Music Man 410-65 Service.

[g1]

So leave R56 in you mean?

I think he meant leave it as a 1W and replace, (not upgrade to 2W) as it can provide fusing action.
Metal oxide or metal film should be ok. Be careful about the physical size, some of the modern resistors are tiny and difficult to work with. When there is a choice between components of the same value & wattage, I go for the larger body size. Larger physical size will also usually have a higher voltage rating.

[Helmholtz]

When there is a choice between components of the same value & wattage, I go for the larger body size. Larger physical size will also usually have a higher voltage rating.

...and means lower surface and wire temperature.

[Sea Chief]

I think he meant leave it as a 1W and replace, (not upgrade to 2W) as it can provide fusing action.
Metal oxide or metal film should be ok. Be careful about the physical size, some of the modern resistors are tiny and difficult to work with. When there is a choice between components of the same value & wattage, I go for the larger body size. Larger physical size will also usually have a higher voltage rating.

Good ok understood g1. I don't actually know what physical size Ive bought, but got my values in 2 w and 3 w.. & the single 10r 1w, all "metal oxide". I think i know what you mean some 1/4 w jobs are so diddy small. Anyway all lumped in with my sprague 150/50 five caps, at 20p each so can discard if n/ g.

thanks sC

[Sea Chief]

Hi Chaps, finally got all my 5 x 150/50v ( sprague ) caps from germany for the board, & my bigger resistors etc. So all set for a full recap.

And for my doghouse, two 20/450v f&t filter caps to do 1st. Between these two, is a CC resistor, & on the readable schematic its a "10.." but would this likely be a k or an r? And is it correct being a 1/2w ( in which case I will leave it be, not replace it ) ?

The filter board & this "10.." resistor in Q, is located just under the central BOLD words 'music man...california.". Just below the F and G test points.
On the second schematic here, even though a 2275-130 I believe both mine & this are identical in this filter board area:

http://pacair.com/mmamps3/sites/defa...d_2275-130.pdf

Thanks for reding, SC

[g1]

two 20/450v f&t filter caps to do 1st. Between these two, is a CC resistor, & on the readable schematic its a "10.." but would this likely be a k or an r? And is it correct being a 1/2w ( in which case I will leave it be, not replace it ) ?

That's 10K. If it measures good, just leave it.
They didn't use the 'R' designation way back then. On this schematic they would just call it 10 if it were 10R. Like the resistors on the cathodes of the power tubes.

[Sea Chief]

That's 10K. If it measures good, just leave it.
They didn't use the 'R' designation way back then. On this schematic they would just call it 10 if it were 10R. Like the resistors on the cathodes of the power tubes.

Hi g1.. excellent thanks. Yes it does measure 10k in circuit in fact, but just thought best to check/ schematic not too clear.

[Sea Chief]

Hi again chaps. Last leg of the service. All electrolytic caps & bigger resistors replaced. Amp working.

What I'd like to do is check voltages & bias, then finally my tremolo is still not right (cross this last).

I see on the schematic some "F" and "G" labels with a voltage figure. Are these service test points? If no bias figure labelled, would this be determined upon the B+ voltage (I believe a huge 725v?) & the known tube type here el34?

I think my DMM only goes up to 600v mind you! Hmm..

Appreciated, SC

[loudthud]

The "F" and "G" labels are a somewhat informal way of saying there are wires going form the Power Supply Filter board to the Phase Inverter board. There is most likely an eyelet on each board where the wire terminates. The Voltages you measure at these points may more around slightly as you change bias, but probably no more than five or ten percent. The source of Voltage is the "center tap" of the Voltage Doubler so expect about half the 725V.

[Helmholtz]

Bias has no effect on the tremolo circuit.

Bias information is provided as voltage drop across the shared 10R/1W cathode resistor. Looks like 0.5V.
Means very cold biasing, i.e class B operation.
If the amp sounds good and there's no redplating I recommend to leave bias alone.

[Sea Chief]

The "F" and "G" labels are a somewhat informal way of saying there are wires going form the Power Supply Filter board to the Phase Inverter board. There is most likely an eyelet on each board where the wire terminates. The Voltages you measure at these points may more around slightly as you change bias, but probably no more than five or ten percent. The source of Voltage is the "center tap" of the Voltage Doubler so expect about half the 725V.

Aha thanks mr.thud. Ok I thought the f,g were test points hence the voltage figures at these junctures.

Right. Now I just need to do a few things here. Set my bias I know reading up all I can on this very amp is ideally 23-25 ma, then test general voltages. (Lastly try & sort the tremolo). I found this info n similar MM's, to read my mA figures " across the 3.9 r ". But Im scouring my board & cannot find a pair.

Re. the B+ voltage. Can I ask, your reply here suggests, perhaps, I can measure my "725v" at a split point getting half this figure.. which I can effectively then double to hopefully tally around 725v. Is that right? Because without a DMM that goes up to 800v.. Im kinda stumped! Be damn useful if so.

Thanks, SC

[Sea Chief]

Bias has no effect on the tremolo circuit.

Bias information is provided as voltage drop across the shared 10R/1W cathode resistor. Looks like 0.5V.
Means very cold biasing, i.e class B operation.
If the amp sounds good and there's no redplating I recommend to leave bias alone.

Great, well it does sound generally fine ( apart from tremolo).. i think. But wouldn't it be wise to check general voltages, & bias if its possible to, as Ive got it open on the bench? It won't have been looked at for maybe 25 years, poss 35 (it has an old looking " tube amps wtd" service looking sticker on the inside cab.. so possibly only just retubed as no evidence of component swaps).

I do have to tackle the trem lastly, so will be on bench for the time being. Thanks HH

[g1]

Bias information is provided as voltage drop across the shared 10R/1W cathode resistor. Looks like 0.5V.
Means very cold biasing, i.e class B operation.

Plate voltage is around 725V. Without allowing for screens, I calculate just over 72%, which I would consider relatively 'hot' ?

[Helmholtz]

Plate voltage is around 725V. Without allowing for screens, I calculate just over 72%, which I would consider relatively 'hot' ?

0.5V across 10R means 50mA. Divided by 4 (tubes) makes 12.5mA. Multiplying with 725V makes 9.1W per tube.

Am I using the wrong schematic?

[g1]

0.5V across 10R means 50mA. Divided by 4 (tubes) makes 12.5mA. Multiplying with 725V makes 9.1W per tube.

His is the 50W model with only 2 tubes and one 10R resistor. He posted the 100W schematic for better clarity of the supply area. The 100W model has 2 cathode resistors. In both 50W & 100W amps, each tube is running 25mA.
The cathode drive models run very cold around 6mA per tube (class B).

[Helmholtz]

His is the 50W model with only 2 tubes and one 10R resistor. He posted the 100W schematic for better clarity of the supply area. The 100W model has 2 cathode resistors. In both 50W & 100W amps, each tube is running 25mA.
The cathode drive models run very cold around 6mA per tube (class B).

You're right. I used the 4xEL34 schematic of post #34 and missed the parallel second 10R resistor.
So it's just normal class AB operation. But should put out more than 50W.

Where's the correct schematic?