preferred bias method for Express

[aaronz28]

i just purchased some NOS Mullard EL34s and want to bias them in my Express - seems to me that the last time I did this, i used the Shunt method, but then a month ago, i read somewhere that unless you have a specific style DMM, you could get false readings??
had something to do with a specific value inside the DMM -
nevertheless -

i'm not much of an electronics guru but i have always biased my own power tubes - can someone clue me in on the appropriate method for best results?

Thanks

Aaron

[martin manning]

Any DMM that can measure current will work, but the shunt method is a bit risky. If you don't have 1-ohm current sense restors at the power tube cathodes, see this post for a very accurate method. https://tubeamparchive.com/viewtopic.ph ... 358#228358
EL-34 is a 25W tube, and you have one per side, so use 25W for Pa Max.

[Tillydog]

Just checking that you're aware of KF's instructions...

From The Trainwreck Pages:

http://www.blueguitar.org/new/schem/tra ... _pages.pdf

It's on page 28 (actually page 10 of the pdf file). I'm sure I've seen -31V somewhere, too).

[leadfootdriver]

I have 1 Ohm, 2-watt 1% resistors wired in permanently. 45 ma's works out to about 70%. Fast and easy.

[shaunf]

Just checking that you're aware of KF's instructions...

From The Trainwreck Pages:

http://www.blueguitar.org/new/schem/tra ... _pages.pdf

It's on page 28 (actually page 10 of the pdf file). I'm sure I've seen -31V somewhere, too).

Would this apply to a clone too and is this safe? I just tried adjusting mine to -30v on pin 5 of V4 and V5. At that setting when I go back and check the mV reading across my built in bias points, I am reading around 60mV. Isn't this too high?

[Tillydog]

Would this apply to a clone too and is this safe? I just tried adjusting mine to -30v on pin 5 of V4 and V5. At that setting when I go back and check the mV reading across my built in biad points, I am reading around 60mV. Isn't this too high?

I don't see why it wouldn't apply to clones, but that sounds too high, even for EL34s

I daren't set my Express (clone) below about -31.5V with 6V6s which geves me 70% max dissipation.

I'm sure I have seen something else from KF with a different voltage on it, but I'm beggared if I can't find it again.

[martin manning]

Ignore the voltage!

Just set the plate dissipation. Maybe someone will want to discuss the "correct" percentage of maximum, but 70% (using cathode current) is a good place to start.

[shaunf]

Ignore the voltage!

Just set the plate dissipation. Maybe someone will want to discuss the "correct" percentage of maximum, but 70% (using cathode current) is a good place to start.

Just so I'm clear, are you saying I should just stick to using my bias points with 1ohm resistors and bias according to the usual formula for EL34's (25 x 0.7) / Plate voltage, which in my case with 403 volts on B+1, so around 41mV converted to mA, or are you saying there is some merit to Ken's suggestion as per that page posted above that one can simply measure voltage on pin 5 of the output tubes and set that voltage to a specific setting? Also, if you're agreeing with Ken's suggestion, but not the particular voltage setting, then what voltage would you suggest, or how do you calculate it?

Regards,
Shaun

[martin manning]

Ken may have had some preference for hotter or cooler bias, within limits, but I don't know. The voltage that it takes to get there is going to vary from one tube to the next, and could be way off for current production tubes. The correct voltage is whatever it takes to get the plate dissipation you are after.

[shaunf]

Thanks Martin, and I understand exactly what you are saying and agree that this would differ depending on the tubes. I just find it strange why he would specifically mention a certain voltage measurement.

I actually went back to those "Trainwreck Pages" documents and read the entire article, which mentions various types of measuring and adjusting bias and describes each. He then goes back in that snippet that was posted and specifically refers to using this "Voltage Measurement" method and the specific voltage.

He also mentions this "Special procedure" done at Trainwreck HQ. I wonder what this could be referring to.

[v846]

Just a guess on my part but the special process might be that Ken was using a specific grade of groove tubes back then which would make bias adjustment unnecessary providing you pick the same grade of output tubes. Maybe this is why the -30 bias voltage for an original Express?

[martin manning]

My guess would be that the special procedure was tweaking the bias circuit to get the bias where he wanted it, correcting it for the component values that wound up in each amp due to tolerances. He must have been convinced that the variation in tubes was insignificant if he thought the bias wouldn't have to be adjusted "for many years." I don't recall reading that any specific tubes were recommended.

[Reeltarded]

Maybe I'm wrong but I check the voltage and go right to the middle of the suggested range then I have someone else play until it sounds the best with the slightest of sweeping around that area..

The hardest part is teaching the monkey brained dumbasses I know how to play a palm mute that actually mutes and stop with the pretend van halen.

NEVER PUT YOUR HANDS IN A RUNNING AMP THAT IS BEING PLAYED BY A MONKEY WITHOUT THE SIGNAL CABLE BEING TIED ON SOMETHING THAT WON'T PULL WHEN MONKEY STARTS BREAK DANCING

I check to make sure I don't see any China Syndrome then I play after scolding monkey for being a monkey.

[redshark]

I'm gonna tell you why Ken wrote to bias the express to -30vdc.

In the 80's groove tubes was labeling and grading siemens EL34's. So Ken would tell his customers: buy groove tubes EL34's grade 4 and bias to -30. Of course that doesn't apply nowadays, so the current method is better.

[tictac]

A note on the Shunt Biasing method which is to connect a DMM set to the 200mA range + lead to OT centertap and - lead to the plate (anode) of the tube.

This really is the most acurate way to read the bias current for each individual tube. A few people may be concerned about the safety of this method but I've seen techs with a DMM and some test leads take a current reading rather quickly without concern of the clicks and pops coming out of the speakers...

I'm a bit more squeamish when it comes to high voltages so I would use test leads with those mini-grabber clips and clip onto the test point amp powered down. Then bring the voltage up with a variac and monitor the current with " hands off the chassis"

This method was a bit more time consuming but it works well. With the resistor method you're reading the current of the plate and screen grid combined so not quite as accurate...

That being said, all my amps have 1 ohm resistors installed and it's the method I prefer if given a choice. What I do is subtract a few milliamps for any screen current present and you're in the ball-park especially when reccomended bias values can range from 50% plate dissapation to 80%.

TT