Music Man 410-65 Service.

[Sea Chief]

As far as I can make out, yes to all the above. Though I can’t read component references on the schematic.

No I realise it's not easy.. frustrating, but in my Pitmann book this lower schematic with the 7025 tube isn't even included, so Im lucky to have it even as a rough plan.

Ok so I get your point, i think, which is to replace all the higher power resistors.

Anyone..

Im still having trouble identifying these high power resistors on my board. Especially the " power dropping " ones which pdf specifically hinted at being important to replace. Ive googled galore for clues as to where these resistors might reside in a tube amp.. but I can't get any clarity other than, I think, possibly close to the power tubes, or filter caps.. but Im just guessing. And Ive scoured both schematics until my brain hurts too.

How do I find where these ".." resistors might be-?



And is it normal to find an odd number of these higher power resistors? (5). Thanks, SC

[loudthud]

I think the two "dropper" resistors are the 2W 620 Ohm resistors near the bias pot. They drop the +/- 46V down to the +/- 16V zener diodes. The math says they burn 1.55W all the time and could go North of that if the Mains Voltage is high. They run hot. I wouldn't be surprised if they melted the solder and fell off the board. The PCB is probably discolored under them. Best to replace them with Metal Oxide flameproof resistors with 2W or 3W rating. Mount them up off the board.

[Sea Chief]

I think the two "dropper" resistors are the 2W 620 Ohm resistors near the bias pot. They drop the +/- 46V down to the +/- 16V zener diodes. The math says they burn 1.55W all the time and could go North of that if the Mains Voltage is high. They run hot. I wouldn't be surprised if they melted the solder and fell off the board. The PCB is probably discolored under them. Best to replace them with Metal Oxide flameproof resistors with 2W or 3W rating. Mount them up off the board.

Great help thanks. Yup colour codes check out, so I'll replace as you & pdf suggest & up a bit. And sprague 150/50 bias caps will go next to them too

Ive read an interesting ditty, from another exact same amp service. Regarding this bias area: it was said on there, that "when these amps introduced the eu voltage was 220v, now its 230v even 240v". This was a french service. The diodes, it was mentioned therefore, might not be up to the job.. or something to that effect.

Would this ring true for mine? Or is UK voltage 230v & with no obvious pcb browning ( diodes or the above fatso resistors, as you can see in my photo ) Im ok to leave as they are? Tried again to add my pcb photo.. but unable to. Its here tho.. https://music-electronics-forum.com/...rem-prob/page2

Thanks, SC

[Sea Chief]

The second schematic down on the link here, is effectively my exact circuit ( certainly in the filter & bias areas, amongst almost everything else ), just the 130 version with 2x additional tubes.

It is though, eligible. But it is sideways! Sorry.

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

[Sea Chief]

Could anyone help?

Im trying to get a list of the larger resistors to replace. I have the two "dropper resistors" identified (620 r, 2w) in the bias area, two more 2w ones identified in the rectifier area ( 150 k, 2 w),

But there is one mystery fatso, I cannot find on the schematic (being hard to decipher.. or on the easier to see 130 schematic I cannot find anywhere). I need some experienced help, to identify it.

It has a colour bands brown/ black/ black/ silver. I think this is a 10r. And same physical size as the other four 2w mentioned above so, I assume it is a 2w.

It is located on only one el34. Between pins 8 and 1.

Does this ring a bell with anyone, a resistor here on these pins of an el34?

Thanks. SC

[Sea Chief]

Pink arrow..

[pdf64]

R56 I think, between the output valve cathodes and chassis common.
Its probably a cathode current sensing resistor, to assess that the bias setting is ok.

[Sea Chief]

R56 I think, between the output valve cathodes and chassis common.
Its probably a cathode current sensing resistor, to assess that the bias setting is ok.

Hi pdf.. ok that great you can possibly id it. Would you replace this with a similar type as you suggested before, metal film flame proof 3w ?

Did you find R 56 on the easier-to-see "130 " schematic in my post #34 ?? If I could tally "10r 2w" to something definitive, I can order one.. if you think it's worth doing. Maybe if so little current &/ or voltage here, at such a low value, maybe its not important to do.

Thanks, SC

[Sea Chief]

pdf64 Aha I think Ive found it.. your 56 reference got me there. Great help that I was going insane looking for it.

Ok so its labelled as a 1w in fact.. would you replace it with a 2w metal film ? (replace it at all?)

thanks SC

[pdf64]

If a valve shorts it may well go up in flames, causing damage.
And CC types don’t do well with constant DC levels.
So replace definitely, using metal oxide or film, flameproof coated.

[Sea Chief]

If a valve shorts it may well go up in flames, causing damage.
And CC types don’t do well with constant DC levels.
So replace definitely, using metal oxide or film, flameproof coated.

Upping them from cc's to metal film ones for heat purposes/ safety measure.. yup principle idea understood.

But as a general rule, it seems, your rule of thumb is also.. to up their wattage by a factor of one as an extra safety measure-?

IE for this sole r56 10r/ 1w.. whack in a 10r/ 2w?

[Sea Chief]

pdf64 sorry, just one more Q ( then I can click 'order' for all my caps & resistors from germany, & pestering over ! ).

My photo above with link arrow.. there's a bigass black 10w 1.5k, looks like a submarine coming towards us. Replace this too or are these ( huge wirewounds i think ) bombproof/ leave it be-?

Thanks, SC

[pdf64]

In the absence of a HT fuse, I suggest to leave R56 10ohms at 1W.
If the big wire wound measures and looks ok, consider leaving it.

[Sea Chief]

In the absence of a HT fuse, I suggest to leave R56 10ohms at 1W.
If the big wire wound measures and looks ok, consider leaving it.

So leave R56 in you mean? Ok will measure the black submarine, hope its 1.5k. Does look fine.

thanks alot, SC

[Sea Chief]

Anyone know if metal oxide resistors are a better choice than metal film?

Or do I specifically need metal film for replacing these old 2w cc jobs?

thx SC