MusicMan 2275-130 Bias question

[Prattacaster]

I currently have one on the bench that is giving me some funny bias readings. A schematic is here:



My reading across the top 39ohm emitter resistor is 36mV. The bottom emitter resistor reads 62mV.

I checked the resistances and they match(within tenths of ohms).

I swapped pairs of tubes to see if the problem followed the tubes and it does not.

There is no way to set the balance side to side so I'm left with thinking it is a drifted JE 1692.

Any other ideas??

[Enzo]

SO far then you have eliminated the tubes as the difference. Drifting transistors? Not my first thought.

First, isolate the problem. MAybe you have funny drive issues, or maybe the drive is fine but the tubes can;t respond. Check ALL sockets. Do you have the same screen voltage on all four at pins 4? And of course B+ at all pins 3. Do you get the same +15v at all four pins 5? What are your cathode voltages?

ANother way of looking at it might be to pull the power tubes, then stick one tube in V1, measure the current flow with voltage across R56. Note that voltage, then move the tube to V3. Reading ought to stay the same, does it? Now move the tube down to V2, and take the reading across R61. Now V4. Are V1 V3 the same? and are V2 V4 the same? and how do the R56 readings compare to the R61 readings now that we have only one tube at a time. If they still are way apart, then I would look down to the drive.

You measures R56 and R61, but those transistors also have four 470 ohm resistors at their other terminals. Measuer across each one looking for any out of step. They are wired in a circle, so ther are funny parallel paths, but those should be the same for each of them, so if one reads only 390 ohms, then so should they all. ANy strangers?

Weak transistor? Sure. Replace those in pairs, if one is bad, replace both. I'd use a common MJE15032, $1.19 each at Mouser.

[Prattacaster]

They are wired in a circle, so ther are funny parallel paths, but those should be the same for each of them, so if one reads only 390 ohms, then so should they all. ANy strangers?

I'll check 'em. However, two of them will test similar but different than the other pair of 470ohm resistors because they do not connect directly to ground, but rather a 20uF 25v cap.

Pins 3,4 and 5 do check out without the tubes in. I'm juggling football, kids, and amp repair tonight so it'll take me a little time to thoroughly test the amp.

I really was trying to get a feel of the likelihood of these driver transistors being the root of the problem.

The kicker with this is that the components are not numbered on the PCB as they are on the schematic, there is no numbering on the PCB so I have to track ithem down by resistance checks. A little more time consuming.

Edit: I tracked down the 470ohmers , and this is how they test: R58 and R59 test at 483ohms. R65 and R68 test about 241ohms. The base to ground resistance is the same for both drivers(483ohms).

I did the "one tube in" test Enzo suggested. It revealed a problem or inconsistency. V1 and V3 tested the same-
Vp=637vdc
Vg2=317vdc
K=47vdc
Voltage across 3.9r = .034

V2 and V4 also tested the same, but different from V1 and V3-
Vp=562
Vg2=277
K=41.5
Voltage across 3.9r= .106

With all tubes in the plate and screen voltage are the same

I am running the primaries at about 95-100vac.

The side running high current with the single tube test is the same side running high with all four tubes installed.

R63 and R64 1.5k screen resistors test very similar.

I'm leaning towards Q2 driver being weak or funky, but I defer to the forum.

Edit again: I was just thinking how the bias is set much too high considering I'm running at 80% primary voltage. But then again the cathode voltage would also increase which might offset the general rise in voltage. Still a little too high.

[Amp Kat]

I'll check 'em. However, two of them will test similar but different than the other pair of 470ohm resistors because they do not connect directly to ground, but rather a 20uF 25v cap.

Pins 3,4 and 5 do check out without the tubes in. I'm juggling football, kids, and amp repair tonight so it'll take me a little time to thoroughly test the amp.

I really was trying to get a feel of the likelihood of these driver transistors being the root of the problem.

The kicker with this is that the components are not numbered on the PCB as they are on the schematic, there is no numbering on the PCB so I have to track ithem down by resistance checks. A little more time consuming.

Edit: I tracked down the 470ohmers , and this is how they test: R58 and R59 test at 483ohms. R65 and R68 test about 241ohms. The base to ground resistance is the same for both drivers(483ohms).

I did the "one tube in" test Enzo suggested. It revealed a problem or inconsistency. V1 and V3 tested the same-
Vp=637vdc
Vg2=317vdc
K=47vdc
Voltage across 3.9r = .034

V2 and V4 also tested the same, but different from V1 and V3-
Vp=562
Vg2=277
K=41.5
Voltage across 3.9r= .106

With all tubes in the plate and screen voltage are the same

I am running the primaries at about 95-100vac.

The side running high current with the single tube test is the same side running high with all four tubes installed.

R63 and R64 1.5k screen resistors test very similar.

I'm leaning towards Q2 driver being weak or funky, but I defer to the forum.

Edit again: I was just thinking how the bias is set much too high considering I'm running at 80% primary voltage. But then again the cathode voltage would also increase which might offset the general rise in voltage. Still a little too high.

Not saying it can't happen but I've never seen one of those transistors go bad in a Music Man Amp. It's always something else when it comes down to the culprit and they really don't do anything power wise in that circuit.

[Prattacaster]

Not saying it can't happen but I've never seen one of those transistors go bad in a Music Man Amp. It's always something else when it comes down to the culprit and they really don't do anything power wise in that circuit.

Ok, Well the components are easy to unsolder and check, I'll check the diodes, maybe a leaky C36.

I should mention that the drivers are TIP41C not the JE1692 or the 2N6488G that were apparently the stock transistors.

[woodyc]

Does anybody know the history of these amp, i.e. why they all have the same model number but some have a 12AX7 splitter and others use the opamp/transistor arrangement? Was one an upgrade of the other?

[JoeM]

Woody,

From what I recall, earlier ones had the tube. I first bought a 65W MM, and then a year or so later, sold that and got almost the same amp, but the 130W model. First one had the tube, and the later one transistor.

[Amp Kat]

I may be wrong but I think the 130 is the only tube model. and I don't think it uses the grounded grid and cathode drive transistors but uses fixed bias.

[Prattacaster]

I am currently working on a 65w with a tube splitter and a 130 with the SS drivers. OF course the one giving me fits is the SS driver type. The 65 and 130 versions have EL34's; earlier versions of either one have a tube driver.

Update:

I swapped the driver/bias transistors to see if the problem would follow. It does not. So I can rule out the driver TIP41C's.

I checked the diodes(D15 & D16) and they give .5 diode drop. There is an added series diode with D15 which is a deviation from the schematic.

Here are some voltages at full power:
I list one set of voltages if they are all the same.
Vp:725
Vg2:357
K: 57vdc
Vg1: 16.1
Bases of TIP41C: .57
Collectors are of course the same as Cathode.

Emitter on top TIP41C: .042
Emitter on Bottom TIP41C: .025 (no change with transistor swap)

I swapped IC-8. There was a JC4558C and I put in different 4558. This did bring the emitter voltages within 10mA or so. I tried a couple and they seemed to be a positive change. But not enough to make me think they are the problem or that the original IC-8 is the issue.

The side that is drawing more current is the side connected to PIN 1 of IC-8 or the top side.

When the amp is in "Standby" the emitter voltage for the top driver is .025 as is the bottom side. However when the HT is flipped in the top emitter voltage rises almost 20mV while the bottom emitter stays the same (+/- 2mV).

I dont know if the issue is with the bottom or the top sets. I would think the emitter voltage should rise when the HT is applied. Once again I defer to the forum.

Thanks.

[Amp Kat]

Don't think those IC's are supposed to be 4558's Do they have the converter sockets that small bear makes ?

[Prattacaster]

The other IC's are not 4558's. They are LM307H's Can-type 8-pin configuration. IC-8 is the only modern style IC and has a socket, not a conversion socket, the socket is original stock.

[g1]

Sometimes 4558 will not work as a sub for 1458. There is a Marshall example Enzo has pointed out before, I don't remember the model but only a 1458 works properly.
I'm wondering if this is possibly an issue here.

Edit: C35 and C36 would also be worth looking at.

[Amp Kat]

^^^ Yeah that's what I was thinking of and not the 307s that are different pinouts. I also don't think the 4558 will sub for the 1458 and could be the problem but they are the same pinout. You do have +/_ 16 volts on the 1458 pin 4 & 8 ?

[Jazz P Bass]

I am afraid that 1458 is not the same as a 4558.
https://www.national.com/ds/LM/LM1458.pdf

[Prattacaster]

Sometimes 4558 will not work as a sub for 1458. There is a Marshall example Enzo has pointed out before, I don't remember the model but only a 1458 works properly.
I'm wondering if this is possibly an issue here.

Edit: C35 and C36 would also be worth looking at.

Interesting... I've seen a couple threads where people have subbed them in, but maybe the same issue occurred with throwing the bias off and went unnoticed.

C35 and C36 have been checked, there is no voltage leaking back to IC-8. There is negligible voltage on the positive sides of those caps, something like .57vdc.

I might just throw replacement for those in there just to rule out another set of components.

I have some 1458's in my order cart. I think those might be the issue since trying a couple different IC's changed the bias voltages.

The 65 and 130 I have are both getting an e-cap job. All the capacitors are original Mallory's including the e-caps. The 65 has brown turd Mallory's for the couplings and the 130 has orange drop Mallory's for the couplings. All those are staying.

The amps arent the greatest sounding, I'm hoping new caps and tubes will make em come alive. Both are pretty ice-picky.

If anyone would like some pics I can take a few.