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Author Topic: Hallicrafters HT-41 Biasing  (Read 7842 times)
OXFORDHAM1
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Posts: 21




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« on: August 16, 2012, 07:34:12 PM »

Hello, I was just wondering if anyone out there has a simple explanation on how to bias the ht-41 there is a bias strip in the rear of the amp . What hooks to this ? the manual is very vague and mentions flashlight batteries. Ive never heard of using flashlight batteries with an amp before. Maybe some of you elmers could help a newbie Thanks, John KC2WTS
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KA5N
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Posts: 4380




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« Reply #1 on: August 16, 2012, 08:36:13 PM »

Well bias is a voltage used to set up the operating conditions of the tube or tubes.
The bias voltage is usually low and very little current is drawn.  One can have a power
supply which produces the voltage or as was often done in bygone years one or more batteries can be used to supply the voltage and since the current is very low the batteries last a long time and are replaced when they are no longer capable of supplying
the voltage and current.  Depending on the voltage one may have to series connect
several cells to get the correct voltage.  Older batteries used to have fahnstock clips
or threaded posts with nuts for the connection.  Perhaps a battery holder would work
in your case.
So what is the reccomended bias voltage? 
Allen KA5N
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KA5N
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Posts: 4380




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« Reply #2 on: August 16, 2012, 08:50:32 PM »

OK I looked online at the manual and it says bias is -3 volts and that is two 1.5 volt
dry cells in series with positive end to ground.  Right there in the manual.
If you are new to amplifiers, I would suggest you get some help with this dude.
The tubes may be difficult to find these days.
Good Luck
Allen KA5N
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W1BR
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Posts: 4196




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« Reply #3 on: August 21, 2012, 10:44:59 AM »

3 volts? I wonder if that amp could be modified to include two or three silicon diodes in the cathode return?

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KA5N
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Posts: 4380




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« Reply #4 on: August 21, 2012, 11:34:49 AM »

The HT-41 uses a pair of 7092's  (Tetrodes) and I predict bad results if a nubie starts
modifying this old amp.  The bias is for cut off and is switched off when transmitting.
I would leave the circuit alone and either used batteries or a wall wart or some other supply.

Allen KA5N
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W8JI
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Posts: 9748


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« Reply #5 on: August 21, 2012, 01:36:14 PM »

3 volts? I wonder if that amp could be modified to include two or three silicon diodes in the cathode return?



I had an HT41 once. It was not a very good amplifier.

The external battery bias, if I recall correctly, was to make the amplifier "high drive power". It simply biased the tubes off without much regard to splatter, and was on a drive terminal strip.

There is another bias that must be shorted to activate the amp. Without shorting that bias terminal, the tubes are in standby bias. This is independent of battery bias to the control grid, used to "reduce gain".

The amp is cathode driven with NO tuned input. Mine had a 2:1 or 3:1 SWR or something. It was pretty high, but worked with an HT32 I had. Probably about 400 watts output.

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K9EID
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« Reply #6 on: August 28, 2012, 08:53:12 AM »

John
Have a well working HT 41 with an HT 37.  Great combination.  There is absolutely no need for batteries, etc.
There are two strips on the back.  If you are driving the HT 41 with a low output transceiver (30 - 40 watts) you
will need to put a jumper across T2 that shorts out the 200 ohm bias resistor.   For normal 100 watt transceiver
outputs, leave the two terminal screws empty. 

Terminal strip T-1 is the bias blocking control.   You will need to short those together from the extra contacts on
your Dow Key antenna relay - or other antenna relays.   In transmit, the two screws on T-1 mus be shorted to
allow the amplifier to work.

Good luck.  It is a great amplifier.    Have fun.   Bob Heil K9EID
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