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Old 28th April 2007, 04:00 AM   #1
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Default Eight 807's in Push Pull?

I have recently developed a strong admiration for 807 tubes and I just acquired a large lot of them!

Can they be paralleled on a push pull output stage to drive a 1900 ohm primary impedance transformer AKA Hammond 1650T?

What valve pre stage is capable of driving such a large set of valves push pull?

Cheers,

Shawn.
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Old 28th April 2007, 04:18 AM   #2
SY is offline SY  United States
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Sure. In triode, you should be able to get 40W or so. You'll need to stick some sort of follower between the gain stage and the outputs to deal with the Miller capacitance. Voltage swing required is the same as a single tube, not at all onerous, maybe 35VRMS.
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Old 28th April 2007, 05:34 AM   #3
BudP is offline BudP  United States
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Tom,

The only way I know to use a transformer as an interstage, that will work with this sort of load, is to step the ratio down from the driver tubes. I use a 1.414 to 1 step down ratio for eight EL 34's in PP parallel triode connection. I also use two paralleled driver tubes in PP parallel triode, a pair of 7119's. Fine amplifiers.

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Old 28th April 2007, 06:44 AM   #4
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Default Re: Eight 807's in Push Pull?

Quote:
Originally posted by TomWaits
What valve pre stage is capable of driving such a large set of valves push pull?

Cheers,

Shawn.
If I was going to attempt something like this, I'd probably go with a MOSFET driver connected as a source follower. You're going to get a whole bunch of Cmiller that will require lots of drive current to charge it up fast enough to minimize the slew rate problem.

Anyway, I wouldn't do that. If you need lotsawatts, two 807's in Class AB2 will give some 80W with 3.5% THD without feedback. Two Class AB2 807's are a much easier load than eight in Class AB1, trioded.

If you have a whole bunch of 807's, sell some off and get the iron that'll work with this.
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Old 28th April 2007, 06:53 AM   #5
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Hi Shawn,

807, like 6L6 is not particularly hard to drive when being used as a beam power tube. It is a little bit harder in triode mode, though, due to the Miller capacitance.

As mentioned already, driving them via a buffer (CF) is a sure fire way to go.

But, there is another feasible way: Just use a beefy LTP, f.e employing triode strapped 6V6 or EL84/6BQ5, which compares roughly to 4-5 6SN7 sections in parallel.

This way, a "Mullard" style amp could be done. Be aware, though, that an LTP using trioded EL84 or 6V6 would be a rather tough load to an EF86 based frontend, as being used in the Mullard circuit. I would suggest 6AU6 or something of its class instead, getting some decent curent going in the 1st stage.

Regards,

Tom
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Old 28th April 2007, 11:23 AM   #6
agent.5 is offline agent.5  United States
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http://www.plitron.com/shopping/shopexd.asp?id=626

why get Hammond when these 400w Plitron are sold as surplus?
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Old 28th April 2007, 01:46 PM   #7
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Quote:
why get Hammond when these 400w Plitron are sold as surplus?
I just couldn't say no. A 400 watt P-P toroidal OPT for $134 USD. Deal! Two transformers are on order, now I just need to figure out what to build. With 2 X 400 watt OPT's it will be big, and loud. Well I have a bunch of 807's, but I also have a bunch of big sweep tubes. Class B screen drive maybe?
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Old 28th April 2007, 01:51 PM   #8
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That might well be the thing to do- with idle currents in the single digits of milliamps, DC imbalance (the thing that scares me about toroid OPTs) becomes less of an issue.

How bad was the shipping?
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Old 28th April 2007, 03:51 PM   #9
rdf is offline rdf  Canada
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Quote:
Originally posted by tubelab.com
I just couldn't say no.
I wrote Plitron for specs more detailed than primary impedance and size and they didn't say anything at all. Curious what you'll find.
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Old 28th April 2007, 03:53 PM   #10
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Quote:
That might well be the thing to do- with idle currents in the single digits of milliamps, DC imbalance (the thing that scares me about toroid OPTs) becomes less of an issue.
CCS in the cathodes keeps this under control, but I think it restricts you to class A (though I have seen it suggested that AB is possible). I am currently running this setup and am very happy with the results. Can be turned into a fully differential output stage.

Shoog
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