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RCA RS-177A console pull refurb

Putt this into my main setup; driving Polk Monitor 10’s and using an NAD C325BEE as a preamp. Sounds great.

I’ve been listening to it a lot this weekend. And for a brief period one of the 6bq5’s (in the center position of the six tubes, on the side of the two small transformers) was glowing very brightly—I am assuming this was “red plating”? Everything sounds normal, it’s appearance has gone back to normal and it has not happened again. Should I be concerned?
 

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Make sure the output tube pins are free of oxidation and the tube socket connectors are clean and tight. May also want to check the solder joints to those sockets. It shouldn't happen again. If it does there is something wrong.
 
So, this amp was sitting around for a while and @noveltone was tube-curious, so I convinced him to take it home and try it out.

He got some red plating on one of the EL84's--and tried to rotate the EL84's and try again a few days later, and the same socket started to red plate.

He is using some speakers that are about 89db efficiency--which I assume wouldn't overwork the tubes enough to cause that to happen?

Any other immediate recommendations for what to look for?

Thanks in advance.
 
I had a red plate on my Pilot 402 after recapping coupling caps too. It happens.
Problem went away so I’m not sure what the root was.

What’s the voltage on the grid?
What’s the resistance of the cathode resistor?
 
I have the am/fm tuner for this, the rc 1211a with glass faceplate and knobs. I can put it on bartertown if you want me to
 
jdsalinger said:
Can you explain why, Gordon?
Click to expand...

not Gordon, but it has to do with draining off stray charges on the grid. Without a resistor to drain off charges, it starts to develop a positive voltage. When that happens more current will flow through the tube, basically the same as a leaky capacitor would. This is actually how grid leak bias works. If you ever run into a tube with a several megohm grid to ground resistor and the cathode is connected directly to ground, thats whats happening.
 
I got this back and tried it out with some university 6201’s in Karlson cabinets—no red plating even after a couple of hours of usage.

Is there any reason inefficient speakers would strain the amp and make it red plate?


GordonW said:
The grid resistor supplies the voltage reference for the grid bias. Too much resistance can allow this voltage to float to levels that affect the bias current adversely.

Regards,
Gordon.
Click to expand...
 
Not really, unless you were running it or beyond max output constantly. Even with pretty inefficient speakers you'd hear the heavy distortion that would involve. One possibility is funky impedance properties on the speakers that might make the amplifier oscillate though.
 
I'd check the problem socket for a dirty or loose contact on the grid. I've seen it more than once cause a red plate scenario. Also get in the habit of using a tube pin straightener. I found one with a tube tester, but you can obtain them on the internet. many of the tubes I acquire come with bent pins, as though someone pulled them out with a tractor... A bent steel pin will deform the heck out of even a brand new socket pin, rendering the pin tension inadequate to maintain contact. The cheapest sockets are the worst offenders I've found...
 
So, I have an RCA RS 177a amplifier that is paired with a RS 179 preamp. Can anyone tell me if I can operate these two together without the tuner? What upgrades would you all recommend?
 
Mikethenewb said:
So, I have an RCA RS 177a amplifier that is paired with a RS 179 preamp. Can anyone tell me if I can operate these two together without the tuner? What upgrades would you all recommend?
Click to expand...

Someone, and quite knowledgeable I might add, tried to help you in the thread you created yesterday morning but you didn't reply.

gadget73 said:
It probably used the switch in the turntable to turn things on and off if there is no tuner. You'll need to connect the proper wires to a switch to manually control it if thats the case. If you've got a schemtic for the RS-179 we can help with that.
Click to expand...

https://audiokarma.org/forums/index.php?threads/rca-rs-177a-help.902245/

Schematic can be found here if I remember correctly you join that site and download the schematic also clarify if there is a tuner or not then that would help in helping you out.

https://www.radiomuseum.org/r/rca_pre_amp_chassis_ch_rs_179.html
 
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The one in the feedback loop? It doesn't see all that much, a 100v cap should be plenty sufficient but higher won't hurt it.
 
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