Dual Gates SA-70 build in an old RCA beat frequency oscillator - reuse OPT, power supply, tubes?

[Urskov]

Hi there

I bought an old, broken RCA beat frequency oscillator model 154. See page 6 here for schematic: http://www.segalandassociates.com/Documents/Radios-Televisions/Schematic/RCA/RCA_m-154-bfo.pdf

The tube complement+voltages are perfect for an old school tube mic preamp. Or actually two mic pres.
It has 2 x 6j7s, a 6C5 and a 6J5.

I’m thinking it’s perfect for a Gates SA-70-type build - the kind of pres I've read user emrr rave so much about.

First question: The 6C5 being a predecessor tube of the 6j5 according to what I’ve read.. Am I correct in assuming that I can use it in place of the 6j5 that the SA-70 schematic calls for for one of the channels with little or even without any modification?
Both triodes, the operating characteristics are pretty similar in the tube charts, same voltages and amplification factor.

Second question: The oscillator has an output transformer with multiple secondary taps, including a 500 ohm tab.
Wouldn’t this be fine to reuse for one channel of the pres?

Last question: The power supply is tube rectified with a 5W4 followed by a choke input filter. Can I just reuse the power supply for the mic pre? Or is a power supply for an oscillator not gonna cut it for a mic pre noise wise?
I should add that I’m gonna need to use a step-down because we have 230v here in Denmark. Am I better off rebuilding the power supply completely with a new PT?

Thanks!

 

[rock soderstrom]

The tube compliment+voltages are perfect for an old school tube mic preamp. Or actually two mic pres.
It has 2 x 6j7s, a 6C5 and a 6J5.

I’m thinking it’s perfect for a Gates SA-70-type build - the kind of pres I've read user emrr rave so much about.

fat sounding mic pre, the American steel tubes are really nice. I highly recommend it!

Am I correct in assuming that I can use it in place of the 6j5 that the SA-70 schematic calls for for one of the channels with little or even without any modification?

The schematic calls for a 6j7 and 6c5 but you can use the 6j5 as well. They are both technically very similar, the more modern 6j5 has a slightly lower internal resistance and slightly higher conductance. It will not make any significant difference in this circuit.

The power supply is tube rectified with a 5W4 with a choke input filter. Can I just reuse the power supply for the mic pre? Or is a power supply for an oscillator not gonna cut it for a mic pre?

In general, you can use the existing PSU, why not? You will want to replace the electrolytic capacitors and maybe add some more RC filtering, the B+ voltage may also need to be reduced a little.

I should add that I’m gonna need to use a step-down because we have 230v here in Denmark. Am I better off rebuilding the power supply completely?

This makes it somewhat more cumbersome to work with an additional mains transformer. Perhaps it would be better to use a suitable transformer alone? You decide, both ist possible. I played a Fender Champ with a step down transformer for years without any problems. If there's enough space, it works well.

Edit: got photos?

 

[Urskov]

fat sounding mic pre, the American steel tubes are really nice. I highly recommend it!

The schematic calls for a 6j7 and 6c5 but you can use the 6j5 as well. They are both technically very similar, the more modern 6j5 has a slightly lower internal resistance and slightly higher conductance. It will not make any significant difference in this circuit.

In general, you can use the existing PSU, why not? You will want to replace the electrolytic capacitors and maybe add some more RC filtering, the B+ voltage may also need to be reduced a little.

This makes it somewhat more cumbersome to work with an additional mains transformer. Perhaps it would be better to use a suitable transformer alone? You decide, both ist possible. I played a Fender Champ with a step down transformer for years without any problems. If there's enough space, it works well.

Edit: got photos?

D'oh! Of course the schematic calls for a 6C5, got them confused.

But that's just the answer I was looking for! Thanks! Can tell from the recycling thread you're the man when it comes to recycling tube devices to pro audio stuff!
I love it, too! Built my first guitar amp a few weeks ago from a signal generator (6X4, 12AX7, 6BM8).

I've started building. Just gonna keep the pt, experiment with the power supply.

Pics attached looks sooo good. Cobber chassis.