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Author Topic: Hallicrafters S53a help  (Read 3422 times)
IZ5PQT
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« on: November 27, 2012, 10:17:21 AM »

hello

I own a nice Hallicrafters S53A receivers, which works well except that it has a bad distortion when I listen to a strong MW local. The obvious cuplrit seems to be the AVC, because if I reduce the manual IF gain it is OK- I tried to replace the detector/avc diode (6H6) to no effect. Looking into the manual I saw that a series resistor drops the 6H6 filament voltage to 4.7 V (the voltage is indicated in the manual).
I never saw before the trick of reducing the filament voltage, and I wonder what was the purpose of that. Perhaps the diode does not operate to its specs and this maybe explains why the AVC action is inadeguate. But why Halli designed the circuit that way?
thanks for your help
Giovanni
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N4NYY
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« Reply #1 on: November 27, 2012, 10:22:19 AM »

How strong or close is the AM station? Is it overloading the front end, before you start tearing into the radio?
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IZ5PQT
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« Reply #2 on: November 27, 2012, 10:56:28 AM »

the station is quite strong, however it's no problem for other receivers. I am pretty sure the AVC develops insufficient voltage.
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N4NYY
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« Reply #3 on: November 27, 2012, 11:14:39 AM »

the station is quite strong, however it's no problem for other receivers. I am pretty sure the AVC develops insufficient voltage.

Every tube set I have worked on that had carbon composite resistors, had some or many that were well out of tolerlance. That would be the easiest thing to look at first.
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AC5UP
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« Reply #4 on: November 27, 2012, 02:11:26 PM »

Here's a similar thread where it's alleged the 6H6 does operate better with reduced filament voltage...

http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=207890

You'll also notice the comment about parallel resistance across a filament. Some sets used a series heater string which worked well as long as all the tubes drew the same filament current and had the same start-up characteristics. I've seen cases where one filament would light up overly bright for a moment then fade back to normal as the other tubes in the string came up to temperature from a cold start. I can't imagine this being good for the hot tube, but there were special 'Globar' resistors made back in the day to even out the cold start sequence and a shunt resistance could slow down a heater.

You'd find series string heaters in AC / DC transformerless sets and radios intended for 12 volt mobile or AC mains operation. Typically they'd run a 12 volt filament string with some 6 volt tubes in series. I should also mention that I have never seen a 6H6 go bad, and that's based on observation and a mil-spec tube tester.

Doesn't mean it can't happen or my tester is infallible, but the 6H6 and modern equivalent 6AL5 / 12AL5 seem to be immortal.   Smiley
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IZ5PQT
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« Reply #5 on: November 28, 2012, 01:00:02 AM »

Quote
Here's a similar thread where it's alleged the 6H6 does operate better with reduced filament voltage...

http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=207890

Thanks AC5UP, so there is some logical explanation. I plan to make a check removing the resistor,
to see if the performance becomes better on strong signals.
 
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G3RZP
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« Reply #6 on: November 28, 2012, 02:43:54 AM »

The lower filament voltage leads to a lower contact potential, so less effective reverse bias on the valve. That maybe why they did it.
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W2WDX
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« Reply #7 on: November 30, 2012, 03:19:00 PM »

I'm curious. If the problem is alleviated by turning down the IF gain, why go and do mods. Is not the IF gain variable for this precise reason. Obviously it is not overloading the front end if the distortion goes away by reducing the gain of the IF. What is being overloaded is the detector, is it not?

Just a thought.

As far as running a filament at lower voltage, would this not contribute to premature failure of the tube?

John, W2WDX

PS: Checking ALL the carbon comps is a good idea regardless.
« Last Edit: November 30, 2012, 03:21:11 PM by W2WDX » Logged

K1ZJH
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« Reply #8 on: November 30, 2012, 05:18:31 PM »

If you haven't replaced all of the wax capacitors, do so!!  A leaky wax cap will load down the AGC bus and cause the problem you are experiencing. Leaky caps will also cause the audio output tube to draw much more current than it should. 

Two other posters explained why the 6H6 is being operated at reduced filament voltage.

Pete
« Last Edit: November 30, 2012, 05:21:17 PM by K1ZJH » Logged
AC5UP
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« Reply #9 on: November 30, 2012, 05:55:06 PM »

As far as running a filament at lower voltage, would this not contribute to premature failure of the tube?

Interesting thought, and here are mine...

The most common causes of tube failure are low emissions, shorted internal elements or an open filament. In my experience an open filament is rare, internal shorts semi-rare, and low emission is far and away the most common failure. Many tubes from the 40's onward used a thoriated cathode and running the heater too hot could damage the coating. Loss of the coating was a bad idea as its purpose was to improve transconductance. In the case of early tubes, running a filament hot did improve emissions. You may recall the old TRF sets had a rheostat on the filament line so the user could crank 'em up on weak stations, but this also reduced the filament and 'A' battery life.

If a 6H6 was intended to dissipate a fair amount of power a cool filament would be a bad idea. Poor emissions, poor efficiency, probably age the internals prematurely, etc. But the 6H6 is a dual diode intended for low-current detector applications and intentionally small so it's good up to 700 MC's.

I was surprised to learn that some folks say they work better at reduced voltage, but as far as harming the tube? I say not likely in this case.......
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K1ZJH
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« Reply #10 on: November 30, 2012, 06:04:54 PM »

The 6U8 first audio stage in the B&W 5100 transmitter was also run at reduced filament voltage. I was told it was done to reduce metal vaporization that could cause leakage over time.  It supposedly also extended the life of the tube. This is second hand info, FWIW.

Pete
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IZ5PQT
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« Reply #11 on: December 01, 2012, 01:27:42 AM »

I would like to thank all of you for the useful answers. It's right that the IF gain is
precisely to reduce the gain on strong stations, but I think that the station is not so strong to justify distortion. Checking for leaky capacitors filtering the AVC is a very good suggestion and
I'll do that. From some of your replies I understand that low filament voltage could be used to improve sensitivity on weak signals, but it could also reduce the AVC action (a kind of delayed AVC which is delayed too much.) So I'm curious to make a comparison with 4.7 and 6.3 V on the filament. I'll post my findings here
73 Giovanni

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