Magnavox 175 - different feedback in each channel

[toxic ingestion]

Hello,

A friend and I recently recapped a Magnavox 175 and also used some nicely matched 6V6s and 12AX7s that we happen to have. The amplifier sounds good. However, in looking at the schematic, feedback on Channel 1 is through a 2.2K ohm resistor (1,000 pF cap in parallel across the resistor), while on Channel 2 feedback is through a 5.6K ohm resistor with a 470 pF cap across the resistor.

Might anyone be able to provide some insight as to why this is done? Wouldn't each channel have different gain, output impedance, etc?

Thanks,
Steve

 

[Sorento]

channel 2 has higher gain thru less feedback to give the balance pot some headroom in both directions;
in other words: signal for ch 2 is first attenuated with balance pot and then amplified roughly 2x more than ch 1 to match ch1 in volume when balance pot is in the middle position;
perhaps the designer wanted to avoid a ganged dual pot for balance;
and yes, output impedance and etc is different ... but you say the amp sounds good, so obviously you can't hear the difference ...

 

[Eli Duttman]

That's not a balance control, as we understand things. It's a trim pot. intended to bring everything from the tuner/preamp into alignment. Remember, the power amp would have been "buried" inside the console and the adjustment inaccessible to the end user. Disabling that control and making the 2 channels "identical" is (IMO) a very reasonable tweak.

That model, along with some similar EL84 based versions, is just about as good as it gets in a console amp. Give "Maggotbox" full marks for using a CLC PSU filter. Definitely not "Bargain Basement".

If 1 of those amps came into my hands, I'd convert it to a triode mode O/P tube "El Cheapo", just to make sure full bass extension was available. Switch the 5U4 to a 5AR4/GZ34 to get a few more B+ volts. The "spare" filament winding would be voltage multiplied to derive the B- rail.

 

[toxic ingestion]

Thanks to both of you for very helpful responses. Indeed, I think getting rid of the balance and putting in identical feedback circuits may be the way to go, just from a purest perspective. I certainly agree that I may not be able to hear a difference.

Steve