RCA RS-177A console pull refurb

[nerdorama]

You might try a different driver tube with higher mu such as 12AT7 to get a bit more gain. Maybe go to 12AX7 like so many others of this topology. The original preamp probably had more gain so they didn't need it in the amp section. Will need to rebias the stages and maybe adjust feedback, but you could keep the same topology.

 

[jdsalinger]

I have to say...I miss mine. I think this is one of the best looking raw console pulls out there.
Enjoy!

 

[darg]

Unfortunately my RCA is still with all my other audio stuff and paperwork in a moving POD so I can't check the real location but when I got mine I had the same issue that the input isn't very sensitive. I remember I replaced a resistor on the input with a pot to adjust it and it was running much better and didn't need full output from my pre. Looking at the schematics I think I replaced R231 and R232 with a pot but don't ask me what value. It might have been 47K or a 100K. This should give you more signal on the input bridges R231/201 and R232/211. Thread about my rebuild and tweeks here http://www.audiokarma.org/forums/index.php?threads/rca-rs177-rebuild.645924/
I remember I checked with an oscilloscope if the output of the input stage tubes was clipping and it didn't so I know it can be done and helps a lot

 

[hrtattckcity]

Purely cosmetic update:
Maple trim in the works, unfastened the transformers to get some of the grime I couldn’t reach before.

 

[buglegirl]

Purely cosmetic update:
Maple trim in the works, unfastened the transformers to get some of the grime I couldn’t reach before.

Looking Great!!

Frannie

 

[Dswankey]

Looking good!!!, what kinda feet will you be using?

 

[hrtattckcity]

Looking good!!!, what kinda feet will you be using?

Just some simple rubber feet from AES, screwed up through the existing holes in the chassis flange into the wood trim.

 

[kvflyer]

What happened to the beer???

 

[Dswankey]

Just some simple rubber feet from AES, screwed up through the existing holes in the chassis flange into the wood trim.

Cool!!, do post pics when the project is complete.

 

[GordonW]

You might try a different driver tube with higher mu such as 12AT7 to get a bit more gain. Maybe go to 12AX7 like so many others of this topology. The original preamp probably had more gain so they didn't need it in the amp section. Will need to rebias the stages and maybe adjust feedback, but you could keep the same topology.

The 6DT8 has the same parameters as the 12AT7, and the same pin-out as the original 6CG7 that's in these amps. This means that you can just plug in the 6DT8.

The increase in gain from the 6DT8 will allow tweaking the feedback loop, to get quite a bit more gain out of the amp. That's probably pretty easy to do- if your amp has an 8200 ohm resistor on the cathode of the input tube (6CG7) to ground, like most of these do- just replace that 8.2K resistor with a 1.5K- or just parallel a 1.8K resistor with the existing 8.2K. Then, just drop in the 6DT8s in place of the 6CG7s. Input gain should be about 2.5 times the original (in other words, it will only take 40% of the original input voltage, to drive the amp to full power), and the feedback wiil be the same ratio as the original amp.

Regards,
Gordon.

 

[hrtattckcity]

Thanks, Gordon. That sounds like a very streamlined fix if it applies.

Do you mind taking a quick look at the schematic and confirming the position—there is an 8.2k resistor at r208/r218 but that may not be where you meant?

Apologies for the handholding.

The 6DT8 has the same parameters as the 12AT7, and the same pin-out as the original 6CG7 that's in these amps. This means that you can just plug in the 6DT8.

The increase in gain from the 6DT8 will allow tweaking the feedback loop, to get quite a bit more gain out of the amp. That's probably pretty easy to do- if your amp has an 8200 ohm resistor on the cathode of the input tube (6CG7) to ground, like most of these do- just replace that 8.2K resistor with a 1.5K- or just parallel a 1.8K resistor with the existing 8.2K. Then, just drop in the 6DT8s in place of the 6CG7s. Input gain should be about 2.5 times the original (in other words, it will only take 40% of the original input voltage, to drive the amp to full power), and the feedback wiil be the same ratio as the original amp.

Regards,
Gordon.

 

[GordonW]

Actually, the schematic in the first post, has the position of the 8.2K and 5.6k resistors reversed (R208-R209 and R218-R219), compared to the schematic I have..

The one I have, has the 8.2K going to ground, and the 5.6K going to the OPT secondary- the one in the OP has the 5.6K to ground, and the 8.2K to the OPT secondary.

In either case- you are dealing ONLY with the resistor going to GROUND (R209/R219). If it's a 5.6K to ground- then you can either replace that 5.6K with a 1.6K, or parallel a 2.2K with the 5.6K. That will give the same results with this schematic, as what I specified above for the other schematic version.

Regards,
Gordon.

 

[hrtattckcity]

I performed the GordonW mod today, installing some NOS rca 6dt8’s and replaced the 5.6k resistors at r209/r219 with 1.6k.

The gain improved significantly; more than loud enough for normal listening when coupled with the preamp section of an old NAD 7250pe. Probably not enough gain for general use when an iPhone is plugged in directly—so if that is your intention you may need to add a gain stage.

Thanks @GordonW

I need to pickup some slightly smaller feet, but otherwise I think I’m done here.
Also wish I would have thought of clamping the unit vertically to the bench like this sooner.

Actually, the schematic in the first post, has the position of the 8.2K and 5.6k resistors reversed (R208-R209 and R218-R219), compared to the schematic I have..

The one I have, has the 8.2K going to ground, and the 5.6K going to the OPT secondary- the one in the OP has the 5.6K to ground, and the 8.2K to the OPT secondary.

In either case- you are dealing ONLY with the resistor going to GROUND (R209/R219). If it's a 5.6K to ground- then you can either replace that 5.6K with a 1.6K, or parallel a 2.2K with the 5.6K. That will give the same results with this schematic, as what I specified above for the other schematic version.

Regards,
Gordon.

 

[darg]

Nice.
I got a second unit last week that I have to tune similar to my first so changing the input tube would mean I would have to do it twice since I need that pair of amps for vertical biamping for my open baffle speakers

 

[hrtattckcity]

Nice.
I got a second unit last week that I have to tune similar to my first so changing the input tube would mean I would have to do it twice since I need that pair of amps for vertical biamping for my open baffle speakers

If it makes a difference, NOS 6dt8's were under 4 bucks a piece at two online retailers--it was a cheap and easy mod.

 

[darg]

Yeah. It might not be the price but rather changing more things on both units to make them even...too much work at first I remember getting the PS caps in a good and safe location was a fun on the first one. Also, I moved and so far have not a confirmed local source for caps and what not. I have a couple of addresses but haven't visit them so far. Not a fan of ordering since I want look at the parts in my hand to make sure they work and have the right size.

 

[GordonW]

It's literally only either changing or adding one resistor per channel, and plugging in the 6DT8...

Regards,
Gordon.

 

[darg]

This discussion made me curious and I checked the amp out. I had received it but left it in it's box since I knew that I had recap it. It still has the yellow bumble bees in it so I'm not starting a fire here
Even when the chassis is marked RS-177A it has one tube more than my RS-177A. The extra tube is a 5963 which is nowhere in the docs I have drawn in. Also the RS-179 schematic that I have does not have this tube on the chassis. Any idea what that is related too? I can remember that some chassis had one tube for the phono input on them but not sure if that was RCA or if that belongs to Magnavox consoles.
Oh, also the pp tubes are russian 6n14n where someone was matching them since they have handwritten markings on the pairs.

 

[darg]

Additional to the extra tube the inputs have 0.05mfd input caps that my first RS-177 does not have.
Also, whoever converted the amp to normal speaker terminals put two 15 Ohm/25W resistors on the 120V power cable which is flying losly in the chassis if one touches the chassis....party is on!!!

 

[darg]

Looking at the power supply caps the whole PS is also rather like a RS179 since the caps are 40mfd instead 80 like for the RD177. I think I have to get parts and compare it to my first RS177 amp that is till end of January in storage, need to move into my new house first before I can really work on it but that mix of things makes it interesting.