Hey Trout,
I am looking at your original scematic with the red and pink markups.
Look at the first stage of the second 12AX7...
The 1500 Ohm cathode resistor has 2.05 volts across it; this means it has 0.001367 Amps flowing thru it (= 2.05 / 1500).
If we assume that this same current is flowing through the plate and the 100,000 Ohm plate resistor; there should be a 137 Volt drop across the palte resistor (= 100,000 x 0.001367) THERE'S THE PROBLEM; you have only a 4 Volt drop. Replace the 100K resistor... it is not 100K!
If it was 100K you would see around 140 Volts at the plate (= 277 - 137).
Either this or the 1500 resistor is bad... doubtful because all the voltages and currents mapped on to the tube curves suggest the tube and the cathode end of the circuit is operating the way one would expect.
I am looking at your original scematic with the red and pink markups.
Look at the first stage of the second 12AX7...
The 1500 Ohm cathode resistor has 2.05 volts across it; this means it has 0.001367 Amps flowing thru it (= 2.05 / 1500).
If we assume that this same current is flowing through the plate and the 100,000 Ohm plate resistor; there should be a 137 Volt drop across the palte resistor (= 100,000 x 0.001367) THERE'S THE PROBLEM; you have only a 4 Volt drop. Replace the 100K resistor... it is not 100K!
If it was 100K you would see around 140 Volts at the plate (= 277 - 137).
Either this or the 1500 resistor is bad... doubtful because all the voltages and currents mapped on to the tube curves suggest the tube and the cathode end of the circuit is operating the way one would expect.
Oops forgot,
The plate resistor has alot to do with the gain of that stage... the more resistance; the more gain... to a point.
good luck and lemme know,
The plate resistor has alot to do with the gain of that stage... the more resistance; the more gain... to a point.
good luck and lemme know,
Hi Gene,
About the pots. Why not just use a couple fixed resistors for each one temporarily? At least you can eliminate that as a problem. Do you think they may be leaky to ground?
-Chris
About the pots. Why not just use a couple fixed resistors for each one temporarily? At least you can eliminate that as a problem. Do you think they may be leaky to ground?
-Chris
Power supply point are now at:
A- 422V
B- 382V
C- 259V
Pins 3 & 8 are tied together so the read the same 1.79/1.80 floats 1/100th.
2nd 12AX7
Pin 3- 1.97V
Pin 8 - 44.5?
Somehow that doesnt seem right? I re-checked several times.
Maybe thats where the problem is.
I am going to check out that 100K resistor, Hey, anythings possible.
10000 + 4000 tubes! Uh-Oh its starting. At this rate you will catch me in a couple of years.
Un heated garage, just wire enough tubes in series to add up to 120 volts, and plug them in. I am sure you have some big ugly tubes that you will never use! I know that I have got plenty.
Un heated garage, just wire enough tubes in series to add up to 120 volts, and plug them in. I am sure you have some big ugly tubes that you will never use! I know that I have got plenty.
Hi Gene,
I'm wondering about the first stage, second tube as well. It looks like 1/2 mA in the plate circuit, and 1.3mA in the cathode. Any chance the cathode resistor is mismarked or defective? The plate resistor could be a low value, like 39K.
-Chris
I'm wondering about the first stage, second tube as well. It looks like 1/2 mA in the plate circuit, and 1.3mA in the cathode. Any chance the cathode resistor is mismarked or defective? The plate resistor could be a low value, like 39K.
-Chris
anatech said:
3 words,
OH MY GOSH!!
Poobah, Can I buy you a beer or three?
In Fact, All you guys!
I pulled out that 100K resistor, It might have Brn-Black-yellow-gold bands on the outside, In fact It IS. But it measures only 1700 ohms! Truly amazing, That was a fresh piece from an Ohmite organizer.
I guess I better check each piece as I pull them from now on.
I put a fresh 100K in, nearly blew the walls down.
So, anyway, after about 10 minutes of plucking, I noticed that I get Way Way more bass than I could ever use. Is there an easy way to lower that a bit? or am I just asking for another headache.
BTW, With the 6L6GC's in here, I am guessing easily over 20Watts. Its to loud? haha My age is showing.
No honestly, I would like to dump a bit of the apparent overdrive and reduce the bass a bit. This thing will ruin my old celestions pretty quick.
I prefer a bit cleaner output, I hit about 20% volume and the overdrive/distortion gets a bit beyound my tastes.
Gene
Oh,, Did I say you guys are awesome???? I know I have been a pain in the,, Well you get the picture.
Well now it's time to check the resistors in the other dead amp.
The simplest way to kill bass is to reduce the value of the coupling capacitors. Try something smaller than .1 for the caps connected to the grids of the 6V6's. .047 or .022
To reduce the gain, try substituting either of the small tubes with a 12AU7, this may kill too much gain though. There are dozens of tubes with the same pinouts as these tubes with different gain and sound characters. I will swap preamp tobes around in a guitar amp depending on the sound I want at the time. In fact with your big transformer, you can swap output tubes around also. If you want to try EL-34's you have to ground pin 1 of the power tube sockets. It can stay grounded with all of the tubes.
The simplest way to kill bass is to reduce the value of the coupling capacitors. Try something smaller than .1 for the caps connected to the grids of the 6V6's. .047 or .022
To reduce the gain, try substituting either of the small tubes with a 12AU7, this may kill too much gain though. There are dozens of tubes with the same pinouts as these tubes with different gain and sound characters. I will swap preamp tobes around in a guitar amp depending on the sound I want at the time. In fact with your big transformer, you can swap output tubes around also. If you want to try EL-34's you have to ground pin 1 of the power tube sockets. It can stay grounded with all of the tubes.
Hi Gene,
Another easy way to bleed of some apparent gain is to tap the cathode resistor and take the cap down lower. Or it could be as simple as using a 47K resistor to the input jack and change to a 50K pot. Either way you can play with it and get the sensitivity you want. So you like the 6L6's I guess.
To roll the bass off, I'm with tubelab. Again you'll have to play with the values. You could also reduce the value of the cathode bypass cap.
I'm happy this one's not burning your tail feathers any more.
-Chris
Another easy way to bleed of some apparent gain is to tap the cathode resistor and take the cap down lower. Or it could be as simple as using a 47K resistor to the input jack and change to a 50K pot. Either way you can play with it and get the sensitivity you want. So you like the 6L6's I guess.
To roll the bass off, I'm with tubelab. Again you'll have to play with the values. You could also reduce the value of the cathode bypass cap.
I'm happy this one's not burning your tail feathers any more.
-Chris
anatech said:
Oh This is such a releif, I planned on building a few of these. I have all the trannies and stuff.
I was looking on the web for possible known tweaks for this model and came accross thisschematic
I kind of like the idea of 1 bright & 1 normal input, So, I am going to try the .022 cap on pin6 of the first 12AX7.
I just wonder if I need to swap out the .1 to .022 in the second 12AX7?
Just for the sake of trying it, I put in the 6V6's to see what effects I would get. No real difference other than the bass seemed a bit less annoying.
Oh Boy, Can My OPT handle that? I have a matched quad sitting here of J&J's 😀
Gene
Hello Trout (Gene),
Glad I could help.
If you are using carbon comp resistors; let me tell you how to "age" them, so things will work right in the future. The carbon comps need to go thru "temperature cycling" to stabilize their value. So... buy a whole big bunch and put them in your garage. Then leave them there for 40 or 50 years. Then, measure each one and repaint the stripes. Then, put them in the garbage. Carbon comps were bad in their heyday... there only worse now... there is no market for them... so who will build good ones?
Seriously, Mouser, Digikey, Newark and others have "metal film" resistors you can buy for cheap. Order with normal ground shipping and you'll still spend more on shippng than parts. All the B.S. about the "sound of carbon comps" is just that... B.S. Carbon comps are noisy.. and it ain't the kind of distortion you want... do you leave the room when the ole lady decides to vacuum the rug?
BTW... I looked at the voltages on the second stage of the second 12ax7... seems that something wasn't right there as well. Does the amp sound good now?
Glad I could help.
If you are using carbon comp resistors; let me tell you how to "age" them, so things will work right in the future. The carbon comps need to go thru "temperature cycling" to stabilize their value. So... buy a whole big bunch and put them in your garage. Then leave them there for 40 or 50 years. Then, measure each one and repaint the stripes. Then, put them in the garbage. Carbon comps were bad in their heyday... there only worse now... there is no market for them... so who will build good ones?
Seriously, Mouser, Digikey, Newark and others have "metal film" resistors you can buy for cheap. Order with normal ground shipping and you'll still spend more on shippng than parts. All the B.S. about the "sound of carbon comps" is just that... B.S. Carbon comps are noisy.. and it ain't the kind of distortion you want... do you leave the room when the ole lady decides to vacuum the rug?
BTW... I looked at the voltages on the second stage of the second 12ax7... seems that something wasn't right there as well. Does the amp sound good now?
Actually its pretty close, I just think Its got a lot more bass than desired, I am hopeing to get to that over the weekend. As it sits, theres enough bass response to actually rattle everything in my work area and that just on a strat. My poor old celections are doomed. You can hear one starting to waffle. On the last test I kinda got carried away, Cranked her up and let her rip. I guess 20 yrs on those babies isnt bad, but hey, I'll stop using them now, maybe get a recone someday before they are ruined.
Gene
Hey Trout,
I would definately take the three beers. But you probably live in Antartirca. Could you just post a listing of the 8,000,000 tubes you bought? Maybe give me a sweet deal on a few?
BTW... I am still thinking about your other amp... the whiner...
Later,
I would definately take the three beers. But you probably live in Antartirca. Could you just post a listing of the 8,000,000 tubes you bought? Maybe give me a sweet deal on a few?
BTW... I am still thinking about your other amp... the whiner...
Later,
Hey Trout,
I would definately take the three beers. But you probably live in Antartirca. Could you just post a listing of the 8,000,000 tubes you bought? Maybe give me a sweet deal on a few?
BTW... I am still thinking about your other amp... the whiner...
Later,
I would definately take the three beers. But you probably live in Antartirca. Could you just post a listing of the 8,000,000 tubes you bought? Maybe give me a sweet deal on a few?
BTW... I am still thinking about your other amp... the whiner...
Later,
poobah said:
I Plan on compiling a full list and putting up a webpage soon. The 10,000 I just bought are still boxed for storage. Soon as I get the racks/shelves up, I will start the long task of them sorted and made more accessible.
I do plan to move a fair portion of them simply because I know theres a large amount I could never possible need or use.
As far as that champ goes, I have no plans to fix/finish it until I get a known better/working PT.
I am going to rethink this carbon composite resistor thing to, Lord knows they can really get you pulling out hairs that at my age I prefer to keep!
I put in the .022 cap instead of the original .1 but Its still a bit more bass than I like.
I feel I am getting right on the border of a nearly audible oscillation also. Raising both volumes to full and tone set at full I can hear a very very low level high E frequency squeel like its right on the edge.
Think I wont be building another like this one, I really think a single tone control just isnt going to get me where I want to be.
The Modded deluxe/marshall thing I built a few weeks back was much closer, I had used a plexi front end, Bass & Treble controls, 1 hot 1 normal channel. Seemed easier to tame.
But Hey, I am learning a ton! and having fun, Well sort of LOL
Gene
If you check the current (voltage across the cathode resistor) and it doesn't go above the recommended max for the transformer you are OK. The transformer that you are using claims to handle "80 mA per side" but it will probably sound better with 50 or so. I have used EL-34's in amps that have a bias adjustment , and found that some re-adjustment was needed.
If you try them, have a voltmeter connected across this resistor when you power up the amp. Divide the voltage reading by the value of the cathode resistor to get the total (both tubes) tube current. If you are below 120 to 140 mA total you are OK. Closer to 100 would improve tube life.
Example; You measured 25 volts across 220 ohms (with your 6V6's) That works out to 114 mA total. You can adjust the tube current by playing around with this resistor value (remember to divide by the NEW resistor value). The tube current affects the tube's tonal characteristics and its life expectancy. Unfortunately the sound that I like (and many other guitarists) seems to coincide with a short unhappy tube life. Just below the melting point. My next guitar amp (a really big SE amp) will have this as a front panel control. That way I can crank up the tube only when I need it.
I noticed in your original Fender schematic that the cap across this resistor is only rated for 25 volts, this is marginal for a guitar amp, due to the likelyhood of extreme use. When this cap fails it can take the output tubes, and sometimes the transformer with it.
Fender was trying to save some money. Put a 50 volt cap there when you have a chance. By the way the value of this cap affects the bass response and the tonal character of the amp. You can experiment with smaller values to kill some bass.
If you try them, have a voltmeter connected across this resistor when you power up the amp. Divide the voltage reading by the value of the cathode resistor to get the total (both tubes) tube current. If you are below 120 to 140 mA total you are OK. Closer to 100 would improve tube life.
Example; You measured 25 volts across 220 ohms (with your 6V6's) That works out to 114 mA total. You can adjust the tube current by playing around with this resistor value (remember to divide by the NEW resistor value). The tube current affects the tube's tonal characteristics and its life expectancy. Unfortunately the sound that I like (and many other guitarists) seems to coincide with a short unhappy tube life. Just below the melting point. My next guitar amp (a really big SE amp) will have this as a front panel control. That way I can crank up the tube only when I need it.
I noticed in your original Fender schematic that the cap across this resistor is only rated for 25 volts, this is marginal for a guitar amp, due to the likelyhood of extreme use. When this cap fails it can take the output tubes, and sometimes the transformer with it.
Fender was trying to save some money. Put a 50 volt cap there when you have a chance. By the way the value of this cap affects the bass response and the tonal character of the amp. You can experiment with smaller values to kill some bass.
Good to hear you got it Gene,
10,000 tubes, woha!!!
I did suggest that lower bias on the output so that you could go back to 6V6s. They're being pushed hard, but that is how Fender did it as I'm sure you know. We can do a quick check of power dissipation if you post voltages again with the 6V6s.
The amp with 6L6s is very similar to P-P class AB1, 400V plate, 6600 ohm OT, 32W out, in that data sheet you show, but we've lowered the bias a lot.
The excessive bass is probably due to the fact that the power amp has no feedback, whereas your Champ amp did. It will have high output impedance without feedback and the speaker's fundamental resonance will not have as much damping.
Is the excessive bass tending to be around 80 to 120 Hz?
Running the tubes in triode mode would lower the output impedance, and also the power. How does push pull triode mode distortion sound for a guitar amp, anybody try it?
Or you could add negative feedback around the power amp.
What do you think of the twin reverb tone stack?
I took guitar as a young kid but I don't play, my son has started and it seems to be holding his interest. You do a nice job on your projects and my son and I are also planning something soon.
Pete B.
10,000 tubes, woha!!!
I did suggest that lower bias on the output so that you could go back to 6V6s. They're being pushed hard, but that is how Fender did it as I'm sure you know. We can do a quick check of power dissipation if you post voltages again with the 6V6s.
The amp with 6L6s is very similar to P-P class AB1, 400V plate, 6600 ohm OT, 32W out, in that data sheet you show, but we've lowered the bias a lot.
The excessive bass is probably due to the fact that the power amp has no feedback, whereas your Champ amp did. It will have high output impedance without feedback and the speaker's fundamental resonance will not have as much damping.
Is the excessive bass tending to be around 80 to 120 Hz?
Running the tubes in triode mode would lower the output impedance, and also the power. How does push pull triode mode distortion sound for a guitar amp, anybody try it?
Or you could add negative feedback around the power amp.
What do you think of the twin reverb tone stack?
I took guitar as a young kid but I don't play, my son has started and it seems to be holding his interest. You do a nice job on your projects and my son and I are also planning something soon.
Pete B.
Hi Gene,
Play around with the coupling caps and keep what you like. Try to find some metal oxide resistors to use, and go with 1 and 2 watt sizes. This is for the voltage rating. These are very stable resistors, Rohm and Sanyo make them (so look for suppliers that sell those brands).
Tubelab is dead on, use a 50V cap in that location. Watch your heater current if you go to EL34's (or better yet 6CA7's). Those heaters range from 1.2 to 1.6 amps compared to 6L6's 0.9 amps. Your power transformer may not like the other tubes. It will run hotter for sure.
You're running hot and have a target. This is where things get fun for you.
PS. Keep an eye for some 7591's and 7199's, maybe some 7189's. Don't throw out any 6EU7's. You may like the 7581A's. They are 6L6GC's all beefed up with a 35W plate - like a KT66.
-Chris
Play around with the coupling caps and keep what you like. Try to find some metal oxide resistors to use, and go with 1 and 2 watt sizes. This is for the voltage rating. These are very stable resistors, Rohm and Sanyo make them (so look for suppliers that sell those brands).
Tubelab is dead on, use a 50V cap in that location. Watch your heater current if you go to EL34's (or better yet 6CA7's). Those heaters range from 1.2 to 1.6 amps compared to 6L6's 0.9 amps. Your power transformer may not like the other tubes. It will run hotter for sure.
You're running hot and have a target. This is where things get fun for you.
PS. Keep an eye for some 7591's and 7199's, maybe some 7189's. Don't throw out any 6EU7's. You may like the 7581A's. They are 6L6GC's all beefed up with a 35W plate - like a KT66.
-Chris
32W?? Ouch!
Right now, I swapped out the .1 cap on one channel of 1st 12AX7 to .022, I also swapped out the .1 cap in the second 12AX7 location, Much better, Now I have 1 basically normal channel, And 1 Very Bright channel. A near perfect scenario. Bass is still strong on the bright channel, But its not overwhelming like before.
I added a grid resistor to pin 2 of the second 12AX7 just for good measure. Now it appears I have moved further from the edge of whistley oscillation I heard at max control settings.
The Bass on the normal channel is very potent, Might even make out well for a bass player. The bright channel is very piercing now especially on the bridge single coil pickup , Reminiscent of the old Birds sound, Or some of the old Beetles stuff on the Abbey Road album, Exactly what I was looking for.
Credit to Triode Electronics on that OPT, I would have never guessed it would have this much range at this power level. And only $19.95? I had my gal order 3 more! I would be willing to bet it would make a great sounding stereo project if not pushed as hard. Is Response flat within 1 dB 50 Hz-20KHz at 20 watts enough?
I'll try to get the bias voltages of the tubes later today, Looking at a couple other fender schematics does show they were real 6V6 eaters or very close to it.
I have to learn more about the math and its applications to valve circuits. The Law Is The Law , LOL
I need to adhere more closely so as not to be such a bother.
Gene
Right now, I swapped out the .1 cap on one channel of 1st 12AX7 to .022, I also swapped out the .1 cap in the second 12AX7 location, Much better, Now I have 1 basically normal channel, And 1 Very Bright channel. A near perfect scenario. Bass is still strong on the bright channel, But its not overwhelming like before.
I added a grid resistor to pin 2 of the second 12AX7 just for good measure. Now it appears I have moved further from the edge of whistley oscillation I heard at max control settings.
The Bass on the normal channel is very potent, Might even make out well for a bass player. The bright channel is very piercing now especially on the bridge single coil pickup , Reminiscent of the old Birds sound, Or some of the old Beetles stuff on the Abbey Road album, Exactly what I was looking for.
Credit to Triode Electronics on that OPT, I would have never guessed it would have this much range at this power level. And only $19.95? I had my gal order 3 more! I would be willing to bet it would make a great sounding stereo project if not pushed as hard. Is Response flat within 1 dB 50 Hz-20KHz at 20 watts enough?
I'll try to get the bias voltages of the tubes later today, Looking at a couple other fender schematics does show they were real 6V6 eaters or very close to it.
I have to learn more about the math and its applications to valve circuits. The Law Is The Law , LOL
I need to adhere more closely so as not to be such a bother.
Gene
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