Fender Bassman 10 70w

[mozz]

Got this to repair from a friend of a friend. It's been sitting a long time, missing a preamp tube and most knobs. Heavy son of a amp. They used it as a 4/10 speaker cabinet for a while after it blew. Appears one of the output tube sockets has arced over, tube is shorted, smoked screen resistor cracked in half, hum balance pot is burnt and looks like the power switch is burnt too. Of course there is a 30A fuse in there now. I can take care of all of that, power transformers DC primary and secondary resistance appears good compared to the Hammond replacement specs. 100 ohm resistors to ground for the filaments should be ok, if it hums i will put a humdinger in there. There is a 30K 20W resistor in the power supply, it was hanging by 1 lead, unhooked. My question is why is this there? Only thing i can figure is to stop the voltage from climbing too high when turned on. I have read these run 500v or a bit higher so i ordered a pair of red base TAD 6l6gc hoping they actually handle 550v as the spec sheet says. I told the owner about $200 in parts already. He said he doesn't mind.

[Helmholtz]

There is a 30K 20W resistor in the power supply, it was hanging by 1 lead, unhooked. My question is why is this there?

Where/how was it wired? Which node? In series or to ground?

[mozz]

Hard to see on the schematic, it is above the bridge rectifier.

[Helmholtz]

Oh, it's orignal wiring. Missed that.

I'd say it's used to pull down the preamp supply.
As the screens are wired to UL taps, the screen currents don't run through the 2.7k "screen dropper".
So without the 30k resistor preamp voltages would be too high.

[mozz]

Yes, all original wiring, doesn't seem like any repair soldering was ever done. Most resistors are in spec except that 2.7k 10w which is reading 2.3k, so i am going to change that but leave most every other resistor alone. New filter/bias/cathode caps ordered. Makes sense on the preamp dropping resistor. Just seems like to use a 20w resistor giving off heat wasn't the greatest idea nor cost effective measure, and soldering it to a terminal lug wasn't too smart. And another question. Should the output tube filaments be wired in phase or out? I have seen them both ways.

[Helmholtz]

Should the output tube filaments be wired in phase or out? I have seen them both ways.

Don't think it makes a difference. Power tubes are not known to cause heater hum (power stage gain is below one).

In case of doubt I would wire the heaters in phase because common mode noise can't pass the OT.

[g1]

I've seen power resistors to ground as 'ballast' in a few other amps. Off the top of my head, the Fender 'The Twin' is the only specific one I can think of.
Field coil to ground was also another method sometimes used but I might be mistaken about that..

[Helmholtz]

Wondering why they didn't just use a higher value dropper (maybe 8.2k).
Would save energy, produce much less heat and improve filtering.

[g1]

Wondering why they didn't just use a higher value dropper (maybe 8.2k).

I always wondered the same. But they must have chose this for a particular reason as they have been doing it the other way forever. The only reason I could figure was that they wanted to put a bigger current load on the winding for some reason or other.

[Helmholtz]

The only reason I could figure was that they wanted to put a bigger current load on the winding for some reason or other.

So it seems.

Without the 30k resistor (but increased dropper) I estimate the total DC current draw at idle to be around 70mA.
The ballast resistor adds another 15mA.
Hard to see how this could make a significant difference - except if someone powers the amp with tubes pulled.
In that case the ballast resistor will prevent the voltages at preamp nodes to exceed the 500V limit.