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Old 27th May 2008, 06:08 PM   #1
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Default Input Volume Pot

In my preamp I have 100k input pot. Wiper directly to the grid (no caps, no grid stoppers). I tried to connect the 1M resistor from wiper to ground. It DOES sound different. With 1M more authoritative sounding, less highs, less detail.

Which one is the right with or without 1M resistor?
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Old 28th May 2008, 12:12 PM   #2
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Actually I ahve 220K input pot and I am bypassing it with different resistors to obtain different pot values.
There is clearly adudible difference between 100k and 105k pot. 105k sounds better to me.
I looked at different schemes of tube amplifiers and value of input pot differs a lot. I have seen lot of 250k, 100k even 50k. Some of them has resistor from wiper to ground (also different values 1M, 470k, 220k), some of them do not has this resistor.

Are these values choosen by math or by ear?
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Old 28th May 2008, 12:36 PM   #3
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How much cathode bias do you have? If the cathode resistor is dropping less than 1,0..1,8V you may have some current flowing out of the grid and changing the bias. The amount of change will be different, especially when 1M is used - you may be hearing different bias settings.

Perhaps also the input valve is old, and has some grid leak at even -3V bias voltage. Try newer valve, if so.

Also, what is driving the pot? Some drivers, like ECC83, or no-so-good opamps, will react better to light load of 1M.
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Old 28th May 2008, 12:39 PM   #4
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I prefer having a resistor from the wiper to ground just in case the wiper doesn't always make good contact with the track of the pot. The added resistor guarantees there will be a path from the grid to ground at all times and can avoid unpleasant noises due to an imperfect pot.

If the added resistor value is much greater than the pot resistance, as in your case, it has very little effect on the behavior of the pot as a volume control. If, on the other hand, it is similar to the pot value, it changes the 'law' of the pot. With the right combination of values, it can make a linear law pot behave like a log law pot, which some people prefer becasue linear pots tend to track more accurately that log-law (aka audio-taper) pots.
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Old 28th May 2008, 12:41 PM   #5
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It is Aikido preamp. I ahev 3.3v accrosthe 470Ohm resistor with B+ 300V.
Valves are new. It does sound better with 1M resistor.

Pot goes directly to the grid of ECC88.
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