[Amps] Requesting opinions on a Drake L-4b's tubes

[John Lyles]

The Collins 828C1, and Continental 314R1 1kW medium wave transmitter 
used a single 3-500Z as a switch tube running at 70 KHz switching 
frequency for pulse width modulated AM. It was very picky of the type of 
tube, and required component changes if switching to a graphite anode 
tube to prevent disasters. So apparently there are differences in the 
tubes. Maybe not subtle when paired in parallel in ham amplifiers?

http://www.jtml.info/

73

John

K5PRO

On 12/24/22 11:27 PM, David Gow wrote:
> The original poster said they were in original Drake packaging.  If 
> they were in fact in original sealed Drake boxes I wouldn't worry 
> about using them together as Drake would have sold them at different 
> times as compatible spares.  I have mixed all sorts of 3-500z tubes in 
> many amps over the decades and my test is to see if under color if the 
> look about the same they are ok to use together.  Your are correct in 
> theory but perhaps not so much in practice.  Right now my Henry 2K3 
> has one original NOS stock Eimac from my shelf bought by me over 25 
> years ago and a brand new RF Parts Chinese graphic tube.  THey were 
> both unused when I installed them six months ago.  They show about the 
> same color, a nice  dull red. when appropriate and have normal plate 
> current, grid current and put out about 1200 watts on 20 meters.  Best 
> practice?,no,  but works fine.  My buddy, a retired broadcast 
> engineer, doesn't approve either so I told him to buy me a nice new 
> matched (at least for a few hours!!) set.
>
> On Sat, Dec 24, 2022 at 8:14 PM John Lyles <jtml at losalamos.com> wrote:
>
>     The author originating this discussion said that they had one 3-500Z
>     that was the vertical thin fin plate, Eimac original, and one 3-500G
>     that has the graphite anode presumably with the horizontal heat
>     radiating structure. This would look like the Amperex/Covimag
>     tubes that
>     are no longer made, but are cloned in China. For that matter, the
>     original 3-500Z from Eimac is long out of production as well.
>
>     I think it would be more risky to pair these two tubes together as
>     they
>     come from different manufacture with different plate physical design.
>     There is no guarantee that the cutoff bias voltage is identical
>     and that
>     the mu is the same or within a tolerance. Operating this way might
>     cause
>     one tube to hog more current, have higher dissipation due to the
>     idling
>     current (Zsac), if they both share the same cathode bias setting.
>
>     It would be optimal to put the older style NOS tube into the
>     amplifier
>     with one of the original (weak?) tubes to see if it works better.
>     Then
>     try it with the other original tube. If the two tests show large
>     difference in power capability, then I would suspect just one of
>     the old
>     tubes. At least start with this plan, and see how it works, report
>     back
>     here.
>
>     If anyone is interested in Amperex/Covimag 3-400Z or chimneys for
>     either
>     triode, get in touch.
>
>     73 and Happy Holidays
>
>     John
>
>     K5PRO
>
>     _______________________________________________
>     Amps mailing list
>     Amps at contesting.com
>     http://lists.contesting.com/mailman/listinfo/amps
>