[Amps] 12 tube 3-500Z

[R.Measures]

On May 25, 2004, at 8:04 AM, Will Matney wrote:

> That 12 holer on ebay using (12) 3-500Z's is a piece of work aint it?=20=

> You would need at least 1 Kw to drive the thing where no standard=20
> radio could ever drive it. Although, it is possible to do this, I seen=20=

> several flaws to this amp. First is the cooling as I only caught  5=20
> muffin fans on the top, and what looked like 3 more blowing in at the=20=

> bottom. I believe it were me, I would have used a couple of large=20
> blowers in the bottom, then chimneys around the tubes.

=F0  Chimneys would reduce the cooling ability unless a noisy high=20
pressure blower was used instead of fans.  To see if the extant=20
envelope cooling fans are adequate, one would need to measure air=20
velocity near the envelope of each tube on the leeward side.  However,=20=

it looks to me like the fans need to be of a larger diameter.

> We all know the heat these things create, especially around the tube=20=

> pins at the bottom.

=F0  Indeed.  To cool the filament pins optimally, air must directed at=20=

pins #1 and #5.  In the photo, this does not appear to be the case.

> Next, if this is what I was seeing, the filament transformer was in=20
> the bottom? If it was it, it kinda looked awful small to me!

=F0  Agreed.  The hipersil=A8 filament xfmr  looks about right for =
perhaps=20
4-tubes.  A 60Hz, 5v, 175A (875w) xfmr would be much larger

> It about had to be it as the leads for the filaments choke was tied=20
> directly to it. Third, I never noticed hardly any bypass capacitors in=20=

> the bottom of this beast. Forth, you can notice in the top that it was=20=

> used in high frequency only with no band switch.

=F0  It looks like a 10/11m monoband amplifier to me.  Also, the tank L=20=

tubing should be larger to handle the approximately. 50A of RF=20
circulating current at 27 to 28MHz.

> Last, the plate choke looks awful small to me too!

=F0  20uH would be plenty for 27MHz.  In SSB HV-RFC service, #22=20
high-temp silicone varnish wire is enough for 5a.  My guess is that=20
such a RFC would be fairly compact.  Also, a bifilar filament choke for=20=

27MHz would only need to be c. 0.5uH. (XL=3D100=BD)

> I think I'm just bad to knit-pick.

=F0  Your assessments seem valid to me, Will.

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org