Help with bias pot- wont adjust!

[mortie]

hey, so i have bought a Marshall jcm 900 sl x 100watt , re tube the amp head with Sovtek 6L6WXT , bought a bias probe kit, plugged in the probe, and the tubes are measuring 70mA! that's hot right?! i tried to adjust the bias trim pot, but the bias reading isn't moving! can anyone on these forums help?

[Hi-Q]

Could be a number of things, see attachment
Les

[M Gregg]

Hi,

If this is a second hand amp you could have a leaking coupling cap to the power tubes! Or the bias supply may be at fault.
As Hi-Q says you need to start by measuring the bias supply!

Are the tubes red plating inside "not the heater" the grey metal plate in side is it red?

As a second thought! What tubes are supposed to be fitted? 6L6? If this is a mod the bias supply may not be high enough for the 6L6. I would have thought 5881 or EL34 would have been the standard fit!

Regards
M. Gregg

[mortie]

hey, the slx is a 5881 tube amp, but thought the 6l6wxt was a 5881 type/ new replacement of the 5881, as for the coupling cap what is that, i have an understanding of tube amps but might need help with some parts of the amps, as for the attachment i will measure the resistors thanks for the schematic, thanks

[M Gregg]

Originally Posted by mortie

hey, the slx is a 5881 tube amp, but thought the 6l6wxt was a 5881 type/ new replacement of the 5881, as for the coupling cap what is that, i have an understanding of tube amps but might need help with some parts of the amps, as for the attachment i will measure the resistors thanks for the schematic, thanks

Do you have any of the original tubes? If you do do they bias correctly?
Sorry if you know about tube amps, however I need to make sure you are aware that 400V + may be inside the amp and that the amp may not have bleed resistors to drop the voltage so it could hold charge after power off for some time. It is lethal!

The coupling caps are C11 & 12 on the drawing from Hi-Q. If this has only happened since a re-tube of the amp then tube type is suspect! Linked to bias voltage.

Regards
M. Gregg

[mortie]

i threw the other tubes away as one of them blew when i used it for 2 hours /the tubes where already in the amp, the amp was bought second hand but the original tubes where tube amp doctor 6l6wgc, as for the measurement of resisters 26 to 27 i could only get a measurement of 140k ohms i dont know what setting to put the multimeter on

[M Gregg]

Its voltage DC that you need to measure.

It is the bias voltage between r26 & r27 to ground that you need to check.

Have you worked on tube amps before?

If you make a mistake and touch the 400v rail with your hand it may kill you!

Just for interest, if you can increase the current through the tube but not decrease it, then the -bias voltage is to low. This would require a mod such as a voltage doubler on the bias supply.

Also check Fs1 fuse is not blown!

Regards
M. Gregg

[mortie]

hey, no i haven't really tried to fix a tube amp, my friend is an electrician, i have done a little studying on electronics, i am guessing you shouldn't touch capacitors with your hand what else is there you should avoid touching inside an amp, i am just learning while trying to fix the amp,


is this bias pot fixable, what are the usualy reasons for an pot to not adjust the bias current, thanks

[M Gregg]

Originally Posted by mortie

hey, no i haven't really tried to fix a tube amp, my friend is an electrician, i have done a little studying on electronics, i am guessing you shouldn't touch capacitors with your hand what else is there you should avoid touching inside an amp, i am just learning while trying to fix the amp,


is this bias pot fixable, what are the usualy reasons for an pot to not adjust the bias current, thanks

mortie,

You need to learn with your electrician friend helping you. Show him this information and post what you find. Some one will pick up what you post and take it further.

Please do not go any further until your electrician is with you. I know you are interested and I want you to learn and be safe!

So please post what you find after you show this to your friend!

I cannot be confident without help with you so I will stop at this point!

Regards
M. Gregg

[mortie]

ok, thanks for your help, like i said i have an understanding of tube amps, and i will not touch the tube amp circuit only with the probes which are insulted, i have not touched the capacitors as they store electrical charge, but can you tel me which parts of the amp shouldn't be touched with a probe at all, i am just trying to see what needs fixing on the amp head need to play some tunes haha!