Tube PA conversion

[weedstoner420]

Now that I think about it, you'd probably run into feedback/oscillation issues doing it that way - having two series gain stages from the same channel sharing a cathode resistor (the 6SQ7 and both halves of the 6SC7 all share that one 470 ohm cathode resistor)...so maybe do as above, but also disconnect the 6SQ7 cathode from that 470 ohm and give it its own dedicated 1.5k cathode resistor...could also change the 470 to a 680 so the bias points of the 6SC7 stages stay roughly the same...

Lol, sorry for the multiple posts. I just realized the 68k feeding the 6SQ7 plate also goes to the 6SC7 plates...so the 6SQ7 will also need it's own dedicated plate resistor (100k should be fine). Here's my final draft, again hopefully you can make sense of it...

 

[Dreaming1]

No problem. Circuits are funny like that. Everything connected. Makes me think I'm smart, then makes me think I'm dumb, then makes me question if I know what is happening at all. I got your drawing.
I am going to hang out with this amp as it is for awhile. But, I will change it in near future.
2x12 had 8Ω speakers, so I will hook up the amp to switch 4&8Ω outputs. Try to tame the gain a bit with a pair of speakers. They are eminence swamp thangs. Low end heavy voicing. I was going to have a pair of brighter ones to complete the cab. So, I will probably use speakers in AC15 and twin reverb for comparison too.
I got it all together, now I just have to get back in there and record some so I can post a video.

 

[weedstoner420]

No problem. Circuits are funny like that. Everything connected. Makes me think I'm smart, then makes me think I'm dumb, then makes me question if I know what is happening at all.

Yes, 100%. And super rewarding when you actually get it working!

 

[Dorian2]

Great thread. Good luck with the amp!

 

[Dreaming1]

Cheap yamaha strat copy. Audix d6 dynamic jammed into the cone of an eminence swamp thang. LOUD! Too loud. Makes this 150w speaker squeal. Need to check the speaker leads again. Pull off the NFB and check that out.
62j7 channels are dirty. 1st ch makes throbbing sound if knob is turned up much past halfway. Ch 2 has a weird interaction with tone knob. Almost more gain, but could just be highs getting in? And when ch 2 knob is turned down all the way, there are pops and squeals.
The tone knob sucks. Doesn't do a lot, and doesn't sweep sound smoothly. Some parts I hear, some parts of the sweep I don't.
The phono ch is clean. It doesn't do anything until turned up past halfway. It also did not enjoy a digitech looper pedal into it. Cut off when I started loop, and came back on with a... slow rise in volume when I ended loop.
Both channels liked all my other pedals. Using a digitech mojo overdrive as a treble booster worked great! They both loved fuzzface and big muff type fuzz. Delay was cool. Makes me wish I had a spring reverb unit to run into it.
I'm playing it, cause it is so damn gnarly, but Im going to open it back up soon. I need to make a wooden brace to hold the chassis while it's on. And I have a twin reverb that probably blew some power tubes and a resistor to fix. So on to pcb stuff again. This was cool as hell.

 

[weedstoner420]

Cheap yamaha strat copy. Audix d6 dynamic jammed into the cone of an eminence swamp thang. LOUD! Too loud. Makes this 150w speaker squeal. Need to check the speaker leads again. Pull off the NFB and check that out.
62j7 channels are dirty. 1st ch makes throbbing sound if knob is turned up much past halfway. Ch 2 has a weird interaction with tone knob. Almost more gain, but could just be highs getting in? And when ch 2 knob is turned down all the way, there are pops and squeals.
The tone knob sucks. Doesn't do a lot, and doesn't sweep sound smoothly. Some parts I hear, some parts of the sweep I don't.
The phono ch is clean. It doesn't do anything until turned up past halfway. It also did not enjoy a digitech looper pedal into it. Cut off when I started loop, and came back on with a... slow rise in volume when I ended loop.
Both channels liked all my other pedals. Using a digitech mojo overdrive as a treble booster worked great! They both loved fuzzface and big muff type fuzz. Delay was cool. Makes me wish I had a spring reverb unit to run into it.
I'm playing it, cause it is so damn gnarly, but Im going to open it back up soon. I need to make a wooden brace to hold the chassis while it's on. And I have a twin reverb that probably blew some power tubes and a resistor to fix. So on to pcb stuff again. This was cool as hell.

Dang that sounds pretty good! I love the sound and feel of a simple no-frills tube amp. Glad it worked out for ya!

 

[Dreaming1]

This would have blown minds in 1954. Speakers and eardrums too. I ran it through a cheap peavey practice amps speaker first. It sounded cool and seemed to be less bothered, but I cant imagine that speaker was anywhere near 150w. I don't get it. A pair of 6v6 tubes and it is as loud as my twin reverb. My wife agrees and suggests it is too loud. I say "what?"
The gain on the mic pre's is too much and the phono ch is too low. Classic Goldilocks problem. I need to open it back up, but it is good fun for now. I will make a video of the sound problems when I get around to it. See if I can't figure some things out.

 

[weedstoner420]

This would have blown minds in 1954. Speakers and eardrums too. I ran it through a cheap peavey practice amps speaker first. It sounded cool and seemed to be less bothered, but I cant imagine that speaker was anywhere near 150w. I don't get it. A pair of 6v6 tubes and it is as loud as my twin reverb. My wife agrees and suggests it is too loud. I say "what?"
The gain on the mic pre's is too much and the phono ch is too low. Classic Goldilocks problem. I need to open it back up, but it is good fun for now. I will make a video of the sound problems when I get around to it. See if I can't figure some things out.

Haha yup, 20 tube watts is indeed quite loud! I've built a few music-listening amps using a circuit very similar to your phono channel - headphone jack from phone or laptop -> 6J5 gain stage -> 6SL7 PI -> 2x 6V6 power amp -> 8" full range speaker - and yeah they are plenty loud. Also the typical fender tone stack has a big mid-scoop that makes it "sound" quieter than something with more mid-range or a flat EQ.

 

[Dreaming1]

I'm about to open it up again. Put in the ohm selector switch and check my OT leads again. Wiring the switch gave me problems for a bit.
Im thinking about trying to reduce gain before I start redesigning too much. Maybe put in two 68k resistors as voltage divider before the coupling capacitor.
If that isn't it, I will remove it and start playing with resistor values. I saw someone say to remove the screen bypass cap and that would lower gain.
I read someone describe guitars in grid leak biased circuits as splatty. That's exactly what I get.
Here is a circuit I found that is another version of what I have...

After exploring this, I will cathode bias a channel and start learning some new stuff.
The adventure continues...

 

[Dreaming1]

Then I saw this which is a Gibson GA-5. A copy of a tweed champ. It only has an input resistor, so I guess I will give that a go too. Then play with resistor values.

 

[weedstoner420]

Hmmm, I can't comment on the overall sound of grid leak bias for guitar because I've never tried it. To me, "splatty" = too much gain across too few gain stages.

I wouldn't hesitate to change that 10M grid-leak resistor to 1M and put a 1k between the cathode and ground (basically changing it to cathode bias), and see what it does to the sound.

And re: putting in a voltage divider to attenuate signal, I'd put it after the coupling cap. If it's before the coupling cap it will affect the DC bias of that tube...

 

[Dreaming1]

Hmmm, I can't comment on the overall sound of grid leak bias for guitar because I've never tried it. To me, "splatty" = too much gain across too few gain stages.

seems that way. Attack clips hard and then the decay cuts off sharply. Like a transistor fuzz box doing the voltage starved velcro tearing sound.

I wouldn't hesitate to change that 10M grid-leak resistor to 1M and put a 1k between the cathode and ground (basically changing it to cathode bias), and see what it does to the sound.

Will do on channel 2. The more diverse sounds, the better.

And re: putting in a voltage divider to attenuate signal, I'd put it after the coupling cap. If it's before the coupling cap it will affect the DC bias of that tube...

Thanks. So much to learn... And Im the one trying to do it.

 

[Dreaming1]

What about in that circuit above? The signal is input resistor-capacitor-resistor to ground-tube grid. And they are saying it is voltage divider function. With a capacitor inside it? How does the cap function in that with relation to the tube?

 

[weedstoner420]

What about in that circuit above? The signal is input resistor-capacitor-resistor to ground-tube grid. And they are saying it is voltage divider function. With a capacitor inside it? How does the cap function in that with relation to the tube?

The cap is just there to block the DC voltage on the tube grid from getting to your guitar. The grid voltage is always gonna be negative with respect to the cathode - with grid leak bias, the cathode is grounded (0V) and the grid is at some small negative DC voltage (they call it -0.8V in that one schematic), and thus the cap is required. In cathode bias, however, the cathode is at a small positive voltage, and the grid is at 0VDC - still negative with respect to the cathode, but then the cap is not required to block DC.

The cap and 10M resistor to ground technically form a high-pass filter, but the resistance is so high that even with a .0047uF like cap in the first pic, the cutoff frequency is still way below 20 Hz (f=1/(2*pi*R*C))...

So the cap blocks DC but is basically invisible to audio frequencies, and then you're left with the 2 resistors forming a voltage divider. But it's not even that much of a voltage divider, like even with the 75k//5M combo, you only lose like 1-2% of the input voltage...so I'm not really sure why the smaller resistor is there...maybe it has some interaction with the impedance of the guitar pickups...? That's probably something where it would be good to experiment with different values for that first resistor (say 0 up to 100k) and see if any of them make a difference.

 

[Dreaming1]

The cap is just there to block the DC voltage on the tube grid from getting to your guitar. The grid voltage is always gonna be negative with respect to the cathode - with grid leak bias, the cathode is grounded (0V) and the grid is at some small negative DC voltage (they call it -0.8V in that one schematic), and thus the cap is required. In cathode bias, however, the cathode is at a small positive voltage, and the grid is at 0VDC - still negative with respect to the cathode, but then the cap is not required to block DC.

The cap and 10M resistor to ground technically form a high-pass filter, but the resistance is so high that even with a .0047uF like cap in the first pic, the cutoff frequency is still way below 20 Hz (f=1/(2*pi*R*C))...

So the cap blocks DC but is basically invisible to audio frequencies, and then you're left with the 2 resistors forming a voltage divider. But it's not even that much of a voltage divider, like even with the 75k//5M combo, you only lose like 1-2% of the input voltage...so I'm not really sure why the smaller resistor is there...maybe it has some interaction with the impedance of the guitar pickups...? That's probably something where it would be good to experiment with different values for that first resistor (say 0 up to 100k) and see if any of them make a difference.

"You know a little." Thank you.

 

[weedstoner420]

"You know a little." Thank you.

Hah, I do like to get down into the nitty-gritty...just don't give me anything with transistors in it, those things still make my head spin.

 

[Dreaming1]

I haven't changed anything yet. Just exploring. Hooked up to a pair of 12" speakers with 4Ω impedance. Running this way altered the amp channels. Changing the amps impedance setting had more effect with the 8Ω speaker load than the pair.
I run a lot of pedals in front of it too. Makes the channels better. So I think an input resistor might calm the gain down.
I bought a nos (if anyone can be trusted with money) 6sj7's and a 6sc7. Sylvanias and a Raytheon. I am about to get set up to test it while live. Then I can check voltages and current and start playing with resistor values.
Saw some info about screen coupling caps should be tied to cathode instead of ground. I will check that out too.

 

[weedstoner420]

Saw some info about screen coupling caps should be tied to cathode instead of ground. I will check that out too.

I just googled this and the first result was from Merlin Blencowe's page. Designing Tube Preamps for Guitar and Bass is definitely one of my primary sources for info on this subject...

 

[Dreaming1]

Got some tubes in yesterday. Pair of 6sj7's and I grabbed a 6SL7. Maybe the 6sc7 will arrive today. Going to enjoy some tube rolling and then have spares.

Update: The original tubes test "good" by operating at all. These new 6sj7 are probably stronger. Ch 1 cleaned up some. Ch2 didn't.

 

[Dreaming1]

Life has kept me busier lately. I got back into the guts of this amp. Peeled back to a single 6sj7 into the phase inverter. A lot nicer. Still getting crunchy around 3 o'clock on the volume knob. Backing volume down on guitar is cool. Might explore a tone bleed on my strat.
Im going to try reducing input signal. Then I will mess with the screen and plate values. I may leave it as is for ch1. Will set up ch 2 as cathode biased and compare. It sounds old. I dig it.
Then I have the 6sl7,6sq7,and 6sc7 tubes to play with. Getting interesting now. I will post some a/v later.