More Fun With Magnavox: The 9300 Series

This IS kidmoe's amp that I recently purchased from him! I included that information for Dave assuming it might have been useful. This is the amp in question, sorry if I led you astray.
 
The 47uf cap up at the top doesn't ground to the same place the other power supply caps do. Not sure if thats it or not, but having current flow across the chassis like that can induce noise in the works.
 
I haven't tried removing the outputs for the right channel yet to see the effect; removing the right channel driver still gave me the same low-level hum. I haven't felt any vibrations and the OPT's are securely bolted to the chassis. Gadget, as you can see the original can cap was replaced with separate caps. I do see that the large, uppermost 47 uf cap is not grounded to the same lug as the three smaller caps which are grounded to one of the twist lugs on the old can cap. If the concensus is that could be a problem, I can easily run a ground from the big cap to the same twist tab. In actuality, I count seven different grounding points to the chassis. I should say that the hum is not excessive, but the left channel is dead quiet and I am taking this on as a challenge to make the right channel just as quiet. It is still a great and very musical little amp. Love those EL84 tubes. We can do this!
 
I see that the caps are replaced. Its just my personal preference to ground all the new caps to a common connection and have one tie to chassis. With the Magnavox specifically, I'd make that single point the one where the transformer center tap connects to chassis. In stock form, it was not on the can cap like most amps did.

I also like my coffee black and I have a thing for mint ice cream. Not everyone agrees with me on that either :)

Have you tried switching the output tubes channel to channel? Poorly matched tubes can cause hum in a push-pull amp. Since you can pull the driver and still get some hum, that tells me the noise is coming in at the output stage.
 
On a temporary basis, I've redone the grounding scheme; not pretty now but functional. Now all the grounds except those for the RCA inputs terminate at the same twist lug of the now nonfunctional can cap securely soldered to the chassis. So I guess this is now similar to a star ground system. I am happy to say things have become very quiet. I have to put my ear directly against the speaker grills to hear a faint hum. I think I'd like to tidy things up and replace the RCA inputs with the grounds run to my single grounding point. So, looks like success for now.

I can go for that mint ice cream but I must have half-and-half in my coffee, NO sugar. In matters of taste there can be no dispute.
 
I was a half and half with no sugar guy until I got a stomach bug one day, then suddenly it was too sweet. No clue why, it was literally an overnight change. I'll take it, its a minor convenience and if I'm in a bad way and have to drink the swill coffee at work I don't have to further torture myself with powdered creamer.
 
After having my Magnavox 9300 sitting on a shelf for years, (eventually moved on to other amplifiers) I spotted this thread and figured why not give it a try...it's not expensive. I've read through the thread quite a few times, and I think I have everything right.
I goofed using a 10uF cap for the screens, and should have read more carefully and could have used a higher capacitance. But I did install the 100 ohm stability (suppression?) resistors if the multi-capacitor is changed down the road with a four section cap. I have a well reformed Sprague three 40uF section capacitor for now.

I haven't powered up the amplifier yet, still scrounging up a quad of 6BQ5 and a few 6EU7. I have the bucking connection set-up...would like to use a 5V4GA...I have a Mullard GZ34, but I really don't want to park my precious GZ34 in a maggotbox console amplifier. :)

I listen to Cornwalls in a near-field setting. I doubt I'll be even be pushing 8 watts a side, if even that...maybe 4 wpc?

Mike
 

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Mike, looks nice. I'm debating about trying to put the inputs on the back, but the original position does reduce signal wiring. Good luck when you fire it up. If you don't have 6BQ5's on hand, I've had good luck with a matched quad of JJ EL84 tubes. I can bias my amp to within a few mV between channels. Plus, they are a very good value. Good used 6EU7's can be had from the auction site inexpensively, as well.

I'd like to give Dave Gillespie a big shout out and thank you. He has been most helpful in my chasing the low-level hum issue. In looking over the wiring, he identified several potential ground loop problems that I have been able to address. The situation is much improved and I am really enjoying this little amp - very musical to my old ears.

Again, THANKS Dave.

Bill
 
A cheap as chips Sovtek 5AR4 will do the job very well. I ran one in mine for a while without any problems. The 5V4 will work, but it drops more voltage than a 5AR4 will. May not be enough to be a problem though.
 
Ok, I ramped up the amplifier with a variac, but only to 117vac. With a 5V4GA, the B+ voltage is only 350vdc.
I'll try it again at 120vac, but I need to pull the rectifier tube and determine the heater voltages at 120vac...I should have done that first thing. It seems if I get an extra 25 volts B+, then everything should measure pretty close to the schematic.

I may have to just pony up for a Sovtek 5AR4, but I can use the Mullard GZ34 for testing/measuring.
If the amp was going to destroy a rectifier, it would've killed the 5V4 by now...
 
So with 120vac input with the bucking connection, and the rectifier pulled I get 6.48-6.5 volts for that heater winding. May as well leave it that way.
With the amplifier powered/warmed up with 120vac input, I get a B+ of 370 volts. The screen voltage is 365 volts, with the 6EU7 filter cap at 335. The cathode voltage between each pair of 6BQ5/EL84 is 16 volts.

Plate voltage for each section of 6EU7 for each side is 185/186 for the left side 6EU7, and 176/175 for the right side 6EU7.
Swapping the right side 6EU7 for another 6EU7, I still end up with a 10 volt plate voltage difference between the left and right side...dunno really why...Maybe a wiring error?
These are used 6EU7 tested on a no-go gauge emission tester.
The cathode voltage for each 6EU7 is at 1.5 volts.

I have a quad of old well worn Mullard EL84 for output tubes, which are sorta (not) matched...well, they looked good on the no-go gauge...
The best I get between each pair, (with some swapping) is 42 millivolts for the left side, and 38 millivolts on the right.
I have other 6BQ5/EL84 to swap in to see if I can get 44 millivolts between each pair.

I'm measuring voltages using 8 ohm load resistors.

Mike
 
Resistors off value would be my guess if the low voltage stays at the socket. If it was a difference in tubes, it would follow the tube.
 
Resistors off value would be my guess if the low voltage stays at the socket. If it was a difference in tubes, it would follow the tube.

The 6EU7 plate load resistors for the left side both measure right at 220K, with the right 6EU7 plate R at 224K, and other at 220K. I swapped out the 224K resistor for one that measured 219K.

Turns out the voltage difference follows the tube...duh...
Everything measures rather close voltage-wise. 374 B+, 370 at the screens, the filter for the 6EU7s (3rd cap) is at 337vdc. 6BQ5/EL84 cathode voltage between each pair is 16.1 volts.

Once I get the 6EU7 situation figured out, I should be good to go. I would kinda like to peek at a 2kHz square wave first...
 
Differences tube to tube would account for that. I wouldn't worry about it all that much if the tube tests good.
 
A cheap as chips Sovtek 5AR4 will do the job very well. I ran one in mine for a while without any problems. The 5V4 will work, but it drops more voltage than a 5AR4 will. May not be enough to be a problem though.

Agreed - a Sovtek 5AR4 is a no-brainer for this amp. :thumbsup:
 
Maggotbox lives!

Yesterday I monitored 2kHz square-waves for each side. Each side looked similar to Dave's posted scope image, with just a little more of a peak on the front left edge. But I really need to invest in a function generator, or try to fix the one I have. The amplitude pot or amplifier in the generator is dodgy, which makes it aggravating and not really something to depend on...if one leaves the pot turned down or left alone it's okay.

Anyways, I've been listening to the amplifier for about 3 hours now without a hitch. I hadn't listened to the amplifier the original way I had it for years, but I don't recall the amp sounding this good. Better than I expected.
I'm definitely going to have to get a different rectifier, but the old Mullard EL84s can hang in the amplifier for now. Maybe scrounge some 6EU7...
I'm using a Heart/Marantz CD player, with a 50K Goldpoint stereo stepped attenuator in a box, into the Magnavox amplifier with Klipsch Cornwalls. Sounds good to me...it all depends on the recording...

Thank you Mr. Gillespie...you made my Maggotbox sound awesome!

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Yup. In hindsight, I wished I had.
I'll eventually do it, because of the commonality of 12AX7 like you mention.
 
I left mine alone strictly because nothing else I own uses a 6EU7, which means I'm not inclined to borrow them for anything. I also have a spare pair.
 
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