Biasing Chinese EL84's

[SmogFighter]

Using this Schematic,
http://www.axlguitars.com/schematics/AA-AK-40.pdf

At TP14 through TP17 It says I should have 20mv on each.

Is that setting a little "cold"? (I was filling my head with Biasing videos over at
Eurotubes dot com.)

The ones that are installed are Chinese "May-pop's" so I was thinking about
installing a different set to change the tone.

Also, I can't seem to be able to find the biasing trim pot in the schematic.
The main PCB has one though.

Edit: found it A-5

[Steve Conner]

The ones that are installed are Chinese "May-pop's" so I was thinking about
installing a different set to change the tone.

Sure enough, if you bias them too hot, they may pop.

20mA per tube seems reasonable, we recently had several threads on biasing, and they came to the conclusion that it's really not super important.

[MWJB]

"Also, I can't seem to be able to find the biasing trim pot in the schematic.
The main PCB has one though." It's called "BIAS 1" & sits betyween R110 & R111 on the schem.

Yes 20mA sounds cold, but if that's what the manufacturer specifies I'd stick to it, plate voltage is quite high. If you want to try more like 30mA per tube (10.8W-ish) invest in some Russian/Sovtek/Reflector military EL84/E84L, they'll take it easily at 13.5W max dissipation.

When biasing up Pro Jrs/Blues Jrs etc. there is a tangible difference between mid 20's & 30mA/tube in terms of tone, I would aim for 30mA and reduce to the point before the tone goes cold/sterile.

[SmogFighter]

I just noticed...

You guys are referencing mAmps... The Schem says mVolts...

Would TP7 be my plate voltage measuring point?

Thanks for your help,
Raul

[Steve Conner]

Same thing. The measuring resistors are 1 ohm, so 1mA through them gives a 1mV reading on your meter.

[SmogFighter]

Thank you for pointing that out!!



I'm a

[SmogFighter]

Another Question...

Since the Amp is rated at 40w and has 4 el84's they are expecting @ 10w per tube?
so to do the math 10w/350v = 28.6mA =100%
70% of 28.6 = 20.2mA

what is the rated wattage of a typical el84?
on this Data Sheet, it shows the wattage output @11w with an Anode Voltage of 250v
and @17w with an anode Voltage of 300v.

But after looking at my Anode (plate) voltage, shouldn't I expect higher
wattage?

[Enzo]

Idle power dissipation is not the same thing as audio power output.

[SmogFighter]

I took some measurements today...

tp7 (Plate/Anode Volts) = 342vdc

tp14 (V5) =25ma
tp15 (V6) =21ma
tp16 (V7) =25ma
tp17 (V8) =06ma

So, where do I look for a problem?

[Jazz P Bass]

Tube V8

Tube V8 is not conducting.
Swap it with V7 & see if the issue follows the tube or stays at the socket.
If it follows the tube, shes a no good.
If it stays at the socket, check the screen & grid reistors.

[SmogFighter]

Thanks...

I will try that after dinner tonight.

Ok, after dinner, I swapped V8 with V6 (Right next to each other) and this is
what I got:

tp14 (V5) =27ma
tp15 (V6) =11ma (suspected bad tube)
tp16 (V7) =27ma
tp17 (V8) =48ma

Then I swapped V6 with V5 (again, right next to each other) and:

tp14 (V5) =11ma (suspected bad tube)
tp15 (V6) =26ma
tp16 (V7) =68ma
tp17 (V8) =26ma

Which shows that it follows the tube. it also shows that it stresses it's
companion tube (evens and odds paired).

SO, I'm off to purchase a new set of tubes.
Thanks for all of your help, Gentlemen.

[ThermionicScott]

Wacky... looks like two of them are matched, but the other two are way out.

[SmogFighter]

New Tubes!!!!

After a new set of JJ's, everything balances out.

tp14 (V5) =25ma
tp15 (V6) =26ma
tp16 (V7) =25ma
tp17 (V8) =26ma

I decided that while I'm at it, I can change out the preamp tubes as well.
Amp sounds great and is considerably louder at any given volume setting.

Thanks All,
Raul