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Maggie 9303: The sound of silence

Moonwatcher

Moonwatcher

Active Member
To anyone familiar with these console-pull amps: My restored, recapped (not by me) Magnavox 9300 series (9303 I think) suddenly has no sound coming from either channel after five years of flawless performance. No hum. No buzz. Only silence.

I know very little about how these work and have little background in the circuitry. The fuse looks fine. I noticed that one of the 6EU7 tubes was not lighting up, so I used the opportunity to buy a couple of Mullards. But replacing the 6EU7s made no difference.

I notice that the rectifier tube (the big, tall one, right?) does not seem to be lighting up, so would replacing that be the next step? I'm afraid to leave the amp on too long, but the transformers don't seem to be warming up either.

The four 6BQ5 tubes are lighting up normally.

What else that does not require extensive knowledge or special testing equipment could I do to figure out what's wrong? There was a smart tech nearby who could work on these, but he is gone.

Thanks for any suggestions!
 
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Dead rectifier makes for no sound. They die sometimes. If it didn't go out in a big ball of sparks or any other scary visible effect its possible that after 60 years of service it has finally just quit.

were it mine I'd stick another rectifier in and see what it does.
 
Great, gadget73, thanks. That is at least something I could do! I wonder if there is a preferred make of rectifier. I think it's a 5U4GB.
 
Moonwatcher said:
Great, gadget73, thanks. That is at least something I could do! I wonder if there is a preferred make of rectifier. I think it's a 5U4GB.
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Yes 5U4GB is more then likely what you currently have.

https://www.radiomuseum.org/tubes/tube_5u4gb.html

As to a preferred make I wouldn't concern myself with that as much as that you'll be purchasing from a reliable seller. The good thing is that they are readily available and not expensive.
 
^ They are and Magnavox used different manufactures too so whatever you wind up getting should be fine as long as it tests well
 
Just for **** & grins, take the original rectifier tube and wiggle it a little in its socket and see if it starts to work. Could have a bad socket or loose pins due to heat over the years. Those things are so rugged there needs to be a catastrophic event to really damage them. This is just a quick check without a tube tester and you won't hurt your amp powered up for testing purposes. Without the rectifier tube, there is no B+ to the plates of the tubes so it will just sit there and idle with the tubes glowing.

Those 6EU7 types are really hard to see glowing but I bet they are both working. If one went out, you would only lose sound from one channel not both. After about 30 seconds, you should see the cathode in the center glow a dull orange.......with the lights out.

The only transformer that would heat up any is the black power transformer and only after being on for about an hour.

My bet is on rectifier tube and / or socket.
 
Thanks, century tek. So just for **** & grins I did remove the original rectifier and wiggled it in the socket, but nothing. It does not light up, although I'm not sure it's supposed to. So now I'm awaiting a $7 NOS Sylvania rectifier. I'll let everybody know how that turns out . . .
 
thats why I like the 5U4, they're available and cheap.

if you have an ohm meter and want a final sanity confirmation, measure from pin 2 to 8 to see if you get anything. A good filament will be a couple of ohms, a bad one will read open circuit.
 
gadget73 said:
thats why I like the 5U4, they're available and cheap.

if you have an ohm meter and want a final sanity confirmation, measure from pin 2 to 8 to see if you get anything. A good filament will be a couple of ohms, a bad one will read open circuit.
Click to expand...
Would a 5AR4 work also? The 5U4 used isn't having to power the tuner anymore. There would be a softer start with the 5AR4.
 
I put my 93xx back in the system last night because I hadn’t heard it in months. Feeling that it didn’t have quite enough juice for the lower efficiency monitors, I put a 5AR4 in. The low end and power seemed to be improved. A year or so ago, I checked voltages and remember more B+ so the amp should make more power. After a private conversation with Gadget at the time it seems like it will be safe.
 
The voltage goes up about 40v if you use a 5AR4. Not a great idea unless you confirm the output tubes are not drawing too much current. I don't honestly know how that shakes out in a stock amp. I do know the driver section ends up with a fair bit more voltage than it should have unless you change one of the power supply resistors.

Or re-work it per Dave's schematic which calls for a 5AR4 for the extra voltage and power that results in. Mine runs a 5AR4.
 
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