Marshall Experts...I'm having "issues" biasing a JCM900...

[BrianS]

...50watter.  Serial number indicates it was built in 1990.

Customer complained of "thin" sound and stated it needs to be biased.  Okay.  Well, after doing the 4-way cleaning, I set to biasin' with my newly wired up Hoffman bias probes.  That led me to discover that my meter had TWO blown fuses in the current measuring circuit.  I'm veering off here...

Problem is that I cannot bias this amp even close to the proper current.  With the original tubes, I could only get about 17 or 18 mA per tube.  They were not matched too well, so I thought maybe they just needed to be replaced.  I took the Groove Tubes out of my Traynor YRM-1 and tried those.  They were matched much closer, but I could still only get up to about 23 mA of bias current through them.

I did a bunch of inter-sluething and schematic browsing, and I find that there are some different resistor values in this particular amp than what the schematic says, but apparently that is "normal" for Marshall.  I did find another reference to someone not being able to get enough bias current for their particular '900 as well (about the same amount that I am getting 17 to 19mA).  They were told by a tech to buy "hotter" tubes, which apparently worked for them.

From what I understand, JCM900s of this vintage never came with 6L6-type tubes.  What is going on here?  Should I change a resistor in the bias circuit so I can get the proper current?  Should I buy "hotter" tubes?  I just find it a bit strange that the amp would not be designed so that you can bias them up properly.

Any insight is appreciated.

[thermion]

JCM900s came with either 5881 or EL34. Has a part in the bias supply drifted way out of spec, like range resistor? Do the bottles from the marshall sound ok in the your traynor?

[jazbo8]

One version of the schematics I am looking at - for 5881 - it says to set the bias voltage (NOT the reading from your bias probe) to about -50v, is that possible with the amp you have?

[BrianS]

This is a MkIII JCM900, made in 1990.  All the research I've done says these did not come with 5881s.  The sockets are wired for EL34s as well.

Regardless, I "piggybacked" the 56k resistor that is in series with the bias pot with a 220k resistor, which gets me closer to the 47k value that is on the schematic.  That worked well and I could bias the tubes where I wanted them, with room to spare on either side.

The amp now sounds fuller and the distortion is "richer", I guess.  Changing out V1 helped the tone immensely as well.

[thermion]

Great, glad you nailed it down!

[EL34]

Any time you cannot get the proper range of bias voltage, find the bias range resistor and change it.
It's usually the first resistor before the bias diode as is shown in most all my layout diagrams

here's a Plexi example
The bias range resistor is in the red box at the right end

A larger value = less negative voltage
A smaller value = more negative