[RTTY] Heathkit SB-1000 On RTTY

[Joe Subich, W4TV]

> REPLY: I disagree. The "carrier conditions" you quote are for Class C
> plate modulated operation. This is not how the SB-1000 operates.

Plate current is plate current, plate dissipation is plate dissipation
no matter what the class of operation. In fact, since an amplifier is
slightly more efficient in Class C than in Class AB2, the tube should
be derated even more in class AB2 than in class C.

> I'll repeat what I said in my original post: The limiting factor in
> the SB-1000 is the power transformer, not the tube or tank circuit.
> If you limit the SB-1000 to about 600 watts RF out on RTTY, you can
> run it all day long with no problem. I ran mine that way for years.

Again, Eimac gives 600W as the useful output for the tube in CW or
SSB service at 2500V on plates which is considerably less taxing than 
constant carrier RTTY.  Applying the same 20% decrease in output power
for a decrease from 3 KV to 2.5KV to the 640W value given for carrier
conditions gives 500W at best.

You were extremely lucky if you pushed an SB-1000 to 600W out in heavy
RTTY use.  The vast majority of those who did so report loss of the
power transformer, band switch and/or tube whilst those who kept the
power to 400W or less (Plate current 275 - 280 mA) suffered no similar
problems.

Is the extra 1.5 dB worth the possibility of losing an expensive
transformer and/or tube?

73,

    ... Joe, W4TV


On 9/25/2012 10:52 AM, Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
> On Tue, 25 Sep 2012 10:01:00 -0400, W4TV wrote:
>
>> I'd suggest the "carrier conditions" rating is the most appropriate for
>> RTTY.  With 275-280 mA at 2500V in AB2 one is still looking at a safe
>> limit of about 400W from a single 3-500.
>
> REPLY:
> I disagree. The "carrier conditions" you quote are for Class C plate modulated
> operation. This is not how the SB-1000 operates.
>
> Referring to the Eimac data sheet, a more appropriate mode for RTTY is Class B
> grounded grid, single tone conditions. Under those conditions the rated PEP
> output is 750 watts per tube. Remember - under single tone conditions, PEP and
> continuous carrier (i.e. RTTY) are the same.
>
> A purist will point out there is an error in the Eimac data sheet. What it calls
> "Class B" is really Class AB2, since there is significant anode idle current
> with zero drive. True Class B would have essentially zero idling anode current
> with zero drive.
>
> I'll repeat what I said in my original post: The limiting factor in the SB-1000
> is the power transformer, not the tube or tank circuit. If you limit the SB-1000
> to about 600 watts RF out on RTTY, you can run it all day long with no problem.
> I ran mine that way for years.
>
> 73, Bill W6WRT
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