intention. Is the value too high, for a TDA1541A?

The value 86ohm is just calculated out of the tube datasheet.

Once we are on the subject of current source loading cathodes, also the first stage may be equiped with a CS (instead of the 68 ohm): This greatly reduces distortion

With a current source instead of the cathode resistor in the first stage, there is a separate I/V conversion resistor needed. Isn't it?

Franz

[Franz G]

What about using a LED instead of the first cathode resistor, as I/V converter?

I remember at the triode festival, a speach from Lynn Olson (or was it Morgan Jones?): LED's are very nice low-Z current sources, instead of cathode resistors.

Could this work? Worth a try?

Another question:

Is it recommended to use a coupling cap at the input? Are the 1.3V DC dangerous for the DAC's output?

Franz