JVM Forum

Hey guys, been reading about how to bias the JVM, seems rather simple, but I would like to make sure my JVM is running properly!
I have been hearing people talk about plate voltage and cathode things etc, i have no idea what this is and how to measure it!

I live in the UK so our voltage is.....not sure, im sure you will know!

Any help would be appreciated
Cheers
-Anthony-

http://jvmforum.com/phpBB3/viewtopic.php?f=7&t=3310

In this topic, there were questions about biasing all over. The last question is mine, but the post before from big dooley is what I used when I biased mine. I have NO electronical knowledge whatsoever and I was able to do it no problem. The picture helped a lot, too.

In a nutshell:
What dooley explains is that the reading you get on the JVMs reading points is cathode current, not plate current. The tubes draw some current on the screens (from 3 to 5mA) and you have to take that into account when measuring.

For example: I biased mine at around 75mA per side (the reading I got on the reading points), which would mean (without taking the screen draw into account) that I have about 37.5mA per tube. But, since my tubes draw 3.8mA each, I double that value (because there are 2 tubes) and subtract it from the JVMs number. Therefore, I get close to 68mA (the numbers I use are give or take a few decimals), which is 34mA per side. That's the value I initially wanted.

Now, when you bias your amp, there is an upward limit you shouldn't exceed. If you remember, Power= Voltage x Current.
Voltage in this case is plate voltage. If I'm not mistaken, the voltage in the UK is 240V, which should give you about 450Vdc of plate voltage (how to measure this? It's in dooley's post.) My plate voltage was 442V, I'll use that in my example.
So, we already know the current (34mA or 0.034A), and now we know the voltage.
442V x 0.034A = 15W (plate dissipation. I have no idea how to explain what that is, since English isn't my first language, but that's how it's called xD)
The max plate dissipation for EL34 is 25W. So, 15 of the 25W is 60% plate dissipation, which is ok. The upward limit I was talking about is this percentage - you shouldn't exceed 70%.

Andy does his biasing by ear, and you should too. Like dooley says in that post, if for example 58% sounds good to you, there's no reason to run it higher.

Hope that helps

yezs, that topic will probably give all the answers, just take your time to read it, and get familiar with it...
one thing to add though:

inside your amp are voltage that'll easily kill you if you're not being careful!!!
touch off the wrong things and you're about to get zapped, or fry a transformer

Its alright i know to be careful, i have looked through one of the other threads already on how to bias it with the little pins etc, ill just make sure all i touch is them! haha

Hmmm i read through it, seems very good and im starting to understand it a little......but now im scared im gonna zap myself haha!

taking only readings from pin 3 and the testpoints is still acceptable
do the calculation and set your amp to 70%... but since you're not including screen current, the error is actually giving you a more conservative setting... more like 60/65%, which is great

if you want it hotter i do recommend to take readings for screen current...
nowaday's i'm measuring pin 4 for that..

example:
pin 3 is 480Vdc
pin 4 is 460Vdc
the difference is 20V which is the same as the total amount of screen current on 4 valves:
so total screen current is 20mA
20mA divided by 2 is 10mA, which you have to subtract from the readings on the testpoints from
the maximum bias setting is 70% of max plate dissipation at idle
EL34's are rated for 25 watt max, so this leaves 17.5 watt on idle or 35watt for a pair of tubes
35 watt / 480Vdc = 0.073mA or mV-reading on your testpoints... now add the screen current into it, which is 10mA in our example
and the absolute limit on the testpoints will be around 83mV

when setting your bias hotter, plate voltage will drop a bit, but screen voltage as well, so double check everything after...

then play the amp AND WATCH THOSE VALVES!!! if one of them gets a red glow on the plates (the outer metal part inside the tube) back off on the bias