Musicman bias question

[Bigdrums]

Hi all: I have a Musicman 130 HD amp I am working on. On the schematic it says to adjust the bias pot to 1/2 volt AC at point "Y"
There are 2ea "Y" points on the schematic
Can this be right, "AC", or just a typo? I can attain 1/2 VDC at that point but not AC.
The "Y" point is right off of pin 8 of 2 of the power tubes (EL34's)

2475-130_and_2275-130.pdf

Here is a different version of the schematic:

musicman-2100-130-schematic.pdf

[Leo_Gnardo]

I can see how it's cornfusing. There's a 10 ohm resistor from cathodes of one pair to ground. And another 10 ohm resistor on the other pair of cathodes. And - - - they're connected together! So, effectively you're measuring voltage across a 5 ohm resistor. Half a volt translates to 100 milliamps when we apply the numbers to Ohms Law, V = IR . I being current in amps. Assuming each output tube is passing equal current that's 25 milliamps per tube. Makes sense for this application.

Something you should be aware of, now 40 - 45 years after manufacture, most of those 10 ohm resistors have drifted. Typically I see 'em 10% high at 11 ohms each, or even more. It's worth measuring them, then applying Ohm's law. Your option to change 'em out to accurate 10 ohm resistors. Or a single 5 ohm. Generally I don't bother, just use the actual resistance at that point & do the calculation. When I do bother, I put in 1 ohm 1% 3 watt wirewounds on each tube, then it's easy to tell which tubes are running hot or cold. Plus the arithmetic is trivial: one millivolt per milliamp.

[Enzo]

I don't see AC, I see DC with a little dot after each letter. it is not clear, but I don;t read it as AC. In any case, a technician knows bias is a DC reading.

[Tom Phillips]

The first questions I would ask is which version of the circuit is in the amp that you are working on. Specifically, does it have a vacuum tube circuit driving the output tubes or is it the solid state version. The bias adjustment procedure is different for each version. I want to be clear that I'm asking about the Phase Inverter / Driver circuit NOT the pre-amp section. Both versions have SS pre-amp circuitry.

Proceed carefully with Music Man amps or things can really get messed up.

[Justin Thomas]

And, make sure your meter is rated for it... My Fluke wasn't, when it was in high-power mode...

Justin

[g1]

Here's the factory info:

[jbltwin1]

It actually says "at" point y. Those two point y's are the same. Those are connection points. HOWVER, I would suggest biasing MM's a little cooler than that. Say, 22-23mA PER TUBE. Go poke around the MM site for guidance. Intro Page Mike