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Old 5th July 2005, 04:01 PM   #11
SY is offline SY  United States
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Six digits, Tim: IRF820.
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Old 5th July 2005, 05:49 PM   #12
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> runing a triode into grid current significantly increases distortion

The output versus grid voltage IS linear.

As a rough approximation: the Cathode-Plate circuit only cares what the grid-cathode voltage is, not how much current flows in the grid-cathode cuircuit. Tubes are very linear in the positive grid range. (Actually, grid current steals cathode current away from the plate, but up to a point this is a small effect.)

The problem is in driving the sometimes-low grid impedance with a steady voltage. You won't do it with the plate of any tube smaller than your outputs. You can't do it very well with old-time cathode followers (though there is a tube that is an A2 output plus a cathode follower). The late-era high-Gm tubes can drive home-size output tubes into grid current if you wish.

Note that with conventional outputs and cathode follower drivers, you need a BIG negative rail to pull-down the cathodes and grids on the negative side. (We can't capacitor-couple because of the asymmetric current waveform.) Between high driver standing current and deep negative rail, driver power consumption can be as expensive as just tossing a couple more output tubes in. Transformer coupling has real advantages: transforming a high-Rp tube down to low impedance, and kicking negative without a negative supply.

BTW: the Macintosh 6L6 amp runs slightly AB2 to meet its power rating. Not a lot of grid current: it only uses a 12AX7 as driver. The original Mac 50 not only runs past the old-6L6 AB2 rating of 47 watts, it uses a transformer driver and takes feedback around two transformers.
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Old 5th July 2005, 07:03 PM   #13
cerrem is offline cerrem  United States
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Taking a quick look...here are some usable numbers I came up with...
A pair of triode strapped EL34 valves in AB2...
Operating at 400V....use a 4.6K plate load you can extend up to the +20V curve in a reasonably linear region, and the load line hits the +20 curve roughly at .320 mA....
This comes out to roughly 30W Power Output....
Figure biasing somewhere abouts -35.....
Make sure you have the full needed AMPLITUDE prior to your driver stage..

Or if you operate at 350 plate volt....use a 4K plate load and you hit the same point at +20V curve which is .320mA ....
This is roughly 26 Watts Power Output.....
I am using plate curves I made a number of years back using a Mullard El34 "bogie" valve.....
The plate load is CRITICAL is AB2 ...if it ain't right...then you ain't going to extend into the + region......unless you LUCKY....

Getting to the driver valve..... There are lots of options..
YOu can use the 6SN7 follower with both sides in parallel, this will give you roughly a 180 ohms for a driver.... or my personal favorite a 6V6 tied as Triode but used as a follower..this is roughly a 160 ohm driver, very linear....Take your pick....

I don't have the magic number for grid current at +20V ...But figure your grid will come crashing down to roughly 1K as pointed out by another poster.....this is a rough but good guestimate....


Chris
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Old 6th July 2005, 04:56 AM   #14
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Wow! Thanks very much for all the responses! My original question was mainly concerned with whether the EL34 could tolerate some grid current, but all the other advice is gratefully received too.

On balance, having read what people have said, I think I'll give AB2 a miss. (Chicken!!) The modeling with LTspice has been fun but, if it's not going to give me a credible result for grid current, then the only way to find the real answer would be to build it. That would involve making extra holes in the chassis for the additional 6SN7, plus the wiring. That's a lot of work for little or no gain (no pun intended).

I already have a negative supply in place. However, it has to supply two channels and wouldn't tolerate more than about 28mA, since it consists of a 50v 100mA winding with a voltage doubler, to give ~110v; from people's comments, I would probably need more than this, especially if I have to use a beefier driver than the 6SN7.

Perhaps the best approach for me, if I need more headroom and don't want to use the EL34s as nature intended (pentode or UL), would be to do what Gingertube has done, i.e. stay with AB1 and add another pair of EL34, triode-strapped, in parallel. My OPTs are big enough to tolerate this, I believe, and I could use the 8 ohm tap for my 8 ohm speakers instead of the 4 ohm tap (which I'm currently doing, to get 7k plate-to-plate load). Just have to find another power tranny!

For anyone interested, I attach the schematic of my present amp, which sounds OK to me. ( Any comments in this circuit would also be gratefully received).

Thanks again for your valued opinions.

Regards,

Ray
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Old 6th July 2005, 05:12 AM   #15
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Ray,
If you want to have a quick and dirty trial of AB2 then use SY's trick and use a IRF820 as a source follower. No hacking up your chassis for a new tube hole or heater supplies to worry about. Sonically it will be VERY close to what you'd achieve with a tube cathode follower, so if you liked it you could convert to tube cathode follower afterward. You've already got a suitable negative rail so its really just one MOSFET and one resistor to add to each channel.

BTW my 4 x EL34 Triode Strapped parallel push pull is into about 2400 Ohms Raa (Plitron VDV-2100 with nominal 6 Ohm speakers on the 5 Ohms secondary). These monoblocks are working quite well with a wimpy front end - ECC99 as common cathode with direct coupled concertina. I'm getting -3dB top at 105kHz at 10 Watts out, despite "inadequate?" drive of the output tube grid capacitances.

Cheers,
Ian
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Old 6th July 2005, 06:23 AM   #16
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> if I need more headroom .... stay with AB1 and add another pair of EL34

Or stop fooling with the small-beer EL34, and use the big-bottle 6550. It is a very powerful pentode and a mighty fine triode. While the EL34 can do amazing things, you have to be heroic with a pair of EL34 to make half what a pair of 6550 will do. I think heater consumption will be less.
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Old 6th July 2005, 07:05 AM   #17
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Quote:
Originally posted by ray_moth
Wow! Thanks very much for all the responses! My original question was mainly concerned with whether the EL34 could tolerate some grid current, but all the other advice is gratefully received too.

On balance, having read what people have said, I think I'll give AB2 a miss. (Chicken!!) The modeling with LTspice has been fun but, if it's not going to give me a credible result for grid current, then the only way to find the real answer would be to build it.
Then get out of the simulator. Start SWAGging some results yourself on pencil and paper!

Tim
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Old 6th July 2005, 08:43 AM   #18
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Project ideas for the future: 6550, IRF820, pencil and paper . . . why not? It's my hobby, after all! I'm heroic enough to try moist things if I can afford them (although the pencil and paper idea fills me with trepidation).

Thanks again.
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Old 7th July 2005, 04:46 AM   #19
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PS I've saved this thread on my PC, so I can take more time to study it. There's a lot of valuable suff here!
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Old 7th July 2005, 07:41 AM   #20
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Quote:
Originally posted by ray_moth
... to try moist things if I can afford them ...
Not a problem; moist things are often cheaper (mildew and whatnot).

...

Tim
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