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Old 7th April 2011, 06:36 PM   #1
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Default Volume Controls

After many tries at troubleshooting my Viking stereo tube amps
balance issue,(one channel much quieter than the other) I think I finally
narrowed it to the volume control having a 1MΩ + difference between the left
and right sections.
I measured this with my DMM unsoldered from the cct.
The overall resistance from the wiper to either tab seems really high @ 4.5MΩ
Each channel has a 6AU6 preamp and a 6BQ5 final.
The volume control is between the input and the grid of the pre-amp tube.
I should mention this is a cheap amp from an early tube phono not a real
high-fi design and I don't have a good schematic.
How is the resistance of the volume control chosen?
Eg. the input Ω impedance of the pre-amp tube grid?
This value of dual section pot 4.5MΩ seems difficult to source.
Is there a way to change it to a more common value?
Put a fixed resistor in series with a lower value pot?
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Old 7th April 2011, 06:44 PM   #2
ChrisA is offline ChrisA  United States
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You can almost certainly replace the pot with a modern 1M pot. I say "almost" because you don't provide a schematic.

But do you even need a volume control. Many times you are using an iPod or something like that as the source and it will have a volume control
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Old 7th April 2011, 06:48 PM   #3
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One of cardinal solutions is to use a ladder type attenuator, like this one:

Index of /maat

It has minimum of precise resistors in the chain on any volume level. Resistors are controlled by high quality relays, relays are controled by a micro processor. Now you may use either a single cheap pot to control volume, or a switch if prefer stepped control.
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Old 7th April 2011, 06:52 PM   #4
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The input impedance of a valve grid is sometimes thought of as being infinite, since under normal conditions no current flows into it, this is a bit of an exaggeration, but it can easily be 10M. Exactly what is going on will depend on the other components at the input, typically there will be a grid leak resistor of ~1Mm, although this may be omitted since the volume pot can serve this function. If there are capacitors these will be dimensioned to match the resistances.

In all probability a 1M pot can be substituted for the existing one without problem.

w
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Old 7th April 2011, 08:43 PM   #5
DF96 is offline DF96  England
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Best not to omit the grid leak, unless you have a very reliable pot with a wiper which will never lose contact with the track.
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Old 7th April 2011, 10:43 PM   #6
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Thanks guys.
a 1 Meg pot will be easier to source for sure.
This amp had no grid leak or caps just straight to the volume control.
I bypassed the volume pot with 1MΩ resistors and it is the problem as both channels are even now.
I would like to bypass the balance and tone control as they are not of a good design and robbing signal and tone.
With the signal chain thus simplified what components would most effect the overall tone of the amp?
Caps coming off plates of pre-amp tubes?
I was hoping to get a flat uncoloured sound.
Whatever goes in goes out but louder.
I am driving this amp with line level signal from Ipod.

Last edited by Glowin Plates; 7th April 2011 at 11:02 PM.
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Old 8th April 2011, 12:44 AM   #7
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Buy 1 of Canada's gems, either a single ganged stereo or a pair of mono volume controls from PEC. DigiKey is the distributor. These hot molded Carbon parts, KA mono/KKA stereo, are milspec tough/reliable and sound very good, without being especially expensive.

FWIW, I'd get rid of everything that's electrically in front of the volume control(s). Your doubts about tone controls are well founded. Use 100 KOhms as the control resistance value. The OEM 1 MOhm value was needed to avoid loading the cheap circuitry down. Get rid of that trash!

A schematic would help the "crowd" make better recommendations. An inexpensive, but good, replacement for the interstage coupling caps. is 400 WVDC 716P series "Orange Drops".

BTW, it's LONG odds the original phonograph used a crappy piezoelectric cartridge.
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Old 8th April 2011, 01:49 AM   #8
Loren42 is offline Loren42  United States
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First rewire the pot so the channels are reversed (left to the right and so on). If the problem really is the pot, then the problem will migrate to the other channel.

If it does not migrate, then the pot is okay.
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Old 8th April 2011, 02:42 AM   #9
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Thanks for the tips.
I have some orange drops in there now .05uF (I think. Wish they would put the value on the things.)
And the bass response is pretty good.
At this point most of the electronic success I have is purely accidental.
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