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Old 18th March 2004, 07:54 AM   #1
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Default EL34 in Pentode Class A?

Has anyone tried the EL34 as a pentode in Class A, using Mullard's recommended operating point?

Plate & screen volts = 250V
Cathode resistor = 106 ohms
Plate load = 2Kohms
Plate current = 100mA
Screen current = 15mA
Input signal RMS = 8V
Power out = 11 watts
Distortion total = 10%

(These figures are for single-ended operation)

My reason for asking is that I already have power and OP transformers that I think would be suitable for this, but in push-pull. OPT is 3.85k Plate-Plate, which is just a bit less than the 4K I would need but I hope would be OK. Thanks in advance for your advice.

Regards,

Ray
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Old 18th March 2004, 09:11 AM   #2
Jax is offline Jax  Sweden
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I have but in PP. The sound became too warm.

I'm now running them in ULPP into 3.5 kohm plate-to-plate with:

Vb=350V
Rk=330 ohm (separate on each tube)

Screen resistor 68 ohm
Rk bypassed
Grid leak 100 kohm (this can be higher)

The tubes will generate some even order harmonics but they are cancelled in the OPT.

Give it a try
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Old 18th March 2004, 01:19 PM   #3
SY is offline SY  United States
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For all that trouble and inefficiency, the distortion is quite high. There are probably better ways to operate the tube.
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Old 22nd March 2004, 12:24 AM   #4
stormy is offline stormy  United Kingdom
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heres the typical operation characteristics for a el34 two valve
push pull with fixed bias.

distortion much less.

Triode Pentode
connected connected

supply voltage 375 400

suppresor grid 0 0

screengrid Res 600 800 *(common to both valves)

controlgrid V -33 -36

anodeload Res 3.5k 3.5k

power output 48W 54W

TOTAL DISTORTION 2.8% 1.6%

HOPE THIS IS HELPFULL

more on request

regards stormy
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Old 22nd March 2004, 12:29 AM   #5
stormy is offline stormy  United Kingdom
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Quote:
Originally posted by stormy
heres the typical operation characteristics for a el34 two valve
push pull with fixed bias.

distortion much less.

Triode Pentode
connected connected

supply voltage 375 400

suppresor grid 0 0

screengrid Res 600 800 *(common to both valves)

controlgrid V -33 -36

anodeload Res 3.5k 3.5k

power output 48W 54W

TOTAL DISTORTION 2.8% 1.6%

HOPE THIS IS HELPFULL

more on request

regards stormy

not quite apears as i typed it, I just noticed its a pentode class A
your building so its second figures in row for you

1.6% distortion sounds better than 10%
and could still be improved I recon.



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Old 22nd March 2004, 06:43 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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You do know that PP trannys are not usable for SE designs? They need to be air gapped.

I would advise going for a PP design, much more power, more bass and able to drive real world speakers. SE can be magical, but its more of a religious thing than a sonic thing. A well designed PP amp can be very engaging and great to listen to.
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Old 24th March 2004, 02:52 AM   #7
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Default Yes, I know it needs to be PP

I have no aversion at all to PP, believe me; in fact, I much prefer it to SE. In my original post, I did try to explain that I was thinking of building a PP pentode design, but using the operating point that Mullard gives for SE pentode. Push-pull just means doubling the currents and the plate-to-plate load, AFAIK, since we're talking class A.

I already know the EL34 does OK at the more usual operating point of 400V @65mA. What I'm trying to find out is how it behaves under conditions of low voltage (250V) and high current (100mA) in class A. I suspect that most of the 20% distortion is 2nd order harmonic, which should largely cancel out with a push-pull arrangement. One respondent has kindly offered the information that he found the sound too warm under these conditions.

Incidentally, you don't need an air-gap in the OPT with an SE design if you use a push-pull OPT and load the two halves of the primary equally, so that there is no net DC. You can, for instance, build a class A push-pull amplifier but only feed the signal to one of the output tubes for SE operation. When you need more power, you can activate the signal to the other OP tube as well, giving push-pull operation. </i>

Regards, Ray
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Old 24th March 2004, 06:31 AM   #8
SHiFTY is offline SHiFTY  New Zealand
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Ok so you do want a PP design! In no way does your first post make that clear.

(Also you can use a PP transformer in a parafeed SE design.)

Back to the topic at hand, why not try it! 100mA is a fairly high current, but well within the 150mA max for EL34. You will need a mains tranny capable of 500mA at 250V, maybe a isolation/stepup transformer?

You could experiment with different loads by swapping the output taps.
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