Hi,
the problem is obviously intermodulation distortion.
Do the tubes point up or down?
The importance of guitar amp input circuits
Moderators: pompeiisneaks, Colossal
-
schaublin65
- Posts: 88
- Joined: Sun Jul 08, 2012 11:59 am
- Location: Ireland
-
gingertube
- Posts: 531
- Joined: Mon Nov 14, 2011 2:29 am
- Location: Adelaide, South Oz
Re: The importance of guitar amp input circuits
The 2 issues seem to be noise in high gain preamps and the frequency response particularly when the volume control an the guitar is wound down increasing the source impedance.
In high gain preamps the gridstop on the input stage can be a significant contributor to noise. Dropping it from the traditioanl 68K to 10K reduces the thermal noise from that resistor (which is proportional to the resistance value). This is normallly shunted by the guitars source impedance which is low except when you turn the guitars volume control down.
There are a number of frequency resonse issues.
The 68K plus the typical Miller Capacitance of the input stage will roll off the highs at around 25kHz. If you reduce the 68K to 10K then you need to increase the capacitance at the grid by adding a cap. The calcs say add 680pF to maintain the same roll off. I have use 100pF and 470pF as the added cap on various amps. The 100pF is more appropriate when you often use the guitar with the volume control turned down (that is, with a higher source resistance). Mostly we just want to make sure we have lots of attenuation at RF.
The gridleak resistor also has some implications. Guitar pickups often have peaky response at about 60kHz. The gridleak will tend to damp those peaks. 220K for Humbuckers and 500K for single coils works well but neither is good for passive XTAL pickups. You could also argue that we don't give a rodents hind quarters what is happening at 60kHz but it may be important with regards ovedrive characteristics (not really likely on the input stage) and with regard to stability.
Swings and Round-abouts, what helps with one issue will often compromise other issues. That is why I decided to go with Merlin's recommendations in the end, cause I was not at all confident I had my head around all the issues.
Cheers,
Ian
In high gain preamps the gridstop on the input stage can be a significant contributor to noise. Dropping it from the traditioanl 68K to 10K reduces the thermal noise from that resistor (which is proportional to the resistance value). This is normallly shunted by the guitars source impedance which is low except when you turn the guitars volume control down.
There are a number of frequency resonse issues.
The 68K plus the typical Miller Capacitance of the input stage will roll off the highs at around 25kHz. If you reduce the 68K to 10K then you need to increase the capacitance at the grid by adding a cap. The calcs say add 680pF to maintain the same roll off. I have use 100pF and 470pF as the added cap on various amps. The 100pF is more appropriate when you often use the guitar with the volume control turned down (that is, with a higher source resistance). Mostly we just want to make sure we have lots of attenuation at RF.
The gridleak resistor also has some implications. Guitar pickups often have peaky response at about 60kHz. The gridleak will tend to damp those peaks. 220K for Humbuckers and 500K for single coils works well but neither is good for passive XTAL pickups. You could also argue that we don't give a rodents hind quarters what is happening at 60kHz but it may be important with regards ovedrive characteristics (not really likely on the input stage) and with regard to stability.
Swings and Round-abouts, what helps with one issue will often compromise other issues. That is why I decided to go with Merlin's recommendations in the end, cause I was not at all confident I had my head around all the issues.
Cheers,
Ian