[Amps] New SB-200 owner

[Roger Halstead]

>
> Why on earth would anyone want to replace 3-500Z's with 4-400's in a 
> SB-220????

Gain?

> First of all they have worse IMD in a GG configuratiion. Secondly they 
> will

GG? I'd probably use them grid driven.

> not be cooled properly by the existing fan.
> I really dont understand your logic.

The whole post was a question was a question as to whether any or all of 
these tubes would work, or be suitable, not a recomendation.

>
> The 4CX250 family is another very poor choice. They were not developed for 
> SSB use and the linearity is only in the mid 20's under the best of

Collins sure liked them for SSB though. As has been mentioned they work well 
with negative feedback, but the KWS-1 didn't use it. I don't know about the 
Collins 6 and 2 transverter.

> conditions. This includes the 250R. Ian, G3SEK/GM3SEK, has written 
> extensively on that subject and even developed a set of bias regulators to 
> at least improve the IMD a bit.
>
> You also have the problem with secondary emission which is prevalent in a 
> goodly number of the used 4CX250's available. As Im sure you are aware 
> hams rarely buy new tubes if they can help it.
>
> A pair of 3-500Z's will easily do 1800W with 100W drive with enough HV. 
> The

That's true, but most of the amps available for conversions or for just 
plain operation were designed and built when the legal limit was 1 KW 
average input. They don't have enough voltage to get that kind of output 
with 100 Watts of drive.  My 2K4 has a tuned input, but on a good day going 
down hill with a strong tail wind it only gives 1200 out and that is what 
the manual claims as well.

> AL-82, PT-2500, SB-200 with Dahl outboard xfmr are just a few examples. A 
> 6M

For me, having to go with an outboard transformer defeats the purpose of a 
desk top amp, or a mod, so in that case I'd start from scratch and replace 
the internal transformer. Also a good, compact plate transformer at 2 to 2.5 
KVA isn't cheap so the conversion price would be going up. Hence I'd still 
go for the old Alpha 76A in that case.

> monoband amp requires tuning just once if you stay within the lower 400 
> KHz. With the proper capacitors that tune up takes only a few seconds. 
> Adding bias control might save pennies on your yearly light bill and do 
> nothing for longevity since a 6M amp is not used nearly as much as an HF 
> version.

Mine is almost always online when getting on 6. If a local answers me it 
gets turned off, but currently being stuck with a vertical as I can't turn 
the big 7L at 115 feet I find 800 out is almost, but not  quite as good as a 
100 on the beam at 115 feet and it doesn't help my hearing a bit. <:-)) 
Course that just shows the good antenna gives a better return than power. 
OTOH I'm currently running solid state for power on 6.  I'd like to home 
brew a legal limit solid state amp, but the logistics of that are something 
else. You not only have to design, or find the proper desing and build, you 
will have more time and effort in building the control circuitry to protect 
the finals than the rest of the amp. Current and voltage limiting, reflected 
power limiting, and frequency monitoring with the ability the shut the thing 
down in a very short time to protect  those very expensive transistors. Then 
there is the 50 or 100 volt power supply running as high as 25 to 50 amps 
peak.  There's a reason those new legal limit HF solid state amps run 6 to 
12 thousand dollars.

73

Roger (K8RI)
>
> I built a single 4-400A 6M amp in the 60's and drove it to 600W with under 
> 10W. I also have  PP 4-125A's that was originally on commercial high band 
> VHF on 2M FM. However those old technology tetrodes have nowhere near the 
> IMD performance of modern tubes and would probably be no better than a 
> used 4CX250 in the hands of most basement builders. I can guarantee that a 
> SB-200/SB-220 will be much better.
>
> Carl
> KM1H
>
>