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Old 5th July 2006, 09:27 AM   #1
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Default Typical plate Voltage for el34

Sorry, I'm a bit of a noob, though I'm reading as much as I can....

Spec sheets say that El34 in class-ab1 push-pull operation can run with a plate voltage anywhere from 350-780 Volts (max being somewhere around 800). I've seen amps that have a switch to change the bias point for 6L6-to-EL34, but they mostly seem to let the plates stay the same (around 450V).

I love the sound of el34 amps I've owned, but never had the opportunity to dig into them before they left my hands. So, I'm wondering what a "typical" plate voltage for a dedicated el34 based amp would be? Is it truly up around 780V?

And, does keeping an el34 around the 450-500V plate range give it a "bad" sound, or upset its operating characteristics?

Thanks in advance for any advice...
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Old 5th July 2006, 09:40 AM   #2
AndrewT is online now AndrewT  Scotland
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the maximum plate dissipation rules.

High plate voltage DEMANDS low current. Lots of room for crossover distortion.

Low plate voltage allows high current. Ultimately ClassA when plate voltage is set low enough.

Anywhere in between is acceptable, just balance ClassA proportion against the sound.

ps. my double push pull runs on 450V with substantial ClassA but changing to ClassAB for about the top 2/3 thirds of output power.
regards Andrew T.
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Old 5th July 2006, 11:00 AM   #3
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The operating voltage is governed by the screen grid's limit of 425v, in both ultralinear and triode mode. In pentode mode, as you say, the plate can be as high as 800v but never push the screen to more than 425v.

Typical plate voltage for class AB1 (pentode or UL) is 400v, with fixed bias and quiescent cathode current of 30 to 50 mA, adjusted to taste.

I'm not a fan of cathode bias for class AB1, because it robs you of some of your B+ voltage. Also, it's rather inflexible, because there's a limit to the ability of the cathode bypass capacitors to hold the bias steady. Quiescent cathode current has to be quite high, about 55mA to 60mA, i.e. close to class A.
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Old 5th July 2006, 02:16 PM   #4
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Most HiFi amps run the EL-34's anywhere form 350 to 450 volts with a few higher. Some guitar amps go over 500 volts. These guitar amps (the old Marshalls come to mind) tend to eat up tubes rather quickly due to the high plate dissipation that they run.
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
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Old 5th July 2006, 05:44 PM   #5
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I ran my SE at 380v for a while then boosted the voltage to 430v, and I noticed a positive change in fidelity, I have read places that 430v is the optimal voltage for the EL34, but I have never heard any evidence other than opinion to back that up
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Old 5th July 2006, 05:58 PM   #6
Yvesm is offline Yvesm  France
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All data sheets from original manufacturers state for a 800V max operating plate voltage.

This means more than 1.5KV swing at anodes, so choose an hi quality socket

Another drawback is that this implies Hi Z load (around 10K P to P) and thus a more difficult to build OPT.

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Old 5th July 2006, 08:32 PM   #7
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I ran my SE at 380v for a while then boosted the voltage to 430v, and I noticed a positive change in fidelity
My EL-34 SE runs 475 volts B+ which puts about 430 across the tube. I am running 60 mA of cathode current, and a 5K load impedance. Yes, I know that this is close to the dissipation limit. This is the best sounding combination yet. Wimpy EL-34's need not apply! JJ's have worked great so far.

I have also ran this amp with 6L6's, KT-88's, 6550's, and a few other tubes. The EL-34's have the sound closest to a DHT, while the 6550's just ROCK. I have 3 pairs of $4 Shuguang "Coke Bottle" 6L6's, that sound great and take all the abuse that I can throw at them. These are the best compromise for a cheap SE amp.
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
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Old 6th July 2006, 12:43 AM   #8
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Most has been said; AndrewT summed it up nicely. Maximum values specified mean just that; you are allowed to use the tube at those values. Different applications will require different combinations of parameters for optimal results, and for amplifiers you will not generally find that EL34s are run with 750V on the anodes, simply because that is away from optimal, although allowable (high ratio output transformer etc. as said by someone else).

To word it a little differently, should you find you need to go to some 700+V on the anodes to get e.g. a required output power, it will usually be better in the overall interest of quality (linearity) to go to lower voltage higher current topologies. Often the better solution may require using 2 tubes in parallel rather than pushing a single tube to extremes, in order to work at an optimal (for quality) configuration.

Even the exact meaning of the max. Va spec is in a grey area. Much of this will depend on the application. You might notice that in sweep applications tubes normally rated at say 500V max can suddenly cope with 2000V under certain conditions. Some miniature tubes are rated at 800V max while larger ones are limited to 500V.

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