Musicman 130 (2275-130) problems

[tubetek]

Hope you guys can help. I am working on a Musicman 130 with the tube PI. It was working fine, then fairly suddenly developed a grainy, distorted tone and low output. (Bass player was using it) I checked it out and noted red plates on the power tubes, consistent with bias faillure. Opened it up and rebuilt the bias supply, and tried it out. No hot tubes, but had the same low output and distortion. ( was biased correctly) Tried new tubes... no change. Checked all the voltages, and all OK, about 720 V on the plates, bias about -34 V, screens at ~360 V; checked all the tube sockets; checked and replaced the 12AX7; changed the 10 ohm cathode resistors; changed all the coupling caps in the PI circuit; changed all the power section caps; removed the .001 caps from the plates to ground, and removed the diodes from plates to ground....all with no change, except a little instability and harmonic distortion, but way down on power (< 5 watts) into a correct load. I then tried wiring directly to the input of the PI, with another preamp, to no avail, so it has to be in the power section. Right?
I have re-heated all the socket and other connections. The OPT is not open and the resistance from center tap to each end is about 80 ohms one one side and about 105 one the other, which I think is OK. What am I missing?

Steve

[Prairie Dawg]

Hope you guys can help. I am working on a Musicman 130 with the tube PI. It was working fine, then fairly suddenly developed a grainy, distorted tone and low output. (Bass player was using it) I checked it out and noted red plates on the power tubes, consistent with bias faillure. Opened it up and rebuilt the bias supply, and tried it out. No hot tubes, but had the same low output and distortion. ( was biased correctly) Tried new tubes... no change. Checked all the voltages, and all OK, about 720 V on the plates, bias about -34 V, screens at ~360 V; checked all the tube sockets; checked and replaced the 12AX7; changed the 10 ohm cathode resistors; changed all the coupling caps in the PI circuit; changed all the power section caps; removed the .001 caps from the plates to ground, and removed the diodes from plates to ground....all with no change, except a little instability and harmonic distortion, but way down on power (< 5 watts) into a correct load. I then tried wiring directly to the input of the PI, with another preamp, to no avail, so it has to be in the power section. Right?
I have re-heated all the socket and other connections. The OPT is not open and the resistance from center tap to each end is about 80 ohms one one side and about 105 one the other, which I think is OK. What am I missing?

Steve

There is a Music Man technical forum that was very helpful to me when I worked on a 50 watt Music Man combo with some nasty distortion.

[acorkos]

make sure you've got the speaker plugged into the main speaker out and not the extension speaker out

[tubetek]

make sure you've got the speaker plugged into the main speaker out and not the extension speaker out

This thing has two outputs that are in series. Meant to use the two cabs that came with the head (a 15" ported and a 2 10") I am going to check out the output wiring. Thanks

[J M Fahey]

Can't you draw a rough schematic of the power section?
I have the transistor driven ones but not the tube driven.
Anyway and just to start with somethingI
If, as I guess, it's basically like most classic power amps, you can split into two sections and check them independently.
You already checked the power tubes: Bias, screen, +B, sockets. I understand there are 10 ohm resistors cathode-ground, what's the cathode voltage at idle?
Now the PI, I guess it's a long tailed pair.
Measure and post the +B available to them, plate voltages, voltage across the cathode biasing resistor and voltage cathode-ground. If all looks normal, inject 1V through .047 or .022x400V directly into the input grid, pulling (for now) one pin of the original input capacitor, just as to discard any possible wiring short. Measure signal at the grid, through a .022x400 or 630V, you *should* still have your volt.
If you do, measure audio voltage at the plates, you should have 30 to 50V RMS (and you should be getting deaf).
All previous tests point to my main suspect:
If you have 30 to 40V RMS at the output tubes grids and you have no (or miserable) output, you have a shorted output transformer.
There's a neon lamp test (search it here) to check that.
Hope to be mistaken.
J M Fahey

[tubetek]

Can't you draw a rough schematic of the power section?
I have the transistor driven ones but not the tube driven.
Anyway and just to start with somethingI
If, as I guess, it's basically like most classic power amps, you can split into two sections and check them independently.
You already checked the power tubes: Bias, screen, +B, sockets. I understand there are 10 ohm resistors cathode-ground, what's the cathode voltage at idle?
Now the PI, I guess it's a long tailed pair.
Measure and post the +B available to them, plate voltages, voltage across the cathode biasing resistor and voltage cathode-ground. If all looks normal, inject 1V through .047 or .022x400V directly into the input grid, pulling (for now) one pin of the original input capacitor, just as to discard any possible wiring short. Measure signal at the grid, through a .022x400 or 630V, you *should* still have your volt.
If you do, measure audio voltage at the plates, you should have 30 to 50V RMS (and you should be getting deaf).
All previous tests point to my main suspect:
If you have 30 to 40V RMS at the output tubes grids and you have no (or miserable) output, you have a shorted output transformer.
There's a neon lamp test (search it here) to check that.
Hope to be mistaken.
J M Fahey

Tried the voltage measurements before, and got about 45 V RMS at the grids of the 6CA7's. I also checked all the PI voltages and outputs with a 1kHZ signal, and they were all normal. I am assuming it is the OPT, but don't want it to be. I am going to redo all the output jacks and wiring, and if that doesn't work, go shopping. Wonder why the OPT would fail? Thanks for all the helps. Steve
.

[J M Fahey]

Hi tubetek/steve.
Before you spend big $$$ on an OPT, just to make sure, unsolder it from the plates and connect *any* OPT you may have around, even a 15W will do, just to confirm suspicions. Even 15W are much louder than what you have now.
The high voltage capacitors and backward diodes connected to the plates are there to protect the OPT, Musicmand are very well built, but nothing lasts forever.
Another possible culprit might be dampness.
Be careful with the typically very high MM plate voltages.
Now that I think about it: do all Power Supply voltages check normal?
Even under load?
That voltage doubler supply isn't very conventional either.

[tubetek]

Trying a different OPT sounds like a good idea. Now, if I can talk my soon-to-be EX into letting me back in the house to rummage through the old iron...

[J M Fahey]

That sounds more dangerous than touching a smoking resistor with your bare fingers "to see if it's hot"

[Enzo]

Both versions of the amp are in the same file at Schematic heaven

http://www.schematicheaven.com/newam...&_2275-130.pdf

[tubetek]

A new OPT did the trick. Need to fit a 10 lb. trans into a 5 lb. spot, though. Thanks for everyone's help. Steve W