I finally convinced myself that i ought to just follow a schematic.
http://www.lh-electric.net/img/nfa.png
It is a single 12ax7a combination pre-amp and phase inverter feeding into a 6l6 push-pull fixed bias power section.
I flipped the standby switch and measured about 6.7 vac on the 6l6 tube sockets but i mistakenly measured between pin 1 and 4/5 when i checked the other base. The reading that I got was initially 6.7 but then it quickly jumped up to about 40v.
I think I know what's happening. I think i must be charging one of the filter capacitors through the test meter.
But i can't assume. Is anyone willing to check the drawing and give me a second opinion?
http://www.lh-electric.net/img/nfa.png
It is a single 12ax7a combination pre-amp and phase inverter feeding into a 6l6 push-pull fixed bias power section.
I flipped the standby switch and measured about 6.7 vac on the 6l6 tube sockets but i mistakenly measured between pin 1 and 4/5 when i checked the other base. The reading that I got was initially 6.7 but then it quickly jumped up to about 40v.
I think I know what's happening. I think i must be charging one of the filter capacitors through the test meter.
But i can't assume. Is anyone willing to check the drawing and give me a second opinion?
The drawing seems fairly straight forward.
I cant see any problems with it an I have designed a few similar amplifiers.
I cant see any problems with it an I have designed a few similar amplifiers.
Well there are problems.
The first problem was found and corrected already. A wiring error. I connected the wrong end of the 150 ohm R14 resistor to ground.
I wonder if there's something wrong with my bias wiring? I don't know why I think that but I've got a feeling like I need to re-check it.
One of the 6L6 tubes gets fairly hot to the touch but if it's glowing at all then it's just barely. I've got tremendous amounts of light in the shop that I can't turn off so it's possible that the tube is just too dim.
The other 6L6 gets warm but not to the extent that the first one does. The 12ax7a barely gets warm at all. I can see some filament glow on the 6L6 tubes but I have not seen any filament glow on any of the 9-pin tubes. Again - don't know if brightness issue or what.
I can say that I measured 6.7 vac between pin 9 and pins 4/5 prior to installing the tubes but I really think that there must be something going on.
As for voltage readings - the article that accompanied the schematic steps you through testing a few voltages.
It expects:
pin 1: 108
pin 3: 1.5
pin 6: 185
pin 8: 109
but I've got more like
pin 1: 300
pin 3: 0
pin 6: 296
pin 8: 5
So once again I'm thinking that maybe I don't have filament voltage on the triodes and they're each acting as an open?
The first problem was found and corrected already. A wiring error. I connected the wrong end of the 150 ohm R14 resistor to ground.
I wonder if there's something wrong with my bias wiring? I don't know why I think that but I've got a feeling like I need to re-check it.
One of the 6L6 tubes gets fairly hot to the touch but if it's glowing at all then it's just barely. I've got tremendous amounts of light in the shop that I can't turn off so it's possible that the tube is just too dim.
The other 6L6 gets warm but not to the extent that the first one does. The 12ax7a barely gets warm at all. I can see some filament glow on the 6L6 tubes but I have not seen any filament glow on any of the 9-pin tubes. Again - don't know if brightness issue or what.
I can say that I measured 6.7 vac between pin 9 and pins 4/5 prior to installing the tubes but I really think that there must be something going on.
As for voltage readings - the article that accompanied the schematic steps you through testing a few voltages.
It expects:
pin 1: 108
pin 3: 1.5
pin 6: 185
pin 8: 109
but I've got more like
pin 1: 300
pin 3: 0
pin 6: 296
pin 8: 5
So once again I'm thinking that maybe I don't have filament voltage on the triodes and they're each acting as an open?
Is pin 1 of your 6L6 sockets even wired to anything? The tube doesn;t need it, and traditionally it is a handy place to wire the other end of a grid stopper resistor on pin 5. If pin 1 is unconnected, then voltage readings from it to other things is just going to be what the high impedance of the mete allows it to sense. Like touching the tip of a guitar cord reveals the ambient electrical fields by making hum.
Fire the amp up with no tubes in it. Go up topside and probe the socket pins. Do you have 6vAC between pins 2 and 7 on the power tubes? I assume the 12AX7 is wired for 6v? So pins 4 and 5 are wired together? Measure from pin 9 to 4 and then 5. Got 6v? If either of those tests fail, then the tube won't light.
With the tubes installed, if the ambient light is too bright, throw a thick towel or a newspaper or magazine over the thing long enough to see if the heaters are glowing.
I;d check for B+ without tubes first. Got 300v at pins 3 and 4 of both power tubes, and at pins 1 and 6 of the small tube? A missing pin 4 voltage will turn the tube off. Power off, measure resistance to ground from pins 8 and 5 of the power tubes. Do you get about 240 ohms and about 471k? Measure from the socket to ground, not the individual resistors. We are checking not only the part, but all the wiring too this way.
I know you have already done some of this.
Power tubes in, and recheck all the socket voltages there. Your schematic wants 19v at the cathodes, do you get something close? That is roughly 40ma per tube, which is fine for 300v B+.
You got a bias probe? You can check each tube current individually.
Finding the full 300v on the plates - pins 1 and 6 - of the 12AX7 means it is not conducting. SO either it is not lit, or the cathodes are not wired to ground.
I have been saying 12AX7, I see it is a 12AT7, sorry, tests are still valid.
Power off, measure resistance to ground from pins 3 and 8. Do you get roughly 970 ohms and 56k on them? Oe about 57k between the two pins. If the ground ends of those two resistors are soldered to some point that is then to be grounded, but isn't, that will stop your tube.
Did you try a different tube? This circuit is made for a 12AT7, but just to see if the socket and circuit works, you can stick a 12AX7 in there for a moment.
Nit pick: You show all the circuit with various point wired to common, and you show the input jacks grounded to chassis. You do not show where those two grounds are connected together.
Fire the amp up with no tubes in it. Go up topside and probe the socket pins. Do you have 6vAC between pins 2 and 7 on the power tubes? I assume the 12AX7 is wired for 6v? So pins 4 and 5 are wired together? Measure from pin 9 to 4 and then 5. Got 6v? If either of those tests fail, then the tube won't light.
With the tubes installed, if the ambient light is too bright, throw a thick towel or a newspaper or magazine over the thing long enough to see if the heaters are glowing.
I;d check for B+ without tubes first. Got 300v at pins 3 and 4 of both power tubes, and at pins 1 and 6 of the small tube? A missing pin 4 voltage will turn the tube off. Power off, measure resistance to ground from pins 8 and 5 of the power tubes. Do you get about 240 ohms and about 471k? Measure from the socket to ground, not the individual resistors. We are checking not only the part, but all the wiring too this way.
I know you have already done some of this.
Power tubes in, and recheck all the socket voltages there. Your schematic wants 19v at the cathodes, do you get something close? That is roughly 40ma per tube, which is fine for 300v B+.
You got a bias probe? You can check each tube current individually.
Finding the full 300v on the plates - pins 1 and 6 - of the 12AX7 means it is not conducting. SO either it is not lit, or the cathodes are not wired to ground.
I have been saying 12AX7, I see it is a 12AT7, sorry, tests are still valid.
Power off, measure resistance to ground from pins 3 and 8. Do you get roughly 970 ohms and 56k on them? Oe about 57k between the two pins. If the ground ends of those two resistors are soldered to some point that is then to be grounded, but isn't, that will stop your tube.
Did you try a different tube? This circuit is made for a 12AT7, but just to see if the socket and circuit works, you can stick a 12AX7 in there for a moment.
Nit pick: You show all the circuit with various point wired to common, and you show the input jacks grounded to chassis. You do not show where those two grounds are connected together.
I will use these recommendations tomorrow. Very grateful.
As for the tubes - I've tried a few different ones - all roughly the same result.
I have a pretty nice selection - I think I'm kinda spoiled actually.
nos rca 12ax7a
nos sylvania 12ax7a
nos sylvania 12bh7a
nos rca 5751
lots of nos sylvania 12au7a
Some other nice things.
On the other hand I only have two 6l6 tubes - both used. nos RCA. They appear to be working.
Oh yeah - I wasn't about to go plugging a guitar into the amp but I did notice that I got a tiny bit of sound just from tapping my meter lead on one of the terminals. I wonder if it's slightly magnetized?
Update after testing:
The circuit seems to be right but I'm now convinced that the filament transformer is screwed up.
(I am using a separate transformer for filament voltage)
I removed the 6L6 tubes and warmed up just the 12ax7. I could see the filaments glowing easily even in the bright light.
I put the 6L6 tubes back and tried to warm all 3. This time none of the filaments were glowing.
So I'll be replacing the transformer and re-testing when I get an opportunity.
The circuit seems to be right but I'm now convinced that the filament transformer is screwed up.
(I am using a separate transformer for filament voltage)
I removed the 6L6 tubes and warmed up just the 12ax7. I could see the filaments glowing easily even in the bright light.
I put the 6L6 tubes back and tried to warm all 3. This time none of the filaments were glowing.
So I'll be replacing the transformer and re-testing when I get an opportunity.
I have faint sound. It's extremely quiet. The guitar has active pickups and volume is maxed. It is slowly getting louder. Very slow.
I am progressing. I uncovered another error. I had multiple OT coils wired up to give negative feedback at the same time. I removed those and I got a lot more volume. It's still a pretty quiet amp. It's not nearly as loud as my 16 watt solid state amp. I won't be able to do voltage checks until Mondy. More details then.
Besides wiring errors, that is not a "Guitar Amp" by any means, no matter what the label says, but a tubed Power Amp.
It's lacking a preamp.
Your guitar, even being active, will barely drive it at all.
I wonder why many get caught in the trap of building very inexperienced "designers" creations, probably never even built once, instead of following the path of commercial designs such as, say, Fender.
I'd add an extra triode or two, and clone some tried and true design, some 60 y.o. classic.
Can't go wrong 😉
It's lacking a preamp.
Your guitar, even being active, will barely drive it at all.
I wonder why many get caught in the trap of building very inexperienced "designers" creations, probably never even built once, instead of following the path of commercial designs such as, say, Fender.
I'd add an extra triode or two, and clone some tried and true design, some 60 y.o. classic.
Can't go wrong 😉
I appreciate the good advice that you've given but i never wanted to do an exact clone of anything. I wanted to learn and then make something unique. This schematic that i followed was selected because it is the simplest possible amp that fit the tubes and OT that i had available. I could have bought any part but i will feel more confident in my purchase now that i've made one.
I also paid very little mind to the wiring and locating parts because i figured that it would be useful to get an idea about how much noise.i can tolerate. I also used the cheap parts instead of springing for the more expensive stuff because i thought it would be useful to hear the bad first, so that if i decide to make upgrades it will be with a purpose rather than just because someone else thinks that you need to have a certain style of capacitor.
I'm comfortable with my decision. Hopefully i'll be able to ask better questions from now on.
So you feel that an additional dual triode would be helpful? I have a few ideas about how i might do that. The one that i've got is currently pretty dark-sounding. A brighter tone would be nice.
I wonder if that darkness is related to the coupling caps that I had to substitute? I think the drawing calls for .01 and i used .022 if i recall correctly. There was an article about that somehwere if i can find it...
I also paid very little mind to the wiring and locating parts because i figured that it would be useful to get an idea about how much noise.i can tolerate. I also used the cheap parts instead of springing for the more expensive stuff because i thought it would be useful to hear the bad first, so that if i decide to make upgrades it will be with a purpose rather than just because someone else thinks that you need to have a certain style of capacitor.
I'm comfortable with my decision. Hopefully i'll be able to ask better questions from now on.
So you feel that an additional dual triode would be helpful? I have a few ideas about how i might do that. The one that i've got is currently pretty dark-sounding. A brighter tone would be nice.
I wonder if that darkness is related to the coupling caps that I had to substitute? I think the drawing calls for .01 and i used .022 if i recall correctly. There was an article about that somehwere if i can find it...
https://www.youtube.com/watch?v=uFuqDk2VYpo
I got rid of the input resistor. Amp seems like a normal cheap amp now. I'm not a very good player. Please excuse my poor playing.
I got rid of the input resistor. Amp seems like a normal cheap amp now. I'm not a very good player. Please excuse my poor playing.
This shouldn't be. Recheck your wiring or post a picture for us to double check your work.
.5v at the input should give you about max clean power somewhere around 30 watts. Do you have a signal generator and a scope?
A 30 watt 6l6 amp is pretty loud, I should know I play one.
As to adding more gain stages it depends on your preference. But I would figure out what is wrong with your build first before thinking about modifying it. And because you used salvaged parts doesn't mean squat. As long as they are close to specification "boutique" stuff isn't going to improve sound.
I wasted a day.
When I get the amp back to the shop I'll need to check voltages again.
I'm still wondering about something that is buggins me - I swapped filament transformer and it helped, which seems fishy to me.
The first xfmr was rated at 20va, the replacement is 25va.
My math suggested that the power required would be about 16va, so 20 should've been fine. Is there something that I don't know about the filament current draw? There's nothing else in that part of the circuit except for 2 6l6 tubes and a 12ax7 tube. Voltage is a little high - 6.7-ish.
When I get the amp back to the shop I'll need to check voltages again.
I'm still wondering about something that is buggins me - I swapped filament transformer and it helped, which seems fishy to me.
The first xfmr was rated at 20va, the replacement is 25va.
My math suggested that the power required would be about 16va, so 20 should've been fine. Is there something that I don't know about the filament current draw? There's nothing else in that part of the circuit except for 2 6l6 tubes and a 12ax7 tube. Voltage is a little high - 6.7-ish.
Hola!
Long time with no update. Long time needed to step away from the project.
So here's the voltages:
My b+ (measured directly from the + side of the bridge rectifier is 305, which is what the schematic calls for.
The cathode side of the output tubes is 19.5 which is also what the schematic calls for.
As for the anode voltages - i did not anticipate the need to measure those so i do not have easy access. I am hoping to hear "that's fine" because the other voltages are good.
As for the pre-amp stage - my values are pretty far from normal.
I'm supposed to have 108-ish on the anode of the first triode, but i have 135.5
I'm supposed to have 1.5-ish on the cathode of same triode but i have 0.72
The other triode (phase-inverter) likewise. I'm supposed to have 185 on the anode but actually have 174. The cathode is supposed to be 109 but i have 134.
One other measurement. I took voltage at u3 on the schematic. Supposed to be 295 but actually only 285.
Here is a fresh link to the article that has the schematic btw:
Guitar Amplifier 6L6 Push-Pull 17 Watts
Now i need to fill you in on some alterations that i have made just because i wanted to see what would happen. The neg. Feedback connection was removed from the circuit. I also omitted one of the input jacks and the 68k input resistor has been jumpered out of the circuit. So the guitar is feeding directly into the gate of the preamp triode.
I am using a 12ax7a instead of the recommended 12at7a valve.
I am using an active set of pickups, so the input signal is relatively high.
With all of the changes made the amp has some noise and feedback problems but i was able to sort it out somewhat by repositioning some wires. At 80% volume the amp is still not as loud as my 15w solid state amp and it starts to make a farty "blat" sort of sound. Doesn't seem like a speaker problem to me but i admit that i don't know how to check that.
It does at least sound like a guitar amp and it's loud enough to hear clearly. No distortion.
Is there an obvious next step? I figure there must still be something wrong with the preamp valve or the biasing circuit around it.
Long time with no update. Long time needed to step away from the project.
So here's the voltages:
My b+ (measured directly from the + side of the bridge rectifier is 305, which is what the schematic calls for.
The cathode side of the output tubes is 19.5 which is also what the schematic calls for.
As for the anode voltages - i did not anticipate the need to measure those so i do not have easy access. I am hoping to hear "that's fine" because the other voltages are good.
As for the pre-amp stage - my values are pretty far from normal.
I'm supposed to have 108-ish on the anode of the first triode, but i have 135.5
I'm supposed to have 1.5-ish on the cathode of same triode but i have 0.72
The other triode (phase-inverter) likewise. I'm supposed to have 185 on the anode but actually have 174. The cathode is supposed to be 109 but i have 134.
One other measurement. I took voltage at u3 on the schematic. Supposed to be 295 but actually only 285.
Here is a fresh link to the article that has the schematic btw:
Guitar Amplifier 6L6 Push-Pull 17 Watts
Now i need to fill you in on some alterations that i have made just because i wanted to see what would happen. The neg. Feedback connection was removed from the circuit. I also omitted one of the input jacks and the 68k input resistor has been jumpered out of the circuit. So the guitar is feeding directly into the gate of the preamp triode.
I am using a 12ax7a instead of the recommended 12at7a valve.
I am using an active set of pickups, so the input signal is relatively high.
With all of the changes made the amp has some noise and feedback problems but i was able to sort it out somewhat by repositioning some wires. At 80% volume the amp is still not as loud as my 15w solid state amp and it starts to make a farty "blat" sort of sound. Doesn't seem like a speaker problem to me but i admit that i don't know how to check that.
It does at least sound like a guitar amp and it's loud enough to hear clearly. No distortion.
Is there an obvious next step? I figure there must still be something wrong with the preamp valve or the biasing circuit around it.
Oh.
I appears that i overlooked something obvious. :-/
Thank you for catching it. I feel like I should've thought of that myself.
I'll go find one.
I appears that i overlooked something obvious. :-/
Thank you for catching it. I feel like I should've thought of that myself.
I'll go find one.
First things first this amp you chose to build will NOT behave like most guitar amplifiers. The voltages you posted with the 12AX7 all look fine.
Lets look at a Fender champ compared to your amp. Open loop gain of the Champ is 47db and for the amp you built it's around 10db.
A Fender Champ with a 20mV input signal and the volume control not even on 2 (1Meg pot voltage divider at 880k/120k, 120k to ground) I get 4.5vrms into an 8 ohm load.
With your amp's volume on full blast a 20mV input only gets you about .7vrms into an 8ohm load.
Basically the amp you built is closer to a line level amplifier......it can still be used for guitar just harder to drive. Pretty much I would just leave the volume on that amp on max and just roll the guitar volume up or down accordingly.
The only way to test if the amp is working properly is to put it on a dummy load and see what you get for open loop gain. All the simulations I did was in open loop. You can get a free signal generator app for smart phones so all you need is a dummy load and a DVM (a scope is better). Input a 20mV 1kHz test signal and see what you get out.
Lets look at a Fender champ compared to your amp. Open loop gain of the Champ is 47db and for the amp you built it's around 10db.
A Fender Champ with a 20mV input signal and the volume control not even on 2 (1Meg pot voltage divider at 880k/120k, 120k to ground) I get 4.5vrms into an 8 ohm load.
With your amp's volume on full blast a 20mV input only gets you about .7vrms into an 8ohm load.
Basically the amp you built is closer to a line level amplifier......it can still be used for guitar just harder to drive. Pretty much I would just leave the volume on that amp on max and just roll the guitar volume up or down accordingly.
The only way to test if the amp is working properly is to put it on a dummy load and see what you get for open loop gain. All the simulations I did was in open loop. You can get a free signal generator app for smart phones so all you need is a dummy load and a DVM (a scope is better). Input a 20mV 1kHz test signal and see what you get out.
I am going to do just what you suggested and measure the output. I infer that the "problem" is the design of the amp - not designed to amplify the signal much - is that right?
What about these active pickups? One of the pickups is an emg81 which supposedly can output up to 4v at max. Is there generally some kind of a balance designed into an amplifier between the pre and power stages?
I had been thinking that more volts in = more volts out, and that the fundamental operation of the amp would be the same in either case.
Now i'm wondering if there's a bad assumption - like maybe if i put too much voltage into the preamp valve then all i'm going to accomplish is to drive the preamp valve to the point where it shuts itself off? The negative bias (cathode bias) is only .7 volts and i'm guessing that the input voltage needs to be within a certain range in order to keep that preamp valve working properly?
What about these active pickups? One of the pickups is an emg81 which supposedly can output up to 4v at max. Is there generally some kind of a balance designed into an amplifier between the pre and power stages?
I had been thinking that more volts in = more volts out, and that the fundamental operation of the amp would be the same in either case.
Now i'm wondering if there's a bad assumption - like maybe if i put too much voltage into the preamp valve then all i'm going to accomplish is to drive the preamp valve to the point where it shuts itself off? The negative bias (cathode bias) is only .7 volts and i'm guessing that the input voltage needs to be within a certain range in order to keep that preamp valve working properly?
Well it's extremely lacking in the number of stages, and then it has negative feedback as well - if you short out R14 that will remove the feedback, and give the first stage it's maximum gain (removing R15 as suggested earlier would still leave local feedback on the first stage).
It will upset the DC conditions a little, but shouldn't make much difference, and certainly not just for a test of the gain you get.
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