I contacted the manufacturer on a related topic some time back. Their
official response (contained in the manual) is for keydown operation, 650W
max, 50% duty cycle, 10 minutes max transmission time (as you know from the
manual)
I inquired how to derate this for a longer transmission time (say 30
minutes). I speculated that since when operating bias is applied (no rf
drive) the amp idles at appx 160 ma yielding a dissipation of 450w,
representing just under 50% of the combined dissipation of the two tubes,
and I asked, is it 'safe' to let the two tubes run with at this level of
dissipation for 30 minutes (the key is up after all).
They responded " In theory 450 watts should be safe for key down operation
but we could not guarantee the tubes would survive this operation."
How does this apply to your AM question. The manufacturer seems to have
some reservations about cooling at 450w steady state so, although I'm not
sure whether they mean 450 w output or 450 w dissipation (possibly the
same).
Others on this reflector may be able to use this data point from the
manufacturer to predict.
Ed
----- Original Message -----
From: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>
To: <amps@contesting.com>
Sent: Tuesday, August 24, 2004 10:13 AM
Subject: [Amps] Centurion cooling improvement ideas for AM service
> Hi folks,
>
> I am the owner of a 3 year old Ten Tec Centurion amp. This unit employs
two
> 3-500ZGs on an elevated socket platform in the rear right side of the
> cabinet (when facing the front panel). On the rear left side is the h.v.
> power supply with a Dahl model 422 transformer which delivers 3.2 kv
(around
> 2.9 under load) to the plates. Between the p.s. and rf deck is a rf
shield
> running down the middle of the chassis. A square hole is cut in this
> between the tubes and p.s. and over that is mounted a Bi-Sonic tube axial
> muffin fan, model 4E-115-S21, sleeve bearing, 115 v. 220 ma 20 w. 2200 RPM
> moving 82 CFM. The fan is on the tube side of the shield, and draws air
in
> through the left side of the cabinet, over the p.s. and exhausts it
> horizontally over the h.v. plate choke and tubes, including the pins and
> anode heat sinks, which are the flat type extending out horizontally. As
> most of you know, this has become the standard cooling method for these
> tubes in amateur amps since Heathkit began using it for the SB220.
>
> This cooling is adequate for low duty cycle modes for which the Centurion
> was designed, but I would like to use mine for brief transmissions (5 to
10
> minutes or less) of dsb carrier AM running a carrier of 200 to 250 watts.
I
> have done this for a couple of minutes and the amp gets pretty darn hot.
I
> would therefore like to boost cooling so as to avoid seal failure.
> Increased noise is not a problem for me.
>
> Bi-Sonic manufactures a fan that is identical to the stock fan but at
higher
> rpm, and 20 more ma, 22 w. and 105 CFM, around a 25% improvement over
stock.
> Is this an increase worth pursuing? Another possibility would be to add
a
> fan to the external surface of the cabinet over the grill on the right
side
> of the rf deck to speed up the draw of air to the right, out of the
cabinet.
> Again, is this an improvement, and would it be worth pursuing?
>
>
>
> Overall, :
> A. Are these mods necessary
> B. Are there any comments on these methods
> C. Perhaps most important, is operating this amp on AM beyond its
efficiency
> and heat dissipation design capability and should I keep it for SSB and
CW,
> and look for something with higher plate voltage and/or tubes with higher
> plate dissipation / more cooling?
>
> Tnx,
>
> Rob Atkinson
> K5UJ
>
> p.s. to save you time and trouble, please no comments to run a plate
> modulated boat anchor.
>
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