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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 25th Oct 2007, 9:29 am   #1
'LIVEWIRE?'
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Default Effect of using lower resistance Volume Control?

I really should KNOW the answer to this, but can't remember. What would result, apart from lower max. volume, from using a 220K Volume Pot. instead of a 1M one? I have had occasion to do this in a Vintage Valve Car Radio, which used a special type of dual concentric control(!M Log Vol + 1.3M Lin Tone), and the only item I had which would physically fit was a similar control for a later model, but 220K Log + !M Lin. The set in question uses A.C. Coupling tto the grid of the 1st. AF Stage. Would reducing the value of the Grid leak resistor(5M6) improve the gain?

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Old 25th Oct 2007, 10:40 am   #2
Studio263
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Default Re: Effect of using lower resistance Volume Conrtol?

In an AC coupled circuit (capacitor from the top of the control back to the anode of the previous stage) the effect would be that there would be a little less bass and a slightly lower volume available generally. You may not notice either effect of course, they would be quite slight.
There's not a lot you can do about the slight reduction in volume but you could cure the bass cut (if it offended you) by increaing the value of the coupling capacitor in proportion to the decrease in value of the control, e.g. if the control was 1M and you've dropped it to 200K then the capacitor could be raised from 10nF (for example) to 47nF to balance it all out.
Obviously these observations only apply in simple circuits where the volume control is AC coupled and isn't used as a diode load or as part of a bias network.
You can always try a quick hook-up by shunting the existing control with another fixed resistor to simulate a lower resistance track before going to the trouble of ordering and fitting a new one.
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Old 25th Oct 2007, 10:54 am   #3
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Default Re: Effect of using lower resistance Volume Conrtol?

I have already fitted a replaement NOS Pot. that I had to hand, but may try changing the coupling capacitors. However there appears to be low gain somewhere in the IF stage as well. (The set is over 50 years old!)
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Old 25th Oct 2007, 12:11 pm   #4
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Default Re: Effect of using lower resistance Volume Conrtol?

If it's a car set don't get caught out like I did, I thought the Phillips 344V that I restored for my old VW was suffering with low gain too but it turns out that the coupling to the antenna is critical and that the antenna only works properly when it is attached (and grounded) to the car body. On the bench these conditions were not met and so it appeared that the gain of the set was (very) low, despite all the suspect capacitors and resistors having been replaced and all the DC conditions being normal.

Following the (very good) advice from forum members I fitted the set and the results were then perfect (no supressor caps needed on the coil or generator either, because the engine is in the back it is far enough away from the antenna and the set not to spoil the reception).
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Old 25th Oct 2007, 1:12 pm   #5
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Default Re: Effect of using lower resistance Volume Conrtol?

Generally, you shouldn't have a problem in using a lower value volume control. The only issue I can see is if the circuit uses the volume control value as a diode load, and by using a different value you alter the ability of the the demodulator to handle high levels of modulation, resulting in a little distortion.

Also, if you use a pickup connected to your set (if it was a mains set rather than a car radio) the volume value might form the load for the cartridge, so altering that can also result in a loss of bass and a little distortion.
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Old 25th Oct 2007, 9:37 pm   #6
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Default Re: Effect of using lower resistance Volume Control?

THanks to Studio263 for reminding me of the grounding problem. In my workshop(which is below ground level) I have a standard Car Aerial mounted high enough up to give an adequate signal in most cases, and the 'earth' side of this is grounded to copper piping, but even so the FM performance is sometimes not what it ought to be
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Old 11th Apr 2008, 4:55 pm   #7
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Default Re: Effect of using lower resistance Volume Control?

------"the coupling to the antenna is critical and that the antenna only works properly when it is attached (and grounded) to the car body. On the bench these conditions were not met and so it appeared that the gain of the set was (very) low,"------
The grounding for the antenna is achieved by an "artificial ground plane".
On Australian Army Landrovers (in the 1970's) that were equiped with two -way radio, two thin wooden battens were tied to the canvas canopy, these being attached to a sheet of metal (aluminium) about 3 feet square, in the centre of which was mounted the antenna.
So if you want to bench test a car radio, just mount a spare antenna into a spare car panel or a sheet of zinc-alum, steel, aluminium whatever is laying about.
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