Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: StevieRayVehkakoski on April 22, 2010, 07:39:32 am
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I guess this one has a EL-84 on the other side and a 6V6 on the other side of the push-pull OT?
http://www.stephensonamps.com/old/lj10head.htm
Has anyone tried something like this? What does it sound like? Separate cathode resistors + caps for the tubes and I guess the input signal to the EL-84 should be cut down a bit to match the output of the tubes a bit closer?
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I´m working in something similar.
http://www.el34world.com/Forum/index.php?action=dlattach;topic=8349.0;attach=13923;image (http://www.el34world.com/Forum/index.php?action=dlattach;topic=8349.0;attach=13923;image)
http://www.el34world.com/Forum/index.php?topic=8349.0 (http://www.el34world.com/Forum/index.php?topic=8349.0)
I have use 6v6 and El34 in this amp, but never mixed.
Right now i´m gonna install split cathode resistor and cap, so i think is posible mixed them.
6V6 and EL34 are similar more than the el84, but i think that PPIV master volumen for each side could do the work.
I,m still working on it. Stay on tune for more information.
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I guess this one has a EL-84 on the other side and a 6V6 on the other side of the push-pull OT?
This doesn't have to be the case, but the 6V6 and the EL84 generally find close to the same optimum load with a given plate voltage. So yeah, they are probably on opposite side of a push-pull OT, judging by the claimed 15w rating (if accurate) You'd get a bit less than that in parallel SE (although you could do it that way too).
Separate cathode resistors + caps for the tubes and I guess the input signal to the EL-84 should be cut down a bit to match the output of the tubes a bit closer?
I'd guess seperate resistors and bypass caps, because cathode bias is the eaisest way to get this done. And yeah, the EL84 will need less bias at the same operating point, so it can handle less input signal before distortion. But I'd imagine having one side distort before the other would be part of the magic recipe for this arrangement.
If I were building it, I'd probably try a master before the phase inverter or a volume put between the phase inverter and 1 output tube only, and see which I liked best. Good balance and identical operation is easily achived and heard in amps using the same tube on each side.
Besides, the Gm of the tubes are different, so they shouldn't respond to input signals the same way, even if you "level-matched" them with the differeing bias in mind. Something that occurs to me is the current contributed by each side is likely to be different, giving the net result of a standing current flowing through one side of the OT or the other. This isn't a problem, but you might want to plan on a larger core than normal or an air gap to help handle this. If you're thinking about a build, you may want to contact a transformer manufacturer and ask aboput a product that can handle the resulting d.c. Exactly how much it will be (which is what they'll need to know) would require at least a basic design and an examination of how the tubes work together.
Or maybe I'm being over-cautious. They might track somewhat well; but a Gm of 5000Umhos in one tube and 8000 in the other makes me wonder.
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And what about mix a EL34 and a 6V6?
pros, contras?
:angel
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The EL-84 and the 6V6 can be biased (and are happy biased) to about same wattage and I guess the OT wouldn't know that there's something going on before there's AC signal? If you bias the EL-34 and the 6V6 to where they usually are biased the EL-34 is going to take something like twice the amps the 6V6 passes through and I would guess the OT wouldn't be happy with that?
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Agreed.
Because of the plate dissipation difference between the 6V6 and EL34, idle current will be very different, unless the EL34 is biased very cold. The only concern is the difference in standing d.c. in 2 halves of a push-pull OT. Push-pull transformers are generally built under the assumption that d.c. in both halves will be nearly the same, which allows the manufacturer to eliminate the air gap and to make the core smaller.
So the only issue would be needing an air gap or a larger core, to allow for the partial magnetization of unbalanced d.c. However, if you ran the EL34 and the 6V6 in parallel SE, this issue is already taken into account, and you don't need any special precautions. The load will be non-optimum for one of the tubes, but you were probably more interested in the tonal qualities more than maximum clean output.
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I built an SE amp with paralleled 6V6 and EL84 with a master on each tube for mixing. It worked great except no one could hear the slightest difference between the 2 tubes in this amp. You may be making a lot of extra complications for very little payback (as I did).
Just a thought.
Dave