Maxamillion
Super Member
I have an emission tester but that's not sufficient to properly match them. Plus, it bothers me that they aren't the same brand. I certainly won't throw then away, but maybe add them to my collection for future use.
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RCA Console Pull Project
- Thread starter Maxamillion
- Start date
I have an emission tester but that's not sufficient to properly match them. Plus, it bothers me that they aren't the same brand. I certainly won't throw then away, but maybe add them to my collection for future use.
Maxamillion
Super Member
Yeah, I couldn't picture my mom and dad (RIP) buying a matched quad for their console that they used once a year to play Christmas albums. If my dad could make it work with a paperclip and a rubber band he would have.
I however am a different breed. I'll use the mismatched set for rough testing then throw in a new matched quad when I'm sure it works.
Maxamillion
Super Member
By the way, I sent some sine and square waves through the OPTs and they came out unscathed on my scope so the OPTs seem to pass a signal nicely.
Full speed ahead!
Full speed ahead!
Maxamillion
Super Member
In the parts accumulation phase now. Have orders in with AES and Mouser for various bits. Will replace the .27uF wax caps with .33uF Cornell Dubilier 940 series - very good and relatively cheap as audio caps go.
Now to start on the cleanup and drilling out the rivets on the caps and wafer sockets...
Now to start on the cleanup and drilling out the rivets on the caps and wafer sockets...
Here in lies the beauty of console amps, my guess is in most cases they weren't used on a daily repetitive basis thus the tubes weren't taxed too much.
Of all the console tube amps I've come across most if not all the tubes in them tested fine.
Maxamillion
Super Member
Started on the chassis cleanup today. After about one hour with brasso, a rag and a dremel with a polishing wheel I have cleaned up a corner of the chassis. Quite the difference, but slow going nonetheless...
It's shiny but there is some pitting on the surface. Once it's cleaned up and all the new holes are cut I may end up painting it with a hammered finish. I tell myself I'm only sticking it in my shop but I can't do a halfway job anyway, I'm too OCD for that.
It's shiny but there is some pitting on the surface. Once it's cleaned up and all the new holes are cut I may end up painting it with a hammered finish. I tell myself I'm only sticking it in my shop but I can't do a halfway job anyway, I'm too OCD for that.
Maxamillion
Super Member
Looking at either of the schematics above, there is a voltage divider (either 390K/470K or 150K/470K) on the input. The series resistor seems very high for a grid stopper, and I know the 470K is the grid resistor. Since I'm putting a 100K volume pot on the input, my inclination is to remove both big resistors and maybe just put a 1K stopper on the input tube (which will be a 12ax7).
Any reason not to? None that I can think of...
Any reason not to? None that I can think of...
If adding a volume pot, that's probably what I’d do as well. But also I’d add a 470k or 1M resistor from wiper to ground. Keeps a ground reference on the grid if the wiper ever disconnects.
Also with a 100k pot and 12AX7 stage you should verify the Miller capacitance isn’t going to eat into the HF response.
Also with a 100k pot and 12AX7 stage you should verify the Miller capacitance isn’t going to eat into the HF response.
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Maxamillion
Super Member
That's one of the reasons I want to get rid of the 390K resistor in series with the grid; it seems that would be a recipe for HF rolloff. Interestingly, the guy in the video on page 1 of this thread didn't seem to have an issue with HF rolloff with the 390K in series with the 12ax7, as he left it in place...
Maxamillion
Super Member
An interesting find as I disassemble the chassis for cleaning...
There is a 39R 2W CC resistor in series with the first 80uF can cap. This doesn't appear on either the 177H or 177J schematics, but is clearly to limit inrush current to the 80uF cap, which is twice the recommended max value of 40uF for the 5AS4.
The earlier 177A model used a 40uF cap, so it didn't need the limiting resistor; it seems to have been added when the capacitance was increased but apparently RCA forgot to add it to the schematics.
That solves the mystery of how they got away with twice the max capacitance without destroying rectifiers.
There is a 39R 2W CC resistor in series with the first 80uF can cap. This doesn't appear on either the 177H or 177J schematics, but is clearly to limit inrush current to the 80uF cap, which is twice the recommended max value of 40uF for the 5AS4.
The earlier 177A model used a 40uF cap, so it didn't need the limiting resistor; it seems to have been added when the capacitance was increased but apparently RCA forgot to add it to the schematics.
That solves the mystery of how they got away with twice the max capacitance without destroying rectifiers.
That resistor may need to be there for proper stability. Before making changes I'd want to run some square waves through it to see what it looks like. Possible the big resistor is there to tame overshoot issues.
The lower gain of a 6CG7 vs say a 12ax7 makes that not quite as bad as it seems on the surface.
The lower gain of a 6CG7 vs say a 12ax7 makes that not quite as bad as it seems on the surface.
FlaCharlie
Super Member
I've never used a 5AS4 so I'm not familiar with them, but I think they're very similar to a 5U4GB.
Here's a data sheet with charts that can be used to determine if the actual (measured) operating conditions of the amp are within the limits of the tube.
If your actual operating conditions pass each of the 3 tests described in the charts you're good.
https://tubedata.edebris.com/sheets/127/5/5AS4A.pdf
The Sams I have are too large to upload directly, and I see you were able to find it, but I noted on mine that it contains errors. Not sure exactly what they are, I probably added the note based on comments I read online somewhere.
The Beitmans schematic in post #9 supposedly does not have any errors.
Maxamillion
Super Member
Fortunately that's easy enough to test. Once I get things put back together I can experiment with that value and see what is really needed.
In the RCA tube data books, under 5AS4 it just says "See 5U4GB".
Maxamillion
Super Member
My project is suffering from serious scope creep. This went from cleaning up an old amp for my shop, to maybe doing a little more, to a complete stripdown and rebuild!
I was just looking at the chassis planning in what order to replace the CC resistors, wax caps, can caps, etc and also paint the chassis when I decided to pull everything out of the chassis and do a total rebuild.
I've got most of the gunk off the chassis, next is to cut the holes for the IEC inlet, switch, binding posts, and rca jacks, then prime and paint. This is going to take a little time...
I was just looking at the chassis planning in what order to replace the CC resistors, wax caps, can caps, etc and also paint the chassis when I decided to pull everything out of the chassis and do a total rebuild.
I've got most of the gunk off the chassis, next is to cut the holes for the IEC inlet, switch, binding posts, and rca jacks, then prime and paint. This is going to take a little time...
Nice project!
Not exactly the same amp but this is the RCA RS-183a that I bought from Squidward about 10 years ago. This is the three channel version of your 177 but has recently been converted to two channel with the larger output transformer on both channels (more recent photo in post #66 later in this thread).
You're going to be so pleased with yours when it's finished!
Not exactly the same amp but this is the RCA RS-183a that I bought from Squidward about 10 years ago. This is the three channel version of your 177 but has recently been converted to two channel with the larger output transformer on both channels (more recent photo in post #66 later in this thread).
You're going to be so pleased with yours when it's finished!
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