Guitar Amplifier Hum Varies with Volume Knob - Page 2 - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th September 2013, 11:46 AM   #11
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Don't feel alone. This is what all amp builders face: "optimal brightness" sound versus volume control noise between rotation extremes. I'm no electronics expert but I have messed around in this area a lot.

What's the guitar's volume pot value? This is often worst when the volume pot resistance is much more than the pickup impedance and the amp's input impedance. You can often just throw in a 250K pot in the guitar and it may go away, but you may lose treble too as the pickup's inductance becomes more relevant to driving into the lower impedance.

It has to do with the input impedance of the amp and the value of the guitar's volume control. You can reduce the value of the 1m resistor and add a series grid resistor and install a lower-value volume pot in the guitar and the problem will usually disappear. You can add a resistor from the volume pot wiper to ground; you can even add a series resistor in the guitar. But you will lose some treble "sparkle" from your pickups (from adding resistance to ground and changing the load the pickups see) and/or some drive pushing that first tube (from adding series resistance). Pickups with a lot of turns of thin wire change sound a lot depending on the load they're pushing into.

It's a tradeoff. Stock guitars come with 250K or 500K pots, but people install 1M or even 2M pots to get the pickups brighter...and live with this volume control problem...after all you can always cut the treble later if it's too bright and some people just hate to lose any treble in the first stage. But when the hot wire to the amp is not loaded by either the pickups or the 1M to ground or the guitar's volume pot, the cable and guitar wiring and pickups become like a big antenna and it seems like the gain of the amp also goes up. Make sure the pickup coils are wired with polarity so that the outside of the coil is the ground side (or closer to ground in the second coil of a series humbucker), make sure all wiring is shielded, shield the wiring cavity. In desperation with single-coil hum, you can try the pickup coils electrically out of phase so they cancel hum and then you have to decide which way you want the magnets. But all that might reduce hum when the volume is full but not affect this off-full problem.

I'd try another 1m resistor from the volume wiper to ground in the guitar and see if it's any better. You can also add hidden screwdriver-pots for that 1M to ground and a hidden screwdriver pot for a series grid resistor, and tune your amp's input to match the individual guitar. That's my favorite fix. You have it wired like some Mesa amps; those had the same off-full volume problem.

You have only one input jack, and no network for mixing two inputs, and no series grid resistor, and a large 1m value for setting the input impedance . Your input will be really bright and not load the pickups, which probably sound great, but it will hum when the guitar volume pot has the input floating far from the ground and far from the pickup load.

Another fix is a push-push switching pot on the guitar, wired to add a resistor from wiper to ground. Some people really like push-push pots on the volume and tone to completely disconnect them from the circuit, then they put a volume pedal in the effects loop.

Good luck; the best thing you can do is experiment, and also try different guitars. This is an area where experimentation or adjustability makes a big difference, and where the guitar and amp have to work together, but individual guitars are very different from each other.

Last edited by cyclecamper; 7th September 2013 at 11:50 AM.
  Reply With Quote
Old 7th September 2013, 12:01 PM   #12
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
If the hum gets bad at the mid-point of the volume control, it would seem to indicate that the problem is located AFTER the volume control. You may need to use shielded cable between the volume control and the input of the first stage following the control.
__________________
Frank
  Reply With Quote
Old 7th September 2013, 07:04 PM   #13
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Yes...the symptom is at the GUITAR volume control, not the amp volume control. So you're talking about the instrument cable which is always shielded, but even more the resistors at the first grid. And thinking more about my recommendation to try a resistor from the pot wiper to ground, that's not as good as the same resistor at the amp, and you'd have to experiment with values as my recommended starting point might not do anything at all.
  Reply With Quote
Old 7th September 2013, 07:15 PM   #14
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
I'm not even sure I fully understand it; the gain seems to go way up when there's no load on the input. Adjusting the grid series resistor and parallel resistor to ground make the problem go away but often at the expense of pickup brightness, or lowering the value of the guitar's volume pot will make the problem go away but again often at the expense of pickup brightness.
  Reply With Quote
Old 9th September 2013, 07:40 AM   #15
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
But he has a hum problem, not a tone problem.
  Reply With Quote
Old 9th September 2013, 03:07 PM   #16
diyAudio Member
 
Join Date: Jan 2007
First off, I'd like to thank everyone for the great discussion! There are a lot suggestions, so it may take a while for me to work through the different implementations.

To clarify, the amplifier - as it stands - has no controls of any kind. The only adjustable parameter is the volume knob of the guitar (stock Squier 60s Telecaster with a 250k potentiometer). As the amplifier is currently undergoing surgery, the guitar cable simply plugs into a free-floating input jack with 12" alligator clip leads connecting signal and ground to the rest of the circuit (there is obviously a lot of room for improvement here with regard to layout). The amp is effectively hum-free at maximum volume and minimum volume, but hums very loudly as the 50% volume position is approached.

To address a few of the suggestions:
- as is to be expected, paralleling the cathode resistor with a large capacitor increases overall gain, but doesn't otherwise affect the action of the volume control
- orienting the guitar differently doesn't substantively make any difference and there is no fluorescent lighting in the vicinity to turn off
- replacing the 1m resistor to ground with a 100k one didn't make a noticeable change of any kind, but it did make it easier to add a series capacitor between the guitar and grid to help make the low-end less muddy

I'll keep everyone posted on any progress. Thanks again for the healthy discussion and suggestions!



-Nick
  Reply With Quote
Old 9th September 2013, 03:24 PM   #17
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
You eliminated the series grid resistor. That has some advantages for tone. But it's the combination of that missing 68K grid resistor in the amp and the 1M to ground that can make problems with the guitar volume pot value and pickup impedance.

Sooo...did you try a series grid resistor with that series capacitor?
  Reply With Quote
Old 9th September 2013, 03:35 PM   #18
diyAudio Member
 
Join Date: Jan 2007
It would be easy enough to try, but if you imagine the internal elements of volume pot set to 50% rotation as part of the amplifier circuit - which they very much are - you already have a 125k series resistor feeding a 125k resistor in parallel with the fixed grid resistor.
  Reply With Quote
Old 9th September 2013, 03:38 PM   #19
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Quote:
Originally Posted by Enzo View Post
But he has a hum problem, not a tone problem.
Exactly, it's a trade-off and he's at the "good tone" extreme. He has very nice bright tone and plenty of gain even potential for a bit of dirt at the first stage because he has no series resistor. He has no tone problem and the amp is not a difficult load for the pickups to drive, mostly just the capacitance of the grid and into a load like that the pickup inductance does not cut the treble and he has plenty of gain. But now the volume pot is the major load on the input, way out there at the end of the cord. When he turns the guitar's volume way down, it short the input, and there's no noise. When he turns the volume pot on the guitar way up the pickups are an impedance to ground, and there's little noise. If you watched impedance of the guitar, or even just the resistance of the guitar at its jack, it goes down at the extreme positions of the volume pot and up in the middle. That difference is amplified. If he limits the gain and lowers the input impedance at the amp, it's harder for the guitar to drive and might affect tone, but the guitar volume pot works without hum when turned just less than full.
  Reply With Quote
Old 9th September 2013, 03:41 PM   #20
diyAudio Member
 
Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Quote:
Originally Posted by SlunkBoy View Post
It would be easy enough to try, but if you imagine the internal elements of volume pot set to 50% rotation as part of the amplifier circuit - which they very much are - you already have a 125k series resistor feeding a 125k resistor in parallel with the fixed grid resistor.
Yes, but that puts it at the end of a long cord. You want it in the amp instead.

And...what is the value of your volume pot in the guitar? It usually says right on them.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Amplifier volume knob noise SatinMill Solid State 29 22nd September 2011 02:26 PM
Guitar amplifier PSU hum woes Arnulf Tubes / Valves 33 7th January 2011 05:01 PM
Amplifier Gain Varies with Frequency gni Solid State 12 8th November 2007 08:16 PM
Volume knob marob Multi-Way 9 15th April 2006 02:13 PM
volume knob Nejc_car Everything Else 3 22nd February 2006 04:25 PM


New To Site? Need Help?

All times are GMT. The time now is 06:40 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2