Re: attn: Chris Hornbeck and Patrick Turner Re: A question (6CA7 bias current)

On Tue, 29 Nov 2005 07:02:05 +0000, Chris Hornbeck wrote:

> On Mon, 28 Nov 2005 01:55:40 -0500, bart <bart@xxxxxxxx> wrote:
>
>>And please don't say "fourty-two". :-P
>
> What's nine times seven?

Umm, an integer between fourty one and fourty three?

>
> Patrick covered a lot of outlying possibilities, just
> in case ('cause cases happen), but first, just
> replace the goll-durned selenium rectifier and retest.
> Can't cost 25 cents, and ain't worth the drama.

That's done. Tube still exhibits the ramp up characteristic. After
reading the comments written into the provided manual, it would appear
that the original owner experienced a bias problem with the original
6CA7's and replaced them, the 30-20-20-20 uf cap and the bias capacitors
in 1978. It didn't solve the problem and he left the cathode voltage at
1.71 volts vs 1.56 volts. I'll be warming up the original chassis
shortly, checking out the bias circuit and seeing what the tubes' behavior
is. I don't want to blow out my existing working one.

Also, this thing has been modded per the Jensen specification (1978).
Dunno if I should go back to the original 5AR4 to drop the voltage down or
not.

>
> Cathode current is completely and solely a simple,
> known and repeatable function of grid voltage and
> plate (= G2, in this case) voltage.
>
> Plate voltage is pretty much independent of the
> particular tubes/ valves personalities. Grid (G1)
> voltage is potentially affected (adversely, for
> our purposes) by the tubes/ valves gassiness.
> (For practical non-zero DC source impedances).
>
> That's it. That's the entire model.

Okay, I'll keep it in mind.

>
>
>
>
> ps: I still don't recommend American 6CA7's for
> Dyna ST70's because their higher transconductance can
> too often cause motorboating. A modern revision
> or rebuild shouldn't have this potential issue.

Revision as in a zener at the -48 volt portion of the chain or something
more drastic? Have you any web available examples of this?

>
> Good fortune,
>
>
> Chris Hornbeck

.

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