[Elecraft] Driving (big) linear amps with the K2?

[Vic Rosenthal]

"phoon.usinet" wrote:
> 
> Has anyone used the K2 either barefoot, or with the KPA100 "cover top"
> amplifier to drive a linear amplifier (Alpha, QRO, etc)?

I've done both!

> The QRO  HF 2500 linear amp can put out healthy RF with surprisingly low
> drive levels, as low as 5, 10 or 15 watts (1000 watts out with 20 watts of
> drive on 40 - 80 meters).  Whether the KPA100 is necessary to drive the QRO
> HF 2500 Linear, is open to debate.

Not to 1500 watts, but to a quite respectable output level.

> In addition to RF drive, the QRO HF 2500 amplifier requires keying output
> from the K2 transmitter, in a shielded audio cable connection, in one of two
> possible ways:
> 
> Key XMT:  Relay contacts of xmtr must be open during receive, closed during
> transmit, and must "sink" +12VDC to ground at 80 MA for CW.

The KPA100 includes an output that would work fine.  For the K2 without the KPA,
it's easy to add.  See <http://www.elecraft.com/Apps/Amp_keying_ckt.htm>

> 1)  Does the keyer circuit of the K2 have such a capability?  I know that
> the K2 has diode switching, but can it handle 80 Ma?

The circuit at the link above will work fine.

> Is there an external "keyer out" jack on the back of the K2 with a phono
> female jack?

You can use one of the 'xverter' holes. 

> 2)  I assume that if the above keyer out put connection to the QRO HF 2500
> can be made, that VOX SSB operation from the K2 to the vacuum QSK relay
> inside the QRO HF 2500 is also doable. Comments welcomed on this point.

Does the KEY XMT input also operate this relay?  If so, yes it will work.  The
K2 amp keying circuit is activated on transmit, regardless of mode.
 
> 3)  The power output of the K2 is 12 to 15 watts "barefoot".  Has anyone
> been able to increase the output to 20 or even 30 watts, perhaps by changing
> the final amplifier transistors, and/or their biasing?  Or, by altering the
> driver circuit that drives the final transistors?  That is, with out the
> KPA100 "cover top" amplifier?

Yes, I went up to 20 watts on most bands.  I do not recommend it, for various
reasons.  I'll send you details off list.

73
Vic, K2VCO
Fresno CA