Magnavox 9300 build thread

[ArchieType]

Just picked up a Magnavox 9304-20 for 10 bucks at a garage sale for 10 bucks. It was attached to a pretty nice mid-century modern style console. I was on my bike, puttering around with the lovely girlfriend, and nearly passed it up, since I couldn't see what was inside (no tools on me), but decided to take a chance and buy it.
Picked it up later in the day and was delighted to find a pair of 6EU7s and a quad of EL84s sitting proudly atop a discrete power amp chassis.
Did some research.
Planning to do the Dave Gillespie mods he described in his "More fun with Magnavox" thread. I'll be posting more as I continue.
Here are the goods:

 

[ArchieType]

Here's a gut shot taken after I pulled out the wires running from the radio and preamp plug
I jumpered the power cord to the PT and hooked it up to my phone/spotify. Sounded nice and warm as is, but one channel has a LOT less volume than the other.
I perused the schematic pretty hard for a while - trying to figure out if there was something asymmetrical about it. There's no balance control in mine, so I stuck a 750 ohm resistor in its place which solved the problem temporarily sorta (ha, not really).
I tested all the Resistors in place. All good. Hmmmm.
Verified everything where it's supposed to be. Yep.

Finally, I took a look at the top end,
just for the heck of it.
Anything wrong here? Um. Yeah.

Hint: rhymes with dead fate.

Ordering caps and other parts this weekend for the Magdallespie.
This will be my first stereo build. I'm a fairly experienced guitar amp guy so there are holes in my basic knowledge.
Anyone recommend some binding posts - cheap?

 

[century tek]

Pull the knob off of the bass control and see if the shaft is hollow. If it is, that is the balance control that you use with a small flat head screw driver.

 

[ArchieType]

Pull the knob off of the bass control and see if the shaft is hollow. If it is, that is the balance control that you use with a small flat head screw driver.

BZZZZZT! Thanks for playing, but that is incorrect. There are no tone controls on a 9300 Maggie power amp. That thing is a hum pot, and it does essentially nothing.
Here's the schematic because I see I forgot to post it above.
Next Contestant? (answer rhymes with "bed-hating").

 

[gadget73]

With the stock preamp, you'd have a balance and tone and all that. If you are not using this with the stock arrangement the balance circuit is not complete. No big deal if you're doing Dave's mods since it fixes the feedback loop, otherwise you will need the two 390 ohm resistors to sub for the balance pot. It requires two resistors to ground.

 

[ArchieType]

With the stock preamp, you'd have a balance and tone and all that. If you are not using this with the stock arrangement the balance circuit is not complete. No big deal if you're doing Dave's mods since it fixes the feedback loop, otherwise you will need the two 390 ohm resistors to sub for the balance pot. It requires two resistors to ground.

Thanks Gadget. I missed that detail in Dave's thread. I'll put those on temporarily, till I do the Gillespie mod.
BTW, The answer I was looking for was red-plating.
If you take look at the Pic above, you'll notice one of the 6BQ5s is doing a pretty good impression of a branding iron. Is that caused by the missing balance resistor? I figured it was a bad coupling capacitor..
But as I said, I'm guitar amp guy - this is my first stereo project.

 

[gadget73]

I saw it but no thats not related to the balance resistor. Caps are probably bad, but if you're rebuilding it it gets new parts there anyway. Clip the white coupling cap and see if it stops. If it does, the cap is bad.

 

[century tek]

BZZZZZT! Thanks for playing, but that is incorrect. There are no tone controls on a 9300 Maggie power amp. That thing is a hum pot, and it does essentially nothing.
Here's the schematic because I see I forgot to post it above.
Next Contestant? (answer rhymes with "bed-hating").

You're fairly new here so I will let that one go!

I know all about these amplifiers but I forgot to mention that I was talking about the speaker balance on the tuner preamp which looks like one I had years ago. Some of those have the balance control hidden behind the bass knob. The balance on the amp itself is a hum balance not a speaker balance.

Red plating is either the tube or the associated coupling capacitor. Does the tube red plate in other sockets? If not, then its most likely that .047 capacitor going to that tube. You should probably replace all four but the disc capacitors might be fine.

 

[EastPoint]

Those white caps are terrible. Disks are fine but you can upgrade to something nicer.

 

[ArchieType]

You're fairly new here so I will let that one go!

I know all about these amplifiers but I forgot to mention that I was talking about the speaker balance on the tuner preamp which looks like one I had years ago. Some of those have the balance control hidden behind the bass knob. The balance on the amp itself is a hum balance not a speaker balance.

Red plating is either the tube or the associated coupling capacitor. Does the tube red plate in other sockets? If not, then its most likely that .047 capacitor going to that tube. You should probably replace all four but the disc capacitors might be fine.

Sorry, CT. I thought I was clear that I'd pulled the Amp out of the console. The preamp is not in play.
Regardless, a balance issue wouldn't cause the red plating in the picture.

 

[ArchieType]

I swapped out the white capacitors and solved the red plating issue. Bucked the additional heater wires from the transformer as per Dave Gillespie's Magnavox thread.
Amp sounds OK now, but it's way too quiet. I'm running my phone straight into the inputs with the volume maxed, and the sound does not exceed quiet conversation level.
I'm using a pair of Infinity Sm 62 bookshelf speakers, which I think are supposed to be efficient.
Dunno if I'm going to go to the effort if this is all there is.

 

[century tek]

I know you want this as a stand alone amp, but maybe try the tuner preamp. Yes, these things do have the power to rattle your eyeballs.

 

[ArchieType]

I know you want this as a stand alone amp, but maybe try the tuner preamp. Yes, these things do have the power to rattle your eyeballs.

Okay, will do. I was under the impression that a pre wasn't necessary with these guys. Dave's thread indicated that. I realize he modded the circuit quite a bit but this thing should be louder as is, right?

 

[century tek]

Yeah, those amps with the resistor mods really do crank by themselves. Can you try something else like a cd player? Tape deck?

Your phone might not have the necessary voltage to drive the amp to full power. With these you need something like 1 1/2 to 2 volts output, powered preamp or no preamp.

I don't own a smart phone but just going by a friends, maybe try some other menu settings?

 

[ArchieType]

Yeah, those amps with the resistor mods really do crank by themselves. Can you try something else like a cd player? Tape deck?

Your phone might not have the necessary voltage to drive the amp to full power. With these you need something like 1 1/2 to 2 volts output, powered preamp or no preamp.

I don't own a smart phone but just going by a friends, maybe try some other menu settings?

What do you mean by "resistor mods“ in the context of the 9300 series?
I'm really only familiar with Dave's rebuild

 

[century tek]

This is the amp that needs the two 390 ohm resistors to take the place of the balance control in the tuner preamp. This is probably why you are not getting the volume.

 

[Dandy]

Yes, with amps of that vintage, the coupling caps should always be checked/replaced before applying power. Hope that EL84 has survived.

Lots of Magnavox experience and threads here in the tubes forum, plus even more on other similar P-P amps. Good luck with your project.

 

[Tubeglowpio]

If I ever stumble upon a tube console in decent condition I would refurbish and upgrade it but would leave it as a console. Especially if it was a concert grand or something, those are a piece of history. Either way enjoy it, they all sound pretty nice.

 

[ArchieType]

This is the amp that needs the two 390 ohm resistors to take the place of the balance control in the tuner preamp. This is probably why you are not getting the volume.

Yes, I have already installed those 390 ohm resistors. Also swapped out the .047 caps.
Almost No volume at all. I believe that many people use smart phones to drive these things.

 

[EastPoint]

Do you have another source to try? You have just plug in a cd player directly-it’ll be loud but not unbearable.