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Old 30th September 2010, 08:22 PM   #11
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You don't say that the line input has to drive the grid directly so... It could be done with a couple of MOSFETS driving the grid of a 100TH with current mode plate feedback.

About 25W at the plate, 20W+ output depending on OPT. 80W dissipation + 30W filament.

michael

OOPS grid is actually +20 wrt the filament so make that +120 from ground.
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Old 30th September 2010, 09:38 PM   #12
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Originally Posted by dannydanger View Post
Ok, going way out on a limb here perhaps but, is there a valve in existence, a large transmitting tube perhaps, that is capable of producing 15-20W in a single ended design, with no driver? I imagine it being some super high voltage monster, is there a valve that could do it or is it a pipe dream?
The 845 can do 20W SE easily. That's what it was originally designed for: AF final for high powered, plate modulated, AM transmitters that could conveniently share power supplies with the RF finals. Of course, that means running these at some serious voltages: ~1000Vdc. It just so happens that the 845 also sounds pretty good as well, so SETs don't always have to be anemic amps that need special speeks, or are only good for headphone duty.

As for as "with no driver" is concerned, what does this actually mean anyway? It will always need some sort of driver, even if it's incorporated into some other device. To get that 20W of output, an 845 will need to see 310Vp-p at its control grid.
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Old 1st October 2010, 02:41 AM   #13
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Any thoughts on an 811A? I've come across a few SE designs utilizing them.
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Old 1st October 2010, 03:35 AM   #14
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The driver stage is needed because it is what does the voltage amplifying. The output tube needs a high enough input voltage on its grid (or however it is fed) to put out the wattage. You may be able to drive a power tube directly from a regular power amp though, through a transformer.

The driver stage is the most important and complex part of the amplifier, but with interstage transformer coupling you can reduce the driver components to get closer to your idea of having no driver stage. Actually a transformer would be sensible on all elements of the tube, that is the grid, plate, and even the cathode (twin coupled, cathode windings, or cathode follower.)

And www.electra-print.com comes close to answering your question as well.
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Old 1st October 2010, 04:12 AM   #15
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Quote:
Originally Posted by dannydanger View Post
Any thoughts on an 811A? I've come across a few SE designs utilizing them.
With the right design, these can sound really good. Being that this was designed to be a "zero bias", RF final, it will require positive bias to set a SE Q-Point, and a stiff driver to supply the grid current it will need. Here, think source follower.

It may also require some assistance from local NFB. This is a high-u triode, and that means a high r(p) that doesn't play well in terms of speaker damping, and, given the higher than normal r(p), the possibility of more h3 and higher order harmonics than you'd expect from low-u, audio type triodes. You may also need some gNFB to clean that up.
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Old 1st October 2010, 01:24 PM   #16
el156 is offline el156  Portugal
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Hi!
The EL156 can give you 25W Class A with Va 450V, and Vin 9V input, wich can be done very easy with a step up transfo! Than you will have 25W output ,with 2V input !
Try It ! and Report for us
thanks
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Old 1st October 2010, 02:48 PM   #17
Pano is offline Pano  United States
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Interesting. Something like the Edcor WSM600/10K? You'd almost get to 9V.
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Old 1st October 2010, 04:47 PM   #18
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I guess one could get almost 25W out of an EL156 with an input stepup transformer, run right on the edge of it's 50W (autobias only) dissipation limit.

There's no way to apply any NFB given the input signal is barely enough, so there will be no speaker damping to speak of.

But it could be done...
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Old 18th November 2010, 12:10 PM   #19
mik190 is offline mik190  Russian Federation
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Here is the single tube single stage 45W amplifier.

http://spbsound.narod.ru/SPbSound/sc...chematic2.html

I read that some people build it in Russian forums.

Some people in Japane did similar as well as I heard.

I would like to repeat it at less powerfull tubes but did not find the scheme.
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Old 18th November 2010, 12:16 PM   #20
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Quote:
Originally Posted by mik190 View Post
Here is the single tube single stage 45W amplifier.

http://spbsound.narod.ru/SPbSound/sc...chematic2.html

I read that some people build it in Russian forums.

Some people in Japane did similar as well as I heard.

I would like to repeat it at less powerfull tubes but did not find the scheme.
So that is an 833A with 2300 Volts on the plate?

WHOA!
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