Biasing a valve amp

nick

Disclaimer:

I am not an expert, do not try this unless you are comfortable around high dc voltages! Do not take what I have written as correct unless it is confirmed by those who actually know whats going on

Right so I've been wanting to learn how to do this for a while, read up a whole ton and apart from the danger factor it seems fairly simple, heres my first go at it.

I removed the amp from the chassis and had a peak (with my eyes not my hands) at the EL34 sockets from the top. Firstly I spotted

The red plate B+ voltage wire isnt looking too healthy and if that touched something else Im sure there would be some fireworks. Cut it short, tinned it and resoldered it properly.

Set your multi-meter to DVC, in my case I used 1000 (as your plate voltage should be 400-500 odd). Connect the ground terminal to a lug on the amps chassis itself and the red wire to pin 3 of your EL34 socket.

Basically 3rd pin counting clockwise just after the locator tab. Make sure the amp has a speaker load, connect power to the amp and turn it on (leaving it on standby). Let it warm up for about 10 minutes or so on standby before switching the standby switch off and you should get

A reading of 488VDC, it was bouncing between 487 and 488 the whole time. Now you use the following formula:

Bias = (Wattage of tube / plate voltage) x 70%

so

Bias = (25w / 488 ) x 70% = 0.0358V or 35mV

Now this is the part I dont understand from the online instructions..

Set your meter to lowest DCV setting (200mV or less) and touch the ground lead to the ground side of the 1 ohm resistor and the positive lead to the pin 1 side. Do this on all power tubes and adjust the tube with the highest value to your desired bias voltage from step 4.

Theres the adjuster, but can someone explain the 1ohm resistor bit?

Gearhead

The 70% ratio you talk about is a rule of thumb I would not be happy with; where did you get that?
Bias is the DC current through the tube at no AC signal on the grid. It gets set by controlling voltages of the grid and the cathode, but there's different methods. One is called cathode biasing, which essentially makes the cathode positive relative to the grid by grounding the grid and putting a resistor between cathode and ground. These amps you normally don't bias since the current through the resistor is pretty much the same with a wide range of tubes of the same type.
The next method is called fixed bias, which grounds the cathode and makes the grid negative to the cathode by a separate negative supply. The voltage on this negative supply will influence the current. They normally put a small value resistor (~1 ohm) between cathode and ground just to let you measure it. Ohms law will let you calculate the current, measure the value of the resistor with the tube pulled and while in circuit measure the voltage.
Next thing is: what is the right current? My advice would be to start off with what it was when you got the amp (you were happy with that sound at the time, no?) and make small changes to see what the tonal consequences are. You can even skip Ohms law that way and stay in the voltage domain ?
Moral of the story: don't measure plate voltage, is besides the point and too dangerous for most ppl.

nick

Gearhead wrote: The 70% ratio you talk about is a rule of thumb I would not be happy with; where did you get that?
Bias is the DC current through the tube at no AC signal on the grid. It gets set by controlling voltages of the grid and the cathode, but there's different methods. One is called cathode biasing, which essentially makes the cathode positive relative to the grid by grounding the grid and putting a resistor between cathode and ground. These amps you normally don't bias since the current through the resistor is pretty much the same with a wide range of tubes of the same type.
The next method is called fixed bias, which grounds the cathode and makes the grid negative to the cathode by a separate negative supply. The voltage on this negative supply will influence the current. They normally put a small value resistor (~1 ohm) between cathode and ground just to let you measure it. Ohms law will let you calculate the current, measure the value of the resistor with the tube pulled and while in circuit measure the voltage.
Next thing is: what is the right current? My advice would be to start off with what it was when you got the amp (you were happy with that sound at the time, no?) and make small changes to see what the tonal consequences are. You can even skip Ohms law that way and stay in the voltage domain ?
Moral of the story: don't measure plate voltage, is besides the point and too dangerous for most ppl.

4) EL34's are 25 watts max and you want to set the bias to be between 60% and 70% maximum dissipation watts. (60% = 15 watts; 70% = 17.5 watts) Divide your desired wattage by your plate voltage from step 3. (example: 15/450 = .033 or 33mV) This is your desired bias voltage (probably between 30 and 40 mV).

Thats from a post on the metro amp forum that I was going through as well as a few youtube videos. I've ordered a new set of JJ's from Mr Valve so will be replacing the EL34s and hence wanting to bias the amp.

RJN

I admire you having the Guts to DIY valve change and Bias .

My amps will be costing 2K for Service and Valve change and Bias :-[

nick

These arrived today from Mr Valve in JHB, now to just figure out the second part of biasing..

nick

Gearhead wrote: Next thing is: what is the right current? My advice would be to start off with what it was when you got the amp (you were happy with that sound at the time, no?) and make small changes to see what the tonal consequences are. You can even skip Ohms law that way and stay in the voltage domain ?
Moral of the story: don't measure plate voltage, is besides the point and too dangerous for most ppl.

Im happy with the way the amp sounds, I just want to make sure its running with in spec so the valves last.

Gearhead

With the old valve in, measure the voltage across the 1 ohm resistor that sits between cathode and ground. Write down. Switch off, roll valves, switch on. Turn little adjust pot until voltage is same as what you wrote down. Switch off, discharge caps, put amp back together. Job done, next time experiment with hotter/cooler until even more happy.

Karel-Mars

Since you go where angels fear to tread, let me chip in too. This biasing thing is quite difficult to grasp, but really simple in principle.

Note about high voltages:
* Check that your life insurance policy is up to date and the funeral plan sorted.
* Try to measure with one hand only; the black probe should be clipped with a crocodile clip onto a convenient earth.
* Stand on a rubber mat or wear rubber shoes.
* I have noted than when I get shocked, I tend to smack myself in the face or on the chest ? So your muscles contract involuntarily when electricity flows through your arm. So always keep your arm free from wires and proximity to sharp metal edges when the voltages are above 80VDC. I was working on an amp one day, while being barefoot. I had the guitar in my hands while probing around the amp; you know... tweaking the values for best tone. I do not know what I did, but when I came by, the guitar already completed it's upward journey, and was coming down, and I could actually catch it.
* After you have been shocked, you should leave the work bench and take some time to recover. Call it a day. Apart from the body effect of a shock, your brain also stops functioning properly (you are in shock). If you were stupid before being shocked, you are more so after being shocked, and might do the same silly thing again. Yeah, been there. Shocked twice in a row :-[

Here's the link to the 70% thing http://www.aikenamps.com/Why70percent.html

Most amps are actually biased quite a bit colder than 70%. 50% is closer to factory preset. But 70% has a better clean tone typically.

bluesman

Biasing an amp is like playing with the pacemaker on someone you care about. There's a lot to be said for self baised amps. I recently repaired a Bassman for a fellow in Kimberley. The cap on the bias supply went south, took the 6L6's with it, actually nuked one of them.

I have a Marantz 8B Hi Fi amp, a quality UL EL34 amp that allows the owner to set the bias on all 4 PP output tubes. Some friends came to visit from Oregon (the state). One of them fiddled with the bias settings and took out the rectifier and 2 of the EL34's that I could tell super conducted. (I never liked the guy's wife anyway). It's fixed now and this was an omen for a long overdue rebuild. My Quicksilver monoblocks make it easy to bias the 6550's and it seems the bias stays set even after many hours of usage.

Bias is a critical part of any audio circuit. On an amp that is not self biased, usage and wear on the output tubes will require bias adjustments periodically but there is no reason to get anal about it unless you hear a difference or the inside of the output tubes resembles a braai.

nick

bluesman wrote: Bias is a critical part of any audio circuit. On an amp that is not self biased, usage and wear on the output tubes will require bias adjustments periodically but there is no reason to get anal about it unless you hear a difference or the inside of the output tubes resembles a braai.

So are you saying stick in the new tubes, let them warm up, play at normal volume, provided they arent on fire or glowing purple the tiny bit it could be out isnt worth it?

bluesman

Did the old tubes fry?

bluesman

Actually Nick that wasn't a fair question. Upon replacing new output tubes, it would seem a prudent idea to check the bias. Keep in mind that when the bias was originally set, it most probably was with new tubes. Legacy tubes have different characteristics than newer eastern european tubes, so aagain it may be a good idea to check. Factors like line voltage differences can also have a profound effect on bias settings. Ive seen the 230v here swing 10% in dips, at least here in the Free State and I have 3 phase where I live and I doube your bias supply is regulated. Go figure huh? Most probably, keeping the bias within range will make your output tube investment last longer which isn't a bad thing, except for a person in the tube business like me.....kidding.
Mark

Attila

If youre going to do this on a regular basis.... might be useful to make one of these jobbies

http://www.el34world.com/charts/biascurrent.htm

nick

bluesman wrote: Did the old tubes fry?

Nope, still good, no purple glowing etc, they just have some rattle when playing an F# which irritates me.

nick

Attila wrote: If youre going to do this on a regular basis.... might be useful to make one of these jobbies

http://www.el34world.com/charts/biascurrent.htm

Ah, Reinhard and I were talking about buying one but I'll give this a bash. Now to find a EL34 socket.. got plenty of tubes to trash ?

Attila

@ Karel, I caught myself in the identical way, completing the ground loop through my guitar strings

hey I also remember reading about this oke sitting on his couch, in shorts, watching TV, flicking through channels , bored shitless with remote in one hand and Taser in the other, next thing he knew when he came round ... he was laying face down on the carpet, gob running out of his nose and mouth, and his dog staring at him as if to say WTF were you thinking :woohoo: