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Before I punch the holes.. A quartet of EL84?

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bullpeters

bullpeters

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Joined 2009
baynuts.com.au
#1
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
 
#3
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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#5
DF96 said:
You will of course need an output transformer which can handle twice the current and present half the impedance.
Click to expand...

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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#8
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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#10
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
 
#11
This is my first post, I have made se 6c33c and pp 1625 amps in the past and have just been given a music angel 8 X EL84 to repair, I have never worked with PPP. the amp is poorly put together and several toasted cathode resistors, I have changed these with similar results. does anybody have a schematic?. I suspect the tubes may be duff, is there a quick test that can be done I do not want to cook somthing else and I have run out of 10 ohm resistors, I tried measuring volts across this but its hard to see four values at a time and it rises very quick, any help would be great
 
#12
Is this going to work?
Click to expand...

It will work. The Tubelab SPP is very similar to the Bevois Valley, except I use a 12AT7 driver tube. The standard board is a 2 channel P-P amp. I have built several PPP amps by pulling one driver tube and running jumper wires to parallel the cathodes grids and plates of the second set of EL84's. The load impedance required is half of what you would use for one pair of EL84's. I have been using 6600 ohm OPT's because I have a bunch, so I simply connect the 8 ohm load to the 16 ohm tap to create a 3300 ohm OPT. I use a separate set of grid stoppers (1 K ohm 1/4 watt), screen stoppers (100 ohm 1 watt), cathode resistors (270 ohms 5 watt), and cathode bypass caps (1000 uF 35 volt) for each tube.

I have tried UL, but usually use pentode connection. On 350 volts the 4 tube PPP amp will make about 35 watts at clip.

For the adventuresome (crazy) person the JJ EL84's and most NOS 6BQ5's will eat over 400 volts on the plate as long as the screen is kept at 320 volts. Four JJ EL84's on 430 volts with a 3300 ohm load will make 50 to 60 watts without red plate or blowing up......and I play my guitar through it, cranked!

I tried wiring 2 boards together for an 8 tube PPPPP but I could never tame the random oscillations. It made over 100 watts when it worked.
 
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