Re: [Amps] 12 tube 3-500Z

[R.Measures <r@somis.org>]

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? 
> You would need at least 1 Kw to drive the thing where no standard 
> radio could ever drive it. Although, it is possible to do this, I seen 
> several flaws to this amp. First is the cooling as I only caught  5 
> muffin fans on the top, and what looked like 3 more blowing in at the 
> bottom. I believe it were me, I would have used a couple of large 
> blowers in the bottom, then chimneys around the tubes.

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

  Indeed.  To cool the filament pins optimally, air must directed at 
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 
> the bottom? If it was it, it kinda looked awful small to me!

  Agreed.  The hipersil® filament xfmr  looks about right for perhaps 
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 
> directly to it. Third, I never noticed hardly any bypass capacitors in 
> the bottom of this beast. Forth, you can notice in the top that it was 
> used in high frequency only with no band switch.

  It looks like a 10/11m monoband amplifier to me.  Also, the tank L 
tubing should be larger to handle the approximately. 50A of RF 
circulating current at 27 to 28MHz.

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

  20uH would be plenty for 27MHz.  In SSB HV-RFC service, #22 
high-temp silicone varnish wire is enough for 5a.  My guess is that 
such a RFC would be fairly compact.  Also, a bifilar filament choke for 
27MHz would only need to be c. 0.5uH. (XL=100Ω)

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

  Your assessments seem valid to me, Will.

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

_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps