The AM Forum

Hi everyone, I decided the other day that I needed to get the ball rolling on the AM setup. I have several good receivers, but the ones that are operational right now will only be makeshift for the time being until I can get my hands on a good signal generator. I dug the HT-32 out of storage that I bought about 4 years ago. The guy that I bought it from had started a restoration on it and I bought it in pieces; thank goodness he kept all the parts togeather. I have already replaced the tube rectifiers with 1n4007's, disconnected the 5v secondaries, and replaced the multi-section filter can in the low voltage supply. I didn't want to take any  chances with the O.E. iron. Is there any more quirks or weak areas that are common for these transmitters that will need attention before I fire this thing up and give it a run? Also, has anyone got any PTT info for it?
Thanks, Phil KC4VWU

Try looking here and clicking on the Hallicrafters portion.
http://mailman.qth.net/
Good luck.

mike

You might want to try the Hallicrafters web site  first:
http://www.w9wze.net/
Go to the right side of the page and scroll to the bottom. Then click on "Search QTH.net Lists"; From the "Select List" Field choose Hallicrafters; then in the "Search for keywords, names, model numbers, etc." put HT-32 and HT32 in each field; click Go. Every thread that had a HT-32 or HT32 discussion from present day back to around 1998 will be listed. If you Google HT-32, you'll also get about 1600 hits.

Phil,

    In general switching from tube rectifiers to solid state takes some application specific study. The HT-32 uses a 5R4 for 750v B+ with a choke input filter, and 5V4 for the low B+ to make 300v with a capacitor input filter. Both those tubes draw 5v at 2 amps or 10 watts each. So by switching to solid state you save 20 watts of filament heat + the power lost in heat from B+ voltage drop in the tube rectifiers X current draw? Not exactly since the B+ will be higher now and the rest of the tubes will run hotter.

    Combine the higher B+ with higher line voltage (125vac today versus maybe 110-115v when this rig was new), and your B+ voltages will be way up there and the filaments might be at 7 volts too instead of 6.3.  So by simply switching to SS rectifiers you need to address some issues. Here are a few:

The B+ will be instant now whereas before the HV took a few seconds, and the LV took 10-15 seconds. The LV capacitor filters might see more than they are rated fore while the rest of the tubes warm up.

The diodes, 1N4007, are rated at 1A, 1000v PIV. Make darn sure you have enough of them. They are cheap. Figure out what you need and add a safety factor, maybe 2X. That might mean 4 in series in each leg for the high B+, and 2 in each leg for the low B+. These are guesses, but I think close to my stated goal. Look at the formulas and multiply by two.

A choke input filter like used on the high B+ need to be suppressed in case the choke gets wound up and dumped. The back EMF will kill your diodes given the opportunity. The W6SAI Hand Books cover R-C Snubber networks for this purpose.

The Low B+ likely needs a series resistor between the diodes and capacitor filter to limit the surge upon turn on, and to lower the B+ back to the desired 300V.

The higher heater voltage can get a bit of a reduction by taking those two unused 5V @ 2A filament windings and parallel them in phase. Then use that 5v in series with the HV transformer primary phased to 'buck' 5 volts away from the transformer primary. Since the fuse is rated at 4 amperes slo-blow, those windings should handle 4 amperes.

Good luck with yout HT-32!

Jim
WD5JKO

    easy PTT can be had by pulling the vox relay tube out and keying the PTT relay to ground through a 22K 2 watt resistor. put the rig in the VOX position if operate it that way. solid stating the HV supply yields nearly 1KV on the 6146's if you have high line voltage, so be careful, and drop the PA screen voltage. then get a milliamp meter in the plate circuit and set the bias to a realistic resting current if you do this. also- the PA is not cut off during standby and just cooks the tubes. i key screen voltage through an added relay to keep it off until i hit the PTT. i have two HT32 (Mark- I) rigs here and they are a reliable good sounding AM exciter that respond well to simple audio mods. you get about 15-20 watts of very clean AM out of them.