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gianmaria 13th February 2006 06:35 PM

Vacuum Tube Pre O.C.L.
Hi everybody!

Anybody knows a schematic of an all Vacuum Tube Pre amplifier, with no capacitor on the signal path?

What kind of solution has been implemented?

Thanks in advance.

analog_sa 13th February 2006 07:38 PM

SY's preamp described on this forum. Any transformer coupled design. It's easy to avoid coupling caps but not necessarily cheap.

SY 13th February 2006 08:46 PM

Actually, my preamp does have one coupling cap internal to it, though I eliminated the troublesome output cap. The input cap could be eliminated, too, by returning the servo output to the bottom of the transformer, but I think that's a cure worse than the disease; the input cap is pretty innocuous, but redoing the servo connection puts any servo errors and grunge right back to the input of the circuit.

One can do some level shifting to get direct coupling all the way through at the expense of complexity. I've played with it in the past and never thought is was even vaguely worthwhile.

Looneytunes 14th February 2006 04:54 AM

Warning: NewB question

What happened to the little chef? All this time I thought you were a child prodigy or something... heretical preamp with a little Frim Fram Sauce on the side. ;)

Seriously (Sy or anyone) if cost weren't an issue, which would be the better choice for a simple preamp: a really good coupling capacitor or a really good input/interstage transformer? And who makes really good input/interstage transformers?

Thanks everyone.

SY 14th February 2006 12:37 PM

I think that an input transformer brings a lot of benefits to the table. I'm much less convinced that there's a similar benefit/cost ratio for output transformers. And I hate big output coupling caps.

So far, I've tested Jensen and Cinemag 1:1 input transformers; both seemed excellent. I'm trying to get my hands on some Magnequest 1:1+1 units to give 6dB of gian in response to some requests. My guess, from testing other Magnequest products, is that they'll be first rate.

Looneytunes 15th February 2006 03:29 AM

Thanks SY

gianmaria 15th February 2006 07:06 AM

Thank you to everybody!

Sorry, maybe I didn't post the question very well.

I meant a preamplifier with NO coupling capacitor e NO coupling transformer on the line path.

So, analog_sa, you got the question: could you help me to find the schematic of the mentioned SY's preamp?

Where do I have to look for?

Thank you in advance, G.M.

pinkmouse 15th February 2006 10:05 AM

2 Attachment(s)
Look here.

A pic of my slowly progressing version...

SY 15th February 2006 10:16 AM

GM, the only way you can avoid having any transformer or capacitor is to have some level shifting. For example, if you use an ECC88 cathode follower with the grid at DC ground, the output will always be riding on a volt or three of offset- under the conditions that I run my CF, the offset will be 1.7V. So you can add 1.7V worth of battery or interpose an LED between the cathode and CCS, then pick the signal off the junction of the two.

In either case, the cure is much worse than the disease.

EC8010 15th February 2006 01:22 PM

One solution is to use a Russian 6XS7 double triode, as its unique cathode emissive material based on unobtainium allows complementary operation from inverted supplies. There is a problem in that the cathode is somewhat radioactive (Pu) and there would be a toxicity issue should the fragilium quartz envelope be compromised, but the minor hazard was considered acceptable by the designers. Besides, the problem is not appreciably worse than some of the Western Electric gas voltage references (cf 313C). Because of the complementary operation, the valve must be biased to stimulate electron emission and the lowest noise option is to do this is optically, using a GaAl laser with a wavelength of 465nm pointing directly into the cathode singularity. Obviously, optically stimulated emission requires that the valve (and laser) is enclosed in a light-proof box, and given the minor radioactivity noted earlier, it is advisable to make the box out of pure lead (999 purity minimum). The original application for the 6XS7 was in reactor control systems (cf Chernobyl).

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