mod transformer substitution for heathkit?

[wb3eii]

Does anyone have an idea if the mod iron from the heath dx 100 and the apache are  interchangeable?
 one has a stock number 51-34, the other has the number 54-31. THEY LOOK THE SAME.
 Was this just some heath ploy to keep the parts stream separate?
 I have a dx 100 with a bad mod x-former, the mod iron for the apache is priced right-what to do?
73 ab3ht

[steve_qix]

A Stancor, UTC or Triad modulation transformer of the appropriate power class would be a _much_ better substitute.  You may have to move something around to fit it, but it would be a superior transformer.

[wb3eii]

possibly so, steve, but the apache iron can be had real cheap, if it will work.
 i have 100bux into this tx, total.i don't want to double the price on one replacement piece.
73 ed

[WA5VGO]

DX-100 transformer is 8000 ohms CT primary and 2800 ohm secondary.
Apache is 11,000 ohms CT primary and 3000 ohm secondary.

Darrell

[W2PFY]

Please delete. Thanks.

[W3GMS]

The mod iron in both of those transmitters is not that bad.  In the Apache, the driver transformer is severely lacking core area and its the first thing I pull out.  I have modified Apaches with the stock modulation transformer and the reports were typically "broadcast quality".  Regulated screen voltage, fixed zener bias, re-do all the low level stages including to removing the clipper and some amount of negative feedback produces  a nice sounding transmitter. 

The plate load impedance are similar between both transmitters so I would not hesitate to try a DX-100 transformer in an Apache.   

Joe, W3GMS 

[wb3eii]

thanks joe,
 it's a dx100 i'm working on, but i think you have told enough to sway me.
73 ed

[K4RT]

In the Apache, the driver transformer is severely lacking core area and its the first thing I pull out. 

Joe,

What do you use as a replacement for the stock Apache driver transformer ?

Brad K4RT

[W2VW]

The Apache mod xfmr can be used with the "turbo" mod connection and should be considered a better piece of iron than the DX-100 with crummy ratio.

A search of this site should show how to click the ruby slippers together.

[W3GMS]

In the Apache, the driver transformer is severely lacking core area and its the first thing I pull out. 

Joe,

What do you use as a replacement for the stock Apache driver transformer ?

Brad K4RT

Brad,
The old part number was PT-20D89 and that part is now being made by Hammond and the current part number is P-T124D. 
Joe, W3GMS

[K4RT]

Joe,

Thanks for the information.  I found the specs for the Hammond P-T124D. Is the purpose of replacing the stock transformer to drive the modulator tubes a bit harder, especially on the lower frequencies?  Have you had any issues with over-modulation?  Also, are you using compression gear?

73,
Brad

[W3GMS]

Brad,

The improved driver transformer will just improve the quality of your audio.  The rig that I designed the mod's for was just using a D-104 and it sounded very good after the audio chain component values were changed along with the new driver transformer.  Of coarse, audio quality is only as good as the weakest link, so a lot of component values were changed in the process.  My recommendation is to take the mods in steps.  I would start with the driver transformer.   Everyone has different goals when it comes to audio.  So take the mod's in steps and stop when you think its met your expectations.  You can go in and change everything or just selectively change thinks to meet your requirements. 

I generally don't use driver transformers.  I make sure I can produce enough audio running the modulators in AB1 service.  In place of a driver transformer, I use a hot cathode phase inverter.  From that I usually change everything else and in the end can either use a D-104 or my normal external audio system.  Its entirely up to you how far you go based on how good you want it to sound.  Keep in mind that the listeners out in radio land all have different ears and each set of ears hears differently, so make it the way you like it and let it alone! 

Its a must to have a good way to listen to your own audio off the air.  I find the "average" listener uses a 6 KHz  bandwidth.  So I always try to set the audio up so it sounds good on that bandwidth.  If your listening to a monitor that does not really have any bandwidth limitation and if you EQ your audio based on that sound, the audio will likely sound "muddy" while listening on a typically bandwidth of 6 KHz.  I use one of Steve's REA modulation monitors and it works extremely well.  It does sound a lot brighter when listening off of that monitor as compared to the average bandwidth receiver.  I know that so when I hear that extra brightness, I realize that most people do not hear it that way.  I have an old 6KHz receiver that I use for setting up the EQ. 

A good scope and audio generation is imperative so you can quantify your changes.  The worse thing to do is make changes and some will be subtly and get on the air and say, "how do I sound".  The opinions will be all over the place.  If you want to go all the way with audio changes, make the audio system as low distortion as possible. With a good scope if you do see a problem, you can start as the 1st voltage amplifier and just go stage by stage until you find the stage that has the problem.  I also try to bias the stages so one stage does not run out of headroom before the other stage otherwise you can start over-driving a subsequent stage down the line.  Proper biasing for low distortion is imperative.     


Its a big topic but do your mods in stages and you will have some fun!

Joe-W3GMS     

[W3FJJ]

I know this thread has been hijacked from mod transformmer to driver transformmer.
Sorry, but thought I add my 2 cents..

I tried a hammond P-t-124d driver transfommer in my valiant. I did a sweep on it
out of the circuit with generator and scope  and all looked great. But when I put it in circuit
the bass response whet down the tubes.. It had same response as the original valiant driver
transformer... and when I measure the valiant original driver transforrmer out of the circuit it
was fine.
I am guessing  that the current in the circuit is saturating the core and causing low freq
response to be poor.. 

I wound up going  with a phase splitter circuit (no driver transformer) and run modulators AB1
and  all is good..

GL  Chuck

[AB2EZ]

The plate current of the driver stage, that is passing through the driver transformer's primary, is relatively small (maybe 10mA or in that ballpark). The B+ supply voltage is higher than necessary to provide sufficient headroom.

Therefore, you can do the following to remove the DC from the primary of the driver transformer... which will a) reduce the saturation of the core of the driver transformer, b) protect the driver transformer's primary winding from excess current in the event of a component failure elsewhere, and c)reduce the plate dissipation experienced by the driver stage.

1. Disconnect both ends of the primary of the driver transformer
2. Attach one end of the primary of the driver transformer to ground
3. Capacitively couple the other end of the primary of the driver transformer to the plate(s) of the driver tube. Use a 10uF (or larger) 450V electrolytic capacitor, with the + side attached to the plate(s) of the driver tube.

4. Attach the B+ (that was disconnected from the driver transformer's primary) to one end of a 10000 ohm 5 watt resistor (wirewound is okay). Let's call this resistor R1.  Attach the other end of the resistor to the plate(s) of the driver tube.

Turn on the transmitter (standby is okay), and CAREFULLY measure the plate voltage (plate-to-ground) on the driver tube. If it is less than 150V, then reduce the R1 in order to reduce the DC voltage drop across R1.

Stu

[W3GMS]

I know this thread has been hijacked from mod transformmer to driver transformmer.
Sorry, but thought I add my 2 cents..

I tried a hammond P-t-124d driver transfommer in my valiant. I did a sweep on it
out of the circuit with generator and scope  and all looked great. But when I put it in circuit
the bass response whet down the tubes.. It had same response as the original valiant driver
transformer... and when I measure the valiant original driver transforrmer out of the circuit it
was fine.
I am guessing  that the current in the circuit is saturating the core and causing low freq
response to be poor.. 

I wound up going  with a phase splitter circuit (no driver transformer) and run modulators AB1
and  all is good..

GL  Chuck

I can't speak for the driver transformer in the Valiant, but I had exactly the opposite effect when I used the previously mentioned transformer in an Apache.  The amount of core material was up to the task.  In my case I used the entire primary rather than 1/2. 

Joe, GMS

[W3FJJ]

Opps, I just checked my transformer stash and the one I used was a hammond 124E not "D"
I looked up the specs and it has a lot more DC resistance,than the d  so maybe a lot more inductance?
and more prone to saturation?
It has dual primary and secondaries, I can't remember how i wired it now.
 I was real surprised how similar the waveform looked to original valiant driver xfrm.
Response was cutting off at 300 hz, and looking nasty, there below.. While everything was great going
into the grid of driver tube..

YMMV

Chuck

[KZ5A]

I have a 124D that was originally intended for a VK2 project.  I could be mistaken but I seem to remember the center tap being on the "wrong side" (primary) of the transformer for use in my VK2.  This not a big problem for AB1 service, since the Modulator bias can be provided thru a pair of moderately high value resistors across the secondary.

I wonder if this might have tripped up the gentleman who had poor results with this change?

73 Jack KZ5A

[K4RT]

Joe,

Thanks for all the information. I agree with doing the mod(s) in stages. I have completed basic mods to the Apache low level audio stages, essentially the W3SCC mods. I have a little more work to do to get the Apache back on air.

73,
Brad