Music Man chassis 2275-65

[Randall]

I've got a 115 Sixty Five in the shop for a tune up. Found a char broiled and broken in two 1 watt resistor from the pin 8 cathodes to ground. I See it on page two of the schematic, but I don't understand why it's there or what it does. The amp is working as is, so what is this resistor doing, and would you agree it is a 10 ohm 1 watt? Soon to be upgraded to a 2 watt. This amp is designated as 6CA7 tubes.

2475-65_&_2275-65.pdf

[Helmholtz]

Well it's a shared cathode resistor and adds a little cathode bias (0.5V) and degeneration.
But its main purpose is providing a bias test point.
Schematic says 10R/1W.
Reason for it opening probably was a shorting tube.
Open resistor might have saved the transformers.

[Randall]

I don't have a replacement 10 ohm, I was hoping I could use a 8 ohm 5 watt I have on hand, but if it's a bias test point, I'm guessing not.

The only reason I can think of as to why it burnt to a crisp is, there must have been a runaway or shorted tube? Interesting that the wires that goes ground from the cathodes are not burned, considering how small they are.

[g1]

Well it's a shared cathode resistor and adds a little cathode bias (0.5V) and degeneration.
But its main purpose is providing a bias test point.
Schematic says 10R/1W.

Considering how cold they run their cathode-drive versions, I'm surprised how hot they run the tube PI versions. I don't think they need to run that hot, but would worry about what B+ might rise to if idle current was reduced to something more like 60%.

Randall, as Helmholtz said, bad tube was likely the cause of the resistor burning.

[Helmholtz]

Considering how cold they run their cathode-drive versions, I'm surprised how hot they run the tube PI versions. I don't think they need to run that hot, but would worry about what B+ might rise to if idle current was reduced to something more like 60%.

Using schematic voltages and considering a screen current of 3mA I calculate a PD of 64%.

The EL34/6CA7 has a plate voltage limit of 800V.
The datasheet example with a B+ of 800V, screen voltage of 400V and Raa = 11k specifies an idle plate current of 25mA corresponding to a PD of 80% (datasheet calls this class B).
Low screen voltage and high Raa seem to be essential.
Works well with good EL34s.

Low PD might be crucial with 6L6GCs when exceeding their PV limit, though.

[Randall]

I got the amp re-capped, and now I have issues. I have double checked my polarities of the caps. But, my B+ is about 125v, and most everything else is low. I have no output

The standby circuit has me scratching my head. In "Standby" position the pilot lamp goes on, and some V's like the =/- 16v are there. In the "Standy Off" position the pilot lamp goes off, the bulb limiter glows bright, and voltages are all low. B+ is very low at under 110vdc no matter what.

What did I do to this amp? It was mostly working when it came here. Worse, I cannot read these smeared MM schems.

This one is one the second page of the second page.

[Helmholtz]

Why was the amp recapped?
Which caps were replaced?
Which caps measured bad?

What's your bulb limiter wattage?
The lower bulb wattage the more voltage it drops .

Pull power tubes and try again.

[g1]

I cannot read these smeared MM schems.

This one is one the second page of the second page.

The high voltage supply uses a voltage doubler. For the HV supply the drawing on pg.3 is very similar and easier to read. Just the standby switch is different (has hi/lo) and it does not have a choke (uses resistor instead).

[Randall]

Hemholz,

The electrolytics were replaced on the filter board as well as 5 - 150uF caps topside because they were original and 50 years old. I routinely do this to decades old e-caps. I did not measure the old caps.

These measurements were made with all tubes removed.

[Randall]

Guys,

I could really use some help with this Music Man amp. It was making sound when he brought it, and just wanted it re-capped and gone over.

My troubles began after I put in new e-caps. Most of them I did topside, but the little PS board needed to be lifted. Now I have problems. Barely any sound at all. I checked as best I could with my bad eyes and this blot of a schematic, but so far the amp wins.

Before I changed the zeners and big 3A 1kV diodes on the PS board, I had my +16v supply, but not the neg 16v. I also had low voltages everywhere. My B+ is around 138v.

Now I have 5.8V on the 16v rail, and 0.8v on the negative rail. B+, or plate voltage is 137v in Hi power mode with no tubes. I am told this amp has a voltage doubler, but in all honesty, I know nothing about that.

Any help please? My customer is also a PITA to add to things.

BTW, the blurry drawing I think is closest (12AX7 PI ) for my driver, and a more simplified PS, such as not doubling up on PS caps and diodes. (D11/D13 and C13/C46) for example.

I have replaced the two 16v zeners (1N4745), two lifting diodes (D9 and D10), and all e-caps. I have not pulled the 1N4003s (D11 - 14) mine just has one going in either way. They test good in circuit. The big 620 ohm resistors measure about the same in circuit, a little high.

I really don't know where to continue. I know someone is going to say "check your connections", but I have done that a couple times already.

[The Dude]

If the -16V rail is not there, it could be because of an open or short. The first thing I'd do is meter from the rail to ground to see which it is. If you have low resistance to ground, you're obviously looking for a short. If not, probably an open. Since you replaced caps, check that the cap on the -16V line is installed correct polarity (+ to ground). It's also possible one of your new caps is bad. FWIW: This is one of the reasons I don't subscribe to the "replace caps because they're old" theory.

[g1]

Do you have the + and - 46V supplies that come off D11 and D12 and feed the 680R 2W resistors before the zeners?

[Randall]

Holy shite this almost damn killed me. This one made me question my own existance. Combination broken wires, and two blunders on my part. I replaced the questionable diodes on the rectifier boar the ones right off the transformer with zeners. I was confused by this arcane schematic and layout with no component designations, and a very next to unreadable print. I walked awat from it several times and went for a ride on my Harley instead.

Got the diodes fixed, then found out I reversed one of the filter caps. DOH. Fixed that, still no. I found maybe three different wires that were broken but not visably so. Tracked those down.

Now I have all my voltages (725v B+!!) back, and no I did not have the +/- 46v supplies when I started tinto this today, but I do now!

Anyway, it's working. The vintage 6CA7 power tubes work, but are very microphonic, and not even close to being a set. I think I am going to try to convince him to go with JJ EL34s, set the bias and get this thing out of my life. This is the blurry schematic with the 12AX7 PI, not solid state.

Ta Ta!

[Jon Snell]

JJs at that sort of HT voltage ... I wouldn't!