"Balancing" bias in two output tubes. (and another thing.)

[Erlend_G]

So.

My Blackstar combo has two EL84's, that will wear out/ need replacing, "sooner or later".

I've already spotted the bias variable resistor, and also the "bias balance" variable resistor- that lets one balance the bias between the two output tubes.

...I want to do the future tube change myself, and also the biasing. (I'm confident it will be OK), but reading about the "balance" resistor got me curious-

Somewhere, on a tube amp forum- someone wrote- that "having two output tubes running with different bias, ("unbalanced") may contribute to a richer, or considered more desired sound from the amplifier.



I'll read up on where to "draw the line" of high and low biasing- but... is there anything to this? My first thought was, that the pair of EL84's should be both bought balanced, and also biased with exactly the same voltage/current.

Thanks.

(and my second question. My combo amp, has a jack plug, going from the "speaker out" section on the back of the amp, to the speaker. I'm afraid this will come loose, or I'll mistake the cord for i.e. the footswitch cable, and then ruin my amp. Is there any way to "semi-permanently" fix this? I'm thinking gaffa tape. I might have answered my own question right now . Sorry.)

-Erlend .

 

[Erlend_G]

The forum wouldn't let me edit my OP;

so i add this: "someone even said, they used two completely different tubes- of the same type. in their amplifiers power stage. "that can't be good?" :? :/

 

[Demanic]

The forum wouldn't let me edit my OP;

so i add this: "someone even said, they used two completely different tubes- of the same type. in their amplifiers power stage. "that can't be good?" :? :/

Do you mean two different brands of the same tube?

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[jeremy]

ive used mismatched tube in an amp plenty of times. almost always sounds fine. once it kinda sounded weird but i think one of the tube was pretty tired. el84 tubes seem less sensitive to not having optimum bias. and im not talking about cathode biased amps. there is nothing wrong with biasing amps yourself as long as you ARE VERY CAREFUL. there are lethal voltages in tube amps so brushing your hand against the wrong thing for a split second could kill you

 

[Demanic]

Edison once demonstrated 500 volt DC by blowing up an elephant. Not a nice thing to do, but certainly demonstrative of what can happen.

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[Erlend_G]

I'll wear thick, "dish washing gloves"- ( i guess they are atleast a few kilo ohms)-
and be careful. . Trust me. I heard about an amp-tech, burning off his lower arm- by resting his elbow on an earthed metal surface, and touching the power capacitors. :/.

Demanic: Yes- the person online, used two different brands of , the same type tube.

-

What will happens, if I match, or "mis match" the bias between the two output tubes?

What is the best to do- match them perfectly- or "stagger" the bias?

...

I know there are different sources I could have searched, to learn about this. But being on the SDUGF, is just so cool .

Thanks

-Erl

 

[Demanic]

Well, if you want to vary the voltage that each tube is getting, set the overall voltage to spec, then vary the balance very slightly to start.
You might want to even try setting the initial voltage for both tubes cooler than spec, just to be on the safe side. After all, the worst thing that can happen running cooler is that it might sound like crap. If so, you can always turn it up.

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[Erlend_G]

Ok

So, I'm asking here- just to learn and make be sure.

When I'm someday changing my power tubes (to a quality brand)-

Am I to bias my amp, "hot, cold or in the middle" ?

And about the "balance" bias pot- should I perfectly balance the two tubes? Or is there "something" to gain- by giving them sligthly different mA.

thanks
-Erlend

 

[Demanic]

I would bias it a little cold, then off balance the tubes just a little and see how it sounds. Then go from there. Like seasoning something.

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[NegativeEase]

Edison once demonstrated 500 volt DC by blowing up an elephant. Not a nice thing to do, but certainly demonstrative of what can happen.

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I think you meant AC

Edison was protective of the dangers of DC -and I don't believe would have killed anything with DC -which is actually a hell of a lot more dangerous with enough DC voltage and lack of resistance to carve a path right through you of course.

Any electrical path that manages to pass completely through your skin tissue is going to almost always be lethal

 

[NegativeEase]

I think a bias probe and volt meter or modern bias tool is worth the investment if you want to do this going forward.

They are not expensive.

I almost never use my Oscope anymore -just not needed.

 

[Demanic]

I think you meant AC

Edison was protective of the dangers of DC -and I don't believe would have killed anything with DC -which is actually a hell of a lot more dangerous with enough DC voltage and lack of resistance to carve a path right through you of course.

Any electrical path that manages to pass completely through your skin tissue is going to almost always be lethal

Thus the hand in pocket rule. 500 volts of AC will fry you. 500 volts of DC makes stuff explode.

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[NegativeEase]

Thus the hand in pocket rule. 500 volts of AC will fry you. 500 volts of DC makes stuff explode.

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Yeah, definitely my Dad used to repair and change out radar systems on all the 60s classic fighters -fighters like century series (104s, 111s, F8s and F4s etc.)..

and bombers like the B-58. and he has crazy injury service stories -and they definitely had the hand in pocket rule when up in the aircraft between all metal bulkheads etc..

 

[misterwhizzy]

Well, if you want to vary the voltage that each tube is getting, set the overall voltage to spec, then vary the balance very slightly to start.
You might want to even try setting the initial voltage for both tubes cooler than spec, just to be on the safe side. After all, the worst thing that can happen running cooler is that it might sound like crap. If so, you can always turn it up.

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Isn't it idle bias current that's being set? The voltage is dependent on the power supply, not the bias adjustment pot.

 

[Erlend_G]

I would bias it a little cold, then off balance the tubes just a little and see how it sounds. Then go from there. Like seasoning something.

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Ok.

I heard, that biasing an amplfier TOO cold- is just as bad- as too hot! :/

But I'll follow the instructions. Right in the middle, is my aim.

(off balancing tubes, sounds exciting to try. Just to listen for any audible difference..! I read some people swear by both one, and the other.)

(Too bad Blackstar has NO customer service, e-mail or contact. Just a stupid "FAQ".)

 

[Erlend_G]

I also read- that in some older amplifiers (was it Fender?)

the bias balancing pot, was called something like the "hum canceling control"-

because the amplfier would start to hum, if the tubes were greatly differently biased.

..my Blackstar is the quiestest amp I've ever owned, included Hi-Fi equipment. You can crank EVERY control, but there isn't even a hiss. It's dead quiet.

 

[dystrust]

Without knowing more about how your amp is designed it's impossible to say how it *should* be set. The claim you read about a richer tone with slightly mismatched tubes is widely held; in fact it's so widely held that a lot of amps deliberately mis-balance their phase inverters to achieve the effect. In amps like that a balanced phase inverter is a waste of money.

If I were biasing your amp I'd try to balance the tubes reasonably close which would insure the most even wear.

 

[beaubrummels]

Thus the hand in pocket rule. 500 volts of AC will fry you. 500 volts of DC makes stuff explode.

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Additionally, my understanding is "While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal." which would include >250v plate voltages.

(FWIW I just read and saw film of the Edison elephant experiment which said it was 6600 volts, but terrible story either way.)

 

[Demanic]

Additionally, my understanding is "While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal." which would include >250v plate voltages.

(FWIW I just read and saw film of the Edison elephant experiment which said it was 6600 volts, but terrible story either way.)

Oh, I obviously miss recalled the voltage.
I was basing what I said on the fact that AC causes you to get hung up on the conductor.

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[Jacew]

I would open the amp first UNPLUGGED to check what kind of bias control is there. Egnater is nice to provide mA values printed on the board next to pots.

Blackstar is usually pretty smart about these things so it might be quite simple.

I've only used balanced/paired output tubes myself, so can't say about that.