The AM Forum

I downloaded a file with DX-60B mods by WC3K and have a question about the Carrier Level Mod.

Since the Carrier Control (shown as a 50 ohm Pot) basically replaces a 33K resistor, I thought it
was a typo (33K to 50 Ohms ??).

Is the 50 ohm value correct ?

Tnx, Dick, W1KSZ

50 ohms would cause too much current to flow through the 6DE7 so I think it has to be 50k.

I can't vouch for another's design but here is the way I upgrade D-60 Modulator's and I know for sure it works.


Phil - AC0OB

That's the circuit I found in the Knight T-150 Mods. A lot more complex
than the one I saw for the DX-60, except for that 50 ohm pot !!

One of the locals has mod'ed his DX-60, I am going over to see what he
did tomorrow.

Tnx for the reply,

73, Dick, W1KSZ

The WC3K mod has some problems:

Changing the power level means the RC time constant of R27 and C36 changes which leads to changes in audio frequency response.

Secondly, the first triode stage does not provide any audio gain. When driving a Cathode Follower you want a good dose of audio in which to drive the screen grid as much positive and negative as possible, because a Cathode follower has negative gain.

Third, the screen voltage will never go negative enough to give full 100% modulation, which is why we use the bias voltage.

The Controlled Carrier system, in my humble opinion, is inferior to a system that has direct screen drive, sufficient audio gain, and grid control of carrier power.

Good luck to you and your neighbor.


Phil _ AC0OB

I was directed to the Arizona AM page, then to the section by W7YAZ.

There is exactly what I was looking for and I have heard a DX-60 on the
air that used it. So I know it works.

I'll go that route.

Thanks for the replies,

73, Dick, W1KSZ

   For what it's worth, I recently applied Steve's mod to my DX60B, and it's superb.  No more controlled carrier, stratospheric positive peaks - if that's your thing - and I get great audio reports.  Unless you insist on maintaining controlled carrier, I highly recommend this:

http://amfone.net/Amforum/index.php?topic=26603.0

The WA1QIX Mod looks simple enough, except for the Audio Input which adds
another level of complexity.

73, Dick, W1KSZ

   I wound up using both stages of the stock 12AX7 audio preamp, because my external audio chain didn't have quite enough output to comfortably drive the 6DE7 with only the one stage.  Still sounds great, though. 

I was just reading what 'QIX wrote about using the Mike Pre-Amp in the DX-60.

73, Dick, W1KSZ

If you want to improve the speech amplifier and modulator, one has to make the appropriate circuit modifications for improvement, because the original circuit was an economical, minimal component circuit to accomplish screen modulation.

Complexity is relative. Personally, I don't see any real complexity in the suggested modifications, just a minor change in the way the circuit operates.

By simply modifying the first stage 6DE7's circuitry, you will have more than enough audio to drive the cathode follower (second) stage. By making simple modifications to the second stage, you can have precise power control as well to drive a linear amplifier.


Phil - AC0OB