Okay this is kinda scary....

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gusfinley

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So I noticed last week that the plates on the EL84's in my DSL401 were glowing. The tubes has been incorrectly set up (biased WAY hot) due to a miscalculation and the fact that the four tubes share the same bias setting.....

I kept putting in different tubes (spares) and finally found a set that wouldn't glow. So I bought a new set of JJ tubes and last night they were glowing too!!! I checked the bias setting and it was correct as per Marshall's recommendation.

At this point I am quite puzzled so I pulled out my multimeter and started poking around thinking that somehow something in the circuitry of the power tubes was messed up... It all looked Okay.....

Then I checked the plate voltages and found that they were at about 415V!!! Now, two weeks ago when I measured them they were at 396V. The voltage somehow increased by 20V in two weeks.

I couldn't figure it out, and then I decided to test the line voltage - it was at 123V AC - a few volts high.. Then I got thinking that the power transformer uses a windings ratio to step up the voltage and so that "few" volts translates to Many volts by the time the amp filters everything to DC.

This morning the line voltage was back to 120V or so, but I'm still getting about 409V on the plates. Luckily I have some equations that will tell me what kind of bias current to set for each voltage.

Looks like I might have to check it every once in a while to see what it should really be at.

Is this type of thing normal, or do you think my apartment just has really bad power?
 
The only thing I can see is that the bias is going to be different for each set of tubes. That's why you get matched tubes, you just have one bias supply, set the same for all the tubes. Marshall's recommendation is a guideline. The actual bias is set for whatever set of tubes is in there.

You put the tubes in, then set the bias, not the other way around.

Overvoltage can definitely be a problem, not sure how much effect 123 vs. 117 would have. I would think that is within acceptable range, though.
 
well....

It seems that from my calculations of "then" and "now" that the power tansformer steps up the votalge by a certain factor. In terms of plate voltage is appears to be about 2.4x. after facoring in the line voltage and solid state rectification.

So for 117V I'd get a plate voltage of about 393V and for 123V i'd get about 413V, so that factor of "a little more line voltage" gives me a differnece of 20V on the plates. So, now a bias setting setup at 393V is going to be 5% higher than it needs to be just due to the rise in plate voltage!! And lets be honest - these amps are already biased pretty hot as it is.... at 415V I could barely get the bias setting into a "safe" value to keep the tube from glowing.....

Just for your info, I did buy a matched set of tubes and I did set the bias AFTER I put the tubes in according to what I thought the plate voltages usually where. Unfortunately they were MUCH higher than I had expected and it DID make a difference because the plates were gloeing for sure!!
 
Yikes

The plates hit 425V today!! at about 123VAC on the line - now its tamed back down to 415......... at 120V

This sucks!! I have to bias my amp so cold so it doesn't ruin my tubes!!! Is there any easy way to regulate the voltage going into my amp?
 
remember that your meter has a tolerance on its readings, I don't know what it is but you are getting an approximation, not the "real" voltage.
 
I agree with dragonworks.... The readings on your meter will drive you crazy if you let it. They will drift slightly almost everytime you check them, unless you have a really expensive unit. You can get a "power conditioner/regulator",
but I personally think it's overkill.
Although once I played a gig at a small place and everything sounded awful. The PA. was distorted and guitar amps wouldn't quite sound like they normally would. I measured it and it was something like 93 volts.Turned out that the circuit we were on also supplied a refrigeration unit in the kitchen.
I found another one and voila! everything was OK.
Getting back to your Glowing condition. Contact JJ's support and find out what the "MPD"(max. plate dissipation) is for the tubes you are using.
Are you using the correct formula to come up with your "safe" settings???
It should be somewhere between 50% and 70% of the MPD. This doesn't mean of course that you couldn't run at 40% or 85% if it gives you the performance that you want without glowing tubes.
For example....Let's say the MPD for your jj's is 35 watts and the plate voltage is 400v.
35 divided by 400 is .0875 x .70 = .06125 or roughly 61ma
35 divided by 400 is .0875 x .50 = .04375 or roughly 44ma
Therefore, an acceptable range would be between 44ma. and 61ma.
The MPD for each brand and type of tube is different.
 
425 volts is a lot for an EL84, no matter what. A lot of modern ones won't take that, no matter what you do to the bias.
 
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