Use of Heathkit SB200 amp

[Jeff-230]

Hi all, this might be a dumb question, but the Heathkit SB-200 has several
modifications that can be made to it, one of which is the reduction of drive
power to excite it.

Does anybody have any idea what the lowest power required to excite an
SB-200 amp is?

Without mods, it is around 100 watts, but I use to have one with the reduced
drive mods, and I drove it with 30 to 50 watts for around 300 watts out.  I
never tried testing the lower end with that mod. Though.

Bottom line is:  if an SB-200 had the reduced drive mod, could a QRP K2 at
10 to 15 watts with an amp key interface safely/adequately drive an SB-200
to say, 50 to 60 watts out?

 

Anyone ever try this?

 

Thanks,

Jeff

KY7S

 

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[Don Wilhelm-4]

Jeff,

If an amplifier (any amplifier including the SB-200) is linear, and has
a gain of 10 dB (for example), then 10 watts will produce 100 watts
output, 15 watts goes to 150, 50 watts will produce 500 watts output, etc.

I am not familiar with the modifications to the SB-200 for the
'reduction of drive', but if the unmodified or modified amplifier is
truely linear, there should be no need for any additional work (see
statement above).  OTOH, if the 'reduction of drive' mod increases the
gain of the SB-200, then it may be useful unless it produces non-linear
operation of the amplifier.

The input power required to drive the amplifier to *full* output is
quite a different matter - I would not expect a 10 to 15 watt input
would be capable of producing full output from the SB-200, but the amp
certainly could be used as a gain block element and provide a good
output signal.

73,
Don W3FPR

Jeff wrote:

> Hi all, this might be a dumb question, but the Heathkit SB-200 has several
> modifications that can be made to it, one of which is the reduction of drive
> power to excite it.
>
> Does anybody have any idea what the lowest power required to excite an
> SB-200 amp is?
>
> Without mods, it is around 100 watts, but I use to have one with the reduced
> drive mods, and I drove it with 30 to 50 watts for around 300 watts out.  I
> never tried testing the lower end with that mod. Though.
>
>  

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[Charly]

Some model amps have a swamping circuit on the input from the exciter to
avoid over driving... Some Alphas have that and many Alpha owners just take
that out to no harm, but are careful, I guess.

The aptly named "swamping" is resistive.  Maybe that is what is in the
SB-200?

Charles Harpole
[hidden email]

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[N2EY]

In a message dated 9/12/07 11:12:05 PM Eastern Daylight Time,
[hidden email] writes:


> the Heathkit SB-200 has several
> modifications that can be made to it, one of which is the reduction of drive
> power to excite it.

What is modified to do this?

>
> Does anybody have any idea what the lowest power required to excite an
> SB-200 amp is?
>

I presume you meant "what the lowest power required to excite an SB-200 amp
to full rated output is"

The SB-200 uses a pair of 572B triodes in grounded-grid. The data sheets list
the 572B as needing 50 watts of drive in that service, so a pair will need
100 watts of RF to drive them.


> Without mods, it is around 100 watts, but I use to have one with the
> reduced
> drive mods, and I drove it with 30 to 50 watts for around 300 watts out.

The tube ratings (for a pair) say that 100 W of drive will give 600 W output,
so 50 W of drive would give 300 W output.

I don't see what could be modified to reduce the drive requirement for full
output, except perhaps changing to grounded-cathode operation instead of
grounded grid. But such a mod would be extensive, requiring neutralization and other
changes.

 I
>
> never tried testing the lower end with that mod. Though.
>
> Bottom line is:  if an SB-200 had the reduced drive mod, could a QRP K2 at
> 10 to 15 watts with an amp key interface safely/adequately drive an SB-200
> to say, 50 to 60 watts out?
>

Since the SB-200 is a linear amplifier, I don't see why not. Of course it's
using a sledgehammer to drive a 3d finishing nail....

The problem is, how do you tune up the SB-200 with such low drive power?

---

I'm curious to know what is changed by the "low drive mod" for the SB-200.

Thanks

73 de Jim, N2EY


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