Biasing a quad (Marshall EL34 100/100)?

  • Thread starter Thread starter Me
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If there's a benefit to checking per tube then I guess those methods are better so I can know per tube (not that there's much I can do if they're poorly matched though).

If it's a 50W at least then I'd be measuring the plate current to a single tube and my method would have to be just as accurate according to Mr Ohms and Mr Kirchoff, if not slightly more so since the meter reading would be more accurate across a bigger resistance and there's no addition 1 ohm added that wouldn't be there in normal running.

So back to the earlier discussion: why 55-60% or 70%? If I get an adapter or resistor and measure the current per tube, should I make the highest tube say 70%, or the average tube 70% (or whatever figure we argue between 55-70%). Any idea how closely a matched pair or quad is matched?
 
Ah ha, found this useful thing which answers the missing questions I had left and also agrees that my proposed method is valid, the easiest and cheapest, and one of the more accurate ones going. Don't need to buy any adapter or bugger about with resistors or soldering...

http://www.aikenamps.com/Biasing.html
 
FourT6and2":3ncj11z2 said:
Like Steve said, the easiest way is to replace the wire on pin 8 to ground with a 1ohm resistor. Like the photo below. Then you can just measure from pin 1 or 8 to ground to get your bias reading. Otherwise, you're gonna have to get one of those socket adapters. All that other stuff is just making it more difficult
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De soldering 8 plates, soldering in 1 ohm resistors, biasing it, then taking them all out is "just making it more difficult". My proposed way is far easier. And I don't have to buy anything. I'm off to the pub to spend my bias adapter money on beer. See link above!
 
Me":lfsiduhz said:
FourT6and2":lfsiduhz said:
Like Steve said, the easiest way is to replace the wire on pin 8 to ground with a 1ohm resistor. Like the photo below. Then you can just measure from pin 1 or 8 to ground to get your bias reading. Otherwise, you're gonna have to get one of those socket adapters. All that other stuff is just making it more difficult
Click to expand...
De soldering 8 plates, soldering in 1 ohm resistors, biasing it, then taking them all out is "just making it more difficult". My proposed way is far easier. And I don't have to buy anything. I'm off to the pub to spend my bias adapter money on beer. See link above!
Click to expand...
Dude! That is ridiculous. You can just leave the 1 ohm resistors in there. They aren't doing anything except making it easy, safe and accurate to bias your amp. So, just solder them in once and you are done! Like others say, you are making things harder than they are...

Steve
 
So you think you should pointlessly modify an amp having resistors hanging off the end of a plate supply wire with an uninsulated part in a very enclosed space in the bottom of a rack mount power amp that get's vibrated a lot being transported around, just to save multiplying a number by 2, measuring the impedance of across the output transformer with a multimeter, and using a simple V=IR equation to make things easier?!?! I wouldn't dream of leaving them there in this amp them unless they were heat-shrinked, and if they were I couldn't bias across them.

Don't get me wrong, I'm grateful for the help with what percentage of plate dissipation power you should adjust to, and getting me down the right track with that, but now I've found someone confirm that I'm not going mad and a simple application of Kirchhoff's current law and Ohms law saves all the buggering about, it sounds like sheer madness to go to all this extra effort for absolutely zero benefit! A tiny bit of theory is SAVING me the messing about. The theory seemed sound. Someone else who wrote an article on biasing confirmed it was sound totally independently from me deducing it from first principles, and the amp now works fine and is biased well within the tolerances suggested for EL34s! :confused:
 
You are seriously over thinking this.

1. The 1ohm resister thing has been a standard way to modify an amp (many amps already come with this resister in place) for decades. Many Marshalls already have this part installed. The photo I posed is from a Marshall Super Lead. Is your Marshall power amp special or something? It's not dangerous to have it installed and not bad to have "uninsulated" resister leads "hanging off the chassis." And you don't install them, then uninstall them later. You put them in once. And that's it. Leave 'em in for the life of the amp. It makes biasing much easier down the line. But whatever man… I guess you know better than us. That's why you're asking us for advice, right? ;)

2. Regarding dissipation: 70% is generally the recommended MAXIMUM per tube. And you can go as low as you want as long as you avoid blocking/notch distortion, which you can hear or see with an O-Scope. But somewhere between 60-70% is usually the norm. And yes, it is a fairly large range.

3. When you bias, you need to make sure that no one tube is over 70% (generally). So you need to be able to check the bias for each individual tube. Otherwise you may wind up with one tube biased at 65% and another that drifts upwards of 70%. This could lead to a blown tube in the future. And that could take other parts out with it, like a tube socket or even a tranny. But you know best I guess. :)
 
Me":3bydmm2x said:
So you think you should pointlessly modify an amp having resistors hanging off the end of a plate supply wire with an uninsulated part in a very enclosed space in the bottom of a rack mount power amp that get's vibrated a lot being transported around, just to save multiplying a number by 2, measuring the impedance of across the output transformer with a multimeter, and using a simple V=IR equation to make things easier?!?! I wouldn't dream of leaving them there in this amp them unless they were heat-shrinked, and if they were I couldn't bias across them.

Don't get me wrong, I'm grateful for the help with what percentage of plate dissipation power you should adjust to, and getting me down the right track with that, but now I've found someone confirm that I'm not going mad and a simple application of Kirchhoff's current law and Ohms law saves all the buggering about, it sounds like sheer madness to go to all this extra effort for absolutely zero benefit! A tiny bit of theory is SAVING me the messing about. The theory seemed sound. Someone else who wrote an article on biasing confirmed it was sound totally independently from me deducing it from first principles, and the amp now works fine and is biased well within the tolerances suggested for EL34s! :confused:
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Dude - you do whatever you want, but many production amps, such as my Henning Cherry Bomb, come stock that with the 1 ohm resistors in place to make it convenient and accurate to set bias. Because all the tubes, even in a matched quad, can measure significantly different current, measuring all 4 tubes will be more accurate and make sure that no individual tube is exceeding 17.5 watts dissipation. This is a very minor mod to your amp. If you don't think it is necessary or you don't want to do it, that is fine, but there is nothing wrong in the least with modding your amp permanently in this fashion.

Steve
 
I just made a long post explaining some of my reasoning but the browser locked up and ate it. :doh:

To cut along story short it's pretty densely packed in that power amp so I'd rather not add the resistors if I can help it. If it was a big empty box like most heads I'd be more tempted. I've also spent far to much time with guitar related stuff in pieces all over the house recently and not enough time practicing for upcoming gigs. It's not like I even use this amp hardly ever.

If the tubes are matched to 5% then I know they're all now definitely within 62.5% to 67.5%. When you consider by my reckoning they were average of 90% based on the current I measured before I touched the pot (so one maybe more than 90%!) it's no wonder the bastard was humming.

Thanks for your help guys. Even though I ignored all your last bit of advice I couldn't have done it without the earlier info! Nullius in Verba! :lol: :LOL:
 
Me":2rhrlda1 said:
If the tubes are matched to 5% then I know they're all now definitely within 62.5% to 67.5%.
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I'm telling you that based on the matched quads I regularly receive that your assumption here is wrong. I've had matched quads where one tube was measuring 34 mA and another was measuring 40mA for a particular B+. There is no way to be sure unless you measure the current at each tube.

Steve
 
Me":3d1r7ncu said:
To cut along story short it's pretty densely packed in that power amp so I'd rather not add the resistors if I can help it. If it was a big empty box like most heads I'd be more tempted.
Click to expand...

Makes no sense. The resistor merely replaces a bare wire lead that is already in your amp. It merely bridges pin 8 to the screw that's holding the tube socket to the chassis. You will have zero space issues. But whatever... do what you want.
 
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