Plate Modulating a DX-40

[W6REF]

Stu and Steve,

Stu - you were right about the need for separate negative grid bias supply for my DX-40 in order to implement screen modulation. I just tried reducing grid drive and it had no effect on RF output until I dropped it below a safe level (less than 1 mil).

Steve - there seems to be subtle differences in the three DX-60 mod articles you recommended. The last one by K4TAX seems to have implemented the best of the other two. Do you know if there is a consensus on this? And...thanks for the recommendation of AES as a supplier for power supply chokes. I'll call them tomorrow.

Regards,

Bob

[kc2ifr]

Man...this is Not rocket science......use a backward connected audio transformer in the b+ to the final and if the screen voltage comes from a separate power supply, put a 15 henry  choke in series with the screen.........WALA u now have a very nice sounding AM rig........
I have done this with many rigs...........
Bill

[KD6VXI]

Actually when I plate-modulate my DX-40 I won't drive the linear....too much drive power. I wouldn't gain much anyway...50 or so watts from the DX-40 versus 100 watts from the linear without exceeding dissipation from the 811A's. However, I was toying with the idea of constructing a 10-watt home-brew plate-modulated rig that would then drive the linear.

But please...as the chorus line of a tune from one of my favorite musicals, "Grease", echoes...."Tell me more, tell me more!...."

Regards,

Bob 

Bob, the AL811 can do a bit more than your giving it credit for.  The 3 811s are good for 600 PEP.  I've run one at 80 to 100 watts of carrier for an extended period of time with no detrimental effects.  Your exciter might prevent this from happening, though...  Some of the riceboxes, although CAPABLE of 25 to 30 watts of carrier, sound HORRIBLE at that setting without mods, etc.

Also, you CAN replace the 811s with 572s if you want to run extended key down AM.  The 572s will make the amp virtually bulletproof (not idiotproof, but bulletproof) for AM, and will allow for 100 watt carrier conditions.

Just my two cents worth.


--Shane

[W6REF]

Hi Shane,

Actually that's how I'm using my AL-811 now. I'm using my Icom 751A cranked down to about 10 watts which is driving the AL-811 to about 100 watts output. It took some experimentation to find the sweet spot on my 751A. There's a tiny pot inside that adjusts the carrier level...but it is VERY critical and touchy...so much so that the 751A drifts from 5 to 10 watts in the first 30 seconds of transmission...and then stabilizes. I've requested critical comments on the audio, and so far most AMers seemed surprised when I told them what the rig was....the consensus is that the quality is exceptional.

It does help, I admit, to have a decent mike...I'm using the Marshall MX990 condenser with mild EQ into a compressor/limiter, all this driving the mike input to the 751A.

Ameritron has told me not to exceed 100 watts AM carrier out on the AL-811, but I don't know if that's because of the dissipation of the 811As or some other constraints on the components or power supply. If it's dissipation only then the 572s seem like a good move.

Regards,

Bob

[ka3zlr]

Hello,

 Fairly rugged little amps and set up properly will do just fine, But what isn't said here or i may have missed, meaning no disrespect, is, When tuning these little gems..run your load up and past peak over the top, noticing a little signal drop and what is gained in PEP when the exciter is tuned for the sweet spot of 100% Mod..is wonderful...it's not all Maximum Carrier power in linear service...save the linear address the audio as well...

my 2cts worth...

73.