Simple SE guitar amp: Tuning the 'gain'.....

[Bill Ashton]

I am wondering if what you are putting together is more like a Vox AC4 than a Fender Champ. Somewhere, somewhere I have several old schematics for versions of the AC4...PM me your email, if I can find them I will send them to you.

 

[billyz]

Billy,

I'm at work and haven't had much time to study your latest contribution (thank you very much). But, in looking at it, I see what appears to be the 'bright' channel....the one with the 470K and 500pf coupling it to the next gain stage. HOWEVER, in looking at that section of the 12AX, it appears it's cathode resistor and Bypass cap are 2.7K and 0.68 respectively. So, that being true, is there some kind of 'compensation' going on where the gain between the two channels becomes roughly equal.....or does the 'bright' channel also carry more gain but does so simply at higher frequencies?

The only level compensation between those channels is the individual volume pots.
The Normal Channel is the one with the 330uf cathode bypass. Originally this circuit was meant to be used for a Bass Guitar. The Bright channel was for guitar. In those days you might have an entire band plugged into the same amp, even a vocal mic. There are two mixing resistors , the 470K's .

 

[triode17]

I've built a little Single Ended guitar amp. Pretty simple thing; one 6BQ5 for the output, with one 12AX7 as a driver. Given the two triodes of the 12AX, I actually have what I guess you'd call a 'series' driver, as the first section is feeding the section which drives the output tube. This is basically a 'spare parts' piece, with a little SE output transformer I pulled from an old 'Hi-Fi' amp made by 'US Audio'. I actually believe this to be an Asian company using the name of 'US Audio' as a USA Marketing ploy. None the less, it came to me with only one good OPT, so I swapped them both out and had this as surplus. I also had an old OPT that I was able to get a HV of about 250V out of after setting up the output section. I don't recall all the details, but I've got a 150K resistor with 100uf / 100V bypass cap, and as I recall I think the bias calculated out to be around 35ma on the output tube. So, moving on to the 12AX, I think I'm running around 120VDC or so on the plates of both triode sections. As, as per what I consider 'typical' Guitar amp 12AX cathode set-up, there's a 1.5K resistor to ground on each triode cathode, along with a 25uf/25V bypass cap.

So, now comes the question: Can I change the values of the cathode resistors on the 12AX section in order to 'adjust' the gain going to the output tube?.....and, if so, is 'more' resistance equal to 'less' gain? I guess I'm just thinking about output tubes here....where increasing a cathode resistor value would result in less current draw by the output tube....and reduce output power......or so I believe. I may be wrong....and if so, PLEASE correct me. Anyway...as my amp stands now, using the 12AX as the 'driver' tube.....it's just too 'gainy'! I've played around with other tubes, including a 12AT, a 12AY, a 12AV, and a 12AU. I found this substitution to be very revealing, as the 12AX provided way too much early gain, where the 12AU on the other end of the scale provided just enough gain to drive the amp to a very minimal distortion level. The AT and AY seem to be the sweet-spot for the amp.....but I'm simply wondering if I can go back to the 12AX and tame the amp through the circuit vs changing tubes? Perhaps the answer is to drop the voltage to the plates? Just looking for some advice here from someone who's maybe played around with this and has some experience with these kinds of issues.

Many thanks,

Tom D.

If you 'up' the cath. resistor, you are raising the Grid to Cathode voltage, and therefore the Q point at which the tube idles. So, you are in effect, lowering the sensitivity which the input signal works with.

 

[triode17]

The only level compensation between those channels is the individual volume pots.
The Normal Channel is the one with the 330uf cathode bypass. Originally this circuit was meant to be used for a Bass Guitar. The Bright channel was for guitar. In those days you might have an entire band plugged into the same amp, even a vocal mic. There are two mixing resistors , the 470K's .

Quote: "In those days you might have an entire band plugged into the same amp, even a vocal mic." Billy, that was me!!! I have a '66 Fender Vibro-Champ. My young band used this with a 4 channel Lafayette Mic. mixer, which used only one transistor and was noisy as hell. But, it worked at the time.

 

[billyz]

I still often use a fender pro reverb for the P.A., ( built reverb !).
Last night we used a Bassman head to drive a single 2x10 fender tweed extension cabinet for the P.A.
vintage shure 55 mics sound fat too.

 

[whoaru99]

Since it only takes one section of a 12AX to drive a 6BQ5 to max dissipation, why use two? How big is the signal input from the guitar? Could you just lower the 1st grid resistor to get the right input level? Is it already just right for what the guitar needs for input impedance? But why two sections of VA?

It is not unusual for some guitar amps to put multiple gain stages in series. You can get different tones by how much signal you push in each section.

I built one with this front end. Three gain stages in series. It can go from quite clean to really grungy. It drives a 7868 P-P output stage with a master volume between.

So, there is gain 1, gain 2, bass, mid, treble, and master volume.

 

[billyz]

Starting to look dumble ish.