[Amps] 3CX1500A vs el cheapo GS-35B

[Carl]

----- Original Message ----- 
From: "Robert Briggs" <vk3zl at bigpond.com>
To: <amps at contesting.com>
Sent: Tuesday, July 31, 2012 10:38 PM
Subject: [Amps] 3CX1500A vs el cheapo GS-35B


>
> Oh boy,  what a bunch of grizzlers...I am having a real chuckle at some
> of the garbage being spouted here regarding the GS-35B tube...We are
> talking about a reasonably cheap to buy tube that amateurs can
> afford....Ok, it's not made in the grand ol' US of A therefore it must
> be junk..

** So much for the typical Aussie anti-American noise. Im glad you cleared 
that air early on.
There are many excellent tubes from Europe and recently from China that 
produce good to excellent IMD signals. There are many unnoticed US tubes 
that do well also. My HB 1500W 2M amp uses a 3CX1000A7/8283 which requires 
33 cfm at 1500W Pd. Its the tube Eimac should have redesigned using the 8877 
base instead of the one requiring a horrendously expensive socket. I think I 
paid $100 for 2 pull tubes and a socket 25+ years ago and the one in use 
still puts out the same power. The spare is a shelf queen that gets cycled 
thru for a contest every 2-3 years.

** The 7213/7214 tetrodes are another underused pair as Ive mentioned before 
and there are plenty of hams around the world still using the old 4-400A and 
4-1000A in GG which are late 1940's designs. NOS 813's are still all over 
the world.

Sorry to burst your bubble gents..I will warrant the majority
> of hams who roll their own are watching this thread with the same
> amusement that I am..


** You didnt even come close. If the Aussies ever developed their own tube 
then I might take an interest.


>
> Ask most of the thousand hams around the world who use this tube if they
> consider it second rate....The majority will be laughing with me..
>


**  Most wouldnt recognize second rate if it bit them on the a** and could 
care less what their signal sounded like.


> In regards to cooling this tube once more you have to look outside the
> box...90cfm is ample air to keep this tube cool...I have my own way to
> achieve this and it is very efficient..


** Compared to 35 cfm full bore with an 8877 its an air hog. Or a pair of 
3CX800A7's at 39 cfm. It is a very inefficient tube both for cooling and RF 
unless the HV is elevated above spec.



You won't have proper cooling of
> a tube without some air noise...Live with it..If you can't hear some air
> noise then you are probably not using enough air or your amp is remote
> somewhere away from the shack...


** I can tolerate some noise but not at a stupid level for the power 
delivered. If I want noise I'll use a 3CX3000A7 which requires 85 cfm at 
3000W Pd
Youre correct about one thing, moving air makes noise.

>
> In regards to connections to the cathode, make up some good brass
> clamps....I seal and pressurise the top RF deck and the same with the
> cathode compartment....It only needs a small fan mounted on the bottom
> cover directly under the tube to keep the cathode assembly cool...I
> machine a socket out of solid 1/2inch thick aluminum to mount the tube
> grid and this is bolted directly to the chassis floor...I have never
> seen any heat failure using these methods..


** Why dont you set up a web page showing the socket details? Most hams 
arent machinists but can hand a drawing to a friend or relative that is 
handy with those tools.

>
> So for all practical purposes the GS-35B tube is a winner and if
> attention is spent designing your layout you will have a good
> amplifier..This also goes for the GS-31, GI-6 and 7B triodes...I would
> guess that most of the nay sayers have never really taken the time to
> check these tubes out or really don't have much idea of what they are
> doing..

** From reports of tests most Russian tubes have poor IMD which is 
understandable since they were mostly designed for military Class C pulse 
service or commercial broadcasting.


>
> Regarding the 8877....A fine tube but generally priced way out of the
> average hams wallet...You can buy pulls I suppose but then it is at your
> own risk if that expensive second hand tube expires...I have a number of
> new 8877 sitting here but I am not particularly interested in using them
> when I can get by with GS- tubes..


** MRI pulls are backed 100% by the respectable resellers. They are pulled 
on a time scheduled basis and not for degraded performance. At $2000+ for a 
MRI its cost effective to always have peak performance and not require 
repeats.
Ive used scores of 8877 and 3CX800 MRI pulls in customer amps and every one 
has produced full power at low grid current and IMD is as expected.

Carl
KM1H


> Bob..VK3ZL..
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