4 x EL34 in A-class

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Leevi
Junior tube assistant
Posts: 109
(2/29/04 8:28 am)
 4 x EL34 in A-class
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 Hi,
Does anyone know if a power amp with 4 x EL34 in A-class
has been built? The circuit could be similar with AC30 power amp but the EL84s are replaced by EL34. Any opinions?
/Leevi
 
HotBluePlates
I only work on Fender's
Posts: 691
(2/29/04 9:56 am)
 Re: 4 x EL34 in A-class
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 Some people on this forum have done it.

The main key is lowering plate voltage for the EL34's so that they can be biased hot enough to be in class A. I think the number they arrived at was 250v on the plates. I don't remember the current figure, but it would be enough that the EL34's were at or near 25W dissipation at idle, or around 100mA.
 
Leevi
Junior tube assistant
Posts: 110
(3/3/04 1:41 pm)
 Re: 4 x EL34 in A-class
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 100 mA on idle sounds very high, the tubes will not last long time. Do you know where I can find schematics for that?
/Leevi
 
HotBluePlates
I only work on Fender's
Posts: 703
(3/3/04 2:56 pm)
 idle current
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 If the amp were being run in class AB, then 100mA would be way too high, and would burn up the tubes.

But Typical class AB voltages are up around or over 400v. And the tubes should be biased for about 70% dissipation at idle.

Since power = voltage x current, as the voltage goes down, more current will keep the dissipation (power) at the same level.

In class A amps, the tubes are biased at or very slightly below 100% dissipation at idle. The current drawn from the power supply stays the same or very nearly the same as the tube delivers power to the load.

However, plate voltage must be lower in class A than in class AB to avoid burning up the tube. So 250v x 100mA = 25 watts, which is the limit one would expect to see in a class A amp.

Class A existed first. Experimenters were trying to find out how to get more power output from the tube, so the raised the voltage and lowered idle current. Overall, the tubes ran cooler and could deliver more power to the load as the exceeded their dissipation rating for brief periods of time.

Class A is where the tube is on all of the time, class AB is where the tube is on more than half but less the all the time, and class B is where the tube is on only half the time. Each class allows the tube to turn off more, cooling it, allowing for higher voltages (with higher peak currents, but lower idle currents), and therefore more power from a given tube.
 
Leevi
Junior tube assistant
Posts: 111
(3/4/04 1:22 am)
 Re: idle current
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 Thanks for your deep explanation HotBluePaltes. I thought that a big current as such is bad for the tubes.
/Leevi
 
 
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