Magnavox 8802 amp restoration and mods

pjrengineer

pjrengineer

Simpler times
Subscriber
Hi

Just wanted to post a brief update to my project I am starting. Here are photos of the stock amp with some mods done. Then another after stripping. Finally I have the sockets installed as well as the choke. The labels were not by me but I’ll leave them. I am using Dave Gillespies circuit provided in the rx for magnavox thread.
 

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Nice work so far. :lurk:
I keep an eye out for a 86XX amp or console locally.
I've had a couple 175's and still have my modded 9303.
One of the 175's I turned into a 6L6 amp. The power transformer barely had enough ompf to do it, but it worked out. I've passed that one along and it's still going strong.
 
Team:

I found a couple issues

Power switch was wired incorrectly. Fixed.

Amp sounds great and adjusts fine.

Amp has hum in right channel nearest to choke.

Read another thread that suggested putting the choke on grommets. That reduced the hum but not to my full satisfaction.

Then I removed the choke and wired a 120 ohm 10w resistor. Hum is louder with resistor.

There is a faint mechanical hum coming from the choke and some quiet hum through the speakers. But when I move to more efficient speakers I suspect it will be louder.

Will do a through voltage testing. I already swapped tubes.

The choke I used was a triad c17x. Is this choke not rated high enough? https://www.digikey.com/en/products/detail/triad-magnetics/C-17X/4915189

I really like the sound of this amp after the mods and hope I can resolve this issue.
 
Things I’m going to try

Remove the choke and connect with jumper (not sure what this helps)

Check grounding of the ps caps

Re-check ripple of first filter cap. Was reading 6-7 volts AC. This sounds HIGH. Could be a new jj can cap is bad but I hope it’s not. I guess I can take it out of circuit and check for leakage next with my sprague to-5.
 
Looking at your wiring layout you don’t want the HV CT going to chassis ground. It should connect directly to the cap common with a wire from there back to ground. This will keep the ripple current at the greatest point in the circuit isolated to the cap and out of the chassis.
 
nerdorama said:
Looking at your wiring layout you don’t want the HV CT going to chassis ground. It should connect directly to the cap common with a wire from there back to ground. This will keep the ripple current at the greatest point in the circuit isolated to the cap and out of the chassis.
Click to expand...
:thumbsup:
 
You may also be radiating an EM field from the choke into the tube nearest the choke. Since the signal isn't the same in both tubes it won't cancel out. Rotating the choke relative to the other parts on the chassis may shut it up. As a quick and dirty check, get a piece of steel, clip it to ground, and stick it between the choke and the tube to see if the hum level changes any.

Removing the choke will add more noise to the power supply, which is unlikely to make anything quieter.
 
gadget73 said:
You may also be radiating an EM field from the choke into the tube nearest the choke. Since the signal isn't the same in both tubes it won't cancel out. Rotating the choke relative to the other parts on the chassis may shut it up. As a quick and dirty check, get a piece of steel, clip it to ground, and stick it between the choke and the tube to see if the hum level changes any.

Removing the choke will add more noise to the power supply, which is unlikely to make anything quieter.
Click to expand...
Unscrewing the choke from the chassis and rotating it and picking it up completely off the chassis doesn’t change the level of noise. Trying the other items suggested next.
 
nerdorama said:
Looking at your wiring layout you don’t want the HV CT going to chassis ground. It should connect directly to the cap common with a wire from there back to ground. This will keep the ripple current at the greatest point in the circuit isolated to the cap and out of the chassis.
Click to expand...
I am going to try it but I suspect it’s something else because the stock configuration had red-yel going directly to chassis ground and it was quiet. Thanks I’ll try it anyways.
 
Doh! I didn’t try the piece of steel trick. Tomorrow’s another day. Been super busy or id hope to solve this by now. One things for certain that using higher wattage parts takes more space and the chassis is tight due to the increased parts count.

Thanks for the teaching moments.
 
pjrengineer said:
Doh! I didn’t try the piece of steel trick. Tomorrow’s another day. Been super busy or id hope to solve this by now. One things for certain that using higher wattage parts takes more space and the chassis is tight due to the increased parts count.

Thanks for the teaching moments.
Click to expand...
The smaller chassis can get a little cramped. I built a different power supply in my 9303. I even added a second choke. The factory choke I mounted underneath, and put the second one up top with three separate filter caps.
I ditched the rectifier tube for a SS rectifier.
 

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Lavane said:
The smaller chassis can get a little cramped. I built a different power supply in my 9303. I even added a second choke. The factory choke I mounted underneath, and put the second one up top with three separate filter caps.
I ditched the rectifier tube for a SS rectifier.
Click to expand...
Very nice. I’m almost there! I have the badge for my wood base too and that will be the icing on the cake. You have quite a bit of capacitance in that PS!
 
pjrengineer said:
Very nice. I’m almost there! I have the badge for my wood base too and that will be the icing on the cake. You have quite a bit of capacitance in that PS!
Click to expand...
Ya, it's one of Batradio's designs. Mark modified a lot of Magnavox amps.
this amp has split cathode too. A separate cathode resistor and bypass cap for each pair of tubes instead of the factory single resistor for all four power tubes.
 
Update. Problem resolved. Amp maybe the quietest amp I’ve ever built now. My mistake was that I grounded the input tube cathode capacitor to the power supply capacitor ground and it was causing dc to leak through the channel.

I won’t be posting pics anytime soon until I get the wiring cleaned up.

listening tests soon.
 
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