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I recently built the WA3TFS linear amplifier featured in the March QST. The build went very well and with my old MFJ 20M QRP CW radio, 3W in gives 20W out with a peak 4A draw. So, I tried to interface my QMX+ to it and was surprised to see the QMX+ stop transmitting after the first dit. It would recover after a time and would transmit fine by itself or through the amp but it didn’t like it all when I activated the amp. The firmware hasn’t been upgraded in awhile so I’m not sure if this has already been addressed. I’m not sure the RF detection is maybe not fast enough ??

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Any ideas appreciated.

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73, Jamie

WB4YDL

If you are using COR function with the amp this will happen as the QMX sees a high SWR while the relays are switching over.? ?If you can use PTT input into the amp and set a TX delay of about 15mS on the QMX it will work.

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Brien - KE7WB

Brien:

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Thanks so much for this. I suspected this was the case. I actually jumpered the diode at the relay in the amp to put in transmit mode, and the QMX+ worked perfectly fine. I need to upgrade the firmware as well. I’ll play with the delay a bit and see if I can get things calmed down.

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73, Jamie

WB4YDL

A mistake on my query - this refers to the QMX NOT the QMX+.

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Question then: can the transmit delay be adjusted ??

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73, Jamie

WB4YDL

Both the QMX and QMM+ share the same software.? "PTT to TX delay "is in the SSB menu.? It appears to be applied in CW mode also.? If you are using CW you might want to turn on Semi QSK and put a value of around 700mS in Custom QSK so that you are not beating your amplifier relays to death.

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Brien - KE7WB

On Sat, Apr 12, 2025 at 04:47 PM, Brien - KE7WB wrote:

If you are using COR function with the amp this will happen as the QMX sees a high SWR while the relays are switching over.?

COR will always result in the transmitter's output being hot-switched with a resulting very high VSWR; transient, but it only takes an instant to blow a FET.? If PTT is available it is always preferable to use it.??

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Hans:? A big Thank You for providing the PTT variable delay function.

Suggestion For Improvement -

Have a separate delay function for PTT_P and PTT_N outputs, this would add more flexibility and possibly obviate the need for a sequencer for e.g. preamp plus power amp switching.

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73, Don N2VGU

I kept looking for a SSB menu and the realized this refers to the new beta firmware. Aha ! I am still on _027 firmware.

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I’ll get on this tomorrow ! Thanks.

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73, Jamie

WB4YDL

Jamie, please keep the group updated on your progress.? I just ordered the WA3TFS amp kit to use with my QMX and am very interested in how to resolve this issue!

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TU & 73,
Jim - AE5JE

Comparing the WA3TFS with the QRP Labs 50W PA it appears that the QRP Labs PA could be easily modified to be a 20W linear with the advantage of PIN diode switching in place of a relay and PPT control.

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Any thoughts?

Hans has stated that he has not tested the 50-watt amp in linear operation. ?It should be doable. ?Adjusting the bias to ensure linear operation would be the key. ?You should have test equipment to verify a clean signal and linear operation.?

With a single bias control it would be important to match the IRF520s for equal gate turn on voltage.?

For a multi band amp, look at the WA2EBY. ?A full kit with enclosure is sold by AmateurRadioKits.?
AmateurRadioKits.in

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73

Evan

AC9TU

OK, so I have the beta firmware installed and I found the PTT to TX Delay in the SSB menu. The value tops out at 99 (I presume milliseconds ?). Turns out that at values up to 98, the QMX didn’t like the timing with the amp and would protect itself and stop transmitting. At a value of 99, it would be OK with the amp but would lock itself into transmit. I had to turn off the power to stop it.

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Now, if I simulate a PTT key down at the amp by jumpering the diode at the relay to ground, the QMX is happy as a lark. No issues at all. Now, I guess I could make a PTT jack on the back of the amp for this, but it’s just another thing.

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Maybe if the PTT to TX Delay was a bit longer (>99), it might behave.

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73, Jamie

WB4YDL