[AMPS] Coil Voltages/Bandswitch

[km1h @ juno.com]

On Wed, 04 Mar 1998 22:25:22 +0000 W7CW Jay E Ostrem <wc7m@vcn.com>
writes:
>Hi Guys,
>
>Here's a question I've always wondered about.
>
>"Is the rf voltage in a final tank coil equally distributed across the 
>coil?"


Only in theory Jay. In practice the variables are to the extreme. 
I'm not BS'ing when I say that layout is the first law in amp design. The
best tube can be parasitic city in a bad layout.

IMO, stick to proven layouts AND tubes. Forget about ancient glass tubes
for the higher bands. Stick with 3-500Z at the low end, 3CX800A7/3PRxx ,
3CX1500A7/8877 and the 3CX3000A7.
Heck, Im using Svetkana 572B's as MODULATORS in my Clegg Zeus on 6 and 2M
AM !

All of the above are showing up as medical pulls at reasonable cost. FYI,
the YU-158 is an 8877 with special testing and a great choice when you
can find one. 

I would be very careful using a rotary coil...the contact surface is
extremely critical and used coils are prone to lots of arcing problems.
They are also prone to instability problems on the higher frequencies.
Me...I would go for a healthy switch and a proper Pi-Net design....you
dont really need 3 variables to play with in the tune up process.
There are many surplus switches from RadioSwitch Corp...I forget their
new name...showing up at fleamarkets for short money. Heck,  only hams
and CBers are building big amps these days with tubes and the CBers dont
need a bandswitch. 

73   Carl   KM1H
.  

>The reason I'm asking this is I'm building an amp that will use
>approximately 5500v B+. I'm probably going with a roller inductor (30 
>amp
>30mH Gates), due to fear of the arcing contact caper in a band switch. 
> The
>amp would be capable of about 5kw with a 500watt driver, to be totally
>honest, so I'm selling my Centaur to avoid temptation.
>
>"Considering the voltages on the coil, and the potential power level, 
>does
>a band switch exist to reliably switch these voltages?"

A single 3CX3000A7 at 4500-5000V or a pair of 8877's at 3500-4000V would
be the ideal amp. The choice is based upon your "needs" and the drive
available. 


73  Carl   KM1H
>                              73 Jay W7CW
>
>                                 1AB Ranch
>                                 Gillette, Wyoming
>
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