A very stupid mistake!!!!!
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 11th April 2003, 03:45 AM #11 Sch3mat1c   diyAudio Member     Join Date: Jan 2003 Location: Milwaukee, WI Nevermind... The point was it's not graphite, it's a powdered graphite coating. And the modern gray coating is more effective. Tim __________________ Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation
 11th April 2003, 03:51 AM #12 fdegrove   diyAudio Senior Member     Join Date: Aug 2002 Location: Belgium 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, __________________ Frank
 11th April 2003, 04:31 AM #13 fragman56 diyAudio Member   Join Date: Jun 2002 Location: Denver, CO 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
7N7
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Join Date: Jan 2003
Location: England
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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 11th April 2003, 03:42 PM #15 Fuling   diyAudio Member   Join Date: Aug 2001 Location: Sweden (Mora) Ooops...! Too bad I didnīt 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 thereīs no 4 ohm tap on the OPT, so I canīt 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? Iīm not going to buy new output trannies, I already have more amps than Iīll ever need and this one was only meant as a funny way to kill some sparetime.
 11th April 2003, 03:57 PM #16 fragman56 diyAudio Member   Join Date: Jun 2002 Location: Denver, CO 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
 11th April 2003, 04:24 PM #17 Joel   diyAudio Member     Join Date: Oct 2002 Location: Brooklyn, NY a-a..... "anode to anode". p-p.... "plate to plate", somtimes "peak to peak".
7N7
diyAudio Member

Join Date: Jan 2003
Location: England
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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 11th April 2003, 06:22 PM #19 Fuling   diyAudio Member   Join Date: Aug 2001 Location: Sweden (Mora) Iīm 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 nīgrunty 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 Iīm a bit tempted just to carry on as planned, ignoring all the technical stuff
 11th April 2003, 06:59 PM #20 Fuling   diyAudio Member   Join Date: Aug 2001 Location: Sweden (Mora) Minutes ago I took a better look at one of the output trannies and noticed something really strange: thereīs no airgap!! Ok, not that strange maybe cause the trannies obviously werenīt 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 thereīs a CCS that drops aome 150V on top of the output triode. Both tubes can tolerate quite high heater-cathode voltages, so there shouldnīt be any problem with the heater windings. Any thoughts, ideas, opinion???

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