Hallicrafters HT-9 questions

[Opcom]

The HT-9 uses a tank circuit with a few-turns coil wound around the plate coil. Tuning is done by the coindenser, and loading is done by  a movable tap on the output coil.

In the manual, regarding loading, section D-2 paragraph (f), it warns about reactive loads.

It says that if the minimum reading, under load, of the plate current occurs at a condenser setting of more than 10 degrees off from  the no load resonant reading, it indicates a reactive load due to improper antenna dimensions. Under such circumstances the efficiency will be low and severe harmonic radiation may take place.

I am not used to this kind of coupling. I'm used to the pi network. Why would a well loaded, resonated PA stage produce harmonics? Am I looking at this the wrong way?

[w3jn]

It's a simple matter of that pesky loaded Q.  The link-coupled ouput in the HT-9 and other similar rigs is only a one-element network (ie one L-C resonant network).  The Q is sensitive to output load; if that load's reactive (ie non-resonant), the Q will drop and the harmonic attenuation will suffer dramatically.  The old west coast Radio Handbook goes into this in detail.

OTOH a pi-network is a two element network - consider it to be two L-networks in series (that's two L-C components).  What most don't see is that the inductor in the pi-network, for the purposes of filter analysis, is actually two inductors in series (yeah, there's only one inductor there, but image it cut in two and wired in series).  So two L-networks do a much better job of filtering and impedance transformation because the first L-network (the one connected to the plate of the xmitter) is better isolated from the load by the second L-network (ie the load doesn't affect the Q of the xmitter tank as much).

Hope that's clear.  I had a hard time visualizing what was going on in a pi-network until I read the explanation in the west coast Handbook.

[KM1H]

I have both the pre war and post war HT-9's here and find they are very narrowband performers with the factory output coils. All I received is a 10 and 40M coil.

Since Ive been unable to obtain any extra coils or even the oddball spaced jack bar Ive decided to build my own and at the same time attempt to build a fairly broadband tank for the phone bands and coax lines. I figure that going to a pi network with a Q of 8-10 with fixed tuned loading is the simplest approach unless somone has details on the Central Electronics networks.

Having about 270# of dead weight to cover 2 frequencies is getting a bit frustrating!

Carl
KM1H

[Carl WA1KPD]

Its a great rig and when it needs work you can just get inside, crawl around and work on it.

I've had both versions but now just have the pre war black model.
I do have a parts unit so if you find the need for something I may have it

I don't have any additional coils but have made my own set for 160. Fairly easy to do
73
Carl
/KPD

[W2DU]

Beginning in 1940 and continuing throughout WW2, all nine FCC monitoring stations in Hawaii used the HT-9s for instantaneous CW comunications between stations. Monitoring stations on the Mainland used land-line teletype, but we had no inter-island land lines in Hawaii to support teletype.

Walt

[KM1H]

Ive been reading WW2 QST's recently and there were some nice writeups on the FCC RID group. They had rather elaborate antennas and recording equipment as well as the dedicated 24/7 landline TTY network and mobile setups.

Too bad they used SX-28's for monitoring, it took 2 hands to keep retuning

Carl
KM1H

[W2DU]

Several weeks ago I posted that as a monitoring officer with FCC's RID, I had taken my precious HRO with me to Hawaii immediately following Pearl Harbor. I initially used it only for fun when off duty, but later on it became the receiver dedicated to receiving info from Russian weather stations, which was sent directly to the Military in the Aleutian Islands for helping plan air attacks.

My Dad bought the HRO for me in 1938, which is same receiver I still use on AM, as shown in the pics at my shack in Florida. (We're in Michigan until October.)

Walt

PS--Forgot to mention that the main receivers used at FCC's RID were SX-28's, which we tuned with one hand. Why would anyone think it took two? One hand on the tuning dial, and the other on the volume control when needed.

[Opcom]

I understand about the Q. Well I guess that clears it up - better have a proper antenna or use a tuner. Thanks!

[W2JBL]

   one of my neighbors here in Brewster NY, Paul W2BP uses a post war HT-9 on 75 meters into a Johnson Match box. he's feeding a 260 foot dipole fed with ladder line, and has absoultely no problems loading the Ht-9 with that setup. I have operated that station many times and it tunes very easily with excellent efficiency. at one time I had suggested he tune the output link in the Ht-9, but after operating it I determined it was not needed. his receiver by the way is an SX-28. what a cool rig!

[KM1H]

Im using a SX-17 with the black one, sometimes I cycle one of the SX-28's thru.

A SX-42 will mate with the gray one.

In any event if I use either on 20-10M it will be with one of the less drift prone brands.

The best thing about the HT-9 is the audio, looks great on the scope and reports usually comment about it. Sure beats having to add another 100# of external junk or hacking up the modulator.

Carl
KM1H

[Opcom]

I was thinking about the audio, considering four 6L6's and the plate load must be around 2200 ohms plate to plate. or less. 420V on the plate, 350 on the screen, going to class AB2 when driven hard, I would say, with the 6J5 driver. I hope I can get arond to it sooner rather than later.