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Old 11th April 2003, 03:45 AM   #11
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Nevermind...

The point was it's not graphite, it's a powdered graphite coating. And the modern gray coating is more effective.

Tim
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Old 11th April 2003, 03:51 AM   #12
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Default CATCH 22?

Hi,

And what's that grey stuff then?

My point is that that graphite stuff is sounding better and...lo and behold... it measures better too!!!

Much can be learned from an old fox,
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Old 11th April 2003, 04:31 AM   #13
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Default Measured distortion

My loadlines of this tube shows a lot of distortion using an impedance of 2000. But, I probably am not doing it correctly.

Is the calculated distortion for this tube as high as I think it is (75 v on the plate as grid goes negative from op point of -17.5 grid and 100 v in the other direction)?

Rick
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Old 11th April 2003, 12:53 PM   #14
7N7 is offline 7N7  United Kingdom
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Default Re: Measured distortion

Quote:
Originally posted by fragman56
My loadlines of this tube shows a lot of distortion using an impedance of 2000. But, I probably am not doing it correctly.

Is the calculated distortion for this tube as high as I think it is (75 v on the plate as grid goes negative from op point of -17.5 grid and 100 v in the other direction)?

Rick


My distortion tests were arranged so as to determine the inherent distortion that the valve produces.

A load of 2k would be far too low - N.B. I was not testing section II of 6EW7 as an output valve.

After all 10W is the absolute limit, so the most that you could screw out of it in SE would probably be about 2.5W

I suspect that something in excess of 4k would be required - perhaps 10k a-a for p-p.

7N7
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Old 11th April 2003, 03:42 PM   #15
Fuling is offline Fuling  Sweden
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Ooops...!
Too bad I didn´t bother to draw a loadline before I bought the stuff...I did it a minute ago and with a 2k load it looked...horrible

Sadly there´s no 4 ohm tap on the OPT, so I can´t cheat that way either...
Maybe it would work better with the output stage connected as a cathode follower? Would of course need a really "swingy" gain stage, but that could be arranged I guess.
Any thoughts about this?

I´m not going to buy new output trannies, I already have more amps than I´ll ever need and this one was only meant as a funny way to kill some sparetime.
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Old 11th April 2003, 03:57 PM   #16
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Default 4k

Hi 7n7,

My mistake, I did use a load of 4k (25 ma:100v) for the distortion drawing.

My drawing shows Pout may be only a little over 1 watt in SE mode at 4k load.

I know that usually one can guess that a tube can produce about 1/4 the plate dissipation in SE mode but how can this be done?

What does "10k a-a" mean? "p-p" means push pull?

Thanks,
Rick
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Old 11th April 2003, 04:24 PM   #17
Joel is offline Joel  United States
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a-a..... "anode to anode".

p-p.... "plate to plate", somtimes "peak to peak".
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Old 11th April 2003, 04:59 PM   #18
7N7 is offline 7N7  United Kingdom
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Default Re: 4k

Quote:
Originally posted by fragman56
Hi 7n7,

My mistake, I did use a load of 4k (25 ma:100v) for the distortion drawing.

My drawing shows Pout may be only a little over 1 watt in SE mode at 4k load.

I know that usually one can guess that a tube can produce about 1/4 the plate dissipation in SE mode but how can this be done?

What does "10k a-a" mean? "p-p" means push pull?

Thanks,
Rick
A-a = anode-to-anode i.e. the total promary impedance of the primary of a push-pull (p-p) transformer.

It is worth noting that the manufacturers of the PX25 (ra = 560 ohms) recommended 10k as the optimum load.

Personally I am inclined to think that 6EW7 is best reserved for driver service rather than as a combined driver and output valve.

I know that some people have claimed success with 6DN7, but again I imagine that they have 105dB/w loudspeakers!

Configured as a diff pair, with cascode CCS and using the second sections as self-biased cathode followers driving a 30k pad, I measured THD of 0.025% (-63.5dB) at +28dB output (19.5v rms).

Since my oscillator's THD is -64.5dB I have concluded that this experimental circuit produced negligable distortion.

7N7
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Old 11th April 2003, 06:22 PM   #19
Fuling is offline Fuling  Sweden
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I´m getting a bit discouraged here...
No opinions about the cathode follower output idea?
No other ideas about what I can do with these tubes and transformers, together or in different projects?
I thoght for a while about using the transformers together with trioded EL86:s. Maybe they would match the 2k load better?

The 6EW7 looks alot nicer with a horizontal load line, which I guess represents a CCS or plate choke load.
Could be a nice n´grunty driver stage for a big triode amp or maybe even a hybrid

The fact remains: This cheap tube/transformer combo does indeed sound quite nice, so I have to admit that I´m a bit tempted just to carry on as planned, ignoring all the technical stuff
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Old 11th April 2003, 06:59 PM   #20
Fuling is offline Fuling  Sweden
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Minutes ago I took a better look at one of the output trannies
and noticed something really strange: there´s no airgap!!
Ok, not that strange maybe cause the trannies obviously weren´t made as SE OPT:s. I thought they where when I bought them...
The weird part is that the amp seems to work quite good with 50 mA DC running through the trannie!

Well well, at least I got a new idea: What about some kind of parafeed amp with a EL86 pentode CCS as a plateload for the 6EW7? A big coupling capacitor would then block all the DC from the OPT, which must be a very good thing, right?

Would also solve the problem with the very different voltages required for the different halves of the 6EW7. The input section asks for a lot higher voltage than the power section, and the PSU design would be alot easier if there´s a CCS that drops aome 150V on top of the output triode.

Both tubes can tolerate quite high heater-cathode voltages, so there shouldn´t be any problem with the heater windings.

Any thoughts, ideas, opinion???
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