Musicman 2475 bias question

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I'm looking at a Musicman 2475 (130 watt 212) here, and am seeing something kinda odd. This is the tube PI version, so I would expect to see 10 ohm resistors off of two of the power tubes' cathodes (according to the schematic) to be used for checking current draw. Not only are there no resistors, but all the cathodes are tied together, and go through a 210 ohm 2 watter (with a bypass cap) to ground instead.

My first guess is that somebody modified it to this layout--no idea why--but a couple of things don't quite click for me. The cathode cap and resistor look replaced, but the cathodes wired together looks stock... and I don't really see anything to suggest where two 10 ohm resistors might have been grounded. Maybe that's just paranoia, but the other thing is the bias supply. I've got about -16v on the grids, which works out along with the big cathode cap, but it's derived from a supply which is +/-16v. On the schematic, it looks like that supply should be +/-46v.

As it stands, the tubes are drawing around 20ma each (well, 80ma total), so at least it was biased about right. I'm wondering if it's possible that this is some transitional design, before they got rid of the tube PI, and relatedly, should I put it back to stock... which looks like trouble, given the lack of enough negative bias voltage.

Anyone seen this kind of thing of one of these?

Thanks,

Tom

[Amp Kat]

I've seen some different current sense resistors like 5 ohms and 1 ohm types too but not Cathode biased so yes it was modded. The voltage on pin 5 would be due to respsect of the Cathode and your calculation would be the voltage across the Cathode resistor divided by in this situation 210 ohms and that will give you the current. Of course we are talking at quiescent point or idle. There is probably some screen currect that should be added also but it shouldn't amount to squat. Mutiply the current by the plate voltage and that's your wattage. The +/- 16 volts should be for the op-amps rail voltage coming off the zeners. I believe the bias supply is on a seperate small board with the orange wires (I think) going to it but I don't recall a bias pot. I thought in it's original state it had two cathodes tied together with the 10 ohm current sense on each pair going to ground. Not sure I'm crazy about the Cathode bias on this perticular amp. Isn't there about 700+ on the plates in high mode ?

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Ampkat,

Thanks for the reply. Re-reading my post, I think I was unclear about a few things. Yes, the B+ is around 700VDC, and the 80ma measurement (total for all 4 tubes) I mentioned was obtained by measuring the voltage across that 210 ohm cathode R. But the -16v bias voltage on pin 5 is actually coming from a bias supply--it's there even with the tubes removed. The grids aren't grounded as in a standard cathode-bias design. As such, this is really set up as a combined grid-bias/cathode-bias design.

The bias supply that I'm talking about is on a separate board near where the AC cord comes in. This board feeds a pot on the main board, which then goes on to a third board to feed the grid resistors for the EL34s. No orange wires, however. I realize I'm spinning a tale now, but it goes like this: The original PT is replaced with one from the later design. When the tech gets it in there, he realizes there's no way it is going to get sufficient bias voltage, so he puts in a cathode-biasing resistor to get it the rest of the way there. I'm thinking this for a few of reasons;

1. the rest of the bias circuit, including resistor and trim pot values, are all accurate to the earlier design's schematic.

2. The tap on the PT that feeds this circuit reads around 12-13VAC, and 16-17VDC after rectification--makes sense for the later circuit, too, but not this one. No tap on the PT reads anything in AC that might translate to the proper range of bias voltage.

3. The taps on the PT look re-soldered, the PT mounting is missing a nut where it's secured to the chassis, and the chassis strap on this side is only secured by one screw, and there are other replaced parts and signs of sodering, some good some bad... basically, it shows evidence of something changed around there.

I guess I just wanted to know if this combined grid/cathode-bias scheme was ever attempted by musicman. Whether it was that or the replaced PT or whatever, it looks like I can't really wire this back to the schematic's design, since I just don't have enough bias voltage, which kinda sucks since it's definitely not putting out 130w.

-Tom

[Amp Kat]

One thing I can say toggle is I'm glad it's you and not me. I have heard of Hi-Fi- amps biased this way and all I can see is the -16 volts clamps the bias at a level not allowing the tube to conduct anymore from the difference of the Cathode resistor and the bias point. Totally different from the Cathode injected grounded grid circuit of the other style Music Man with the 1692 transistors. What I would do is tap off the AC plate supply and build a new bias circuit and use the existing pot and revert it back to it's origin and your done.

[Enzo]

The shematic shows the two cathodes wired together with a common 10 ohm to ground, not individual cathode resistors. REplace the 10 ohm with the 210 and you have what you have.

If someone replaced the power transformer with something else, that might explain things.

If I were to wager, I might bet on this: There were two versions of the 2475-65. One with the tube PI and the one with the transistor drive. The transforemer you have sounds like the one for the transistor drive version. It has the high voltage widnings, but there are not 46v windings, the extra secondary is rectified and heads right to 16v zeners. SOmeone lost the power tranny and came up with this one as a replacement.

They added the cathode resistor to take the place of the missing bias voltage capability.

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Enzo,

Yeah--the more I think about it, the replaced PT story (using one from the later design) makes the most sense. As for the EL34 cathode R, this is a four tube model, so I think there were two 10 ohm current sensing resistors--one for each pair.

Taking another look at it today, I'm also noticing that the supply for the opamps is coming out at around +/-9.5VDC (measured at either end of the zener pair). I'm guessing that this should be +/-16VDC? That's what it is on the later schematic--the earlier one doesn't make note of it.

So, with this amp modded as it is, not only do the power tubes get a lower voltage bias supply, but the ICs get a lower voltage, too. What do you think of deriving a 46VDC supply as a tap off of the HV winding, as AmpKat suggested? It seems like that's my best bet for getting this amp to operate as intended throughout the circuit.

-Tom

[Enzo]

Sure, why not?

[xtian]

I have a Music Man 212-HD 130 (chassis 2475-130) with a tube PI. I'm installing a new quad of JJ EL34s. I'm wondering how to evaluate the plate voltage, in order to choose a bias point. The plates have ~700v, and the screens have ~350v. So, in the bias calculator, do I enter 700v or 350v plates?

[Enzo]

Hi, xtian.

Please start a new thread for your amp, this thread has been dead for twelve years.

Your plates don't have 350v on them, so I wouldn't use that.

Schematic says adjust bias for exactly 1/2v at "point Y", which is cathodes of tubes.