Schematic of ID of this 6V6/6U8 stereo amp

[Sparky OR]

Looking for schematic on this console amp, no markings, think it is Magnavox. Chassis has enough room for a little larger OPT's so I'd like to tweak it a little.

 

[Tom Bavis]

There's a Stromberg amp with those tubes on my web site - might be close.

http://www.audiophool.cjb.net/MadeInRoch.html

 

[Sparky OR]

Stromberg-Carlson P-P 6V6 amp

Thanks for the ID and schematic. Did you come up with diagram on your own? Just wondered about the feedback notations, if that was your addition to factory. Were you trying to cut back some of the higher frequency? My own amp could use some new tubes, but doesn't seem to have the power of a P-P EL84 amp I built using Dyna ST-35 iron. I'm going to try better OPT's and play with a little less feedback. I do like the tagboard component board that this chassis uses over some of the rats nest wiring found on a lot of other less expensive similar amps.

 

[Tom Bavis]

The feedback was originally set up for considerable bass boost, as the simple passive tone control that was used with it could only CUT bass. So if you're using it alone, the changes I show will flatten out the frequency response.

 

[Sparky OR]

What program do you use to generate schematics such as this. I have downloaded Eagle, but am not up to speed on it yet, maybe never, my brain is full...

 

[Tom Bavis]

I did this one in Visio, not the best for the purpose, but I have it at work, and did it on my lunch hour.

This does look exatly like the one I sold on EBay - I put RCA jacks into existing holes and the 6U8 is not original - I am told it had 7199s originally. I'll bet the previous owner couldn't find any and rewired for 6U8s. 6V6s were gone too when I got it.

I put a matched set of 6V6s in it and made the other changes listed. If you change the 6U8s, check the voltages - direct coupling in the first two stages. I might have used 6BL8s (ECF80)myself, more linear, and they're same pinout as 6U8. But don't know how they'd bias in this circuit, might have to change cathode or screen resistor.

 

[Sparky OR]

So I was comparing this amp against your schematic, and it looks identical to your schematic, in regards to the feedback. Maybe the last owner changed it to your schematic? Anyway, I'm a little fuzzy understanding series capacitor in line with the feedback resistor, as well as paralleled. I'm used to seeing the feedback resistor, sometimes with no parrallel cap, usually with,...wouldn't having one in series effectively cut off all lower end feedback which would boost bass?

 

[Sparky OR]

This must be the unit you sold on Ebay....

 

[Tom Bavis]

Series cap WAS used to boost bass - increased it rather than just shorting it, so it didn't affect DC bis - one change instead of two... the 56 pF cap was need to keep it stable when open circuited - Stromberg left it out since the speakers are never disconnected in a console?