JCM 800 2204 Voltage "mystery"...

[AtomCap]

Working on a JCM800, all the voltages are present and correct in the psu, bias is @ 10 ma (should be around 36), replaced the bias pot, 15K, 47K, both 10uf/100V, (shotgun'd the whole bias circuit minus diodes..) have the right voltages too, even see the right voltages on all pins @ power tube sockets- yet no bueno on the bias..

anyone ever come across such strangeness??


thanks..

[Phil_S]

Tubes that are used up?

[AtomCap]

Brand new JJs, test fine on the amplitrex, even tried my go-to good tester El34's, same result....

in the process of checking all continuity

[pdf64]

Is the voltage on the screen grids good?

[AtomCap]

all voltages are within spec as posted in the schematic..
just checked the choke (fine..)

screens at 470V (withing spec of 464 in schem...)

the mystery continues.

[Phil_S]

Assuming you have a good measure at 10mA, we know that is way too cold. You say you have the correct (or reasonable) negative voltage at the socket pins of the grids. So something is off (literally), by that I mean shunted to ground or not making a connection. Have you stuck a voltage probe in the tube side of the socket to see if the socket pin(s) are conducting through to the tube pin(s). Is this amp passing any signal at all the to speaker?

[AtomCap]

After much confruckstrations on this, I decided to go cross reference the marshall book.. The schematic I was working with was laid out for 6550 operation but not labeled as such.. (bias values different..) the amp is/was right.
Its just I didn't know I had the wrong tubes in, because I foolishly trusted the mislabeled gif.... I popped a set of 6550s in, biased right up 40ma, with a range up to 74ma on tap, So after all the fun - I "interneted" myself and a book saved my ass.

the big kicker is, this amp is for a customer and he was gigging it with EL34s, biased @ 10MA, which is 25ma short, so he's gonna shit bricks when he hears this thing now


I did check continuity within the sockets, I made a gizmo for doing that out of an old tube! just because I work on so much old shit that having a malfunctioning good looking tube socket isn't a improbable reality, what a mouthful that was..

anyway, thanks for the helpage Gonna go pass out..

[Phil_S]

Persistence got you there. Congratulations!