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Transistor switch using negative base voltage

Jan 2nd 2013, 04:43

WA2P

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hi all, I have a question regarding transistor switching.

Background: I'm trying to modify an old tube transmitter (HT-32) to switch the bias on the finals from the normal -49 volts for transmitting to something around -125 volts when idling so the finals are completely cut off. I've found that the control grid of the driver tube goes from -60 volts to -1 volt when the code key is pressed or the microphone is used. I've tried powering a 12 volt low coil current relay by tying it to the -60 volt wire through a resistor, but it pulls the voltage down to 0 and the relay doesn't close.

The question: I know a trasistor can be used as a switch to close a relay. Can a transistor's base be connected to a negative voltage source, in this case the -60 volt wire via a resistor, and not fry the transtor? I would want the transistor to drive the 12 volt relay, and of course not pull the -60 volt line to 0.

Thanks -- Pat Ciferri (WA2P)

Jan 2nd 2013, 15:22

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

You are looking for an electronic bias switch--you may want to copy an existing design.

http://www.arrl.org/pubindex/search/page:1/Merged.pubs:QST;QEX;Ham%20Radio;NCJ/keywords:bias%20switch/model:Merged

Zack Lau W1VT
ARRL Senior Lab Engineer
Jan 3rd 2013, 05:09

WA2P

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks for the link to the QST articles. I've looked them over, but I can't figure out how the circuits shown would fit into my HT-32. The rig uses -49 volts for bias everywhere, and the -49 volts are on the control grids of the 6146's. I know I have to disconnect the current bias line from the grids and then switch between -49 volts when transmitting and a larger negative voltage when idling, and that I have to add a new bias supply to the rig to provide the higher negative voltage.

Unfortunately, I have to admit that I have little technical circuit knowledge, and can't figure out how to adapt one of the designs from the QST articles to work with my rig. I would very much appreciate some assistance in coming up with an appropriate circuit as I know I could easily do something to damage the rig.

Thanks, Pat Ciferri (WA2P)
Jan 3rd 2013, 13:57

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

You may want to visit the AM Fone web forum to locate a vintage radio expert who can assist.
http://amfone.net/Amforum/index.php
You have to be very careful in modifying vintage gear--not only are there hazardous voltages, but a mistake could easily destroy the power transformer, a very costly repair!

Zack Lau W1VT
ARRL Senior Lab Engineer
Jan 3rd 2013, 16:57

WA2P

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks for the lead to the AM Fone forum; I will check it out. Before I do, I would like your opinion on a possible solution.

From looking at the schematic again and measuring voltages in the bias supply, I've found that it produces -130 volts before going to the components, primarily an 18K resistor, that regulate it down to the required -49 volts for the entire rig. The control grids on the 6146 finals are connected to that voltage, and the output from the driver is fed there also via a capacitor in it's plate circuit.

Am I on the right track in saying that I could remove the current bias connection to the 6146's grids, duplicate the few components required to get to generate -49 volts, and use a bias switch to bypass the new 18K resistor to put the full -130 volts on the 6146 grids to cut them off? If so, can I also tap into the plate circuit from the driver, via a capacitor, to control the bias switch so the resistor is not bypassed when transmitting? I would think that by modifying only the line that goes to the 6146 grids, the chances of damaging the rig would be small.

Assuming I'm correct, I would look for an article on how bias switches work to begin with, since I currently can't figure it out from looking at the QST articles.

Thanks, Pat Ciferri (WA2P)
Jan 3rd 2013, 17:08

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I'd second W1VT's cautions. If you're new to electronics design, this kind of modification is fairly easy to screw up unless you're following proven step-by-step instructions that apply to your particular rig. And even then, the mod is only "cosmetic" (reducing power dissipation during standby). It's a question of risk vs reward and your tolerance for risk.

On the other hand, it will be an educational experience, hopefully not involving smoke.

73 Martin AA6E
Jan 3rd 2013, 18:42

WA2P

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
I wish this was only a cosmetic change to reduce power consumption. Unfortunately, the original issue is that when the rig is turned on, no matter if in standby, operate, CW, SSB, or AM, the finals glow orange all the time. The rig transmits fine, it's just that the finals are going to fail if something isn't done to cut them off. If the idling current was not so high and the tubes wouldn't overheat, I wouldn't worry about it.

I've posted to the Hallicrafters Users Group forums, and the consensus was that the bias should be more negative when not transmitting. That's led me to searching for a bias modification solution since the finals will fail if nothing is done.

Thanks for everyone's input, I appreciate it. I'll check out the AM forum next. As an aside, if anyone would like the "joy" of looking at the rig and is within reasonable driving distance from Poughkeepsie, I'd like to hear from you... HI

Thanks - Pat (WA2P)
Jan 3rd 2013, 19:51

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I have no direct experience with the HT-32, but glowing orange can't be normal in standby. The bias must either be set wrong (is there a pot to adjust?) or some components have changed value (like leaky or open electrolytic caps, overheated resistors, etc.).

The owner's manual (or the maintenance manual if you can get one) should tell you how the bias should have been set when the rig was new.

I think I would work to try to get the rig back to its original bias condition before adding an upgrade.

Good luck!

73 Martin AA6E
Jan 3rd 2013, 20:56

W1VT

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
http://www.amwindow.org/tech/htm/ht32resto.htm

Notes on Putting a Hallicrafters HT-32
Transmitter Back on the Air
by Ken - K6FC

This page suggests adjusting the bias voltage to get a bias current of 28 mA. He ended up with a bias voltage of -53 volts for his particular rig.

Zack Lau W1VT
ARRL Senior Lab Engineer
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Jan 3rd 2013, 21:58

WA2P

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks, thats the most promising lead I've seen. Maybe the -49 volt value isn't as sacred as the manual suggests.

There's one other symptom I forgot to mention before. At one point, I removed ALL of the tubes except the finals and the two power supply rectifiers and the voltage regulator 0A2. The finals STILL glowed as orange as before! Of course, increasing the voltage with the bias adjustment pot made the glow disapear. Maybe it needs to be neutralized...

Pat (WA2P)

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