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Old 5th February 2008, 11:38 AM   #1
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Default EL84 fixed bias PP Triode

Some time back I was the recipient of an amp that had seen better days but had good output and power trafos.

Given that I have well behaved Fostex single driver speakers of around the 95dB/W ballpark (and access to a local tube amp guru) I've decided to dip my toe into an el84 tube amp.

What we have on the cards is a silicon rectified B+ in the ballpark of 320v with less than 40mV of ripple, a separate bias supply for the output tubes enabling me to run them all in fixed bias. I'm wanting to run them in triode mode for starters, can always do the ultralinear thing later on if needed as the output trafo is sorted for that if I desire but the purist in me would rather not and it's not like I need the power.

Also planning to see how things sound with zero neg feedback, again an easy retrofit if necessary.

Input stages are an Alps 100k switched attenuator into a 12ax7 gain stage, cap coupled to a 12ax7 concertina phase splitter (very similar to the input stages on a Rogers Cadet but minus the neg feedback connection)

Comments? Brickbats?

For the moment I'm planning on limiting the silicon content to the diodes in the psu for the heaters, B+ and output bias, no CCS or LED arrays.

As much a learning exercise and first step as an end point. Gotta start somewhere I guess and if you already have the mains tranny, choke and outputs then you're most of the way there. (other bits on hand include a matched quad of el84's and the alps attenuator)

TIA, Drew
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Old 5th February 2008, 12:04 PM   #2
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Nice setup! With 30mA Iq and 8k primary you should be able to squeeze 4W A and 7W AB out of it. Add 10 ohm resistors between each catothde and ground. But use a better tube than 12AX7 as driver. 6DJ8 would be a far better choice. I would prefer paraphase but that is a matter of taste.
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Old 5th February 2008, 12:15 PM   #3
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First off, do you know if your OP trannies are the right impedance, coupled with your speakers, for EL84 in triode? They need ot be 10k P-P. For UL, they need to be a bit lower at 8k P-P, so the load for triode is a singificantly higher impedance than for UL. If they suit UL, then you'll probably get more distortion than you really should in triode mode. I any case, the most you're likely to get from triode mode is about 3 to 4 w and since EL84s are especially good in UL, that's the way I'd go if there's any choice.

Secondly, EL84s behave so well (in both triode and UL) using cathode bias that I doubt you'll do any better with fixed bias, though it won't do any harm to try if that's what you want.

Thirdly, unless you're stuck with 12AX7 for the voltage amp and splitter, you could do better IMHO. It's OK for guitar amps but I don't think much of it for Hi-Fi use. I'm also unsure if a 12AX7 concertina splitter would do very well driving EL84s in triode mode. A better tube, with 60% of the gain, would be a 12AT7.
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Old 5th February 2008, 12:46 PM   #4
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Quote:
Originally posted by ray_moth
They need ot be 10k P-P.

Secondly, EL84s behave so well (in both triode and UL) using cathode bias that I doubt you'll do any better with fixed bias, though it won't do any harm to try if that's what you want.

A better tube, with 60% of the gain, would be a 12AT7.
Hi Ray,
I do not agree with you about that 10k for triode unless we are talking about pure class A but then B+ should also be lowered to ca 250V. 8k will work well in triode AB-mode.

My example is based on AB, B+300V, really on the limit for the tubes.

About the two other matters I totally agree even if I consider 6DJ8 better than the 12AT7. But that might also be a matter of taste.

Cathode bias will drop the available voltage over the tubes so this will make everything a little more reliable.

Brgds
Lars
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Old 5th February 2008, 02:58 PM   #5
have' is offline have'  Netherlands
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My Simple EL84 (PP)are using opt with 6,5k primary.
I see that there are EL84 amp using 5k up to 10k.
How does the sound vary if we go from the lower to the higer
output impedance??

Thanks
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Old 5th February 2008, 05:30 PM   #6
kevinkr is offline kevinkr  United States
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I've used output transformers with primary Z of 7.8K - 10K PP with triode connected EL84 and have noted only very small differences in performance with real speaker loads and indeed on the bench as well. I would think most recycled EL84 UL/Pentode output transformers will provide more than adequate service in triode connection. I generally run in class A/B, but have built at least one amplifier that was class A.

You should expect no more than about 7W in AB1 and perhaps 5W in A.

I built a 7Wrms pp stereo amplifier on a Scott 299B (7189A) chassis nearly 18yrs ago, it is the only amp I still have from those days - and it still sounds great on the occasions where I use it.
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Old 5th February 2008, 06:20 PM   #7
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As I said in my first post you can expect 7W AB and 4W A with 320V and 8k.

If you want to try different Iq and Raa you can use my calculator:

www.revintage.se/PPABAMP.xls

Use it together with an Ua/Ia-diagram to find your specific working point. Just remember to plot Pamax in the curves before you begin. You can find an Ua/Ia for triode-connected EL84 in Mullards datasheets at www.duncanamps.com
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Old 5th February 2008, 08:16 PM   #8
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here's an amp I built recently that uses EL84's in triode, with a negative voltage supply. I like the way it sounds

Click the image to open in full size.
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Old 6th February 2008, 01:38 AM   #9
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Quote:
Originally posted by ray_moth
First off, do you know if your OP trannies are the right impedance, coupled with your speakers, for EL84 in triode? They need ot be 10k P-P. For UL, they need to be a bit lower at 8k P-P, so the load for triode is a singificantly higher impedance than for UL. If they suit UL, then you'll probably get more distortion than you really should in triode mode. I any case, the most you're likely to get from triode mode is about 3 to 4 w and since EL84s are especially good in UL, that's the way I'd go if there's any choice.

Secondly, EL84s behave so well (in both triode and UL) using cathode bias that I doubt you'll do any better with fixed bias, though it won't do any harm to try if that's what you want.

Thirdly, unless you're stuck with 12AX7 for the voltage amp and splitter, you could do better IMHO. It's OK for guitar amps but I don't think much of it for Hi-Fi use. I'm also unsure if a 12AX7 concertina splitter would do very well driving EL84s in triode mode. A better tube, with 60% of the gain, would be a 12AT7.

As the amp was UL to begin with I expect the trannies are 8K, not 10k. I'll have a play and see which I prefer. Not chasing anything above 3w.

Concertina phase splitter has a gain of less than 1 anyhow, irrespective of the tube used if I read the theory correct. It's the input tube that gives the voltage gain to drive the outputs.

As I haven't purchsed the input/splitter tubes yet and my power trafo has dual heater wingings it's open season for input gain stage and splitter duties, 12a?7, 6sn7 or whatever I want really.

Same, I'm not fixed with any particular B+ aside from the fact that my secondaries will only give 120 and 240 AC (well, I guess that means that if was going to use a voltage doubler then anything from 600 odd volts downwards is achievable) point is, B+ certainly doesn't need to be at 320.

The output section of Sorenj's amp is pretty much precisely what I was planning on doing, solves all those issues with how cathode bypass caps and cathode resistors of different types "sound" by just biasing the grid instead. But, as people have said, cathode bypass is simple, has been widely used, plus it gives an element of failsafe as the tubes age.

With a room of 3.5m x 6m and a listening distance of about 2.5m with a pair of 95dB/w efficient single driver speakers, more than a couple of watts will be plenty sufficient to aggravate my wife, cats, 2 year old... Not chasing megawatts.
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Old 6th February 2008, 03:57 AM   #10
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Not chasing megawatts, true, but it doesn't hurt to have some headroom so that transients don't get distorted. That's my main reson for suggesting you use UL, plus the fact that I doubt if you'll be able to hear any difference between triode PP and UL PP a low levels.
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