Rectifier for this RCA EL84 amp console pull?
- Thread starter BmWr75
- Start date
Yep, including the cathode resistors in the input stage. That's one part of the feedback loop, that many people don't always think about (including me, sometimes, until I sit down and go through things methodically)...
Regards,
Gordon.
You know what is rewarding?
Working on an old amp and it actually sounds good after some restoration work (unlike the frustrating prior 2 weeks spent on another amp).
Need to see what is involved to remove the tone controls next, and maybe the balance pot.
Working on an old amp and it actually sounds good after some restoration work (unlike the frustrating prior 2 weeks spent on another amp).
Need to see what is involved to remove the tone controls next, and maybe the balance pot.
atmasphere
Active Member
I know this is an old thread.
I came to this party a bit late. I've taken an interest in the RCA RS199 but its been hard to find. The RS193 has been a bit easier. Its not the same amp by any means but I figured it would make a good platform in which I could start by reproducing the RS199 circuit and proceed from there. I found one which showed up in the mail yesterday. It took an hour or so to completely strip it; its off to paint. The chassis was shedding a yellow green dust that suggested cadmium, which is really toxic so I figured I had to paint it if I was going to spend any time on it. Oddly, the tubes were shipped with it and tested very good on my tester.
Tomorrow I can start reassembly as the powdercoat finish is already done.
I'll retain the tube rectifier for now. My plan is to replace the Voltage amplifier position with a 12AX7 as that's a lot easier to source than a 6EU7. The new socket for it is the shielded type in ceramic. Not sorted just yet where the input jacks and speaker terminals will go. One thing interesting about the output transformers is they appear to be an ultra-linear style. But the screen tap is doing hum bucking duty in this case. My plan is to install a choke in the power supply since there's a fair bit of room for it, thus allowing the taps to be used for the screens of the EL84s. Otherwise the amp will have the same circuit as the RS199, (so no tone controls, although the volume and balance controls are staying for now) although I'll have to change the feedback slightly to deal with the reduced gain caused by UL operation.
The amp employs a loudness control rather than volume. Having worked on a lot of lower powered amps, one thing I've found is the loudness controls seem to make the amp sound muddy. So the volume control will not have the loudness bits.
I came to this party a bit late. I've taken an interest in the RCA RS199 but its been hard to find. The RS193 has been a bit easier. Its not the same amp by any means but I figured it would make a good platform in which I could start by reproducing the RS199 circuit and proceed from there. I found one which showed up in the mail yesterday. It took an hour or so to completely strip it; its off to paint. The chassis was shedding a yellow green dust that suggested cadmium, which is really toxic so I figured I had to paint it if I was going to spend any time on it. Oddly, the tubes were shipped with it and tested very good on my tester.
Tomorrow I can start reassembly as the powdercoat finish is already done.
I'll retain the tube rectifier for now. My plan is to replace the Voltage amplifier position with a 12AX7 as that's a lot easier to source than a 6EU7. The new socket for it is the shielded type in ceramic. Not sorted just yet where the input jacks and speaker terminals will go. One thing interesting about the output transformers is they appear to be an ultra-linear style. But the screen tap is doing hum bucking duty in this case. My plan is to install a choke in the power supply since there's a fair bit of room for it, thus allowing the taps to be used for the screens of the EL84s. Otherwise the amp will have the same circuit as the RS199, (so no tone controls, although the volume and balance controls are staying for now) although I'll have to change the feedback slightly to deal with the reduced gain caused by UL operation.
The amp employs a loudness control rather than volume. Having worked on a lot of lower powered amps, one thing I've found is the loudness controls seem to make the amp sound muddy. So the volume control will not have the loudness bits.
All of the RS-193 use the same "Hum bucking" winding on each output transformer. This is not an Ultralinear, "tapped screen" type of connection. The hum buck winding is like a "tail winding" connected after the plate is energized. In genuine UL windings, the screen is energized by the high voltage before the plate.
Furthermore, these sweet sounding amps do not have any feedback loop.
If you need more gain, consider losing or changing the tone control circuitry. You could also consider adding another gain stage per channel.
The original power transformer supplied some power to an AM/FM tuner in some consoles.
So, there is certainly no danger in adding one more gain stage, per channel.
Furthermore, these sweet sounding amps do not have any feedback loop.
If you need more gain, consider losing or changing the tone control circuitry. You could also consider adding another gain stage per channel.
The original power transformer supplied some power to an AM/FM tuner in some consoles.
So, there is certainly no danger in adding one more gain stage, per channel.
you could in theory flip the leads and feed B+ to the end to get something amounting to a screen tap but I suspect its going to be pretty close to the end of the primary. Depending which way you wire it, it'll end up very nearly a pentode or very nearly a triode rather than a UL tap.
Also the impedance may end up somewhere not so helpful.
Also the impedance may end up somewhere not so helpful.
I'd change any disc caps hanging off those pots. Bad ones are notorious for causing all kinds of volume and balance issues. It's a cheap try, anyone.
Good luck!
Problem ended up mainly being a bad resistor. See Post 23.I'd change any disc caps hanging off those pots. Bad ones are notorious for causing all kinds of volume and balance issues. It's a cheap try, anyone.
Good luck!
Good deal!
atmasphere
Active Member
I finished up my RS193:
I hogged out the chassis at the top right to allow an IEC connector- the kind with a fuse inside.
I hogged out the chassis at the top right to allow an IEC connector- the kind with a fuse inside.
Nice work!!
Dswankey
Lunatic Member
Excellent!! how does it sound? what speakers are you using with it?
atmasphere
Active Member
It seems to sound fine. I think I like the monoblock SEP EL84 amps I built a little better though. That's likely due to the impedance of the speakers. The RS193 is supposed to drive a 4 Ohm load and the shop speakers are 16 Ohms. I put it on a small set of 8 Ohm JBLs though and it did much better.
Yeah- in retrospect I should have pulled the end shells of the power transformer and had them painted too.
Lavane
Lunatic Member
Eh, some blue painters tape, some news paper, a razor blade, and you can isolate that power trany and paint it in place. I've done that and it works well as long as you take your time with the tape around the trany.
I even managed without the added risk of the razor blade, but used a little bit of sand paper. Just the bells and mounting tabs, left the rust on the laminations.
Looked much better after.
Transformer Rust — Friend or Foe?
I heard that rust is bad for my transformers. Rusty transformers will also hurt my tone, so I must remove the rust or replace the transformers to make my amp operate properly and extend its life.
www.mercurymagnetics.com
I would NOT sand off too much surface rust from the laminations. That may increase the eddy currents discussed in the article.
An old tech used say it may look good, but it's bad.
An old tech used say it may look good, but it's bad.
I don't touch the surface oxidation at all.
None has overheated or had other funky issues.