• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

RCA hum cancelling OPT feedback

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
RCA used a hum cancelling OPT on many of their SE consoles. It looks like an UL with 3 primary connections but rather than connecting B+ to one end it is connected to the "middle" and the plate and screen connected to the ends.

Would this also not inject some FB (pos or neg?) Into the screen?

Also in the last one I got both OPT screen supplies were connected to a shared dropping resistor which feeds both screens. Wouldn't this introduce some cross talk?
 
Last edited:
RCA used a hum cancelling OPT on many of their SE consoles. It looks like an UL with 3 primary connections but rather than connecting B+ to one end it is connected to the "middle" and the plate and screen connected to the ends.

Would this also not inject some FB (pos or neg?) Into the screen?

Also in the last one I got both OPT screen supplies were connected to a shared dropping resistor which feeds both screens. Wouldn't this introduce some cross talk?
Look closer, the "middle" and the screen have both rather big capacitors to ground.
So where do you see FB :D
Mona
 

PRR

Member
Joined 2003
Paid Member
> Can you post the schematic? That connection should give positive feedback to the screen.

I got one in the garage. Come on over!

I attach a tease.

Assuming they did not bother to change wire gauge at the tap, the tap is 5%-6% along. True, that would be positive feedback, but so little that it hardly matters. And little magic if you re-rig to be NFB. But as said, there's fat caps too. New, it let RCA use slightly less fat caps (a cheap trick). Old, it makes the radio's bad-cap HUMM a little squirrelier, but the fix has nothing to do with the buzz-tap.
 
Last edited by a moderator:
It's a scheme to balance out the output tube's plate resistance against R15 (the 1200 ohm resistor) that loads the other end of the primary winding. The tap is where the main B+ connects, and has some ripple hum. If we assume the same wire used in both sections of the primary, the turns ratio between them is about 17 to 1. The spec sheet on the 50C5 says its plate resistance is 10K. The filter resistor (R15) that is used to get B+ for the output tube's screen and the rest of the radio will act, along with the second portion of the filter cap C1B, as a counterbalance to the output tube's plate resistance. Crudely put, the turns ratio divided by the plate resistance should equal 1 divided by R15 (this 1 is the other part of the turns ratio). Here this doesn't quite come out right, so maybe the resistance measurements in the diagram are off, and/or there may be a complication due to some ripple still present on the screen grid, and/or some ripple getting into the audio driver stage. Still, you can get ripple reduction as heard at the speaker down around 6dB or so. A minor downside is that you'd waste some audio output power, but that would be less than a dB's worth...
 
Last edited:
Isn't it just the chance to add a screen/preamp/radio circuitry supply choke, and to counter the SE bias DC current to allow less core for the same audio low frequency performance before core saturation becomes noticeable?

I don't think there was any inherent transformer related hum cancellation. Given the half wave rectifier, they had to alleviate hum/ripple a bit, but at minimum cost.
 
Last edited:
Ex-Moderator
Joined 2011
An example of the "hum cancelling" OPT wiring (from a Magnavox console):

Screenshot - 12_29_2016 , 6_34_40 AM.png
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.