Music Man 2275-150 HD150 Reverb Head

[retrorod]

Hey all! I am trying to troubleshoot the titled MM head dated 10-07-80 using 6L6GC tubes...
scematic http://www.webphix.com/schematic%20h...6_2275-150.pdf

I do not have an O-scope for diagnostics. The amp powers up but has hardly any volume and the tone is thin and distorted. I get very low volume from either channel. The amp has new power tubes and it seems that the voltages are correct by the schematic. I am questioning the bias voltage.
Has anyone encountered this amp with low distorted volume? I would like some points to check and test....or any ideas, please.

[Jazz P Bass]

As always start at the power supply.
There are opamps that the rail voltages must be steady (no Vac)
Without an o scope your best bet is the output tube grid pins.
You can measure the bias voltage & report back.
Also look for a steady bias voltage by flipping your meter to read Vac.
Insert a front end signal & see how hard you can run the grids (Vac)

[g1]

What is the history of the amp? Was it working properly recently, and if so, what happened when it stopped working right?
"it seems that the voltages are correct by the schematic". This is pretty vague, it would be better if you list voltages you have measured so no one here has to assume anything.
As far as the bias goes, make sure you have +22V on pin 5 of the power tubes. Then check the DC voltage across each of the 3.9 ohm resistors R73 and R81. It should be around 25 millivolts across each when the amp is idling.

[retrorod]

As always start at the power supply.
There are opamps that the rail voltages must be steady (no Vac)
Without an o scope your best bet is the output tube grid pins.
You can measure the bias voltage & report back.
Also look for a steady bias voltage by flipping your meter to read Vac.
Insert a front end signal & see how hard you can run the grids (Vac)

My plate voltage is 698VDC pin4 is 348VDC and have 22.4VDC on pins 5. The same on all tubes.
Reading across the 3.9 resistors measures 25dcmv and 19 dcmv

[retrorod]

What is the history of the amp? Was it working properly recently, and if so, what happened when it stopped working right?
"it seems that the voltages are correct by the schematic". This is pretty vague, it would be better if you list voltages you have measured so no one here has to assume anything.
As far as the bias goes, make sure you have +22V on pin 5 of the power tubes. Then check the DC voltage across each of the 3.9 ohm resistors R73 and R81. It should be around 25 millivolts across each when the amp is idling.

The owner claims that it worked well at one time.....no clear history.
My Big mistake that I just discovered is that I had used a bias-rite meter. After removing it the amp has full clean power. Apparently the bias arrangement on the Music Man does not corellate with the Bias-Rite meter.
Previous to this, I had replaced the 6488 transistors. It had one original and one subbed TIP41A. I speced out the rest of the board components and everything checked as good and within tolerance. I had also replaced the J-fets and op-amps (all socketed)

What about the difference in bias voltage between the 3.9's (25 & 19)....?
At any rate, removing the Bias Meter cleared up the tone and volume.

[g1]

Very odd about the bias-rite, I wouldn't have thought it would affect it, it works fine in other amps?
The difference in the readings across the 3.9 ohm resistors is due to the tubes not being perfectly matched. I think if you swapped the tubes around the voltages would probably follow.
You mentioned changing the IC's. IC8 in particular should be the original type (1458) not any substitute number.

[retrorod]

Yeah, g-one, it was rather strange. The bias-rite has been working fine. As soon as I pulled it off to re-test voltages, I heard the familiar hum of a live amp. It was like the bias probe was 'shunting' the amps power. Possibly coincidence.... but the amps sounds great and all seems well!

I am not familiar with Music Man amps....I know the history but have not played or worked on one. I like it. It's not a Fender but.....?!?
I read on the MM site to bias the lowest side to 25mv....as long as the other side does not exceed 44mv. And yes IC8 has a fresh (1458) as well as new 2N6488 transistors. I will install fresh power filter caps (100mfd/500V) as soon as I get them. The originals tested OK with ESR meter, but they ARE old!
Thanks for the help of everyone-
Rod

[Jazz P Bass]

No Vac ripple readings
And a signal reading at the grids.

[retrorod]

Have not tried that JPB....Perhaps tomorrow. Exactly what 'front-end signal'? And signal readings from grid to ground? I know some, but assure you I have much to learn....But I am willing

[Jazz P Bass]

'front-end signal'
A signal into the input jack.
'And signal readings from grid to ground?'
Yes.
You will be looking to see if there is enough drive signal to make full power.
Or a difference in signals at different grids.
Not having a scope you will have to be creative.

[g1]

Jazz, don't know if you caught this, if so pardon the interruption.

My Big mistake that I just discovered is that I had used a bias-rite meter. After removing it the amp has full clean power. Apparently the bias arrangement on the Music Man does not corellate with the Bias-Rite meter...
At any rate, removing the Bias Meter cleared up the tone and volume.

[Jazz P Bass]

Missed that !
Thanks.