The AM Forum

I'm considering plate modulation of my DX-60B.  When tuning, should I follow the same procedure as with controlled carrier or, is there some leeway as far as input current?

You should tune it up as if your going to operate CW.  I forget the exact plate current they recommend, but load it up accordingly.  If you want to ease back on it a bit you can lower the plate current a bit.  I use to modulate my DX-60 with a pair of 807's.  I think I loaded it up  to around 120 MA or so.   Leave it in the CW position when your using it with your external plate modulator.     

Joe, W3GMS

That's what I was thinking.  I have a cherry XTR with the VFO.  It works great but the audio sucks and I don't want to screw with the wiring.  Seems to me that just making a provision for an external modulator through that connector on the back should be easy. A pair of 5933's p/p into a UTC S-20 should really rock!

Check out a previous post by Wa1qix in regard to DX60 audio.  The best audio I have heard on a dx60.  I plan to do Steve's mod on a dx60 I have. Not a big project at all.   If you want to change the mod scheme, you have to make circuit changes anyway. 
Peter

Peter,
I have heard Steve mod's which change it from carrier control to screen modulation.  It does sound very good, but I believe the original question was about tuning the DX-60 while using Plate Modulation.  

The original Carrier Control modulation does not have the ability to drive a non linear load that the screen presents.  Screen modulation fixes that problem and there are many ways to do it.  The simplest is to get rid of the impedance between the cathode follower and the screen of the 6146.  Tim, WA1HLR recently came out with his screen modulator scheme that include his version of veri power.  Like Steve's, it does a very good job and sounds great on the air.  The typical resting power of that scheme is ideal for driving most linear amplifiers.

Plate modulation will allow the DX-60 to run more carrier power and depending on what you want to do is a great way to go as well.  

Joe, GMS  

one of the electric Radio issues had a plate modulation scheme that was self contained in the dx60 by adding another 6146 as a Heising modulator

I think 2ZM did a plate modulation modification (say that three times fast) using a 6DZ7. Sounded very FB.

A nice outboard option (if you don't want to build your own modulator) would be an Eico 730.

I did more research and discovered a flaw in the DX-60's function switch  It has a bad reputation for arcing over because the HV B+ runs through it.  I found a modification converting those switch contacts to operate a relay which in turn switches on the B+ via a break between the HV secondary and the rectifier.  Well and good.  That seems to be a good modification but has little to do with modulation.

Further research didn't give me any sources for circuit changes to add plate modulation.  I saw the pix of the modulator shown in the June 2010 "Electric Radio" article but there was no schematic.  Anyone have a copy?  I've also looked up all the other modifications to the existing modulation circuitry but I'm not going to do those mods.  If I decide to sell this thing in the future, it won't be modified further.  As it is, I think someone put in a voltage regulator tube for the VFO (I have yet to check that out) and it works great with the VFO, but plug in a crystal and no go.  Somewhere along the way I'll modify the receiver because it sucks.

Building a modulator is no problem.  I'm putting together a speech amp / driver using a single 6V6 into a UTC S-8.  I'm building a couple of modulators starting with one for the DX-60B.  I can use either 807s or 5933s into a NOS UTC S-20.
As I recall the accessory socket has a couple of unused pins.

Of course, if anyone wants to trade this station for a functioning Viking Ranger...  LOL, Mike - W9ZSL

Referring to your original question on tuning up a 6146 for plate modulation see this:

http://www.r-type.org/pdfs/6146.pdf

Be sure to have the correct grid drive for AM plate drive - a 6146 is a bit fussy on this.  Basically, Joe is right - tune up as for CW operation but observe the numbers as found in this data sheet

Al

PS:  Those are max numbers.  I use 100 ma on the plate and 3 ma on the control grid

I looked for the WA1QIX mods and can't find them.

Try this:

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

Here is a modulator that I had, which was added to a DX-60. The modulator was a small chassis that someone build (SK). Perfect size for DX60. Attached schematic of the modification needed in that case.

Would it be a good idea to put a choke in the screen circuit for self-modulation?

John's schematic of a plate modulated DX-60 is very well done...

However, I would suggest a 30k ohm 10 watt screen dropping resistor. Also, the RF bypass capacitor between the screen and ground should be 0.001uF (not 0.005uF).

The two different screen voltages shown for CW and AM are not correct. Those two different screen voltages might have been correct when the transmitter was screen modulated. However, with plate modulation (as per this schematic) the screen voltage will be the same for both CW and AM. I.e. around 200V.

A 30k ohm screen dropping resistor works extremely well in the Johnson Ranger (6146 final RF amplifier tube); particularly if one uses an external modulator (but also with the internal modulator). There is no need for a Heising choke.

The power dissipation (at carrier level) in the screen dropping resistor will be around: [400V x 400V/R] watts. If you use R=10k ohms, then you will need to use a 25W resistor.

As far as the modulator is concerned:

If you don't care about whether you are using a classic tube type modulator or a modern solid state design, you might consider a modern solid state amplifier: rated at 200W peak output into an 8 ohm load, in combination with a backward-connected single ended audio output transformer: rated for at least 100mA of unbalanced DC on the high impedance side, at least 30W of audio power, and designed to be used with an amplifier with an output impedance of 5000 ohms or larger. The backward-connected output transformer will serve the function of a modulation transformer.

Stu