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.