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Old 15th June 2006, 10:04 PM   #1
brownbr is offline brownbr  United States
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Default Who's right re bias adjustment versus plate voltage

I'm going crazy with all the conflicting internet info on who's right about the best bias setting for tubes. I'm trying to set the bias for a Marshall TSL 602 with a pair of Mullards El34s. When I set them to say 70%, the plate voltage shoots up to 471. Should I be concerned about the high plate voltage, or does it not matter. I thought the amp was supposed to be 450 plate voltage, but I'm beginning to wonder if it's 460. Either way the plate voltage still jumps high and even worse at 50%. Also If I run them at 100%, the plate voltage nears 450.
Some say worry about the plate voltage, others say don't worry about the plate voltage. I'm hoping somebody as some insight or an opinion as I don't want to abuse these new Mullards.
thanks brett
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Old 15th June 2006, 11:24 PM   #2
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I'd be more concerned with over dissipation. According to the spec sheet, EL34s can take more than 470Vdc. Set them to 70% of Pd.

Of course, you could always fix that regulation problem with the PS.
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Old 16th June 2006, 01:01 AM   #3
Enzo is offline Enzo  United States
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If you regulate the B+, then it won't sound like a MArshall anymore.

Of course you watch the plate voltage. How can you set it to 70% if you don't measure the plate volts. Yes the more current through the tube the more the B+ sags. SO they are interactive. Once you get it in the ballpark, you then recheck the B+ and readjust to what you want.
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Old 16th June 2006, 02:10 AM   #4
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Of course you watch the plate voltage. How can you set it to 70% if you don't measure the plate volts.
You need to know what the plate voltage is but you normally measure plate current, not voltage, when setting the bias. A conventient and safe way is to measure cathode to ground voltage, using a low value cathode resistor. Of course, this also measures screen current but you can allow for that.

470v is nothing to worry about, the EL34 can take 800v plate voltage (according to its specs, anyway). The plate voltage variation you are seeing, as the plate current changes, is simply due to a combination of OPT primary resistance and sag in the power supply. It is to be expected, especially with a guitar amp.

To get 70% dissipation, which is a good starting-point in most cases, you need about 21w total plate+screen dissipation. So, at 470v plate voltage, start with a quiescent cathode current of around 45mA. Then, you will probably want to adjust it for the best sound.
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Old 16th June 2006, 04:13 AM   #5
brownbr is offline brownbr  United States
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Thanks you guys. I'll set them to 70% and I will take into account the plate voltage measurement in my calculation. I can rest easy again.
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Old 16th June 2006, 07:25 AM   #6
cerrem is offline cerrem  United States
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Personally I do not agree with this 70% rule...Doesn't make much sense..
But more importantly it is imperative that you properly measure the current for each valve...
The best and absolute method is to put a ammeter in SERIES with pin# 3 .... The "shunt" method is non-sense and will give eroneous results..So try to avoid that..
When you are dealing with guitar amps in Class AB, there is a wide range of acceptable bias ...SO in that case it's should be done by ear... I use both scope and ear to get it where I like it..
Hi-Fi gear you need to dead on..but for Rock-N-Roll it's all about making it sound good when it is cranked...Just as long as the valves are safely within plate dissipation...

Chris
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Old 16th June 2006, 10:17 AM   #7
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Personally I do not agree with this 70% rule...Doesn't make much sense..
It's just a "rule of thumb", a reasonable place to start that is safe, as far as dissipation is concerned, and leaves you room to manoeuvre. As you say, in the end it's best to find the optimum bias by ear.
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Old 16th June 2006, 10:21 AM   #8
Merlinb is offline Merlinb  United Kingdom
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Cerrem, I think you misunderstand the bias measurement method Ray Moth mentioned. You measure the voltage across a cahode resistor (usually 1ohm) divide by resistor = current.
What you certainly don't want to do is to use the 'crossover notch' bias method- that will only work for amps handling perfect sine waves, and there can't be many of those!

Mullard EL34's are like tanks, they were run over 800V in some older amps, and would take the punishment. 470V is peanuts to them.
As was mentioned already, it's the anode DISSIPATION that you don't want to exceed. In AB amps, max anode dissipation happens somewhere on the positive input cycle, so biasing at 70% ensures that when you hit max dissipation, it is within the allowable wattage for the valve.
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Old 16th June 2006, 11:47 PM   #9
Enzo is offline Enzo  United States
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And Ray, you are right it is the current we are measuring, but without knowing the plate voltage we have nothing to base our current target upon. The point was you had to watch both. The original poster didn't seem to realize that at first.
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