Musicman trouble shooting has caused hum, and only hum.

[Gaz]

A friend of mine decided to hand over his Musicman 2275-130 to me, which was only producing severely distorted tones at low volumes.

First, I checked the plate voltages, which all seemed normal. I decided to wait on checking the other voltages, and went to check the bias. I inserted my homemade bias checker made out of a tube base and socket, which has a 1 ohm resistor in series with the cathode pin (pin 8), but when I turned on the amp my meter read nothing. A slight hum was coming out of the amp, and the heater of tube attached to the bias checker wasn't glowing. I checked for shorts in my checker, but everything seemed normal.

This is where I should've tried the amp again to make sure it was still doing the same distorted/low volume thing, but instead I went to test the main filter caps. They tested good, and I reinstalled them, but when i turned on the amp... only hum.

At first, one pair of tubes started red plating. I turned off the amp, and tried again, and no more red plating, which really confused me. The pair now glows slightly blue/purple inside the plate structure when I turn on the amp and take it off standby. I know this glow is normal, but in this case I'm wondering if it's symptomatic of something else.

The plate voltages are all low now, and I can't figure out what's going on. Any help would be great.

BTW, I'm guessing the initial problem was probably leaking coupling caps, but I can't really test until I solve this other huge issue of an amp that's only humming and not passing signal.

Thanks,

Alex

[Don Moose]

You have to start with the power supply - everything depends on a solid PSU.

With the power amp tubes out, at the filter caps, compare the DC volts with the AC volts (assuming your meter can strip off the DC bias - if not, put a cap in series with the + lead)

You should have something pretty high for DC - over 300, I would think - and something pretty low for AC - less than 10V (much less). High AC on top of high DC suggests filters. High AC and low DC may mean rectifiers or a lost ground connection.

Hope this helps!

[Gaz]

Thanks a lot, I'll see what I get without the power tubes in. To just to make sure I don't fry my meter, because I've never checked AC voltage on the filter caps before, what do I set my meter at?

I've checked the DC volts on the PS many times safely, so don't worry that I don't know what I'm doing on the safety side of things, I've just never had to check for AC volts where so much DC volts are present. Clarification would be great.

Thanks again.

[Enzo]

Have you looked over the schematic? DO you have the version with the phase plitter tube? Or the version with the cathode drive transistors?

[Gaz]

Enzo, I have the cathode transistor version as well as the schematic. Any first steps to figure out what's causing this horrible hum? I've found other threads with similar problems, but never any answers. Thanks.

[Enzo]

Your standard bias methods then have nothing to do with this amp.

OK, tubes are redplating - have you tried other tubes? That is the most likely problem, a bad tube.

Power up WITHOUT power tubes. OK, is there +700v more or less on pin 3 of every socket? Is there 350v more or less on pin 4 of every socket? Is there +15v more or less on pin 5 of every socket?

Power off. Check the two drive transistors with your meter on diode test. Are they OK, either one shorted? Check their emitter resistors, R56,61. Both measure 3.9 ohms more or less?

Al that OK, install tubes you KNOW work by testing in other amps. Now what happens?

[Gaz]

Good to know about biasing. Having only dealt with all tube amp thus far, you can imagine how confused I was when I traced the cathode pins back to a transistor. So I guess that's taking the place of the phase inverter then...

Well, the tubes actually stopped red plating after the first problematic power up, and now one pair glows purple in the plate stucture. It didn't do this previously. As I said, I know glow is normal, but it just started happening along with the humming.

Thanks so much for the step by step - THAT IS REALLY HELPFUL. I will do so and report back.

[Don Moose]

to check for AC volts where so much DC volts are present. Clarification would be great.

In series with the red (+) meter lead, put a cap - if polarized, point the plus at the filter. Value should be at least a couple of microfarads. Voltage rating should be AT LEAST 1.25x the DC voltage you expect to find.

Suggest soldering the tip of the positive lead to the point you want to measure so you don't have to try to hold something in place against 600V.

[Gaz]

Enzo,

All voltages on the tubes look good without the tubes in. They looked bad with the tubes in. Am I too dumb to realize this clearly means bad tubes?

Transistors and emitter resistors were good.

One pair of the tubes are still red plating, and when I switch pairs, the same pair still red plates. Is that still indicative of bad tubes?

Thanks.

[Gaz]

Still confused...

I tested the tubes, and they all tested good, but I'm not sure if testing for shorts or transconductance is really conclusive in this case.

I tried to check the bias across the 3.9 ohm emitter resistors: with the tubes in one read 2 volts, and the other close zero volts. According to the Musicmanamps page, one should read 25MV and the other should about 45MV. What happening here? The bias pot seems to do nothing.

I don't have an extra set of EL34s at my disposal. Grrr.

I'm not sure what to try next this amp is so strange compared to an all tube amps that I'm used. If I can figure this one out, all tube amps will seems easy

[Gaz]

...and those bias measurements were made with the tubes in. Without the tubes in, bias, when measured across the emitter resistors of the driver transistors measured zero.

Thanks for any more help.

[Enzo]

Without tubes teh emitter resistors have no path for current, so they will read zero.


You have a tube that redplates no matter what socket it goes in? Or is there a socket that any tube stuck in it red plates? Obviously if the tube redplates wherever it goes, it is bad.

[Gaz]

Thanks for the clarification about the puzzling emitter readings.

Yeah, it's the sockets that cause the red plating.

So the only anomaly between the the red plating pair and the other are the cathode voltages. The cathode pins on the RED PLATING tubes read 8 volts, the 'functioning' pair reads 30 volts (can't remember exactly) in the "low" setting. In both settings, "low" and "high", the cathode pins of the red plating tubes was much lower.

Only being able to trace the cathode wire back to the perplexing drive transistors has left me stumped. I'm still fuzzy on how to check these in circuit accurately with my diode test.

Thanks for the response, Enzo.

[Enzo]

So replace the drive transistors. They may test OK with you hand meter, but that doesn't tell you what they do under real world voltages and currents.

DO a search, seems to me we sub something like MJ15030 OR SOMETHING SIMILAR.

[Gaz]

I will do. Perhaps when I naively plugged in the bias checker with the 1 ohm resistor on pin 8 something went bad with the drive transistor preceding it...

Stupid question: how do I locate the coupling caps in this amp?

Thanks a lot.