Upgrading the RCA RS177A - RS177J Console Amps: Three Stages of Mod Recipes

[pac1085]

Thanks for the post. I bought one of these from a member on here as my first tube amp project but quickly moved past it onto Dynaco, Scott, etc and never finished it. This helped motivate me again ;-)

 

[grindfix]

Here is my implementation in Knight 6BQ5 amplifier. The diode section was later updated with "bullet proofing" option

 

[grindfix]

I can't say enough about these RCA output transformers- They're just console transformers right? But RCA really knocked it out of the park- I have never seen a console amp with transformers this good- getting very low frequency performance (only down 1dB at 30Hz =at power=), and a barely existent resonant peak at like ~190kHz.

Very nice work and great documentation. I think I have one of those RCA chassis stashed away in my basement. The Fisher Philharmonic chassis transformers are even smaller but measure fairly similar. The amp I built with them sounds very impressive for its tiny footprint.

Power Output 8W into 8ohm load
Input Sensitivity 0.9v
15.5dB of Global NFB
Frequency response is flat 20Hz-35KHz

Full power THD:
50Hz = 1.3%
1KHz = 0.34%
15KHz = 0.8%

 

[wparks]

Here is my implementation in Knight 6BQ5 amplifier. The diode section was later updated with "bullet proofing" option

Thank you for sharing that. It seems the STF16N50M2 is a very capable newer device, that has the Zener protection between the gate and source now built into the device. Amazing stuff.

Did you build this on a hand wired perf board or PCB, or point to point?

I always wonder about both the 4.7uF at the ADJ of the LM337, and similarly the 0.47uF at the gate of the MOSFET- are these purely for noise abatement, or do you think there's a time constant for shaping dynamic response involved? I wonder, since the screen is lagging the B+ if it would be interesting to see the effects of varying these RC time constants on distortion. One would probably need to use either an arbitrary waveform or sweep different frequencies?

 

[wparks]

Very nice work and great documentation. I think I have one of those RCA chassis stashed away in my basement. The Fisher Philharmonic chassis transformers are even smaller but measure fairly similar. The amp I built with them sounds very impressive for its tiny footprint.

Power Output 8W into 8ohm load
Input Sensitivity 0.9v
15.5dB of Global NFB
Frequency response is flat 20Hz-35KHz

Full power THD:
50Hz = 1.3%
1KHz = 0.34%
15KHz = 0.8%

Thank You for the kind words. Very nice work on that! What model and do you have a build thread on that?

I have the ability to measure distortion, but it's using a Focusrite USB audio interface and REW- always seems like such a huge effort and challenge to get the amp set up on my table where the laptop lives and get everything connected up and configured correctly. P.I.T.A really, so I don't measure it very often. Looks like you have done a very thorough job. Again, Nice work!

 

[grindfix]

This is all Dave's work. I can't take credit for designing any part of it; I only connected two circuits to work together. I believe it was done before me as well, but I either couldn't find a reference to it, or it simply was designed for something else and wouldn't work for my amplifier. I don't remember.
Both SS devices are plastic case version without a metal tab and are screwed to the amplifier chassis. All wiring is done point-to-point with longer T-strips.

 

[grindfix]

That amplifier was initially built as an exercise to test the output transformers' performance with 6BQ5 tubes in place of the original ELL80.
The thread is here.

Then, I decided to try EL95, and it worked better. The thread is here.