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Old 14th Nov 2022, 10:15 am   #1
AdrianH
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Default How to form an ALC circuit in a conventional Audio amp?

I am just wondering about how one would form an automatic level, or gain control for an audio amp, this would be for modulation amp on a AM TX home build.

I am wondering if I could use say an EF92 as the first stage, rectify some audio from the output transformer to feed back to the EF92 variable gm grid to control the output so it does not go past a certain point, a bit like the agc on radios.
Not sure if there is a more suitable variable gm audio valve or not, of if there is a better way altogether, using valves.

I am working in Kcad on the circuit at the moment so could put an idea up in a while.

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Old 14th Nov 2022, 10:19 am   #2
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Look for Valve Audio Compressor circuits. There are many out there.
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Old 14th Nov 2022, 10:56 am   #3
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Default Re: How to form an ALC circuit in a conventional Audio amp?

In times past there was a cunning circuit which used a Neon bulb wired to the modulator transformer, and one of the contrast adjusting ex-tv LDRs / photocells in the grid circuit to one of the low level audio stages, when the Neon lit its light illuminated the LDR whose resistance fell, introducing audio attenuation to reduce the loop gain.
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Old 14th Nov 2022, 11:04 am   #4
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Default Re: How to form an ALC circuit in a conventional Audio amp?

I have done a search on the internet, but I admit to not having a lot of success getting simple circuits, lots of talk on very expensive units for music, I have found one article on a constant output broadcast unit but the finished article was 6 valves plus rectifier although it does show a single valve idea.

The neon with an LDR, is novel would not have thought of that, I will just continue with Kicad for a while and let people pull the idea to bits.

I also know that in simple am transmitters like the AT5 there really was no compression, just something like a neon/resistor divider to indicate peaks of a certain level.

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Old 14th Nov 2022, 11:10 am   #5
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Have you considered using a Vactrol ?

https://en.wikipedia.org/wiki/Resistive_opto-isolator

Loads of circuits around for these.

Cheers

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Old 14th Nov 2022, 11:18 am   #6
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Here's a 'classic' valve circuit from a RSGB handbook, where it was used to control the audio gain of a FM transmitter to prevent over-deviation. A bit of the audio is taken and amplified by one half of the ECC83 triode, rectified by the two diodes, and used to back-bias the EF92 first-audio stage.

Should work just as well with an AM transmitter to control the audio and let you talk it up without overmodulating and causing splatter.
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Old 14th Nov 2022, 12:31 pm   #7
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Quote:
Originally Posted by G6Tanuki View Post
Here's a 'classic' valve circuit from a RSGB handbook, where it was used to control the audio gain of a FM transmitter to prevent over-deviation. A bit of the audio is taken and amplified by one half of the ECC83 triode, rectified by the two diodes, and used to back-bias the EF92 first-audio stage.

Should work just as well with an AM transmitter to control the audio and let you talk it up without overmodulating and causing splatter.
Thank you that is very good, I did not think to look at my old books, one always thinks of the internet! I kept away from this thread for an hour to work in Kicad, anyway this was my initial thinking: -

modstage.pdf

I have the mod transformer from an old Aircraft TX and it was meant for two EC90's, unfortunately the driver transformer was open circuit as the very fine wires must have corroded over years.

The idea was to wind an extra winding on the output side to give me a feedback winding and using a diode rectify the voltage to drive the EF92 grid negative.
R18 R17 would provide a potential divider between HT and ground as a sort of threshold I have to get past before limiting action takes place with R12 tying the diode to 0 Volts.

R11 to R9 would be another potential divider to give some control of the limiter after the action point and C5 R9 being the time constant. trying to run the EF92 at reduced voltages and current, I really did not wish to have around 10 mA through the cthode.

Anyway I would appreciate if I have flaws in my thinking above, but I do believe I will be trying the VHF/UHF book idea, thanks again to G6Tanuki for that.

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Old 14th Nov 2022, 12:46 pm   #8
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Neon bulbs are highly non-linear.

Variable-mu valves distort (it's where their name comes from)

It may offend but a high efficiency LED or two from a half-dead light bulb, shining onto a classic ORP12 will do the deed. This can cover a wide dynamic range. Audio distortion is low.

LEDs have a pretty linear current to light intensity curve without jumps and negative resistance characteristics.

LDRs can be a little slow at low intensities, though.

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Old 14th Nov 2022, 12:49 pm   #9
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Default Re: How to form an ALC circuit in a conventional Audio amp?

That should work OK, but I'm not sure if you need the two 100K resistors and the capacitor to the overwind on the mod transformer; I would simply feed the signal from the mod transformer to one end of a potentiometer [100K or so] with the slider to the diode; then you can use the pot to set the point at which the drive to the diode gets high enough for it to conduct and so start the back-biasing of the EF92 to reduce the gain.


Just for reference, attached is the LDR-and-neon circuit I mentioned [from Pat Hawker's Amateur Radio Techniques book]. I guess you could use a very small incandescent bulb driven from the overwind on the mod transformer in place of the neon; that might work better because of the non-linear voltage/light-output curve of a filament bulb, and its thermal inertia, would stop it having too much effect at low signal levels.
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Old 14th Nov 2022, 1:04 pm   #10
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Here's an old thread from a couple of years ago when i dabbled with a valve based compressor limiter.

I'm not convinced i managed to get it working properly and other jobs took priority.

https://www.vintage-radio.net/forum/...d.php?t=167214

73s

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Old 14th Nov 2022, 1:28 pm   #11
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Quote:
Originally Posted by G6Tanuki View Post
That should work OK, but I'm not sure if you need the two 100K resistors and the capacitor to the overwind on the mod transformer; I would simply feed the signal from the mod transformer to one end of a potentiometer [100K or so] with the slider to the diode; then you can use the pot to set the point at which the drive to the diode gets high enough for it to conduct and so start the back-biasing of the EF92 to reduce the gain.
Yes, a potentiometer would enable the action point to be set and I guess if R1 was variable that would then allow some control over the limiting. I was trying to keep what is potentially a 500 Volt peak to peak waveform from getting back in the circuit, if I got say 20 Volt pk-pk max from the overwind it would be safer.

I am assuming that with a neon requiring say 70/80 Volts to strike and the variable resistor oyou are then setting at what point the limiting action is taking place and at ll levels below that the amp is running as normal.

If I was to use Davids idea of a LED, or back to back LED's I would have to come up with some back to back zener diodes to also provide a suitable striking voltage point.

It seems I have now a few alternatives. Time to get the copper clad out drill some holes and start mounting things for a trial.

This is where the fun starts.

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Old 14th Nov 2022, 1:37 pm   #12
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Hi Aub just seen that post and read the thread, it does seem to require quite a high initial input. If you did not get any further with it I will continue with the ideas I have for the time being.

It is all part of the fun. Trying to keep it simple.

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Old 15th Nov 2022, 11:58 pm   #13
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Things have moved on a bit, I have a stage with EF91, ECC83 and two EC90's driving the mod transformer from the above circuit. I still have to add a bit to the circuit, as in a LDR, two LEDS and two Zener's and a few resistors to provide level control, if I have it correct by merging David's and G6Tanuki's comments/posts. that lot courtesy of discussions on the other forum.
I am not sure if the mod stage will actually work in real life, but it is fun building and trying things out.
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It is being powered by 12 Volt to 250 and 6.3 V switching PSU's while I play, the end idea was to mod a small valve 5 Watt TX for either 80 or 40 metres, which I would still have to build.
This is the circuit so far before the compression bits are added.
modstage017.pdf
The mod transformer and EC90's were part of the Mod stage in a TR998, or STX-9x series aircraft transceiver and would provide the mod for a TT15 giving out about 5 watts at 130 MHz using anode and screen modulation, I have tried to figure out how much power I am getting from this unit now and think I must have something wrong as I seem to calculate round 450 milli Watts, this is just by connecting the output across some resistors and measuring the pk-pk voltage before clipping, the load is 11 k Ohm and I get 200 volts Pk-pk.
But I learn from building and figuring so for me it is still good.

Adrian
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Old 23rd Mar 2023, 6:35 pm   #14
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Default Re: How to form an ALC circuit in a conventional Audio amp?

Closed at OP's request

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