Music Man 410-65 Service.

[g1]

Where's the correct schematic?

Post #19 (pg.2)

[Sea Chief]

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).

Hi chaps. Thanks for helping on this. Yes g1 is right.. mine is the 410-65 (four 10".. irrelevant/ 65 watts, two el34). Sorry for any confusion with the 130 schematic (all the same, apart from two extra tubes.. readable schematic you see hence using for reference).

So in terms of measuring the bias mA on each tube, Im still not sure where to do so. My single 10r 1w cathode resistor (just replaced with a MF 10r 1w) I can measure across. But this is a low voltage figure. I find some dedicated MManservice service info (definitely for my GB-1 main pbc, with my 12ax7) the same info as someone kindly found in a post yesterday:

--------------------

Section II GRID BIAS CALIBRATION (This applies to all models "using 12AX7 DRIVER TUBE")

A. This applies to all models using a grid drive. (12AX7 driver tube).


B. Adjustment is as follows:

1. Turn the amplifier to "on" with the HI/LO Switch in the HI position.
No signal.

2. Using a volt meter, measure the voltage from cathode (pin 8) to ground
on the output tubes. Across the 10 OHM cathode resistor is a
convenient measuring point.

3. Adjust the bias trimpot located farthest to the right (looking from the
front) on the GB-1 and BB-1 circuit boards until you read .5 volts DC
across the 10 OHM cathode resistor.


------------------

So this is all I have to follow. Is this all I need to establish, just to see 0.5v at this resistor @ two pins on one of the el34's?

Thanks, SC

[Sea Chief]

Where's the correct schematic?

Hi HH. Schematic ( correct, but a bit blurry ) is the FIRST/ UPPER one on this page:

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Sea Chief]

The original Music Man service manual here

( 3 "Sections", only Sections 2 & 3 are relevant to my amp: 2 for bias adjustment, 3 for tremolo adjustment)..

https://music-electronics-forum.com/...etch?id=843983

Thanks, SC

[g1]

So in terms of measuring the bias mA on each tube, Im still not sure where to do so. My single 10r 1w cathode resistor (just replaced with a MF 10r 1w) I can measure across. But this is a low voltage figure.

Yes that is where you measure it. It is supposed to be a low voltage figure, 0.5V DC.
Solving for current, 0.5V divided by 10ohms gives .05A, or 50 milliamps. That 50mA is shared between the two tubes as they both run through that same resistor. So 25mA per tube is the current when you set bias trim pot for 0.5VDC across that resistor (assuming they are well matched power tubes).

[Sea Chief]

Yes that is where you measure it. It is supposed to be a low voltage figure, 0.5V DC.
Solving for current, 0.5V divided by 10ohms gives .05A, or 50 milliamps. That 50mA is shared between the two tubes as they both run through that same resistor. So 25mA per tube is the current when you set bias trim pot for 0.5VDC across that resistor (assuming they are well matched power tubes).

Hi g1, ok great thanks. Bu would if seem more ' accurate to adjust to correct 25mA' actually measuring the mA, as I recall doing on other amps/ never this way, as it would be a bigger number by a factor of 50 (?) compared to 0.5 though? Or for this it doesn't really matter/ just as long as you hit 0.5vdc via the trimpot.

[Sea Chief]

My sprague 150/50 recap job, & fatso resistors: two mint green 150k charlies top rhs, two 620r 3w are raised 3/4" above pcb..

[Sea Chief]

Just redone my bias trimpot to 0.500v across the 10r cathode resistor. It was at 0.320v.. so that means they were running "cold" at 16mA per tube? If so maybe I'll hear a bit of difference.

If I've upped the mA, isn't it more important I (somehow) now check the B+ V isn't too high? Especially if we're talking 725v.

Thanks chaps, SC

[Sea Chief]

Got the trem sorted too.. turned out it needed another 1/2 trimpot rotation to find another small area on the trimpot ( dead between two 'areas' of the dial where the effect worked, gave me impression Id done all i could).

Ideally I need to check the B+ lastly: is there no way of measuring " half " of say 725v anywhere?

Anyway service done. Extremely grateful for the help, pdf, g1, HH & mr.thud especially. Here she is with a new lease if life..

[Helmholtz]

Ideally I need to check the B+ lastly: is there no way of measuring " half " of say 725v anywhere?

Half B+ can be measured at the closed standby switch.

[Sea Chief]

Half B+ can be measured at the closed standby switch.

Really? Ok that's great then. Will have to get her up on the bench again tomorrow. Let's hope I get 360v. Cheers SC

[g1]

If I've upped the mA, isn't it more important I (somehow) now check the B+ V isn't too high? Especially if we're talking 725v.

It's good to check it, but if anything, raising the idle current will bring the B+ down a little. (tubes doing a bit more work at idle, more of a load on the supply)

[Sea Chief]

It's good to check it, but if anything, raising the idle current will bring the B+ down a little. (tubes doing a bit more work at idle, more of a load on the supply)

That's interesting- so I've learnt a good few things on this thread. Good I'll leave for now & just monitor the tubes visually/ heat on amp top might give me an indicator.

One last thing: the tremolo Ive got working as best I can via trimpot.. but still puzzled by this: during searching for service info I came upon this clip of a 210-65 (same amp, just two 10" etc) for sale. Good Tremolo example. Now this one's effect -does- happen with the preamp distortion, getting a fairly decent 'supro' sound. IE the effect, unlike mine still, doesn't diminish/ muddy incrementally as you turn up the 'volume' increacing gain until its a mush at 10.

So mine is only useable clean. If this clip's -65 amp can do it, so could mine. But what would be hampering mine I wonder?

You have to get through a link wray impression Im afraid . Then he tries to play ' baby shark ' or something. Thankfully only 2 mins. If you could bear it..

https://www.youtube.com/watch?v=sdm_Zd_e9d0

Thanks, SC

[loudthud]

It would be nice to see the oscilloscope pictures that were missing from the pdf file (post 64) where it tells how to adjust the tremolo trim pot.

[Sea Chief]

It would be nice to see the oscilloscope pictures that were missing from the pdf file (post 64) where it tells how to adjust the tremolo trim pot.

What is the benefit of using the scope over playing/ listening/ tweaking trimpot as I'm only able to do (without a scope) ?