Magnavox 169aa help!

[Spansky]

Hi all! New here. Working on tube stuff at a low-level for a long time.

Rebuilt a couple guitar amps, converted a number of bogen pa's, that kind of thing. Mostly guitar oriented, some hifi.

This has me scratching my head, as I've not dealt with pentode input stages much.

I've a 1958 Magnavox 169aa pp 6v6 amp with a 6an8 input/phase inverter. Great cabinet and speakers. Lp deck is shot, looking to run the unit as a streaming amp so I need something close to line input.

I was about to start re-capping, then realized I should consult for any ideas on the following:

Decrease pentode input gain?

Retube to something else? I think I've got some 6v dual triodes, 6eu7 maybe?

Anything else I'm not thinking about? (Not an expert by any means!)

Basically the amp is, I think, going to have wayyyyy too much gain and wrong input impedance, and since I'm a guitar dummy I'm lost in line level/pentode input/actual linearity stuff.

Plus I plugged an RCA into it from my MD player and holy cow, lots of noise. Really just noisy all around though, I did some voltage testing and preamp values were way off.

I have a schematic which I looked over with the unit pulled apart, looks spot on.


Any help? It would be sincerely appreciated.

 

[TriodeLuvr]

The gain should be about right. There's quite a bit of attenuation due to the negative feedback loop. The turntable must have had a high output ceramic cartridge.

Jack

 

[Spansky]

Thanks Jack! I'm sitting in front of it and it's a Friday so I'll go ahead and replace some capacitors and check resistor values.

The voltages I got were not as specced, we'll see if I can get it back in line. I'll update later.

Thank you again for your insight!

 

[kward]

The input impedance is trivial to change if needed by simply changing R6 grid resistor on the pentode grid. That has consequences though as it interacts with C8. So changing R6 probably also means changing C8.

I’d leave the frontend gain alone if you can. It’s not as easy as it may seem to just reduce the pentode gain because you would also need to adjust the resistor divider on the input into the triode stage to re-balance the stage for push pull drive.

All that could be done certainly but I don’t think you’ll need to given your desire to convert the amp into a stand-alone unit and drive it from a line source.

Check the plate resistors of the pentode and triode stages for accurate resistance values and also for noise. If they are carbon composition resistors they have a higher tendency to be noisy and to drift. May need to replace them. You may also have a noisy 6AN8.