Musicman 2475 - 150 questions

[pontiacpete]

This amp has 4 6L6s and in 'hi' mode plate voltage is 740vdc scr. 370v Grid 23v and cathodes 68v
I think I read it here from one of the well respected members that the hi mode is just going eat tubes with plate voltages this high and its better to avoid it. Or maybe I imagined that.. . .But anyway, how does a 6L6 survive 250v over its max?

What about biasing? The trim pot TR1 looks to me to be more like a balance pot for the 2 sides. Is that right?
Usually I bias using a 1R resistor on pin 8 to ground or use the shunt method or I divide the voltage of each side of the OT but the resistance of each.
What is the best way to go?
Thanks for any help.

[J M Fahey]

This amp has 4 6L6s and in 'hi' mode plate voltage is 740vdc scr. 370v Grid 23v and cathodes 68v
I think I read it here from one of the well respected members that the hi mode is just going eat tubes with plate voltages this high and its better to avoid it. Or maybe I imagined that.. . .But anyway, how does a 6L6 survive 250v over its max?

Can't vouch for modern tubes, but in general 6L6 can stand a lot on plates, K6MZ who used to build radio transmitters said it's a derivation of a Radio tube which used to have its plate exiting through the glass top, so the plate structure by itself handles a lot, just worry about the socket for arcing.
The trick lies in keeping the screen close to 300 V and biasing it cold.

What about biasing?

Cold.
I'd keep it between 5 and 10 mA per tube, not more.
Of course forget that Forum nonsense about "70%" .

The trim pot TR1 looks to me to be more like a balance pot for the 2 sides. Is that right?

No, see that it biases both sides at the same time.

Usually I bias using a 1R resistor on pin 8 to ground

Don't !!!!!!! , or you'll witness

on your bench.
You would be grounding the cathode (which now stands at over 60 volts positive) while the grids are held at steady positive 22V

or use the shunt method or I divide the voltage of each side of the OT but the resistance of each.

Very dangerous, you'd have both multimeter probes, one in each hand, some 750V above ground. [shiver]

What is the best way to go?

What the factory uses: measure DC voltage across R73 and R81 and do the math for, say, 10 mA per tube max.

So if it's 10mA per tube, two would be 20mA , passing through 3.9 ohms you'd expect 78mV across each.

[Tom Phillips]

What about biasing? ...What is the best way to go?.

First let’s make sure that everyone understands that the Musicman utilizes a class B power amp. There have been discussions in the past about weather the output stage is “True Class B”, class B1, B2 or other. Suffice it to say that when properly set up there is very little plate current flowing in the output tubes when the amp is at idle. The power tubes are at, or very close to, cut off. The signal drive is via the cathodes not the control grids.

Following is some Musicman information that I copied from a document titled “Music Man Bias Adjustment. Music Man Amplifiers SERVICE BULLETIN #1”
Section I DRIVER TRANSISTOR BIAS CALIBRATION PROCEDURE

A. This applies to all models containing the following circuit boards:
DB-2, DB-3, DB-4, GP-1, GP-2, GP-3, GP-3A,GD-1,GD-2 AND GD-2A.

B. Adjustment is as follows:

1. Turn the amplifier to "ON" with the HI / LO Standby switch in the HI
position. No Signal.

2. Using a voltmeter measure the voltage from emitter to ground on each of
of the two driver transistors. Across the 3.9 OHM emitter resistors is a
convenient measuring point.

3. Adjust the bias trimpot (TR-1) until you read 25mv DC across the 3.9
OHM emitter resistors. If there is a difference in voltage between the
emitters of the two driver transistors, set the lower of the two to 25mv.
The higher of the two should not exceed 55mv DC.

NOTE: Use only a 1458 op amp to drive the output section.
This applies to all models "not using 12AX7 driver tube".


Hope that helps. At least it is information directly from the source.
There are dedicated Musicman forums too. If you Google “Musicman bias adjustment” you are sure to get many good hits. I also remember several discussions on the old Ampage Forum. Maybe you can find them at AMPAGE Archive

Cheers,
Tom

[pontiacpete]

Many thanks for the help. I was able to bias it to 25mV on one of the 3.9R resistors and on the other read 29mV.