switching channels

9am53

Member
2005-02-18 6:59 pm
Ontario
Hi, I have been working on my guitar amp for a while, and I finally have a great high gain overdrive sound I was looking form but I cant have any sort of cleaner tones for rhythm without loosing volume. The way I got the gain was by raising the plate voltages on my input stages to 190V from their prior settings of 65V and 145V. Can I create a second "Channel" for lower gain applications by having an alternate route for my input signal with lower plate voltages? ie have the 100k resistors for high gain and lets say 200k resistors to switch to?
 
If I understand you correct you want the distorted/overdriven sound to have the same volume as the clean without having to adjust the master volume when you switch between the two?
The way I do this is to have a voltage divider at the output of the preamp. The divider is switched in when overdrive is selected. This enables the pre to be overdriven and distort big time, but the volume level is pretty much constant.
Since a tone stack does the ame as a divider (in terms of bringing the original signal down in amplitude) you can also have a simple tonestack that you switch in when overdrive is selected. That way you may shape the overdrive differently than clean at the same time as you keep levels equal.
 
That is why in many amps with both channels there is a "post" control for the dirt channel but none for the clean. The GAIN control sets the distortion levels and overdrive, while the POST controls the volume of it. This is separate from the MAster VOlume. Post is just a volume control near the end of the channel.

I visit www.ampage.org daily, and I don't think ther is excessive attitude there. Any forum has its twits, but overall there is a lot of guitar amp knowledge there and a willingness to dispense it. Just go to the Guitar Amps section first. The Open Forum part does get into stupid stuff.
 

9am53

Member
2005-02-18 6:59 pm
Ontario
thanks guys, so my idea of lowering the plate voltages for a cleaner channel is not so good? is there anyway of controlling the gain while maintaining volume AND headroom? as it is the amp is just loud enough to practice with my band, but if I have to play quieter I will have to mic it, and that is kind of immasculating :xeye:
 
It seems you want the clean sounds to be louder and the distorted to be about where you have them, (volume wise)? In the first posting it seemed like you just wanted to lower the distorted sounds to be equal the clean.
Lowering the plate voltages will not lower the gain enough to be noticable. The lower plate voltage will result in a stage with less headroom and more distortion.
If you want less gain in the clean, you can switch in paralell resistors in the plate so as to lower the plate load and thus the gain. To keep the cathode current at the same levels you could at the same time switch in a series resistor in the cathode which will lower gain even more. But I dont think this is what you want?

It seems to me your amp is at it's limits and if you crank the clean any more up it will start to distort? In that case the only way to get the clean and the crunch to be at equal volumes is to lower the crunch. That means you use your amp for monitoring, and mic it up for all the rest to hear. Unfortunately that is the norm these days. It seems clubs are taking more and more control of the sound. Most bands are limited in how much they are allowed to turn up their amps, and instead everyone is mic'ed and a sound dude sits in the back with all the controls. Understandable that the clubs want the best sound for their customers, but it also ruins some bands since they often sound better when they are in control of their own sound.
 

9am53

Member
2005-02-18 6:59 pm
Ontario
hey thanks a lot! I do pretty much max the amp out, and that is why i get the overdrive (and why I blew an output tube yesterday:) but before I raised the plate voltage to 190 I was getting nowhere near the amount of gain I get now, so I thought I could lower the plate voltage to lower gain, but maintain headroom that way.. you're saying I should try to switch in a parralel plate resistor for the lower gain sound? should it be same as the other one? what should the cathode resistor be around...like 50K? or is there no way of telling without the circuit infront of you...? thanks for your help.
 
Check out
http://www.harmonicappliances.com/tonerator1/tonerator_i_schematic.pdf

for a schematic of a channel switching guitar amp with a clean channel and a dirty channel. It uses shunt switching to reduce transient (switching) noise. You can use relays (as this amp does), JFETs, LDRs (like Fender and Boogie sometimes do) or even BJTs to do this.

You can learn everything you ever wanted to know about guitar amp switching by buying and reading TUT1 and TUT2 from http://www.londonpower.com/. You can also get kits here.

You can also see how the "pros" do it by checking out guitar schematics. There are a number of sites where you can find these schematics. I like http://www.mif.pg.gda.pl/homepages/tom/schematics.htm -- it has all the usuals plus some schematics you don't usual see (Bogner and Soldano, for example).

Have fun!