Octal 6g3

[jbones1]

Hey guys,
So I recently got around to building this project. I've made a couple of modifications from the earlier design (see layout). Also a couple of things from the layout below have changed: I swapped out the 220K V1 plate resistor for a 100K as per the previous comments, and I also swapped around some of the bias circuit resistors to get the bias dialed in.

At any rate, the amp works and passes sounds fairly well. The problem so far is that the output volume is a lot lower than I expected. Its about equivalent to a small champ circuit I built a couple years ago if not a bit quieter (though the same 50w speaker). I tone is also kind of muffled, flabby, and fairly heavily distorted from about 2 on the dial on up... :/ Hmm. I was hoping to get 30-40 watts out of this amp, and have it be able to keep up with a loud band easily. Got a ways to go on that front as it stands!

Here are all the tube pin voltage readings:

V1 (6sl7)
1. 1mv
2. 191.7v
3. 1.65v
4. 3mv
5. 183.1v
6. 1.54v
7. 3.2vac
8. 3.2vac

V2 (6sc7)
1. NC
2. 243v
3. 16.7v
4. 17v
5. 237v
6. 23.6v
7. 3.2vac
8. 3.2vac

V3 (6l6gb)
1. 168mv
2. 3.2vac
3. 407v
4. 407v
5. 38.5v
6. 38.5v
7. 3.2vac
8. 46mv

V4 (6l6gb)
1. 65mv
2. 3.2vac
3. 403.2v
4. 406.8v
5. 38.5v
6. 38.5v
7. 3.2vac
8. 45mv

I'm getting about 720vac from the power transformer which rectified down and filtered is giving me 425v before it hits the power tubes. I have the bias set at 45mv at 410v = 18watts per tube?

Again, I am a rote beginner at some of this stuff although I do have a few kit builds under my belt! I'd really like to get this thing running better and at a better volume! Any thoughts?

[jbones1]

[loudthud]

Are the connections on the output transformer primary correct? Usually red goes to B+, blue and brown go to the output tubes. Check the ohms between the output tube plates and then between each plate and B+. Between plates should be maybe 300 ohms and maybe 140/160 from each plate to B+.

Connections to the impedance switch are unclear. It looks like the green and yellow wire get shorted by the switch. Small cap on tone control should be 500pF, not 50pF. I'm still not through the whole thing...

Tip: Ground pin 1 of the 6SC7.

[jbones1]

Are the connections on the output transformer primary correct? Usually red goes to B+, blue and brown go to the output tubes. Check the ohms between the output tube plates and then between each plate and B+. Between plates should be maybe 300 ohms and maybe 140/160 from each plate to B+.

Connections to the impedance switch are unclear. It looks like the green and yellow wire get shorted by the switch. Small cap on tone control should be 500pF, not 50pF. I'm still not through the whole thing...

Tip: Ground pin 1 of the 6SC7.

The OT wiring was going to be my next step. Looking at the wiring diagram that cam with the OT the Red wire is in the center Blue on top brown on bottom. The blue and red wires have dots by them.


Ok so I swapped the OT leads around (I'm using Webber's 1484 silvertone OT btw).

Red to B+ Blue and brown to the tubes.

Now I have way less output than bafore. :/
B+ to tube is 86ohms. Tube to tube reads 175ohms.

[loudthud]

Red to B+ Blue and brown to the tubes.

Now I have way less output than bafore. :/
B+ to tube is 86ohms. Tube to tube reads 175ohms.

That should be correct. Now, to prevent oscillatons, disconnect the 56K from the speaker jack until the amp starts working better. What kind of switch is being used for impedance selection? If you are unsure about it's connections, disconnect either the yellow or green wire. Tape up the disconnected wire to prevent a short to ground. One position of the switch should connect the remaining wire to the speaker, you should hear a little hum or hiss. Don't play through the amp unless you are sure a speaker is connected.

The voltages look good. But there is a problem in the layout.

Remove the 0.022uF cap across the 220K resistor connected to pin 2 of the 6SL7. That should clean up the muffled sound. If there is not enough gain (crunch), move the 0.01uF cap from the junction of the 100K and 15K to the side of the 100K connected to pin 5 of the 6SL7.

If the sound is not crazy loud, turn the power off connect a guitar cable to the input jack. Check the ohms from the tip of the guitar cord to pin 1 of the 6SL7. It should read 68K ohms. From the tip to the ground of the guitar cable should read 1 Megohm. If these are not correct, the input jack is not wired correctly.

[jbones1]

Ok let me restate that.
Red to brown or red to blue is 87.6ohms
Brown to blue is 175.6ohms

These are values when not connected to anything.

[loudthud]

Ok let me restate that.
Red to brown or red to blue is 87.6ohms
Brown to blue is 175.6ohms

These are values when not connected to anything.

IT's correct!

[jbones1]

Ok sorry we were posting at the same time.
Input resistances look good.
You are talking about the .022 resistor on pin2 correct?

[jbones1]

Ok I removed the 56k from the speaker jack and hardwired the OT to the jack. The switch is completely out of the circuit. Also removed the .022 cap from pin 2 of v1. Still no joy. It passes signal but at whisper quiet volumes.
Bummer.

[loudthud]

The voltage on pins 5 and 6 of the 6L6 tubes should be negative. Is this correct?

What are the voltages on the four pins of the filter capacitor?


I suspect the signal is shorted to ground or the wrong value resistor has been used somewhere. Check ohms to ground (power off). Pin 4 of 6SL7 should be 220K with volume control all the way down and 1.22 Meg (+/- 20%) with volume all the way up. Pin 1 of 6SL7 should be 68K with no guitar cable plugged in and about 1 Meg with a cable plugged in but not plugged into a guitar. Ohm from the "A" wire to the "B" wire. It should be 440K ohms. Ohm from pin 5 of the 6SC7 to pin 2 of the 6SC7. It should be 182K ohms.

[jbones1]

Thank you so much for the help with this, it is truly appreciated!

I had to go answer the dinner bell, but I will check out those Ohm readings and report back tomorrow.

On thing noticed while poking around the speaker jack earlier is that all three leads on the output side of the OT (4ohm tap, 8ohm tap and ground) seem to have reference to ground. When they are disconnected from the circuit they show up as a closed loop to the chassis on my meter. Is this normal?

-Jon

[loudthud]

The resistance of the speaker windings inside the transformer is very low. Usually too low for most meters to measure, a few 10ths of an ohm.

[jbones1]

Here is a gut shot for those of you following along at home

[jbones1]

IT LIVES!!!

So, I went ahead and did all the troubleshooting as suggested by Loudthud. I finally stumbled across a silly little jumper that somehow ended up between points "A" and "B" on my schematic. When I double checked it against the original Fender schematic low and behold it wasn't supposed to be there. Was reading 0ohms instead of the 440k it should have been. Hmm... not sure how that happened. Pulled the jumper out and the amp came to life!

At any rate I now have WAY more than enough volume out of this little beast and I couldn't be happier. Again, big thanks to Loudthud to walking me though the troubleshooting!

So....

Does anyone have any advice about how I might be able to bring down the gain / increase headroom a bit? The distortion coming from the amp is absolutely beautiful, however it starts to come on about 2 on the dial and tends to break into gnarly feedback when turned up to usable "band" volumes. After all the problems with this circuit this week it seems to be a nice problem to have at this point...

[loudthud]

Headroom is kind of a subjective thing to someone without a strong technical background. A guitarist tends to think of it as how far you can turn the volume control up before you get audible distortion. In the technical sense, the position of the volume control doesn't matter, it's how big the signal at the input is when you get distortion. One of my suggestions was to move the 0.01uF cap that was connected to the 15K and 100K resistors and to pin 3 of the 6SC7. Did you do that? Moving it back to where it was will allow you to turn the volume control up farther before distortion. That circuit acts like a master volume control. This change won't make the amp any louder (true headroom), it will just change the position of the volume control.

If your Volume control is a linear taper, that will change where distortion occurs as you rotate the pot. You want an "Audio" or "Log" taper pot. Many pot manufacturers call this the "A" taper, linear pots are called a "B" taper. The way to tell for sure is to rotate the pot half way between full on and full off. Now measure the resistance from the center terminal to each end terminal. If the resistance is about equal, it's a linear pot. If one side is 3 to 10 times the other, it's a log or audio taper. In this circuit you don't need to disconnect any wires from the pot to do this.

Edit: Forgot to mention the 56K that goes to the speaker jack. When you re-connect it, the power amp may oscillate. You may or may not be able to hear a very loud tone in the speaker, prehaps just really bad sound from a guitar. The cure is to swap the blue and brown wires going to the output transformer. Another way to accomplish the same thing is to swap the "A" and "B" wires between the board and the 6L6 tubes. When properly connected, the 56K resistor will reduce the voltage gain of the power amp meanning you will have to turn the volume control up farther to get the same volume.