Music Man 2475-130 Repair

[RWood]

That's encouraging; thanks.

I'll update the thread after I've put in the new transistors, etc.

[RWood]

SUCCESS but there is more work to be done

You guys were right on the money with the diagnosis, parts needed, etc. The collective knowledge here is phenomenal and I would have been dead in the water without you.

I put in the two new driver transistors, new 3.9R resistors and just because I bought one, a new opamp. I didn't put in the zeners although I did get some.

The foil side doesn't look petty - maybe the backing material heats up - but the traces, which were roughed up a bit from the previous repair, took solder very well. It all went smoothly (perhaps the toughest part was inserting the new opamp!)

(having trouble attaching pics from my work computer - will add later)





I brought voltage up slowly to about 4.5vac on the heaters so I could tell which way the bias pot adjusted. Once I figured that out I brought the voltage up the rest of the way and very easily dialed in 25 to 30mV across the 3.9Rs. Very stable current draw. I left it on for an hour and the current crept up maybe 2 - 3 mV across the resistors.

The amp was also very quiet, no surprise since all the filter caps are new BUT now there is this: a lot of hiss when I raise the Master Volume. No guitar plugged in, and it is not effected by the other controls, just noticeable hiss that starts with the MV at less than 1 and increases more as it is raised.

Ideas on this? Thanks, in advance.

[Enzo]

Hiss is hiss, typically from a preamp. It may or may not be normal what you are hearing. Otherwise, we track it down like any other place - isolate the source.


Always let an amp cook for 20 minutes or so before setting the bias - normal to drift initially.

[RWood]

Or.....or..... I could have had the amp upside down and gotten mixed up about what was on 0 and what was on 10.

Which I did. Doh.


Amp works fine. I will just back out gracefully now, if I can.......

[g1]

I was going to say that these amps are kind of noisy anyway.
A bit about the reverb tank, I figured Enzo's gut instinct would be correct as usual, and it sounds like it is all working well. You can ignore the following, but I figured I'd outline the differences between yours and the stock tank (in case anyone wants to get OCD about the tank for this model amp).
Stock tank is 4FB3A1B. You found in your amp a 4FB2B1A. The impedances are the same, yours has medium decay where the stock unit has long decay (4th digit of part #). Stock unit has grounds of input and output jacks connected to shell of tank. Yours is spec'd for input grounded to shell, output insulated (5th digit of part #). It could be that the output being insulated does not matter, or someone may have reconfigured it and connected it to the tank shell. In any case if there was a problem, you would either have no reverb, or a lot of hum when reverb turned up. Finally, the last digit of part # designated the position the tank should be mounted in. Again, it doesn't seem you have any problem, so you can probably ignore this. Stock unit is open side down, yours is open side up.
Explanation of the codes can be found here: https://www.amplifiedparts.com/tech-...d-and-compared

[RWood]

g1, thanks for the detail about the tank. It seems to work as it should. It is a little subdued compared to what I am used to, and that could be the medium decay. I have never heard of a tank mounted open-side up. This one was loose in the bottom of the amp with no bag. I got a bag for it and put it in open-side down. And there is no hum from the input or output jacks. I was not aware of most of these quirks. I have a few tanks I pulled from Hammond organs; I will be interested to see how they are spec'd.

[Pedro Vecino]

A curiosity: does this particular circuit have any precedent, in which the signal is injected to the cathodes of the power tubes?
Does the circuit have any advantage over traditional methods (power, tube life...)?

I have observed on an HD130 that following the bias instructions with 25mV across the resistors (3.9R) at the base of the transistors the current on the tube plates is about 3mA each at idle which corresponds to the division of [25mA : 3.9R] : 2, but I don't quite understand how it works.

Thanks!

[Enzo]

Yes, others have done it. The Peavey VTX series of models uses this approach.

It gets the final stage closer to class B.

[Helmholtz]

BJT and tube(s) are wired as cascode, where the BJT determines/controls the common current and the tube takes most of the voltage. Tubes are operated in class B.
It seems that this arrangement allows stable operation at higher than normal plate voltage and consequently more output power, but screen voltage is typically much lower than plate voltage.
It also seems that this circuit provides less crossover distortion than typically associated with class B operation.

[Pedro Vecino]

Thanks to both.

A peculiarity that I have observed is related to the adjustment of the master volume.
With the master volume at zero, setting the bias to 20mV on the 3.9R resistor I get 20.5mV with the setting to 10. But if I lower the master volume to 5, I get 25mV. I have repeated the operation several times. How can the bias setting depend on the final setting in the master volume? Makes sense?

Master volume potentiometer is 10K linear.

Thanks again.

[g1]

With the master volume at zero, setting the bias to 20mV on the 3.9R resistor I get 20.5mV with the setting to 10. But if I lower the master volume to 5, I get 25mV. I have repeated the operation several times. How can the bias setting depend on the final setting in the master volume? Makes sense?

Have you checked for increased hum or noise at these settings? Or change in the zener voltage at the power tube grids?

[Helmholtz]

Without signal the MV should have no influence on idle current as it's AC coupled. Otherwise consider a leaky C34.
With signal, average DC tube current increases with all class B or AB amps, but that's no longer idle current.

[Pedro Vecino]

g1: The amplifier is very quiet. It has no noise whatsoever, except the natural noise from the preamp on high settings. I get a stable 16.1V in the grids and it shows no signs of drift.

Helmholtz: Yesterday I changed capacitor C34. I just changed now C35 and C36 on the IC outputs without results.

I have observed that in one of the resistors it rises 5mV and the other 3mV when I go from a setting of zero to 5 in the master volume.

On the other hand: I remember reading somewhere that the master setting influenced the idle bias setting.
I did not give it credibility but it is what I have in front of me .

[Helmholtz]

Helmholtz: Yesterday I changed capacitor C34. I just changed now C35 and C36 on the IC outputs without results.

I have observed that in one of the resistors it rises 5mV and the other 3mV when I go from a setting of zero to 5 in the master volume.
.

I can only imagine that the change is caused by some leakage of C34. Even new ecaps have some leakage resistance in the high MOhm range.
If you care, replace with a film cap.
OTOH, the rise in idle current is completely insignificant.

[Pedro Vecino]

I thought the same: it is not very relevant but in search of spiritual relax I just tried a 22uF / 63V MKP WIMA capacitor and the same thing happens. I'll put the electrolytic back on.

Thanks again.