67 Bassman Hum

[The Dude]

Since you initially had a shorted tube and smoke, I'd also check socket pins for charring. You could have a socket to tube pin problem. Cleaning and re-tensioning socket pins might be a good idea.

[Valvehead]

[QUOTE=g1;n990483
If you swap the tubes around, does the lower value (18mA) stay with the socket, or follow the tube?

[/QUOTE]

Follows the tube

[Valvehead]

Since you initially had a shorted tube and smoke, I'd also check socket pins for charring. You could have a socket to tube pin problem. Cleaning and re-tensioning socket pins might be a good idea.

Will do

[Helmholtz]

Follows the tube

So tubes are quite mismatched.

Please still post the voltages at the lower ends of the 220k grid leaks.

[Valvehead]

So tubes are quite mismatched.

Please still post the voltages at the lower ends of the 220k grid leaks.

Post #29 shows that ...yes? If not , where exactly do you mean lower ends?

i did put a set of matched tubes in and it made zero difference in the hum

[Helmholtz]

, where exactly do you mean lower ends?

Look at the schematic.Upper ends connect to 1.5k grid stoppers.

[Valvehead]

here ya go

[Helmholtz]

It is strange that one of the 220k resistors drops much more voltage than the other.
Did you check the resistor values?

What is the input resistance of your meter?

[Valvehead]

It is strange that one of the 220k resistors drops much more voltage than the other.
Did you check the resistor values?

What is the input resistance of your meter?

234k / 236k



I dont know input resistance of meter - its a Extech ex505

[pdf64]

here ya go

And just to be clear, if the 6L6 are swapped around, does that ‘large V DC across the grid leak’ thing follow the valve or stay with the socket?

It was s struggle to find it, but the meter’s input resistance is 10M, see p15 https://flir.netx.net/file/asset/492...nal/attachment

[Helmholtz]

So the voltage drop across the 220k (caused by the meter current) should be about 1.1V.
Higher voltage drop could be due to a leaky coupling cap.

[Valvehead]

So the voltage drop across the 220k (caused by the meter current) should be about 1.1V.
Higher voltage drop could be due to a leaky coupling cap.

ive got some new .022 around here i think

[pdf64]

I think it’s a good idea to use 630V caps to couple to output valve grids. As at power up, they might have perhaps >500V across them. And even the slightest DC leakage can spell disaster.

[Valvehead]

Changed those 2 .022 caps . No change in hum

[Helmholtz]

Changed those 2 .022 caps . No change in hum

Are your grid voltages equal now?