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Old 16th July 2007, 09:39 PM   #11
JoshK is offline JoshK  Canada
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Btw, I am watching this and other threads on the 813 with interest. I have some 813's in my possession, but I am not yet ready to work with this kind of voltage. I gotta cut my teeth on some more manageable projects.
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Old 16th July 2007, 11:37 PM   #12
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hey-Hey!!!,
Don't hold off for a 1kV 813 amp. It just isn't needed. That valve doesn't need its plate at full operating temperature to function as its getter. Run them conservatively, manage the filament voltage carefully and step up the B+ as you're comfortable with it.

Try a full voltage plate TX with a tapped autoformer in its primary to lower its output voltage. Feed it 70-80VAC instead of 120. There are some wound with taps every 5V, so 120 across the whole thing, and taps in 5V increments down from there...
cheers,
Douglas
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Old 17th July 2007, 01:49 PM   #13
JoshK is offline JoshK  Canada
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That is a good idea. I also have some 500VA isolation transformers (huge EI ones) that have multitapped primaries and secondaries that you can adjust voltage with. I could put those in front of the pwr tx.

I bought the 813's as I wanted to try some big bottles and they are cheaper for good old stock than 845's and 211's.
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Old 17th July 2007, 01:59 PM   #14
nhuwar is offline nhuwar  United States
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They are cheap and easy to drive compared to the 845's and the 211's

Thats why I picke it for my first amp.

Nick
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Old 17th July 2007, 08:13 PM   #15
7N7 is offline 7N7  United Kingdom
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Bandersnatch & JoshK,

Again interesting points - obviously these pentodes are special. However it is also said that pentodes are noisy (see Jones - Valve amplifiers 3rd ed.) owing to partition noise.

813 at 600V? quite low power I would guess; at 1kV you should be nowhere near cut-off if you run 90mA - I know that the PSU is expensive - I have been there!

And JoshK I agree the 813 is a splendid valve and will offer great results.

Good luck!

Paul

Edit: Indeed I did run 813 at almost 1.2kV, but later decided that 900V at 90-100mA would give better results. On the subject of feedback I had an output transformer with four equal secondaries, so I connected it for 4 ohms and connected the centre point to the cathodes of the 813s and enjoyed a distinct improvement in the mid-range (I cannot do feedback maths!).

On the subject of high voltages it was once said that to the steeplejack, above a certain point, all risks are equal. That may be so, but even 600V is a high voltage, take great care. I blew up carelessly constructed diode bridges due to tracking. These days I am a little more cautious.
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Old 17th July 2007, 09:48 PM   #16
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Quote:
Originally posted by 7N7
Bandersnatch & JoshK,

Again interesting points - obviously these pentodes are special. However it is also said that pentodes are noisy (see Jones - Valve amplifiers 3rd ed.) owing to partition noise.

813 at 600V? quite low power I would guess; at 1kV you should be nowhere near cut-off if you run 90mA - I know that the PSU is expensive - I have been there!

And JoshK I agree the 813 is a splendid valve and will offer great results.

Good luck!

Paul

hey-Hey!!!,
Cut off is also dependant on the load. With a 600V/150 mA idle piont, a low a-a load will allow cut-off and be an AB1( assuming no grid drive ) amp. Supporting that same idle point with a proper load is needed to achieve the Class A operation.

I would suggest adopting the automotive definition of high voltage----anything over 50V. Low impedance suplies that work so well for audio amps are not to be trifled with, or taken lightly.
cheers,
Douglas
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Old 1st October 2008, 02:56 AM   #17
JoshK is offline JoshK  Canada
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I am been meditating on the e-linear design.

I was under the impression that the e-linear plate (or g2) to plate feedback was specific to a pentode output tube. But this approach could be used with a triode output too couldn't it? I mean not through UL mode but from plate to plate. So for example, if you were driving a 300B with a D3A (in pentode) you could to plate to plate feedback right?

I was just thinking that this approach is a bit more applicable than some of the applications we've seen. I get why it is used with pentode in UL as output for more power while lowering output Z, but you could use it with triode and just further lower output Z.
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Old 1st October 2008, 02:58 AM   #18
JoshK is offline JoshK  Canada
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Oops, I just realized that I posted that in the wrong thread really. I was searching some e-linear threads, while this is one, its not the one I intended.
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