Restoring ReVox G36 MKIII - Expert Opinions Solicited

[kirikoo]

Hi Martin,
Thank you for the reply.

In the mean time I replaced all capacitors in the power supply and others elco's in the audio circuitry.
The result is really excellent, a very dynamic sound, sweet highs, never harsh.

You mean that the new "A77" heads have a lower output than the original G36's ?

Best regards,

Kirikoo

 

[MartinR]

Hello kirikoo,

The result is really excellent, a very dynamic sound, sweet highs, never harsh.

I never liked the sound of the original playamplifier. But what do you expect from three ECC83 in a row?

That´s why I built my own reproamplifier.

You mean that the new "A77" heads have a lower output than the original G36's ?

No. The new G36 head and the old head have the same output voltage. 3,8mV 1KHz 0dB 19cm/s 320nWb.

The A77/B77/A700 head has an output voltage of 1,9mV. 6dB less.

Best regards,
Martin

 

[Marco Fasolo 1980]

Depending on the input impedance of the device you are connecting your ReVox to all you may need to do is replace the output coupling caps with larger capacitance values. This should be ok if the input impedance is 47K or greater.

In the case where you are talking about 10K then your simplest recourse is a unity gain buffer using either an op-amp or a tube based cathode follower using something like a 5687. One other possibility would be to revise the CF in the ReVox's playback amplifier to use a 12AT7A and run that at a few mA higher than the stock 12AX7A in order to get the drive for low impedance loads. Note that if you do that the extra current has to come from somewhere, and the best place to get it is by deleting the speaker amplifier or the first two stages of the record amplifier. I've done both as I find neither particularly useful in this day and age - and those extra 2 stages ahead of the record level pots clip like crazy with modern digital sources. In addition the additional circuitry reduces transparency and is completely unnecessary with any modern source unless you plan to use dynamic mics with this deck, which I don't.

Transformers aren't really on for this application because the required impedances are high enough that the use of most affordable transformers will result in a loss of transparency, both leakage inductance and stray capacitance are major issues at this impedance. (Reflecting a 100K load impedance to the deck is non-trivial at these signal levels.)

Hi kevin , can you describe in detail the procedure to change the ecc83 at the cathode follower output with a 12AT7?
How do I increase the mA? Please let me know. All the best Marco

 

[kevinkr]

Hi Marco,
I got rid of this recorder about 9 years ago so I really don't have anything to refer to at this point.