After I replaced the capacitors and installed a polarized plug, I had no hum at all. The wiring in my 9301s did not take the center tap of the balance pot to ground, for some reason. Anyway, I have zero hum and I even used a scope to verify.
Console stereos: 1958-84
- Thread starter cfz2882
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phillymatt53
Well-known member
You've got me a bit confused there, I assume you are talking about the HUM balance pot.
You say that the balance pot wiper doesn't go to ground - THAT would be of course EITHER the HUM balance, AND the input level balance.
Regardless of whether there's hum or not, this is my answer....
The HUM balance pot wiper DOES go to a 100 ohm cathode bias resistor which IS grounded, and as I mentioned, keeps those cathodes and the filaments NEAR ground potential, which is what Magnavox did on purpose.
The reasoning, if you can understand it is - it's to prevent the input and output tubes from any "flashover" or arcing issues inside the tubes from happening, along with raising the filaments above ground potential for the 6EU7's.
A lot of people (mainly amateurs/hobbyists/internet guru's) don't fully understand this type of design or why it is a necessary thing to do with tube amplification.
Magnavox didn't just put it in there for looks.
I assure you I am not an amateur, but my background is in mobile communications, so I do assume you are correct. But, I love fiddling around with stuff just to see what would happen. Now if I could only find a blank chassis or a place near me that does sandblasting, I would remove all the parts and rebuild, I'd also replace all the passive components I could. Since both my 9301's came from less than clean environments, (barn and landfill) I'd like to get them all spiffed up like I've seen others do.
phillymatt53
Well-known member
"Spiffed up" is worth it if it's going to be a visable piece on display.
But if hidden inside the console, I wouldn't get obsessed with appearance other than some reasonable cleaning.
The 9304-20 that I pulled from a console in 2002 was in decent shape, just dusty.
I've done extensive modifications to it to bring it up to the best performance that it's capable of.
Normally, a pair of EL84's in push-pull can put out about 10 watts before clipping.
And the 6EU7's are configured in a simple Paraphase splitter design to feed those outputs.
But a more stable and balanced design with lower distortion is a cathodyne phase splitter, AKA Split-Load Inverter, which was my choice.
In addition, the original output transformers have their frequency limits too, and I replaced them with the Dynaco Z565 outputs, which are of a "Ultralinear" design and a wider more accurate frequency response.
On top of those mods, I went for the Dave Gillespie modification of biasing the outputs from the original cathode bias to Fixed Bias controlled by a regulator.
The result of all this is substantialy higher power (17 watts/channel), with much less chance of distortion at higher volume peaks/levels, Superior frequency range, and increased tube life.
That amp now can drive my power-hungry Advent Maestro floor speakers to window-rattling levels with clean audio and solid bass that gives me goosebumps, and crystal clear highs.
I housed it in my own custom case
Some pics...
Damn! You do good work! That component is beyond beautiful. Mine is in the console, but the problem is that I know it is in there, not being as pristine as it could. Thanks for all your help and info. I once had the skill to create something like this, but that was way back during Disco.
phillymatt53
Well-known member
Thanks Eugene.
I'm like an old, experienced Surgeon, and insist on performing my work to extremely high standards - mostly.
Being in the electronics repair business has given me this blessing.
Plus, I'm picky about doing things the best way, no shortcuts, no slop.
A lot of tediousness is involved, which doesn't much bother me, hell, it kills time and keeps me off the streets. LOL!
