Guitar Gear Forum

I'm trying to check the bias on my Jet City head i recently purchased and i'm having some trouble with it for some reason.

I've worked it out to be 40mA. Plate voltage is 522V, runnign 6L6 for 70% plate dissapation.

Now i assume i want to be measuring the mA at pin 8 of the power tube sockets?

On the 2204 i built i have a 1ohm resistor from pins 8&1 to ground and i measure from the resitor, using Ohm's law converting the mA to mV.

But when i measure the current at either pin 1 or 8 it starts out low and increases and doesnt stop. Turning the trim pot didnt make a difference. I can't see a specific bias test point on the board...

Any ideas, probably a noob question but i can't figure it out.

Thanks in advance.

if it doesn't have a 1 ohm resistor - you need one to measure this way.

Is there another way to measure bias without that?

Can you measure the voltage at Pin 5?

Or do i need a bias probe?

Multimeter set to read 200mA (!) DC.

Red to Fuseholder (OT in, if you followed the wiring of a standard 2204), black to either of the PIN 3s of the power tubes.
Attention: 500V on your multimeter here !!!!!

You'll read parallel to the OT - good DMM needed.

Don't use the fecking 70% max. plate dissipation.
Way too much for modern tubes and modern amps.
60% is absolutely enough, no matter if you use EL34s or 6L6s.

Hold the phone!

I think i got it, asked another amp tech and help clarify.

They said:

"Connect red lead to output transformer centre tap (HT) and connect black lead to either side of the transformer primary (anode connecions) pin 3. This shorts out one half of the transformer and gives you how much current is flowing through that side of the transformer, so you half the reading for the individual bias current."

I think that is the same thing you explained Olaf

Thanks, i worked it out for 60% dissapation: (30x0.6)/533= 34mA

Used i bais probe in the end, my friend had one. Much safer as you dont need to worry about you you multimer effectively being a wire with HT supply though it.

Yup, thats the same method, Liam.
The difference between this way and using adaptors/1 Ohm Rs is, that on the cathode (adaptors) you also read the screen current, which adds 5-10% to what you read, so at the cathode 60-65% is ok while at the plates (again, good multimeter with low internal resistance needed) 55-60% is IMO ok.

Duesentrieb wrote:Don't use the fecking 70% max. plate dissipation.
Way too much for modern tubes and modern amps.
60% is absolutely enough, no matter if you use EL34s or 6L6s.

Thank you for the verification! I can't tell you how many arguments I've been in over this... Running at 70% sounded horribly hot to me, I just tweaked them back to where I liked the tone, and it ended up being in the 50-60% range.

jet66 wrote:

Duesentrieb wrote:Don't use the fecking 70% max. plate dissipation.
Way too much for modern tubes and modern amps.
60% is absolutely enough, no matter if you use EL34s or 6L6s.

Thank you for the verification! I can't tell you how many arguments I've been in over this... Running at 70% sounded horribly hot to me, I just tweaked them back to where I liked the tone, and it ended up being in the 50-60% range.

Pretty sure I remember a conversation where Dave at Baron also prefers right around the mid 50's. Olaf knows the goods