Pin 1 6550 tie point

[Mick Bailey]

Just working on a Blankenship 'Twinplex' head that that a runaway bias issue. Interestingly, pin 1 on each of the 4x6550 tube sockets is used as a tie point for the grid stoppers and so connects the tube metal shell to the signal and bias voltage. Not something I'd want to do myself.

[Leo_Gnardo]

Just working on a Blankenship 'Twinplex' head that that a runaway bias issue. Interestingly, pin 1 on each of the 4x6550 tube sockets is used as a tie point for the grid stoppers and so connects the tube metal shell to the signal and bias voltage. Not something I'd do myself.

I've seen that done in a very old amp that had metal 6V6 output tubes. Its owner was wondering why he got a mild shock when he touched the tubes. Heck I'd be more worried he'd get burned. I guess he did his poking around before they warmed up much.

Should be easy enough to undo, just stand the stopper resistors up on pin 5 like Marshall used to.

[Mick Bailey]

Do you mean pin 6?

[Enzo]

Was the amp originally intended for 6550? or for 6L6?

[g1]

From what I've read, the Dynaco MKIII's also used pin1 as a tie point (w/6550's).
Seems some 6550's have pin1 connected to the base while some do not, no universal standard.

[Enzo]

And we should never rely on someone checking which tubes they use. Poor practice to use that pin on a 6550 amp.

[Mick Bailey]

The amp was supplied new with 6550s, but if it was originally designed for 6L6 then that would explain it.

[catalin gramada]

From what I've read, the Dynaco MKIII's also used pin1 as a tie point (w/6550's).
Seems some 6550's have pin1 connected to the base while some do not, no universal standard.

I feel also is not a rule. Tying to whatever potential is sort of screening method. I found on a pair of kt120 the ring conected directly to plate what is more dangerous of course, specialy if want to swap the tubes and are not sure the caps are completly drain !!! Worst,there is not mention into datasheet