[Amps] Building an Amp, Selecting a Tube

[Roger (K8RI)]

Radio WC6W wrote:
> Hi Roger,
>   You wandered in several dimensions with that query.
>
>   
Oh, yah...<:-))
>   For 2 KW out, I'd recommend going with a single tube, unless providence brings something else, with sockets, your way. The obvious choice is an 8877.  Somewhat rarer, and typically less pricey surplus, are the old 4CX1500A, and 5CX1500A/B which are also good for 2 KW out but, are filament power hogs.
>   
<sigh> about 15 to 20 years ago I cleaned out a bunch of stuff as I was 
so busy and seldom on the air. I had three, new 4CX1500Bs.  The A 
version is the real current hog at 38.5 amps (nominal) while the B runs 
between 9 and 11 amps. Now I wish I had those back. Of course the B is 
the one that takes the expensive socket. 
>   Covering 160-6 meters is a challenge.
That's not a necessity. I'm just spoiled with the auto band switching 
between 160 and 6 on the HL-1.5Kfx. I've also entertained the idea of 
just a 6-meter amp.  I was ordering one from Alpha when the 1600B became 
unavailable.
>   You might consider either switching to a separate output network for 6 or... there is an old trick of inserting a small (adjustable) coil, which will only marginally affect low band operation, between the tube's plate and the tune capacitor.  On 6M, the tube's output C then becomes the input capacitor of output network and the low band plate capacitor becomes the load cap with a separate small L coil from this point to the 6M output connector.  If the pi section of the low band output network is simultaneously set to 160 Meters, it will have enough reactance (circa 5K ohms) to be irrelevant.  A separate plate choke will be necessary for 6 or... the through the coil plate feed trick may be employed.  See: http://wc6w.50webs.com/wc6wamps/index.html?fr62.html
>
>   Finding the tube may not be the hardest part of your quest.  The tank L's, C's and bandswitch seem to be somewhat less available nowadays.  And let's not forget the matter of some largish power supply parts.
>   
If need be I can wind the coils and probably find the capacitors, but 
and adequate band switch could be a problem.
>   Transistors would be a whole 'nother can of worms.  2 KW reliably suggests 16 devices plus a bunch of splitters and combiners, a bank of output filtering, a 4 KW power supply, and if running into anything less than a perfect 50 ohms a 2KW rated tuner.  It ends up being lots more stuff.
>
>   
Everything except the tuner is built in, but it makes for a complex 
design. Except for the tuner it's quite compact and easily lends itself 
to liquid cooling.  I've actually taken a pair of 160 watt, 2-meter 
amps, using splitters and combiners tweaked them into well over 300 
watts out.  (BTW step one is disabling the RF keying mechanism in one of 
the amps and slaving it to the other. Otherwise it can get interesting 
as they take turns keying each other<:-))
>   Now what was that about buying a commercial amp?  :-)    I believe that old Henry 4K Ultras are going in the low $3K range; Not all that much when you consider that those prices are in inflato-dollars.  You'd only need to add a few parts for 6 & 160 to meet your goal.
>   
But man are they big and heavy! <:-))

I just got rid of a 2K4 with heavy duty Peter Dahl plate transformer and 
a spare set of Eimac tubes.
I might just get another 76A and put a pair of 3CX800s in it as a  
6-meter monoband amp.  I'd make up a new front panel and have a neat, 
compact, rugged, and relatively inexpensive amp.  The 76As are going for 
bargain prices most of the time at usually less than $1000 with full 
output although the prices cover quite a wide range.  If I could get a 
good 3-holer, I'd use it for HF and convert my old 2-holer to 6-meters.  
That would be the least expensive and neatest route with the least 
work.  Once the 8874s gave out I'd convert the three holer to a pair of 
3CX-800s as well. That should run the legal limit, key down for hours.
> 73 & Good evening,
>    Marv WC6W
>
> http://wc6w.50webs.com/
>
>
> --- On Sat, 7/26/08, Roger (K8RI) <sub1 at rogerhalstead.com> wrote:
>
>   
73 and thanks for the ideas.

Roger (K8RI)