Reworked a RCA RS177F Amplifier

[MEur]

I bought this unit from eBay and someone had tried to get it working but didn't complete it. It was in rough shape and one of the two large caps was bad and broken off during shipping. I rebuilt the power supply with new caps and replaced all the coupling caps between stages with new 22 nF 600-volt caps,(6x film caps). I also cleaned up some wiring, and after some testing on a dummy load, connected it to speakers. It sounded great, but with very little gain (as everyone on the forums said). The gain measured around 1, which seemed terrible even for 6Gu7 tubes.

I took a risk and removed all the components from the gain stage/phase splitter and redid it with a different circuit. Since this was my first time with a blank sheet design, I consulted ChatGPT and came up with a boosted gain stage that will work with a 6GU7 or a 6DT8 and give more gain. I also simplified the phase splitter into a Concertian style with 100k loads. This produced a gain of around 18 with the 6GU7 and more with the 6DT8. With the Wiim set to output 2 Vrms, it should be able to drive the amplifier to full wattage.

It sounds great and has plenty of gain with the existing 6GU7s (which I like the sound of better than the 6DT8s). If you change the heater wiring, this circuit should work as is with a 12AX7 too. I posted the circuits below for anyone contemplating this project. I highly recommend this change the RS177 is a great sounding unit! I hope this helps someone in the future.

Notes:
I designed a 22uF bypass cap for the driver cathode but removed it because it sounded too bright in my system.
I added a 10k Pot to the feedback circuit and I like having the adjustment.
I put the speaker terminals and RCA input where it made sense from the perspective of the wiring, but would put them on the back behind the transformers.
I ended up Biasing the gani stage with 1.2k resistors, which is a little hot but sounds good.
I removed the 330k resistor on the input completely and left the 470k grid leak.
I didn't add an input coupling cap

 

[macyjrm]

Thanks for posting this. There are abundant RS177s around but not a lot of interest in them. I sold one years ago as found in an old RCA console because I don’t have the skill set to pull off these mods.

 

[gadget73]

Lots of ways to do this though, all depends how you want to go about it. Two thoughts I had without putting much deep effort into it is

1) 1/2 of some twin triode per-channel as a voltage amp, and re-purpose the 6CG7 into a long tail pair
2) dissimilar dual triode with a high-mu and a low or med-mu section. Keep the split load on the low or med mu section and the high mu for gain

for #1 not sure which twin triode, but I'm thinking something lower than a 12ax7. Maybe a 12AY7, its pretty linear and mu of 33 I think, somewhere in that zone. Should give a real-world gain around 25, and the 6CG7 as an LTP will give ~8. That should be plenty of open loop gain to apply a reasonable level of feedback and get acceptable input sensitivity. If thats still too hot, a 6CG7 will get you ~15 or so for gain.

for #2 you could go 12DW7, its 1/2 a 12ax7 and 1/2 a 12au7, but something a little more out there and cheaper is maybe a 6CY7 or one of a couple of other TV vertical oscillator / output 9 pin tubes. The high mu side is 68, and a split load inverter always ends up somewhere around 0.9 grid to either output so total open loop gain won't be crazy high but it still may be enough. 6bq5 tubes aren't real hard to drive.

mostly it would depend on if you felt like popping another hole in the chassis or not, or what inverter tickles your fancy. I like 6CG7 tubes though, nice and linear and they can be had cheap. Also as an LTP they can generally be subbed with a 6GU7 or a 7AU7 so long as pin 9 isn't grounded. Not sure there is a lot of advantage. Having rolled those tubes in my 6L6 amp that runs that setup I can honestly say I don't really notice a difference. Those are all cheap tubes though so it lends itself to experiments without breaking the bank.

 

[Maxamillion]

Nice job! I did one a couple years ago with a 12AX7 conversion, and a 100K stepped attenuator. Sounds great and is in regular use as my basement gym amp.

Details here:

RCA Console Pull Project

Picked up this amp for peanuts at a flea market a few years back; the seller even included a chunk of the old console so I could ID it. Seems like a nice, simple push-pull 6BQ5 amp. Anyone ever heard one of these? Impressions? Watch-outs? I am using an old Lafayette SS receiver to power my...

audiokarma.org

 

[Maxamillion]

Quick pics of the final outcome in case the thread is too long to read...;-)

 

[MEur]

Quick pics of the final outcome in case the thread is too long to read...;-)

Wow! That looks really great. I was thinking of building a chassis around mine with some old barn wood and maybe going for a little bit of a steampunk look. I have to get some wood.

 

[MEur]

Lots of ways to do this though, all depends how you want to go about it. Two thoughts I had without putting much deep effort into it is

1) 1/2 of some twin triode per-channel as a voltage amp, and re-purpose the 6CG7 into a long tail pair
2) dissimilar dual triode with a high-mu and a low or med-mu section. Keep the split load on the low or med mu section and the high mu for gain

for #1 not sure which twin triode, but I'm thinking something lower than a 12ax7. Maybe a 12AY7, its pretty linear and mu of 33 I think, somewhere in that zone. Should give a real-world gain around 25, and the 6CG7 as an LTP will give ~8. That should be plenty of open loop gain to apply a reasonable level of feedback and get acceptable input sensitivity. If thats still too hot, a 6CG7 will get you ~15 or so for gain.

for #2 you could go 12DW7, its 1/2 a 12ax7 and 1/2 a 12au7, but something a little more out there and cheaper is maybe a 6CY7 or one of a couple of other TV vertical oscillator / output 9 pin tubes. The high mu side is 68, and a split load inverter always ends up somewhere around 0.9 grid to either output so total open loop gain won't be crazy high but it still may be enough. 6bq5 tubes aren't real hard to drive.

mostly it would depend on if you felt like popping another hole in the chassis or not, or what inverter tickles your fancy. I like 6CG7 tubes though, nice and linear and they can be had cheap. Also as an LTP they can generally be subbed with a 6GU7 or a 7AU7 so long as pin 9 isn't grounded. Not sure there is a lot of advantage. Having rolled those tubes in my 6L6 amp that runs that setup I can honestly say I don't really notice a difference. Those are all cheap tubes though so it lends itself to experiments without breaking the bank.

iI ran the numbers and I can put a 12AX7 in mind and get more gain but I have to change the heater wiring. The advantage of reusing the 6GC7 tube was that the amp came with the tubes so I only needed to buy a rectifier. I think I have $50 into the whole thing.

 

[Dswankey]

..I was thinking of building a chassis around mine with some old barn wood and maybe going for a little bit of a steampunk look. I have to get some wood.

Do it!!! they can look quite good

My RS-177

 

[gadget73]

iI ran the numbers and I can put a 12AX7 in mind and get more gain but I have to change the heater wiring. The advantage of reusing the 6GC7 tube was that the amp came with the tubes so I only needed to buy a rectifier. I think I have $50 into the whole thing.

all just depends on how you want to do it. Its a 6bq5, they aren't that hard to drive. Lots of options and there is enough space on the chassis to do pretty much anything you want. I happen to like 6CG7's so making use of them doesn't bother me.

 

[albusark]

Quick pics of the final outcome in case the thread is too long to read...;-)

Wow that's so impressive, and what an awesome way to set up the bluetooth. Killer build.