Before I punch the holes.. A quartet of EL84? - diyAudio
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Old 13th October 2012, 07:49 AM   #1
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Default Before I punch the holes.. A quartet of EL84?

I have all the lovely matching Iron (chokes as well) for about 325 - 350v and 8 or 4k Ot's. I want to upgrade my current EL84 amplifier and I plan to make "Bevois Valley" clones in monoblocks but with a quartet of EL84's. If you are unfamiliar then its a concertina without a driver stage into UL Push pull EL84's (2) From the Morgan Jones Book
So before I start punching the holes, Is this going to work? Can I Just parallel the valves and adjust the cathode resistors etc, or am I going to need to swing more voltage and add a driver stage? I cant find another schematic to get a hint and I have never done "parallel" output valves (cant even spell it!)! Please throw me bone!
Mick
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Old 13th October 2012, 09:09 AM   #2
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I've changed my Leak Stereo 20 to PPP 6S4 tubes, and that works nicely. These are genuine triodes with 9 pin bases. Cheap as chips. I've exchanged emails with Morgan Jones about using these and he approves.
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Old 13th October 2012, 05:13 PM   #3
rongon is online now rongon  United States
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My understanding is that with pentodes (and Blumlein/ultralinear connection), the input capacitance of the EL84 output tubes remains low enough that it's not that big of a deal for a 6DJ8 cathodyne to drive.

Mr Jones even "slugged" the grid resistors on the EL84s in the Bevois Valley amp with small value capacitors to reduce the bandwidth. So I doubt you'll lose appreciable bandwidth by driving push-pull-parallel EL84s with the Bevois Valley cathodyne (concertina) phase splitter/driver.

The voltages won't change, so long as you bias the tubes to the same operating points. Each EL84 will still have -11V grid to ground.

What I can't tell you is if there are other issues involved, like current limiting. I don't have the expertise to figure that out.

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Old 13th October 2012, 06:19 PM   #4
DF96 is offline DF96  England
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You will of course need an output transformer which can handle twice the current and present half the impedance.
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Old 13th October 2012, 06:52 PM   #5
rongon is online now rongon  United States
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Quote:
Originally Posted by DF96 View Post
You will of course need an output transformer which can handle twice the current and present half the impedance.
Hey, that's pretty important!

The OPT would need to be rated for > 160mA current load, about 4k or 5k ohm primary to 8 or 4 ohms secondary.

Come to think of it, wouldn't any UL OPT designed for PP EL34s work pretty well for PPP EL84s? I know the Dyna A470 from the ST70 amp had a 4k3 primary, with the EL34s biased to 50mA each quiescent. I don't know how they'd do with 160mA of standing current.

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Old 13th October 2012, 07:14 PM   #6
DF96 is offline DF96  England
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Yes, an EL34 OPT would not be far off what is needed for paralleled EL84s. Some transformer makers make OPTs for paralleled valve; others don't.
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Old 14th October 2012, 06:39 AM   #7
disco is offline disco  Netherlands
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How's the slew rate affected?
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Old 14th October 2012, 01:12 PM   #8
rongon is online now rongon  United States
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According to that wiki, the slew rate is a function of the load (input) capacitance and the current needed by the preceding stage to drive it. If I did the math right, the highest frequency the 6DJ8 cathodyne can drive is still very high, even when presented with approx 500pF Miller capacitance from a paralleled pair of EL84 in triode. The Miller capacitance should be much lower from a parallel pair of EL84 in ultralinear connection.

I suspect the above may be correct because, as mentioned previously, Morgan Jones considered the bandwidth to be so wide in his Bevois Valley amp that he slugged the grid leak resistors of the output EL84s to introduce a low pass filter. Double the output tubes will present double the input capacitance, but with that being low in the first place, twice something very low is still pretty low.

I'm a complete novice, so I may have that entirely wrong. Hopefully someone who is more confident with this will chime in.

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Old 16th October 2012, 01:34 AM   #9
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[QUOTE=
Come to think of it, wouldn't any UL OPT designed for PP EL34s work pretty well for PPP EL84s? I know the Dyna A470 from the ST70 amp had a 4k3 primary, with the EL34s biased to 50mA each quiescent. I don't know how they'd do with 160mA of standing current.

--[/QUOTE]
Funnily enough the OT was using EL34's. There is only about 350v available for the max B+ so i didnt think that EL34 would be the way to go. Plus I like EL84, they are cheap and sound good
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Old 16th October 2012, 10:32 PM   #10
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I've built a Bevois Valley. To convert to parallel push pull EL84 will not be a problem. The additional load on the concertina splitter of 2 off 470K grid leaks per side (220K total) is not a problem and as Rogon says above the original design had the input capacitance of the EL84 slugged with a cap across the grid leak resistor (this is where the dominant pole of the circuit is). You probably won't need that cap, its function will be provided by the additional capacitance from the parallel output tube. Build it and check stability with the usual square wave test.
I would use individual cathode bias resistors and bypass caps for each of the 4 output tubes.
My Bevois Valley became the mod platform for my first Baby Huey.
Cheers,
Ian
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