Egnater Tweaker Issue(s)

[fdesalvo]

Happy Friday!

Another Tweaker has landed on my bench. Owner states amp emitted a puff of smoke and he switched it off. After taking some voltage measurements with the amp unloaded, I found the heater circuit and its CT to be @ ~48vdc when measured to ground, but ~3.2V from either side to its CT. Hard to read, but is this due to the way the CT goes to ground? Is it elevated above ground, which could be causing the readings I mentioned?

The resistor that caused the smoke was R35, the 470R screen/grid resistor. There's no charring or discoloration and tit still measures @ 470ohm. After changing the bias resistor to 500ohm (from 220), I"m not getting any heating issues with R35 after installing new tubes. What I'm not getting is any sound at the output and the heaters on V5 are 4x as bright as V4. The only schematic I've found online is the one I found here, which I'll attach below.

TIA for any wisdom

[Helmholtz]

After taking some voltage measurements with the amp unloaded,...

Very bad idea with a tube amp. Big risk for the OT.

[fdesalvo]

Unloaded as in no tubes? I found the issue - F1's fuse holder was apparently damaged by the previous owner. When I wiggle it, the heaters dim to uniform brightness and the amp makes sound. TY

[Helmholtz]

Unloaded as in no tubes?

Ok, forget my comment (to me unloaded means without load).

[fdesalvo]

Ok, forget my comment (to me unloaded means without load).

My fault for the ambiguous vernacular

[g1]

Generally, 'unloaded' to means no speaker or load connected. Otherwise I usually just say 'power tubes pulled'.

The schematic is missing several bits, so not sure exactly where the heater CT elevation is coming from, but you can see at the power tube heaters there is a connection to R25 and CN5 which must be connected to some DC source.
There are two 'F1' fuses shown, one is 1.6AT and the other is 250mAF, which one has the fuse holder issue?
Maybe I'm missing something, but how do you know it was the screen resistor that gave off the smoke if it looks good and measures good?
If it was for sure the screen resistor that smoked, usually screen resistor issues are caused by bad power tubes. You lowered the idle current, so that may be why the power tube is behaving for now, but it may not be reliable.

[fdesalvo]

Generally, 'unloaded' to means no speaker or load connected. Otherwise I usually just say 'power tubes pulled'.

The schematic is missing several bits, so not sure exactly where the heater CT elevation is coming from, but you can see at the power tube heaters there is a connection to R25 and CN5 which must be connected to some DC source.
There are two 'F1' fuses shown, one is 1.6AT and the other is 250mAF, which one has the fuse holder issue?
Maybe I'm missing something, but how do you know it was the screen resistor that gave off the smoke if it looks good and measures good?
If it was for sure the screen resistor that smoked, usually screen resistor issues are caused by bad power tubes. You lowered the idle current, so that may be why the power tube is behaving for now, but it may not be reliable.

In my haste I left so much detail out. One of the original tubes did short and began overheating R35. I've never seen one of these resistors fail to show show evidence of overheating prior, but these are crazy times. He must've caught it just in time, too. I suspect the owner started pulling fuses and mis-inserted the 250mA fuse with enough force to bend the cap and break the solder pads. The cap that holds the fuse has really thin and brittle metal ears, which deformed. What a strange design this amp has; a hodgepodge of decent construction with cheap parts throughout. I re-tubed the power amp, and with the new bias resistor, the amp is back in service.

I keep taking on PCB amps for the challenge and to force myself to become a better tech and reader of schematics. I've a long way to go! TY!