[Heath] Concerning the SB-200 and tubes...

[Kenneth G. Gordon]

On 18 May 2014 at 21:44, George Collins wrote:

> Hi Ken,
> 
> What a neat project!  I would appreciate information on the fan you used and any
> photos or other info you have available in electronic form.

I have taken a number of photos with the idea of writing an article for Electric 
Radio Magazine on it. I can send you the photos I have, and a .JPG of the 
chassis, but I have not written anything on it yet. The fan is a fairly common 
one. I'll get that info to you too.

>  I have an SB-200
> that I was planning on selling, but this sounds like something I would like to
> try.

The first thing you want to do with the SB-200 is replace all the caps and 
diodes, especially in the power supply. If the filter capacitor in the bias supply 
is bad, all kinds of really strange things can happen if you attempt to use any 
form of "soft-key" unit with the SB-200.

The SB-200 is a very, very relible and robust amplifier in my experience. If it 
weren't for the 572Bs, it would be completely reliable.

>  Have you any information on the IMD performance to expect from the
> conversion?

That was one thing I specifically investigated. Although I have forgotten the 
exact figures, in SSB service (either AB1 or AB2), 4CX250B's IMD is low and 
that is one reason I chose it.

The 4X150A is not particularly good in that regard, so although I have a 
number of NIB ones, I chose not to use those. BTW, that tube fits the exact 
same socket and chimeny as the 4CX250B.

I'll try to find the URL where I found the IMD figures for that tube and for 
several others and will send that to you as soon as I find it.

BTW, many years ago, I used first one, then a pair of 304TLs in grid-driven 
class AB-2 service as an SSB linear amp for AFMARS work. I only learned 
recently that the IMD of a 304TL is NOT suitable, i.e. good enough, for 
modern SSB work.

I was very disappointed to learn this. They are one of my favorite tubes.

Ken W7EKB