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 Post subject: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 8:09 pm 
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Location: Port Dover, Ontario
I would like to teach myself to do this and after reading what I can about the technique, I understand that I should do these things:

1) use an unmodulated RF signal;

2) keep the signal generator level as low as possible;

3) get the AVC voltage as negative as possible whilst doing the various adjustments.

Am I on the right track here?

If any member of the forum has other tips for me, this would be very helpful.

Thank you.

Joseph


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 8:34 pm 
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I think you are on pretty solid ground.

Not sure if modulated vs unmodulated signal is critical when using this method. I usually like to monitor the demodulated 400 hz audio while doing an alignment with AVC, just so I can detect any noise or distortion that may increase during the alignment process.
This would not be possible using an unmodulated signal, so I use a signal modulated with a test tone.

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Last edited by processhead on Sep Mon 03, 2018 12:45 pm, edited 1 time in total.

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 9:35 pm 
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Location: Austin, Texas
I also use a modulated signal to get a better indication of something that isn't working correctly.

I check the AVC level with no signal and then increase the generator signal until the AVC is in the range of 0.5V to 1V more negative. If the AVC gets more than 2V more negative than with no signal, I decrease the generator signal to get back to the 0.5V to 1V range.

Jay


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 9:38 pm 
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Location: Near Brandon, Iowa
For this technique to succeed, you will need to measure the AVC voltage with a DC voltmeter that has an extremely high input impedance, such as a vacuum tube voltmeter. An analog (needle-type) VTVM is desirable because you are focusing on assessing relative indicator movement as you make the adjustments rather than dealing than the absolute numbers provided by a DMM.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 9:44 pm 
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Yes... an analog DC meter is good because you can tweak the IFs and wath for the meter needle's maximum dip.

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 11:37 pm 
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I use my DMM (Fluke 87lll) all the time for this. You can add a 10 meg resistor to the end of your probe for a 20 meg input, then multiply your reading X2. I lock my meter into the 40v range. Easy to see the numbers go up and down, don't bother chasing 1/100 of a volt and it works fine.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Sun 02, 2018 11:39 pm 
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You can practice this: attach the meter to the correct points on the radio. Tune the radio across the dial for over-the-air signals. The meter should move as you tune into stations.

This is also how some S-meter and tuning eye tube work.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 12:48 am 
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I always use the AVC for alignment using the following technique:

1 - Connect a 1.5 or 3 volt battery across the AVC integrating capacitor with its positive end going to ground.

3 - Connect the VTVM to the detector end of the AVC resistor.

3 - Align for max negative voltage. The signal level will not change the gain of the receiver because the AVC line is clamped by the battery. Modulated or not does not matter.

4 - Disconnect the battery.

Works every time.

- Leigh

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 5:11 am 
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The battery fixing the AVC voltage to the tubes at a constant value is important. Otherwise, if the AVC is actually working, it will serve to smooth out the peak you are trying to see as you make the tuning adjustment.

Note that Leigh's instructions have you connecting the battery to the end of the AVC line that feeds the grids of the RF tubes, so the high value resistor separates the battery from the point you connect the meter. On many sets, the frame of the tuning capacitor will do for the battery connection, and the hot end of the volume control for the meter. Check the specific schematic to see if this works for the particular radio you are aligning.

Ted


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 12:39 pm 
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Interesting discussion, I have to admit that I have never tried using this method. I will pull one of my radios out that has had an alignment done by the non-AVC method, and try this method out. I am interested in seeing what advantages/improvement come about.

Russ

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 12:49 pm 
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Location: Utica, NY 13502 (USA)
Alignment should to be performed when the circuits are at maximum gain. Therefore I do not use a battery which will set the gain at less than maximum. Then I keep the signal generator low enough so that only a slight deflection of the meter is observed as each stage comes into alignment.

I also use the audio meter method but the procedure of keeping the signal generator output and resulting AVC voltage at a minimum is the same.

Dave


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 1:09 pm 
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I have never tried the battery method, but why some of these methods were likely not common early, lies in the meters. Most were in the order of 1000 ohms per volt, & better instruments came at price which many could not afford. The low impedance saw some data sheets here with the comment on AGC / AVC, "Do not measure" as the low impedance meter loaded that line down to zero, which also upsets the bias.

I would expect more volts from modulated signal: However, its something else to explore, if we want to have fun fiddling: Its a learning thing. Don't forget there is a commonly used AGC, & then there is "delayed" AGC & these are often sourced differently. E.g. One from the secondary of the IF coil along with the audio, the other from the Plate of the last IF tube, it & audio using separate diodes.

In response to Dave's comment: I have mentioned that "Astor" brand / manufactured sets here often had a plug for a meter probe, already wired up to the plate of the OP tube, just for aligning.

Marc


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 1:19 pm 
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I agree with Dave, no battery needed. Just keep the signal low enough so you don't max out the negative AVC voltage.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 2:13 pm 
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I can see the idea behind the cell. It would tend to lock the bias at a fixed value. But I would be interested in comment re say 6U7 (6D6) & many others that have a recommended bias of -3V.

Marc


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 2:14 pm 
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Lou deGonzague wrote:
I agree with Dave, no battery needed.

The battery clamps the radio's gain at max so you get nice sharp peaks rather than the muddy peaks you get when the AVC system is working, regardless of generator level.

- Leigh

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 2:15 pm 
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Marcc wrote:
I can see the idea behind the cell. It would tend to lock the bias at a fixed value. But I would be interested in comment re say 6U7 (6D6) & many others that have a recommended bias of -3V.
Marc
Hi Marc,

That's why I originally said to use a 3-volt battery if appropriate.
Other voltages could be used if required.
The exact value does not matter, but it should be as low as possible.

- Leigh

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 2:37 pm 
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Ok, I'll try this on the next set and compare the results.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Mon 03, 2018 2:54 pm 
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Leigh wrote:
Marcc wrote:
I can see the idea behind the cell. It would tend to lock the bias at a fixed value. But I would be interested in comment re say 6U7 (6D6) & many others that have a recommended bias of -3V.
Marc
Hi Marc,

That's why I originally said to use a 3-volt battery if appropriate.
Other voltages could be used if required.
The exact value does not matter, but it should be as low as possible.

- Leigh


Much as I expected, however I rather feel that if the no signal bias is quoted (like -3); going below it may be adverse, by causing the tube/s to operate outside its / their intended operating point. "Appropriate" is perhaps: Not below recommended / quoted bias. Something to toy with in practice.

Marc


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 04, 2018 12:16 am 
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Thank you everyone for all of your help with my first attempt at this. I am encouraged to try now.

To keep my first attempt somewhat simpler, I will try it without the battery.. Should this work out, I will use the variation including a battery. For the sake of completeness, what voltage would be recommended with a 5 tube set using 12SA7 12SK7 and 12SQ7?

Thank you again.

Joseph


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 04, 2018 1:31 am 
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What Leigh said....
Quote:
The battery clamps the radio's gain at max so you get nice sharp peaks rather than the muddy peaks you get when the AVC system is working, regardless of generator level.

+1. One of the functions brought about by the AGC action is the broadening of the bandpass giving better fidelity on strong stations - exactly what you don't want when tuning for the peaks - the battery has virtually killed the gain of the RF and IF stages. One of the first things you do when tuning up a communications receiver is turn the AGC off.

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