Which do you prefer - EL84 UL PP or EL34 Triode PP - and why? - diyAudio
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Old 23rd February 2007, 01:22 AM   #1
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Default Which do you prefer - EL84 UL PP or EL34 Triode PP - and why?

Some very nice amps have been built using EL34 triodes in PP, to give around 16W. However, very nice amps have also been built to give almost as much power more economically with EL84s in ultralinear PP, using 100v less B+, less current and occupying less real estate. Is EL34 triode PP worth the overheads involved?

Also, if it has to be EL34s, would it be better to use them in ultralinear PP, getting double the available OP power for no additional overheads?

I realise that 'one man's meat is another man's poison', but I'm interested in hearing people's preferences and their reasons.
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Old 23rd February 2007, 04:32 AM   #2
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Default Re: Which do you prefer - EL84 UL PP or EL34 Triode PP - and why?

Originally posted by ray_moth
...Is EL34 triode PP worth the overheads involved? ...
I realise that 'one man's meat is another man's poison', but I'm interested in hearing people's preferences and their reasons.
One advantage of considering EL34s over EL84s (other than the power issue) is that (depending on the topology and driver/splitter tubes used) the same amp can be used with KT66, KT88 or even 6550. This offers a bit more flexibility in soud and power options. I am not used if the EL84 offers the same.

Some 1 year back I heard my Brother in Law's Ella Consonance. He origianlly had EL34s in them and when I heard them with El34s I felt "something" was missing. A few days later he had changed the output tubes to KT88 (I had no knoledge of this) and when I heard the KT88 version it sounded much more real. It was only after I asked him did I know what the change was. This expereince is what drove me to try and build a tube amp. I wont soon forget that Eureka moment. Thanks Mikey!
...still looking for the holy grail.
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Old 23rd February 2007, 06:06 AM   #3
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Assuming a 'normal' EL34 PP NFB amp i never liked the triode conversion. It could well be the original drivers could not do a proper job once pseudo triodes were involved. At a risk of sounding seriously out of fashion i would rather have a proper pentode mode - regulated (or separate supply) G2.
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Old 23rd February 2007, 11:26 AM   #4
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For a slightly lower input capacitance and significantly lower B+ and drive requirements, you can use two sets of EL84 instead of one set of EL34. The optimum load will be about 5k p-p, which is a pretty common transformer and will work fine with other tubes.

I am in full agreement with analog_sa.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 23rd February 2007, 12:22 PM   #5
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Hi Ray_moth,

the questions is what you regard as "overhead", since nothing comes for free.

For example, using EL34 pure class A triode PP, you will get about 10 W Po at a damping factor (before or w/o applying any gNFB) that will be hard to beat, also a very uncritical response to load mismatch (think real world speakers) and excellent linearity (low THD) to boot. At a lousy efficiency, of course.

Of course you can get that same Po from a pair of EL84 in either straight pentode PP or UL connection - but not from pure class A, of course, hence you will have less DF (much less w/ straight pentode mode), more critical response to deviations from ideal load behavior, and more THD to boot. You partly can fight those drawbacks by applying loop feedback - probably tons of it - to come anywhere near were EL34 class A triode PP already is. And this means added complexity and effords and parts count.

Always remember, many roads eventually lead to Rome, but nothing comes for free. Hence you will have to decide for yourselve which way to go - according to what what you accept as being acceptable for yourself.

Besides that, PP pentode UL with a decent ammount of class A within AB and a sensible ammount of gNFB applied certainly is a common, generic and proven way to do things - just like a generic middle class car.

If you want something better than that (whatever you define as being better for you) you will have to pay for it one way or another. If better = cheaper, then go straight pentode PP w/ lots of gNFB, and have fun to get the THD down while keeping the amp unconditionally stable. If better = more DF, less critical to load, less gNFB needed, less distortion to boot (!), go triode PP class A.


If in doubt, just measure.
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