Finally got around to doing some serious detective work on an 8802-00
here. As-received, the amp did some STRANGE things... audio, but at a low level, accompanied by a background "thump-thump-thump" motorboating noise.

Did some research, determined that there was a RESISTOR MISSING on each channel... in a very important location! In the feedback circuit/cathode circuit for the first input stage, there NEEDS to be a resistor to GROUND, to provide both voltage-dividing for the feedback circuit (the lack of such causing the low gain), and to provide a proper DC path for the cathode of the first tube stage (the lack thereof probably causing the motorboating. I've posted a pic of a similar schematic to the amp I'm working on, showing the position of the missing resistor... though the schematic shows EL84/6BQ5 output tubes (my 8802 has 6V6 outputs), the basic topology of the circuit is the same.

Got the 390 ohm resistors installed, and it SEEMED like it worked. Not quite. After being on for a while, it started up with hum and a strange squeal/Theremin type noise. Strange thing, was that you could wave your hand around the output tubes, and MODULATE the noise!

Turned out to be the bias cap... the one in parallel with the 180 ohm power resistor wired to the cathodes of all the 6V6 tubes. Frankly, it wasn't a great idea to have the bias cathode (basically, a FEEDBACK circuit of sorts, at least in theory) in the same can as the power supply filter caps... and it's DEFINITELY a problem with 40 year old components!

I decided, if I was going to replace it, I was going to go "whole-hog"... instead of using a standard 30uf electrolytic, I used three Panasonic 10uf mylar 100v caps, and a 3.3uf Panasonic poly cap (all available in the "surplus caps" section of Madisound... or at least something similar, at this point). Basically, I wired one of the three 10uf caps to the yellow-wire terminals (cathodes) of the three 6v6s farthest away from the power transformer (grounded the other ends of the caps to the available un-used terminals on the 6V6 tube sockets... each socket has 4 unused terminals, stock), and the 3.3uf poly directly across the 180 ohm bias resistor.

Boy, did THAT work! No more noise, and the power transformer runs at just above room temp now (it got considerably warm before)! There was definitely some oscillation before, but it's GONE NOW.

At that point, I decided to basically update or replace all the caps... put two series .01uf 630v Wondercaps (2 .01s in series is .005uf) in place of the .0047 coupling caps between the first-stage 6EU7 anodes and the grids of the "positive" 6V6s, and used .15uf GE 41L poly (850v, also available from Madisound) caps to replace the .047s between the output of the paraphase inverter (second 6EU7 stage) and the grid of the "negative" 6v6s. Also, I used GE (from Madisound)
10uf 400v polys, in parallel across each of the three power supply cap sections (raising them to 50, 50, and 40 uf respecively).

While in there, I decided to make some convenience and safety upgrades as well... power switch, fuse holder, 5-way binding posts for speaker outputs, and a pilot light (wired into the 6v heater supply).

Makes for a crowded house underneath the amp (you have to be careful to place the caps to leave adequate space between the new caps and the power supply power and bias resistors!), but it's easily do-able, without having to modify any of the existing layout (other than slightly re-bending a few resistor leads, to move stuff out of the way). Nice thing, was the way I did it (with the ABS plastic inserts for the switch, fuseholder and such) I was able to install ALL of the new parts WITHOUT cutting or drilling ANY new holes in the amp chassis! It's still FACTORY under there, so it could be put back STOCK, if desired, just by pulling out all my mods and reinstalling the Molex jack for the preamp/tuner and the terminal strip for the speaker outputs. I like "low-environmental-impact" mods, ESPECIALLY when they turn out THIS good!

In all- yep, it's better. Highs are really smooth, bass is more substantial (it seemed a little weak before, but now it's definitely THERE). As I have alluded to elsewhere, this series of Magnavox amps requires more input gain (voltage) than the AMP185/AMP175 series, but the payoff is a TOTAL LACK OF ANY BACKGROUND NOISE WHATSOEVER. I mean... I had the amp connected to 95 dB sensitivity speakers (Tannoy SRM12X), and even with my ear INSIDE THE CONE OF THE DUAL CONCENTRIC, RIGHT AT THE OUTLET OF THE TWEETER HORN, I could hear NO hiss, NO hum, NO NOISE WHATSOEVER. How many amps (regardles of price!) can claim THAT?

My only possible further plans, are maybe to make a set of wood side and front panels (a hardwood 1x2 board makes GREAT planks for this chassis, they're PERFECT in width!), possibly add a bottom panel (expanded metal) and feet. Maybe better input jacks, as well... but, as good as the amps sounds now, I dunno if I want to upset the apple cart!

Regards,
Gordon.

Great thread, Gordon.
I wish I had some electronics knowledge to be able to do something like that.

I, for one, would say yeah to some wood pannels.

Jorge

Yes, the panasonics are indeed great caps.
Great job, Gordon.
Tanx for sharing, and...have fun.

I, also, wish I knew more about the electronics. I have two local CLs that don't seem to get much activity (I was the only one to call on my KG4s), and I see Magnavox and other brand console units for sale fairly frequently, but don't know which are which, or what they're worth.

Take a peek at the electronics inside.

If you see tubes, probably worth a few bucks, provided you can get yourself a repairman.

Jorge

Nice work!

John