Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DanielG on August 02, 2020, 03:18:54 am

Title: JCM 900 SL-X Preamp tube voltages?
Post by: DanielG on August 02, 2020, 03:18:54 am

I decided to go outside the box a little and I've ended up putting together a JCM 900 SL-X Preamp with a single ended output stage.
It's actually working surprisingly well, and after I remembered to add the ground reference, it's surprisingly not very noisy considering how much gain it has.

However, I do not know what the right voltages for the preamp are supposed to be. It isn't cleaning up as well as it should so it's clearly not biased right.

I found an old repair thread on another site where someone listed the voltages of their amp while it was broken, but never followed up with the values after he got it working. The nightmarish schematic available for it doesn't have any voltages listed at all.

So, I'm wondering, do any of you guys and gals have a 900 SL-X you could throw a multimeter on real quick? Thanks all!



Thanks all!

Title: Re: JCM 900 SL-X Preamp tube voltages?
Post by: Jon Snell on August 03, 2020, 06:28:20 am

Ideally, we look for about 1/2 - 3/4 HT voltage for the anodes and if cathode follower, just north of 1/2 HT as long as there is room for the signal to pass without clipping, that will be fine.
Some people rely on anode current to get the gain curve of the triode, others are not fussed.
The DC values will of course vary with selected valve and age of valve, so pretty much a non starter to collate voltages.