Music Man 112RP 65 - Intermittent Noise

[tmhanser]

I still have a noise problem with my Music Man 112 65RP and I've had it serviced by three reputable people now. This amp is from 1979, and these Music Man amps have solid-state pre-amp sections.

 

It still produces a buzzing or "sizzling" sound, even at moderate to low volumes. It sounds like a bad speaker, but I took the speaker out and had a speaker repair technician bench test it. He says it's fine, so I put it back in.

 

It still makes this buzzing-distortion sound, so I took it to repair tech #2. He put new tubes in it and re-biased it, but made no other changes. He DID indicate that he could see an "instability" and he said it could be caused by mis-matched power (driver) transistors, but he doesn't do solid state work.

 

So, then I took it to repair tech #3. He changed some caps, resistors and fixed some kind of AM radio interference circuit on it, but he says that it's unstable, too. I guess you can see this on an O-scope? He said it was putting more than the rated power after he changed some components but I can STILL hear this distortion on single notes, especially the low E and A strings. Play the E string, 12th fret and you can hear this distortion on top of the ringing note. It seems to decay BEFORE the notes stops ringing, if that means anything.

 

What can I do? I love this amp and I considered dumping it for a Fender Deluxe Reverb, but I still want to fix it. The fault has to be there, somewhere. It seems to come and go, but I know I'm not the only one who can detect this noise.

 

Is it normal for vintage amps to have intermittent noise problems that are hard to nail down? Would replacing the two power (driver?) transistors remedy this problem?

 

Thanks so much and I appreciate any help you can offer.

 

 

 

[HoobaStunk]

Would replacing the two power (driver?) transistors remedy this problem?

 

 

Why not? Those are fairly inexpensive components and should be easy to replace. Also, I'd sweat all of the solder joints since this is a 30+ year old amp. There's probably a touch of corrosion beginning to creep in to a few of the solder joints after 30 years.

 

I used to have a 65 Model 4x10 combo from 1975. Mine had a pair of 6L6s.

 

[onelife]

Your speaker may have tested okay but I believe it would still be worth your while to try your amp through a different speaker and, if possible, trying a different amp through your speaker with everything else being the same.

[WRGKMC]

I have the head version of that amp. Mine is a 1976 with the inverter tube. They changed to a soldi state inverter later because they had a habit of catching on fire when that tube went bad. If it has a 12AX7 phase splitter tube, then it falls between 1974 and 1977. If it doesn't have the 12AX7, then it falls between 1977 & 1980. Removing the chassis from the cabinet and finding a handwritten date is the only way to date it to a particular year as far as I know.

 

The bade plates are another sign. Black on white or white on black. I believe the older ones like mine are white on black. The newer ones were black on white but I'd have to do some digging to confirm the years.

 

Ernie ball just reissued the heads too. I was wondering how long that would take. A mint condition one like mine will probably double in price in a few years then I'll sell it.

When I got mine I had a blown speaker sound too. That's why the guy sold it to me for $150. I did some digging and quickly found a bad solder connection on one of the power tubes cathode resistors. I also found a loose connection to the deep switch.

 

One thing I did learn was the documentation for the power tube bias is wrong.

 

If your techs don't know the amp really well, they may not have set the power tube bias right. The original documentation tells you how to set the bias for 6CA7 tubes which had become obsolete. The only other tubes that would work in the amp and handle the very high 800V bias are JJ EL34 tubes. If you're using any other EL34's they are likely the cause of your problem.

 

On top of that the Bias setting adjustment for the EL34's is only 1/10th as high as the 6CA7 tubes. The setting is off by a decimal point and you can be running the tubes way too hot if you don't know this documentation issue.

 

I did allot of digging to find this because the settings I found weren't making sense with the EL34's.

 

Luckily Electroharmonix has reissued the 6CA7 Tubes designed for those heads so it might be best for you to get a set of the 6CA7's especially if you aren't using JJ's and have them set right.

 

I put a set in mine and it has killer overhead now. Dead clean tones when the channel preamp is turned down. The Preamp volume is what gives the head drive and that's normal. Its solid state drive that gradually increases as you turn it up. If you keep the Main volume 50% or more it sounds pretty much like a driven tube amp. I'm not a huge fan of its driven sound but it does take pedals well so I just dial up a small amount of drive and get the rest with pedals.

 

Other then that mines been rock solid. The guy I bought it from kept it in mint condition. Its got no rust and no tears in the vinyl. Not even a scratch on it.

 

You can dig for some of this information here. http://www.pacair.com/cgi-bin/discus/discus.cgi?pg=topics This is what I did and came across the bias reference some place there. Just realize the manual references the higher powered tubes and EL34'sbias is set to like 5 voltes where as the 6CA7 are set to .5 volts if I remember right. Its real easy to check too. .

 

[tmhanser]

Thanks, everyone. And thank you, WRGKMC, for your detailed reply. I'm familiar with that web site you suggested, also.

 

The tubes are new (EH) and recently biased and it's been thoroughly inspected by at least three techs now. My only other alternative is to go out and purchase a replacement speaker - I just don't any other ways of isolating the problem.

 

Maybe there's a chance this speaker makes a buzzing sound when it's in the amp, but not on a technician's bench?

 

Would mis-matched or worn out driver transistors cause this buzzing or sizzling sound I can here? It's not distortion from being over driven, trust me. This is a clean-sounding amp, with no tubes in the pre-amp section.

 

Why can't a tube amp repair tech replace two power transistors? Good question.

 

What's causing the "instability" that two techs have found? The two power transistors? Good question.

 

Thanks again, everyone.

[WRGKMC]

There's no reason to replace parts if they test good. Its true some parts can develop partial faults, but that would be very hard to determine on these heads because the transistors are "supposed" to overdrive. The amp will only sound clean with the preamp turned down below #3 or so. Above that its supposed to overdrive. The amp is not like a Fender that's designed to run clean as you turn the preamp up, it's supposed to overdrive.

 

Of course there's always the possibility something was missed, Like I said, the info on the proper bias is "extremely" difficult to find and I wouldn't be surprised by having 3 techs miss it. I'm a certified tech myself and only found it with allot of reading.

 

If you suspect its the speaker, then remove the speaker and carefully move the cone by hand. if the speaker coil overheated, it may have become deformed and is rubbing on the magnet. you can usually detect this moving the cone in and out by hand. You can even have some stamped steel speaker baskets get warped or deformed when they are mounted with uneven pressure or aren't mounted on a flat surface. Sometimes you can just twist them a little and get the proper piston movement without the coil rubbing. You can also have a cone that sees too much moisture and begins to warp. Sometimes you can work the cone to flex it and get rid of the rubbing.

 

Any kind of cone rot or paper cracking can cause misalignment. If you find a cracked cone you may be able to save it with some rubber sealant used to repair the surrogated surround area that commonly cracks when the paper gets rotten. In all cases you have to remove the speaker and check it out to be sure. Checking the head with another speaker of the correct impedance will be the true test.

[WRGKMC]

These two adjustments from the manual are for amps that have the original 6CA7 power tubes only "NOT" for EL34 substitute power tubes. The only EL34 tubes I suggest running in these amps are JJ tubes which can take the high 700V plate voltage. If an amp tech attempts to bias these tubes like a Marshall they will run way too hot at around 37ma and break up way too much. These tubes must be biased between 22~27ma max.

 

These heads also tend to have the boas creep. Setting the main volume to max when setting the bias can prevent this from occurring.

 

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 trim pot (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".

__________________________________________________

Section II GRID BIAS CALIBRATION (This applies to all models "using 12AX7 DRIVER TUBE")

A. This applies to all models using a grid drive. (12AX7 driver tube).

 

B. Adjustment is as follows:

1. Turn the amplifier to "on" with the HI/LO Switch in the HI position.

No signal.

2. Using a volt meter, measure the voltage from cathode (pin 8) to ground

on the output tubes. Across the 10 OHM cathode resistor is a

convenient measuring point.

3. Adjust the bias trimpot located farthest to the right (looking from the

front) on the GB-1 and BB-1 circuit boards until you read .5 volts DC

across the 10 OHM cathode resistor.

NOTE: Be sure tubes are warmed up before adjusting the bias voltage.

 

 

When I was digging I came across this in one of the Forums.

 

If the amp has 100 ohms resistor instead of 10 ohm at pin 8 of power tubes the bias calculation for 5V across for 100 ohms, .5v for the 10 ohms resistor bias.

 

[tmhanser]

Revisiting this conversation...

 

I haven't made any changes to this amp yet but I still occasionally here this buzzing distortion that to ME sounds like a speaker buzz, but I've had the speaker out and bench tested by a local speaker repair guy, who profoundly said there was nothing wrong with it. I could go out and drop $100.00 on a new Eminence or Jensen, but what if it's NOT the speaker?

 

WRGKMC: I wish you were in the Seattle area so you could listen to this amp for me. I find it hard to believe that the transistors in this amp distort at any volume, I really do.

 

I've considered trading it in for something else, but I don't want to give up on it. It's too nice of an amp and I've invested too much into it at this point.

 

Two different tech's indicated they "saw" an instability and I can here an intermittent buzzing distortion at moderate volumes (not cranked). What could cause this? The only thing left are the two (paired) power transistors. Who can I get to change these in the Seattle area?

 

Thanks, everyone.

[onelife]

Have you tried your amp with a different speaker or your speaker with a different amp?

[tmhanser]

To be honest with you, I have no other speaker or amp to test with. I may just spring for a new 12" and give it a shot if I can't find someone that will let me try their speaker. Or, I may head in to Guitar Center or some other local gear store and ask if I can try a different speaker.

 

The "instability" issue seen by two techs - that part I can't figure out. How something can be intermittent and not constant is beyond me. Old, vintage amps can be a real pain, apparently.

 

What do I have to lose by changing out the two power transistors?

 

 

[onelife]

... How something can be intermittent and not constant is beyond me...

 

What do I have to lose by changing out the two power transistors?

 

 

One of the issues with electronics is heat. Circuits can heat up causing resistors to change their value or poor solder connections to become unstable.

 

Changing transistors usually involves a soldering iron. Again the issue is heat and semi-conductors (transistors) are susceptible to damage from heat from the iron. Unless you have experience with soldering and using heat sinks there is a risk of damaging the power transistors - and they may not be the source of the problem - so there is something to loose there.

[tmhanser]

Thanks, onelife.

 

I'm not going to attempt these fixes myself. I had one tech mention that this "instability" could be caused by a mis-matched pair of power transistors. And those have yet to be changed. Tech #3 changed a few caps, resistors and diodes, and he also indicated there was some "instability."

 

I could buy a new speaker but there's no guarantee it'll fix this buzzing-distortion.

 

Thanks again.

[onelife]

I could buy a new speaker but there's no guarantee it'll fix this buzzing-distortion.

 

The best thing would be if you could borrow a speaker in a cabinet and just plug your amp into it. Perhaps you could get together with a friend who has another combo amp so you could try your amp through a different speaker and another amp through your speaker - that way you could determine if the amplifier or the speaker is causing the problem without taking anything apart or spending any money.

[tmhanser]

Thanks, onelife.

 

I'll have to nail down another speaker to test this problem further. Maybe it IS a buzzing speaker.

 

[Mr.Grumpy]

Crossover distortion sounds a lot like a 'rattly' speaker. I turned down buying a used bass cab once (that was probably perfectly fine) because my bass head had crossover distortion from mis-biased output transistors. Just a little tweak of the trim pot (WITH A VOLTAGE METER ON THE APPROPRIATE TEST POINTS) and it was fixed. If your tubes are biased too "cold" this could cause the problem you describe. Or it could be something else, hard to tell over the internet.

 

I replaced the driver transistors on my MusicMan HD-212, the 150 watt, four 6L6 version, it was not a big deal, but then again I was working as an electronic technician at the time. Pretty standard bipolar power transistor in a TO-220 package.