Simple volume control circuit

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Simple volume control circuit

Post by tuberadiogeek on Tue Jan 13, 2015 7:42 pm

I have a good full range amp I'd like to use on occasion but it lacks a volume control. I have a almost complete chassis that was once a large radio but was too far gone, so I stripped it of some parts but it still has all of it's control knobs, resistors, disc and mica capacitors all in tact. I was thinking I might be able to use some of those parts to make some type of volume control. So is that possible? if so can someone provide me with a simple design? thanks
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Re: Simple volume control circuit

Post by Wildcat445 on Tue Jan 13, 2015 7:46 pm

All the volume controls I am familiar with are variable resistors. Could you use the volume control out of the parts radio chassis? What am I missing?

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Re: Simple volume control circuit

Post by tuberadiogeek on Tue Jan 13, 2015 8:26 pm

That's what I was wanting to do, but do not know how to do that.
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Re: Simple volume control circuit

Post by Ragwire on Tue Jan 13, 2015 8:26 pm

What WC said.
Perhaps Google up some schematics of tube radio or amplifier volume control circuits to look at...just mind the voltages and stuff, and do not run DC through the volume control.
Typically, the audio is coupled through a DC blocking capacitor to the potentiometer, and the leftover audio goes through another capacitor to the grid of a preamplifier.

Typically again, the potentiometer shunts a variable amount of the audio AC voltage (only) to ground. Buy an audio curve potentiometer, not a linear curve one. 500K ohms to 1 megohms are common values for tube equipment.
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Re: Simple volume control circuit

Post by tuberadiogeek on Tue Jan 13, 2015 8:45 pm

So far every schematic I've looked at requires an IC and is 1 channel. I'm not trying to buy parts. I wanted to use what I had on hand. I have some tubes too, but only a few select ones. I figured it would be simple and not require any power source.
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Re: Simple volume control circuit

Post by tuberadiogeek on Tue Jan 13, 2015 8:54 pm

After a quick inspection of my parts chassis, it looks like I only have single gang pots and they are all in the mega ohm range. So it doesn't look like i'm building a volume control any time soon.
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Re: Simple volume control circuit

Post by Ragwire on Tue Jan 13, 2015 9:22 pm

I missed that is was solid state. The volume control would likely be much lower, like 5-10K.
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Re: Simple volume control circuit

Post by 35Z5 on Tue Jan 13, 2015 11:45 pm

Assuming the amp already has enough gain no additional amplification stages are necessary, just a pot, jack(s) and a cap or two... Since it's SS you'll need a 5-10K control as mentioned, be sure it is audio taper(If nothing else RS has them)...

Here's a schematic of one of my transmitters, volume control is R3, not a lot more necessary(again assuming amp already has enough gain)...

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Re: Simple volume control circuit

Post by Wildcat445 on Tue Jan 13, 2015 11:56 pm

Ragwire, there was nothing I can see that specifically mentioned solid state. I assumed tube. What do they say about assuming? Tom's on it.

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Re: Simple volume control circuit

Post by willy3486 on Wed Jan 14, 2015 8:32 am

I have rebuilt a few of these pulled tube amps from big stereo units. Most don't have a volume control built in. The best circuit so far I have used is this
amp

On the left side of the image the 6eu7 tube has circuitry on pins 8 and 5. The 10k,1m and 100k volume have all been added. I usually and this circuit and then add a epoxy cap to the input of it. I think a .05 cap maybe. The control is from radio shack and its a dual 100k volume control. There are all types out there but this one is the last one I used.

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Re: Simple volume control circuit

Post by terrydec on Wed Jan 14, 2015 2:13 pm

I'm confused- Is it tube or solid state?  If solid state just hook it up like an "L" Pad.  If tube here is the info from Alfred Corbin's book-
One side goes to ground, the other side goes to the AVC line, (look for a 1 meg in series and a.0005 mfd cap to ground)), then the wiper goes through a .005 mfd cap to the AF or or control grid of the Det/AVC/AF tube, like a 12SQ7.

Did this help?
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Re: Simple volume control circuit

Post by neali on Wed Jan 14, 2015 2:21 pm

Make sure you use an Audio taper (logarithmic) rather than a linear taper pot. Volume control belongs in the preamp after the AF stage, not power amp portion of the amplifier. If you want to get fancy, look for audio multi taper pots and google loudness control. Here is a good link to get started:

http://www.angelfire.com/electronic/funwithtubes/Amp-Volume.html

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