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Old 10th October 2007, 07:52 AM   #1
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Default Needed : ECF83, can anyone help ??

Hi there,
I need a single or pair of ECF83's to try out a preamp project I am thinking of. They are quite tricky to lay hands on. Has anyone got a single/pair which they would be willing to pass on to a good cause.

Shoog
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Old 10th October 2007, 04:03 PM   #2
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$8 at vacuumtubes.net ( http://www.vacuumtubes.net/p5.pdf ), $8.50 at AES ( http://tubesandmore.com/ )
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Old 10th October 2007, 06:05 PM   #3
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At a pinch I will go to America for parts, but its not my first choice due to distance.

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Old 10th October 2007, 06:24 PM   #4
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I've never been dyslexic before, but I read Eire as Erie.
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Old 10th October 2007, 06:50 PM   #5
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Try, www.chelmervalve.co.uk they might be able to locate. They have often rescued me out of a tricky spot.

Are you sure you want to design around a rare'n ?

richj
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Old 10th October 2007, 10:11 PM   #6
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Thanks for the heads up on the Chelmer, I will take a look.

I know its risky going with such an obscure valve, but it just looks as if it will fit the particular requirements of a project like no other. I need a low low gain pentode (x10 in triode) that is happy to operate with both the plate and screen at about 80V, and pass reasonable but not excessive current. The pentode in the ECF83 fits the bill like no other, and has a free triode thrown in to boot (which will make a reasonable CCsink for a cathode follower stage). The only fly in the ointment is that the pentode is remote cutoff. Still i will bias at -5V and only expect an input sensitivity of about 2Vrms. I hope this will keep it linear and out of its cutoff range.

If anyone has any useful ideas of a suitable alternative I'm all ears.

Shoog
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Old 11th October 2007, 04:16 AM   #7
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Shoog,

I would carefully look at the ECF80 (6BL8) to see if will do what you want. Specs are very similar.
Why?
Coz there are huge quantities of these things out there (common as mud) and I know they were often used in audio (often as a tone stack driver and recovery and sometimes as a pentode preamp + triode concertina splitter).

I know one hi end designer who is currently designing a line stage using 6BL8 - he's using the pentode section as a cathode follower output driver and using the triode section as cathode follower, ahead of the pentode to drive the screen

ASIDE: with a pentode cathode follower its important to keep the screen at AC 0V with respect to the cathode, if you simply set the screen voltage to a DC level then it is the triode characteristics which apply, it does'nt work like a pentode any more.

Cheers,
Ian
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Old 11th October 2007, 03:04 PM   #8
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I think I may just try the ECF80. I am going to use the pentode in a UltraLinear type configuration with the screen referenced to the cathode of a totem CF. So effectively it will be triode strapped. Does anyone know what sort of gain I can expect from the pentode as triode.

Shoog
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Old 11th October 2007, 08:00 PM   #9
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Quote:
Originally posted by Shoog
I think I may just try the ECF80. I am going to use the pentode in a UltraLinear type configuration with the screen referenced to the cathode of a totem CF. So effectively it will be triode strapped. Does anyone know what sort of gain I can expect from the pentode as triode.

Shoog

I regulary use 80's as triodes. Expect 26dB gain (roughly half of pentode) as circuit shown and I also use an input tranny for some step up and isolation. The screen res can be omitted and circuit typ for front end p-p amp with NFB via undecoupled cathode resistor

I find pentode with full 40dB gain too microphonic.

The med mu triode section is excellent for cathodyne function. They nearly always are.

hope this helps.
richj
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Old 13th October 2007, 02:44 AM   #10
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Quote:
Originally posted by richwalters



Expect 26dB gain (roughly half of pentode) as circuit shown richj

Actually the ECF80 gain as triode should read 4.5dB without cathode decoup cap and I make the gain up by using an input tranny. I'm somewhat against using electrolytics around critical nfb signal paths. The gain may seem quite low but I make this up with a high gain driver stage 28dB using video tubes.

A high power amp 2x 150W using octet 6550's also using ECF80's, is currently in the assembly/ build phase. The electro design is complete and assembly as a challenging winter project.

richj
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