Quieting amplifiers

W8JITom at aol.com W8JITom at aol.com
Wed Apr 10 14:44:25 EDT 1996

In a message dated 96-04-10 11:20:40 EDT, you write:
>     Air doesn't like to turn corners either.   While what Fred sugests 
>     might well reduce the air system noise, vigorous application of this 
>     solution reduces the air flow to the tube by increasing the back 
>     presssure and intake resistance.  It is not a good idea to restrict 
>     the air flow through an amplifier unless you have the facilities to 
>     measure the results against the tube manufacturer's air flow 
>     requirements.  No amplifier I ever saw failed due to excessive noise.

Dick's advice is VERY good, especially with the large stuff he works with.
With air velocities used by tiny little tubes like 1200's, air is pretty

I have full air flow measuring capability here. The easiest way to measure
airflow is by the pressure differential across the anode. Eimac recommends
.5" for 1200 watts CCS dissipation with the 3CX1200, and the flow can be
reduced for amateur duty. Remember CCS is 1200 watts without ANY

With the AL1200 on the slowest speed blower tap:
no cover-.38",  cover .38", cover with 50% of exhaust holes blocked .38",
cover with 50% of inlet holes blocked .37", cover with a baffel box
(described earlier) on inlet and outlet .37+ ".  Cabinet inlet fully blocked
0.1"-, cabinet outlet fully blocked 0.2"-.

Blower on fastest tap and normal cover  .52".

An amp with a pair of 3CX800's "especially noted" for being quiet. (3CX800's
require .5" of pressure drop at 800 watts CCS per tube):

No cover .13" , cover .12"  

That's still enough for amateur service, since it allows ~800 watts total CCS
dissipation with a pair of tubes (at sea level).

Baffle away according to description on those 1200's, but for hammering away
during contests, use a faster blower tap. If you aren't sure of your amp
(especially if the amp is really quiet) buy some temperature sensitive paint
or crayons. Test the tube seals and other critical components. Like Dick
said, not may tubes or components fail from noise!

73 Tom

>From David B Curtis <David_B_Curtis at ccm11.sc.intel.com>  Wed Apr 10 17:45:00 1996
From: David B Curtis <David_B_Curtis at ccm11.sc.intel.com> (David B Curtis)
Date: Wed, 10 Apr 96 09:45:00 PDT
Subject: Moderation -- why is it?
Message-ID: <Wed, 10 Apr 96 11:24:34 PDT_1 at ccm.hf.intel.com>

Why is it that everyone who wants a moderated version of cq-contest 
wants someone else to be the moderator?

Why is it that everyone who wants a moderated version of cq-contest 
wants someone else to pay for the hardware? (except one -- he knows who 
he is)

Methinks it sounds a lot like SSTV'ers that want everyone else to vacate 
"their" frequencies "just because".

Pony up or quit griping.

In that spirit: I will volunteer to moderate 1 week in 5.  I will read 
every single post on cq-contest and forward the best to some other forum 
within 48 hours.  Don't like my version of moderation?  Read the 
unfiltered version.  All flames directed at the moderator will be 
construed to be a statement of one's willingness to take a turn in the 
barrel as moderator.

No way will my employer allow me to run a reflector on their hardware, 
and I won't put up the hardware myself, so some other person needs to 
volunteer for that part.

73, Dave NG0X
david_b_curtis at ccm.sc.intel.com

(This is the last time I "lecture the pile-up" on this topic -- promise)

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