RCA rs-199b console amp

[nerdorama]

I bought the guts (amp, tuner/preamp, turntable) from an RCA console yesterday. I have read about this amp and it seems worthy of restoration. Dave Gillespie's Magnavox SE rebuild is an option, the RH84, and Kegger's scheme as posted in another thread as well. I am inclined toward Kegger's circuit but am interested in thoughts and opinions on the subject. I have the chassis stripped and cleaned. I have all the parts on hand to do something very close to Kegger's circuit or the RH84.

Would also be interested whether the tuner/preamp is worth any effort.


Thank,
John

 

[s-petersen]

If you are planning to replace the output transformers, I would go with Dave's circuit and use the recommended transformers , if one of the other circuits are specifically designed for the original output transformers, or do not use negative feedback I would use one of them. Changing the original circuit may have stability or frequency response problems unless you can tune the circuit yourself.

 

[nerdorama]

I can tune it but I have a notion that the Kegger circuit with local feedback might be more to my liking sound wise. I'm just curious if anyone might have some personal experiences with one or the other to share. I don't really want to replace the OPT's. They seem to be decent size for a small SE console amp and probably do a decent job.
Thanks,
John

 

[s-petersen]

I've built the Dave SE1 (for someone else)and it is awesome, I have another in queue (for me)as soon as I get some free time

 

[Dswankey]

If you haven't already read this thread you might find it interesting.

https://audiokarma.org/forums/index.php?threads/rca-rs-199b.251342/

 

[gadget73]

I'd use the stock circuit and simply move the voltage amp stage from the tuner onto the main chassis. Its pretty darn close to the DGSE1 anyway. There are really only so many ways to do single ended 6bq5 amps with a 12ax7 driver so they're all pretty much the same. The only real difference is whether you do global feedback or not. I'm a fan of it because it helps fix any failings in the output transformer.

if you're changing the transformers, use Dave's circuit since it was tailored for the specific transformers he used.

 

[nerdorama]

Hi Dswankey,
Yes, I read that thread. I like his cosmetic treatment. This is the one with the Kegger circuit toward which I tending.
https://audiokarma.org/forums/index.php?threads/rca-rs199b.441949/#post-5668807

Since I have the parts on hand I think I will try it. If I don't care for it then I will probably convert to something like the DG-1 and tune it for the RCA outputs.
Thanks everyone.
John

 

[tubeactive]

These amps are worthy ! Iron is good to very good, but bandwidth gets very, very good with a bit of overall loop feedback.
Look at the amp, before stripping it, please ? The voice coil feedback arrangement can simply get changed to "more normal" values. Simply, and I mean simply, rearrange the 100 pf cap across the 3.3K cathode resistor. Keep the 3.3Ks as the 6CG7/6FQ7 cathode resistors, but twist the 100pf cap to become the feedback cap toward the speaker voice coil. Across those caps, parallel a 10K to 33K feedback resistor. So, that now unconnected end of the 100 pf/10-30K Ohms then gets connected to the white leads "directly" from the voice coil. Of course, simply lift the white lead (some versions have yellow leads for the voice coil/feedback) from those unneeded 47 Ohm/2W resistors.

One of the units I built, I used the 47 Ohm/2W at the "bottom" of each 3.3K now 6CG7 cathode resistor. That way, their junction, 47 Ohms and 3.3K Ohms, can be a great point for adding more feedback.

In fact, the feedback arrangement of the stock unit is what reduces output level. Next time I build or rebuild one, I will install a variable, 25-30K ohm pot for an easy, adjustable feedback. My user RS-199 unit, which I demo at meets all the time, sounds incredibly good with my reference Norelco(Philips) full range 8 inchers in TL boxes. That amp uses 22K feedback resistors from the voice coil, above the 3.3K cathode Rs. While some lurkers have posted the feedback is minimal, it certainly makes a big sonic improvement over stock. Plus, the sensitivity of the input is now much more usable and can even benefit from an input pot. We do not need an extra gain stage as some folks recommend.

 

[nerdorama]

A bit of progress. Finished up the final wiring check this AM and a slow power up with the variac but no tubes. Voltages came up ok so I put in the tubes. It plays music. Got it up to voltage with 120VAC at the power plug. Sounds pretty good with my sacrificial test speakers but voltages are a bit off on the 6BK7 driver. Need to tweak the cathode resistors for a better op point. The EL84's both test on the weakish side so I'll dig for some better tubes.

 

[mcheinstein]

I had a very similar RCA pull that I converted into a DGSE1. I used the same PT with a voltage doubler and did not switch to tube rectification. Later, I switched out the AES outputs for edcor GXSE and I used Miflex copper foil couplers, which both miraculously fit on the little guy. Sounds fantastic.

I'll be watching this one to see where you take it! here's a pic of mine with the phono pre I built for it.

 

[Dswankey]

The chassis cleaned up nice!!, looking good

 

[nerdorama]

I gave the chassis a couple coats of hammered silver. Covered a few blemishes and I caused a few myself after the paint job. Oh well. It has a very quiet background on my 90dB speakers. I will get a few scope measurements made one of these days. And see if I can rustle up a fresher pair of output tubes.
John

 

[nerdorama]

Listened for an hour or so on a couple pair of speakers. Not rendering an opinion yet. I'll play it a couple of days and then start deciding about it. If I had to render judgement now I would say it's a bit recessed in the treble and wooly in the bass. I will run it more and try to get better output tubes into it. The 6BK7 is NOS.

Forgot to mention, I managed to do this entirely with parts on hand.

John

 

[nerdorama]

Bit of an update on Easter. Listening to the little remodeled Kegger circuit RCA with my Lynn Olson ME2 satellites. It's a nice paring within it's volume range. The speakers have a fairly flat impedance curve specifically designed for tube amps. Voices are really nice. Glad I did the update to the RCA amp.
John

 

[nerdorama]

Coming back around to this console amp. I was never really happy with the results from the Kegger take on this amp. It had a very rolled off treble which was a bit over -5dB at 20kHz.


Started looking at using a bit of GNFB and found a decent result with about 5.2dB which counteracted the -5dB of rolloff quite nicely but made the input sensitivity rather low.


Trying to get a little input sensitivity back I increased the value of the local feedback resistor from 100k to 200k. This actually further improved the HF response as well which is about -1dB around 40k. Sound is now nice and airy. Sensitivity is still on the low side. The limit of the sig gen in my Picoscope is 2V and not enough to get to clipping. My Aikido line stage doesn't seem to have trouble driving it to distortion but not sure how much voltage is needed. Thinking about one of the higher mu options like 6DT8, 6AQ8 etc. which are pin compatible with the 6BK7B that I'm using. Pretty sure I have some 6DT8's around. Will have to figure new plate and cathode resistors. For now I'll listen to it for a while.


The 10kHz square waves look decent for a low feedback SE amp. The left channel being slightly cleaner than the right.