Guitar amp idea

[Naterbater]

Why not just start with one of the recommended configurations and then go from there?

I have tried this. The problem that I am having is that the voltage drop across the bias resistor is not as high as the data sheet indicates that it should be. That's why I have been experimenting with different configurations.

It exactly mans that tehegrid must be negative compared to the cathode. If the cathode is at e.g. +4V and the grid at zero (as it should be when "leaked" to ground via a high-value resistor) it's exactly what is needed.

My problem is that I am not getting anywhere near the correct voltage. If I follow the typical setup on the back of the tube data sheet, I come up with less than 1 volt positive on the cathode.

You should have zero at the grid. If you measure different, you made in mistake either in your implementation or in your measurement.

Of course I have zero at the grid. I am trying to understand what I am doing wrong. Measurement on the cathode comes from ground to cathode. Measurement on grid 1 comes from grid to cathode.

Think it over.

Helpful. Something wrong with the second grid?

 

[abbey road d enfer]

. Measurement on grid 1 comes from grid to cathode.

That is your mistake.
When you introduce your meter between cathode and grid, the cathode voltage leaks into the grid circuit, which is a very high impedance; it disturbs the operating point.
You must measure the cathode voltage in reference to ground.

 

[Naterbater]

If you look back at my comment, I was measuring cathode voltage to ground. I was also measuring grid voltage to cathode separately. If this is unneeded I will not measure it.

I did some more learning on my end and I was able to figure out how to plot the bias point on the graph. My 12av6 is biased a little weird according to the graph, but its in the realm of possibly working I'm going to let it ride. The 12au6 graph is throwing me off though. If I have 200v on the plate with a 100k plate resistor that gives me 2mA on the vertical scale. If I draw a line from 200v to 2mA it doesn't give much of the useable curve to work with. What should I be aiming for on the 12au6 graph?

 

[john12ax7]

One thing to keep in mind with tube curves is that the plate voltage is the actual voltage at the plate, not the B+ supply voltage.

Are you running it in triode or pentode connection?

 

[Naterbater]

I am running it in pentode connection.

 

[abbey road d enfer]

If you look back at my comment, I was measuring cathode voltage to ground.

That's all you need to measure.

If I have 200v on the plate with a 100k plate resistor that gives me 2mA

Wrong assumption. Ohm's law. I=V/R, where V is the voltage across the resistor, not the voltage across the tube.
The V you need to put in the equation is (B+ -Va)

 

[Naterbater]

Success! I plotted a bias point for the 12au6 and just used the values I came up with. After that I ran power to the output transformer and the power tube. It was biased cold to begin with, but I hooked up a guitar anyway and it made noise! Check out the video. Obviously I have to finish putting things together, bias the output tube, and do some tuning but I think it has potential to sound good. Ignore my playing. Hopefully this project will give me the ambition to practice more.

 

[Naterbater]

Ok, well I biased the output tube to about 75% at idle and I wasn't quite happy with the output volume so I started experimenting. I plugged in hundreds of different combinations trying to figure out the right one for the sound I was going for. I didn't really look at the bias graphs, I just went by what my ear liked. I ended up raising the plate resistor on the final preamp so that it would send more signal on to the output. At the same time, I lowered the plate on the first stage be cause it was driving the 12au6 into full distortion by about half way through the potentiometer travel. Of course I also experimented with bias resistors, coupling capacitors, bypass caps, etc. In the end I think it came out sounding amazing! It is dead quiet when cranked full volume and loud enough to be uncomfortable when you play.

I went with two gain/volume pots, one after each of the preamp stages. It does nice clean sounds with the gain turned down on the first stage. It also does really nice crunchy overdrive with the gain maxed out. The second pot acts as a master volume. It allows me to drive the second stage (12au6) into distortion without blasting the output at full. The overdrive distortion tone suffers a little when the master volume is turned down, but will allow me to play without blasting the kids out of bed every time I want to dial in some crunch. As far as tone is concerned, the only thing I wish it did better is the transitional breakup distortion. Clean sounds good and crunch sounds good, but the mid gain half clean half dirty blues tone can sound a little staticky. I could probably do more if I were to build a tone stack, but I don't think I'm going to do that on this build.

Does oscillation hurt the tubes or other components? Sometimes when both the pots are maxed, the output will lightly oscillate. If I scooch the master volume (or the first gain stage for that matter) off the top rung just a hair it goes away. I actually don't mind it. It has a dirty tremolo effect that sounds nice on some single note picking.

In the end I am really happy with the way this turned out. I have attached a copy of the full schematic. Feel free to point out errors or areas I can improve. Thanks for all your help.


Edit- I am leaving this morning for a family Christmas trip, but when I get back I will finish assembling the chassis in the enclosure and I will do some proper recording in the studio to give you all some sound samples.

 

[musipol]

If you're not shielding the input to the 1st stage and maybe to and from the 1st vol pot, I would suggest it. Should help with regard to the oscillation.

 

[Naterbater]

I could do that. Will it harm anything if I leave it? It only does it when both pots are fully cranked and if it's not hurting anything then I'd just as soon leave it as an effect.

Next question, I have ordered 2 sets of knobs that say they are 1/4" D shaft, but but when I try to install them on my pots they are too small. I want something that resembles old television tuners. Are there any out there that have actual 1/4" D shafts?

 

[musipol]

Honestly, whenever I ended up building an "oscillator" unintentionally, I have always fixed it. It was usually layout related. That being said, tubes will put up with a lot of abuse before giving up. If you're not over dissipating the power tube when it is oscillating.......maybe it's ok. I don't know if the output XFMR would be harmed?

 

[scott2000]

I have ordered 2 sets of knobs that say they are 1/4" D shaft, but but when I try to install them on my pots they are too small. I want something that resembles old television tuners. Are there any out there that have actual 1/4" D shafts?

Do you have a picture of the knob style you're after?
Is there a datasheet for your pots?

 

[Naterbater]

They are just 500k pots with on/off switches mounted to the back. No data sheets, but I might be able to dig the boxes out of the trash bin. I know they said 1/4" D shaft on the packaging.

 

[scott2000]

If you're not against set screws, getting knobs for 1/4" smooth shafts could open up more options too.

 

[Naterbater]

Do you have a picture of the knob style you're after?

The set screw might work. These are the ones I have that I wish would work. I will try to drill them out and add a set screw.

 

[abbey road d enfer]

The set screw might work. These are the ones I have that I wish would work. I will try to drill them out and add a set screw.

You shouldn't have to drill the knobs, unless they are 6mm D-shaft. There are many knobs with a 1/4 " D-shaft imprint.
Can you measure the shaft of your pots?

 

[Naterbater]

I ended up drilling out the knobs and tapping some set screws. It worked out great. I will attach a couple of completed photos and a couple sound clips. I am not a good guitar player, but I think the amp sounds pretty good. Keep in mind that this thing has a 6 inch speaker and probably less than 2 watts of power.

 

[abbey road d enfer]

I will attach a couple of completed photos and a couple sound clips.

What guitar is it? I can't figure out. Neither Gibson nor Fender as much as I can tell...

 

[Naterbater]

Fender Strat style copy made by a company called Fretlight. It's a learning guitar with led lights embedded in the fret board. When connected to a computer and running their software it will light up the fingerings for chords, scales, and eventually entire songs. It was a nice idea, but it didn't work great for me. I found rocksmith to be a better learning tool.

Clean clip was single coil middle pickup. Distorted was bridge humbucker.

I have owned some nicer guitars over the years, but when I realized that I was not going to be a good guitar player I sold them. Currently I have a Yamaha acoustic, a Squier P-bass, and the Fretlight strat.

 

[abbey road d enfer]

Clean clip was single coil middle pickup.

Funny how it doesn't sound like a Strat. Most Strat copies sound like like a Strat, even more so than some real ones.