Biasing an Amp and Voltage question

[Catalyst77]

Ive just got a secondhand ceriatone plexi 50watter. :D

But I can't play it yet :(

The tubes came seperately from the amp so i need to bias it.  Its got a pot for this ontop of the chassis between the tubes and two red holes either side of the tubes.

Do i simply get a multi meter put the negative in one end and the positive in the other and fiddle with the pot till it reads 40mVs?

Also i noticed on the back there is a voltage selecter for 120, 220 and 230 volts, however all my other amps are set to 240 volts

Does that mean i can use it ? :?

[PhilKing]

230 volts will be fine.

For the bias you put the tubes in and put the amp on standby.  take the reading on each tube (if it is like mine there is a black comon and 2 reds probe sockets).  You will only be able to set the bias to the mid point for both tubes,  but if you got a balanced set, that should be OK anyway.

[Dmoney]

can i ask how you get the 40mV value?

EDIT: i miss read some stuff

[Catalyst77]

can i ask how you get the 40mV value?

EDIT: i miss read some stuff

Here's the link where i got the 40mv from; i know absolutely nothing about this sort of thing!

http://reviews.ebay.co.uk/Valves-Tubes-Do-I-need-to-Bias-my-Valve-amplifier_W0QQugidZ10000000000987756?ssPageName=BUYGD:CAT:-1:LISTINGS:1



I think i understand the mid way point thing about balancing the tubes; for some reason i can't seem to get any readings with the meter at all!

Doh

[Dmoney]

dude if you really dont know i wouldnt suggest trying to bias it.

so a ceriatone 50watt plexi is 2 EL34 power tubes.

you need the plate voltage of the amp, and the tube type, and from that you can work out an arbitrary idle current that your tubes need to draw to be working around 70% plate dissipation.

say your plate voltage is 490v and you are using EL34's, then idle current would be around 35mA, you can go hotter and colder according to taste, but within the operating tolerances of the tube itself.

(please someone correct me if im wrong)

in order to measure those voltages and current (especially if those red points dont seem to be doing anything) unless there is an actual bias measurement point in the ceriatone, you need to know a few techniques, and sticking your multimeter in the wrong place is a really bad idea! which is why i would suggest being super careful if you dont know what your doing. or maybe even taking it to an amp tech.

[hamfist]

Ive just got a secondhand ceriatone plexi 50watter. :D

But I can't play it yet :(

The tubes came seperately from the amp so i need to bias it.  Its got a pot for this ontop of the chassis between the tubes and two red holes either side of the tubes.

Do i simply get a multi meter put the negative in one end and the positive in the other and fiddle with the pot till it reads 40mVs?

Also i noticed on the back there is a voltage selecter for 120, 220 and 230 volts, however all my other amps are set to 240 volts

Does that mean i can use it ? :?

If the amp was already biased for that set of tubes, and they were just removed for transport, then you can just plug them straight back in and start playing.

  If not, then you need to find out what those red holes are, next to the power tubes, and how to use them. If the previous owner doesn't know then email Nik@ceriatone.com who will be more than happy to explain to you.  It certainly sounds like they might be biasing points.

  As for what bias current to set the tubes at, then you cannot just set them at an arbitrary 40mV (which actually will equate to a 40mA current, the way it is probably set out). You do need to have some idea of the plate voltage - which you DO NEED TO KNOW what you're doing to measure it, without risking a very nasty shock.
  Once you know the plate voltage, then you can work out what current is required at that voltage to get your tubes using 60-70% of the max power for a EL34 , which is 25W.

65% of 25W is 16.25W and the equation you need to use then is Watts = Volts x Amps, so if you have a Plate voltage of 450V, to find your current to achieve a bias of 65%, you need to re-arrange that equation W=V x A  .......  A = W/V  (current in Amps (not mA), power in Watts, and voltage in volts). So in the case of a 450V plate voltage, going for a 65% bias, A = 16.25/450, which equals 0.0361 A, which equals 36.1 mA (which, as I said earlier, will probably be measured as 36mV on the bias points you have on your amp.

As for finding out the Plate Voltage, Nik at Ceriatone should also be alble to tell you pretty much what it should be. Amps are usually quite consistent on this. Can't remeber exactly what my Ceriatone Plexi 50 used to run at. Think it was about 450V. It won't affect your calculactions much if you're out by +/- 10V anyway.

Good luck, and remember be very, very careful. Double check everything.

[PhilKing]

This is what Nik told me on my JTM45:
As for the bias, correct. We already install the 1ohm resistor onto the tube socket (pin 8/1 to ground). You just measure across this resistor, with meter set to DC Volts. Do one at a time.
 
Your reading will be milivolts. Shoot for 35-40mV.

Remember that this is for KT66's though, so your voltage will be different.

[HTH AMPS]

some rough calculations at 70% dissipation...

43mA @ 400v
38mA @ 450v
35mA @ 500v

If you shoot for 35mA you'll not go far wrong - look at the plates, are they glowing red?  bias them cooler (30mA).

[Catalyst77]

Thanks Guys

I cant get the tubes to balance perfectly, but ive got one tube at 40ma and the other at 38ma.

Ive played it a low volume and it seems fine, but i dont really want to crank it til i know its right.

[Dmoney]

did you find the plate voltage?

its normal for tubes to be a couple of mA out i think*. should be ok as long as you checked out the plate voltage.
I dont think you can assume that the pV is gonna be the same as the JTM45 mentioned previously, and those suggestd bias readings are for KT66 tubes which have different operating conditions to EL34's, which im guessing is the kind tubes you have.

*unless of course you own sound control music in manchester and you think its cool to have 6L6 tubes at 35mA and 67mA with 496v plate current!

[HTH AMPS]

Thanks Guys

I cant get the tubes to balance perfectly, but ive got one tube at 40ma and the other at 38ma.

Ive played it a low volume and it seems fine, but i dont really want to crank it til i know its right.

unless the valves are matched perfectly or you have individual bias control for each valve, you won't get them exact.  furthermore, it really doesn't matter - I'd consider 3mA appart to be closely matched.  any more than 6-8mA appart and I'd be looking to use other valves.

leave the amp cooking at idle for 15 mins, if the plates aren't getting any red spots then you're good to go - crank that baby.

[hamfist]

Thanks Guys

I cant get the tubes to balance perfectly, but ive got one tube at 40ma and the other at 38ma.

Ive played it a low volume and it seems fine, but i dont really want to crank it til i know its right.

If your plate voltage is about 450V then you've got those valves biased a tiny bit hot really (72% of max). To be honest, you won't be getting any improvements in tone by biasing them hot, but just shortening their lifespan.  Try reducing the bias and seeing what it sounds like. The best advice is always to have the valves biased as cool as possible, but still getting the best tone.
  If you're guessing at your Plate Voltage, and it ends up being nearer 500V than 450V then you really could cook those valves in a very short time.