Screens tied to Secondaries? (Webcor design)

[nashvillebill]

I'm converting an old Webcor reel-to-reel to guitar amp duty. But I'm rather curious how/why they've tied the cathodes (I know, I fat-fingered and wrote screens in the title) to the OT secondaries as shown (I cleaned up the original schematic to just show the bare-bones stuff I'm keeping)

I sure am curious about this design approach,any thoughts would be welcome! Seems like a version of ultra-linear mode, or am I out in left field again?

[loudthud]

That's a form of local negative feedback. You see it on some MacIntosh amps. There has to be a cathode resistor somewhere though. Usually the 4 ohm tap is grounded or connected to the cathode bias resistor and the cathodes connect to the common and 16 ohm taps. Yes, the 4 ohm tap is the center tap, do the math.

[Enzo]

I think you are missing a ground connection in the center of the speaker winding.

No, not ultralinear, this is just another way to introduce negative feedback.

[Enzo]

OOps, LT beat me to it.


Some amps even had a separate secondary winding just for feedback in that manner

[J M Fahey]

Yes, it's cathode feedback and you definitely need a center tap plus a cathode resistor (which may be bypassed) to ground *or* send it straight to ground and apply fixed bias, your choice.
The drawing looks edited, part of the transformer turns look "sliced" to the right.

[nashvillebill]

Yes, I did edit part of the drawing, there was one other tap on that part of the secondary, but with the convoluted switching arrangement, it did not appear to be connected when the internal speaker was selected.

Here's more of the schematic, hope this helps.




edit....ah crap, yep I see the red tap in the center is indeed grounded all the time with a 820 resistor and bypass cap...I must've spent too long at the soldering iron when I was cutting up the schematic...

[Enzo]

So next time, don't bother to edit the drawing, we can read them when they are whole.

[J M Fahey]

-"Hey, Son ! why did you nip 6 pins out of this poor 12AX7 ?"
-"Weeeellllll, I just wanted to keep it simple, that's why !!! .... isn't it a triode after all?"





With all due respect and remembering : "been there ... done that " ..... (or worse)

[imaradiostar]

Without really study the schematic (or seeing all of it) it appears that they're using the power amp as the tape record bias oscillator or electric eye driver or somesuch thing in record mode. The 820 ohm resistor is the cathode bias resistor- note that the whole secondary is floating from that point. It's exactly as stated above- common, 4 and 16 ohm taps are used.

It reminds me of my conn strobotuner. It's a cute little 6aq5 push pull amp...driving a motor!

It's common practice in some hifi circles to connect the cathode bias cap's ground lead not to ground but to one of the output taps for negative feedback. I've been meaning to try it on a small EL34 amp I built but haven't had time.

Jamie

[nashvillebill]

Thanks guys, for both catching my mistake (cutting out the resistor) as well as explaining it.

Incidentally, I had edited the schematic merely for my own use, as I was guting the amp and cutting out all the pieces parts I wasn't going to re-use. But I got carried away with the white-out I guess.