DSL-100 tube problem

[jezzzz2003]

I have a DSL-100 that has one tune redplating, which is strange,
Ive found one 5w resistor for that tube open circuit so need to replace it, my question is, what might cause this to happen?

[jezzzz2003]

Update, Ive replaced it and now the two tubes on the same side are redplating?

[JasonP]

what is the bias set at?

[Blue Strat]

Try moving the red plating tubes to the other sockets. If they still redplate the tubes are shot. If not, there's a problem with the bias feed to the first 2 sockets or your bias setting is way off.

Check the bias of all power tubes in the amp.

[jezzzz2003]

Ok Ive just checked the bias again, it was set to 85ma on both halves,
Now, Im measuring 85 on the opposite side to the redplating tubes (I swapped them but same side redplates) and I cant get any lower than 125 on the other side which isnt redplating? unless theyre set the opposite way around on the bias pins?

[Blue Strat]

Not sure I understand. If you're reading 85mA per tube, that's WAY too high. Back it down into the 30s. If 85mA is for both tubes, per side, that's probably ok.

If the higher reading tubes (125) read the same no matter which sockets you put them in, there's either a problem with those tubes or they are mismatched with the other 2.

You might want to start with a fresh MATCHED quartet and try again. You're going to need new tubes pretty soon anyway since the ones that have been replating aren't likely going to last too long.

[jezzzz2003]

Thanks Blue,

Well I read them again today, each tube reads around 40ma
but, when I try to bias from the back, the lowest reading I can possibly get at the bias pots lowest setting is 102ma per side, the redplating has gone now but why cant I read lower?
It read 90 at first reading but 10 mins later it had changed by just over 10ma?
Im using a Fluke meter on the ma setting.

[rockon1]

When you bias from the back 3 pin molex connector it is measured in mv not ma. Dont know if that helps. This link gives the procedure. If you have a probe you could just go by that reading. Also like Mike said perhaps a fresh set of tubes to start with. Bob.


http://www.marshallampforum.com/forum/viewtopic.php?t=17


..

[Carol-AnnAmps]

You are reading the combined plate current of two tubes per test point on the back. There is one test point per side, and two tubes perside in a 100W. It reads in mV, but thats just a reading across a 1 ohm resistor, so it's the same what the mA reading would be if you were measuring the current in line(Ohms law).
The two pots are very interactive and there is actually a video somewhere on the web that demonstrates how to carry this job out. I usually aim for 80mA per side on a 100W D/T SL. Anymore is a little enthusiastic for those amps and current power tubes.

[Backstage Kent]

I've seen several DSLs and TSLs with throughplate issues on the output tube sockets--I'd recommend soldering the sockets on the non-solder side of the double sided board. Make sure the bias supply is stable on both sides also--I've seen a couple with issues there as well.