Strange bias reading

[496dart]

Im getting a weird number on my meter when checking bias on a jcm800 -100w amp. Using the shunt method im getting -059.01 ma on one pair and -00.82 ma on the other pair . Ive checked all the parts in the bias supply and swapped the tubes around. The tubes are new matched Sovtek 6550.

What gives

[Enzo]

OK, on the bad pair, are the screens at B+ voltage? If not, check for iopen screen resistors. Have you recently had a tube failure? That could have damaged the screen resistors. Also verify that the cathodes are indeed grounded.

And try reversing your meter leads. 59ma is 59ma, who cares if the polarity is backwards. SOmetimes the meters respond funny. When I do that, for some reason on one side the reading settles right down to whatever it should be, and the other side always has to "charge up" to its reading over 10-12 seconds. Reversing the meter leads makes a difference.

Also check to see if the transformer has an open side. Is the plate of the cold tube getting B+ at all?

[496dart]

You arent going to believe this. I was working on it this afternoon-checked everything again, and i finally found the problem. This brand new Marshall DOES NOT have a ground wire for 2 of the tube cathodes !! They simply forgot to put a wire in !! Hard to believe.

I installed a wire and now im getting about 59ma per side--thats as high as it will go. I still think thats a little low for 6550 's with 450v at the plates......

[Enzo]

I believe it, that is why verify the cathodes to ground was on the list.

Was the amp originally set up for 6550? You might have to tweak the bias circuit values a bit to lower the bias voltage if you want more current.

[496dart]

yea Enzo--this is a stock amp-6550 model about 1-2 years old

[Carl / Zwengel Amps]

I'd keep a close eye on that amp anyway. Even Marshall wouldnt be THAT stupid as to leave off a pair of cathode grounds...it'd never make it through QA! If it were the inside or the outside pair my guess would be that somebody, maybe even the factory, changed trannies or the stock transformers couldn't reliably handle a quad of 6550's, or something else along those lines. I'd guess that someplace in the chain of posession somebody messed with something for some reason!

-Carl

[496dart]

That could be, the trannys are Dagnall . Is that the factory brand they use?

[Carl / Zwengel Amps]

Yup thats who mgf's their iron

-Carl