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Author Topic: Interface to Grounded-Grid Tube Amp (FT857D)  (Read 703 times)
WU6X
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Posts: 12




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« on: November 02, 2007, 04:49:01 PM »

I have a classic Heathkit SB-200 tube linear that I wanted to interface to the new solid-state 857D radio. Unfortunately, you can't simply key the amp with the Transmit-Ground on the CAT/Linear jack, as the amp keying connector measures about +135VDC (tied to the tube grid), and the 857D XMIT-GND is only rated for 50VDC, 400ma, suitable for keying a solid state amp. Trying to do so holds the amp keyed ... ask me how I know this? And, if you're really lucky, doesn't burn up the radio! I was lucky.

The easy solution is to source a 12VDC relay with normally-open (NO) contacts. The contacts on the relay should be rated to at least 400ma. Then wire things up as follows:

CAT/Linear Jack:
 *GND to Common pin on the relay
 *13.8V to one side of the relay coil
 *XMIT GND to the other side of the relay coil

Relay Contacts:
 *Normally-Open contact to amplifier relay, or center pin.
 *Common ground to shield on amplifer relay connector.

Menu No-020:
[CAT/LIN/TUN] setting changes to "LINEAR"

Now, when you key the 857D, the external relay is pulled-in, and the contacts ground the amp keying circuit (or grid) and everything works like it supposed to.

Optional:
I put a small switch in the XMIT GND line as well, so I can disable the external relay when not using the amp. Otherwise, the relay will be working away everytime you key the radio.
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