Heathkit DX-40 tube transmitter

[Antennae]

Does anybody happen to know if the Heathkit DX-40 will transmit outside of the ham bands?  An acquaintance of mine who is into ham radio has found one and they want to test it.  But they only have a crystal that is around the 43m band to use on their dummy load. 

Also, they want to test to see if the "phone" mode works but they don't have a microphone that fits the older style mic out jack. They were wondering if they could just temporarily wire up an mp3 player to the + and - leads of the mic.  Would that work? 

[Beerus Maximus]

The first question I can answer confidently for you. Yes, you can transmit outside the ham bands with the DX-40. Unlike modern transceivers with digital control that can prevent you from doing so, the DX-40 is a simple radio from a different era, at a cost efficient price point. A crystal for just below the 40m band (or above the 80m band, say) should work just fine for you.

Audio will be a different story. What you have there is a screen modulated transmitter designed primarily for CW. AM was achieved with a high impedance microphone like a D-104. Wiring the headphone output of an MP3 player to the mic jack may modulate the radio to some degree, but it may not sound great. It certainly won't hurt to experiment but watch out for ground loops on the audio, etc and you may find you'll need to pursue some kind of impedance matching audio transformer, or a different way to modulate the radio entirely.

Also, when working on equipment of this vintage be extremely mindful of high voltages that are present, and that can remain present even when the radio is OFF. Take precautions to bleed power supply caps before working on the radio.

[Pigmeat]

A fella I knew used one of the things to pirate after a lot of mods. Not only did it work, but it sounded great. I heard it on the air several times. It seems to me he used an audio transformer, but that's been a long time ago.

[redhat]

Max Carter has some notes on his website about 'guilding' small ham tube transmitters for better audio.  Notes here http://maxmcarter.com/hg303/index.html

+-RH

[Antennae]

Thanks fellas.
I love old websites like that.  I like the style of that globe transmitter, I've seen a Globe V-10 VFO Deluxe of the same style. 

Wow, look at the fan on that thing.  So do tube transmitters benefit from muffin fans like that?  Whats the best way to use a fan, blow the air onto the tubes or suck it out of the case? 

[Charlie_Dont_Surf]

Yes, certainly will transmit outside the ham bands. The Heathkit DX-40 & DX-60, the Johnson Ranger I & II, Viking I & II and similar boatanchorage were the main tools of the trade decades ago.

A lot of the time people were using crystals to get to those frequencies as the VFO would not cover it. There was a healthy trade in used out-of-band crystals but companies like JAN Crystals (now out of business) would make new ones for you for little money.

In the 1980s, the US activity was in 7300-7450, more or less, with some in 48 meters too. I know of people that used these rigs at frequencies in the tropical bands, for example 3200-3400 KHz. This gives you some idea of how out of band they could be used.

A few things to note:

1) If the DX-40 has been sitting around unused for decades, be weary of the big electrolytic capacitors. Consider replacing the big electrolytic capacitors before even applying power for the first time.

2) I'm too lazy to search but I'm sure that there are webpages of recommended modifications for a DX-40 for better audio.

3) As Beerus Maximus mentioned, audio modulation may be problematic. MP3 players and computers these days are low impedance outputs, often < 100 Ohms and the DX-40 wants something much higher.


One possible alternative is an output transformer "turned around" so that the low impedance side connects to the MP3 source and the high impedance side connects to the DX-40. Output transformers from tube guitar amps is one source.

These guys make new ones: https://www.hammfg.com/electronics/transformers/classic/1750?referer=1101

You won't need a big, power handling transformer for this. EDIT: Which means that these might be a better option: https://www.hammfg.com/electronics/transformers/audio, especially the studio or broadcast quality transformers, for obvious reasons.

[Pigmeat]

Those Johnsons sounded good. The Rolls-Royce's of pirate transmitters w/ lots of info available on how to modify them or blow them up on the internet.

I thought Hammond went out of the transformer business years ago? There was a panic over finding replacements.

[Charlie_Dont_Surf]

I thought Hammond went out of the transformer business years ago?

Hell no. They even bought out or are repping Peter W. Dahl now: https://www.hammfg.com/electronics/transformers/classic/pwdahl

Though they are also in the enclosure business.