I built a version of an 18 watt Marshall (actually it's very similar to the brown note schematic based on the lite IIb, though I don't think that's relevant today)
Two 12ax7 into two 6l6gc in push-pull.
The amp sounds kinda cool, but I'm not sure how cool because it is quiet. It should be a 20+ watt amp but I'm not sure if it would out-perform my cell phone.
B+ was measured at just over 326 prior to installing the tubes.
I didn't go back in for more measurements yet.
All of the tubes are used, so they might be bad tubes. I strongly suspect that old tubes and improper bias are the answer, but I don't have any known good tubes that I can use. I'm going to find some new tubes somewhere and swap them in, but in the meantime I thought I'd ask you fellas about one other thing that's bugging me.
The OT primary (tube side) measures 110 to center on one side, but 125 to center on the other side. Is that too much of a difference? Can I or should I try to even those numbers out?
Thanks.
Two 12ax7 into two 6l6gc in push-pull.
The amp sounds kinda cool, but I'm not sure how cool because it is quiet. It should be a 20+ watt amp but I'm not sure if it would out-perform my cell phone.
B+ was measured at just over 326 prior to installing the tubes.
I didn't go back in for more measurements yet.
All of the tubes are used, so they might be bad tubes. I strongly suspect that old tubes and improper bias are the answer, but I don't have any known good tubes that I can use. I'm going to find some new tubes somewhere and swap them in, but in the meantime I thought I'd ask you fellas about one other thing that's bugging me.
The OT primary (tube side) measures 110 to center on one side, but 125 to center on the other side. Is that too much of a difference? Can I or should I try to even those numbers out?
Thanks.
Ohms. I don't really know. I don't know if you can add resistance in series with the lower winding but my gut thinks it probably won't work. I don't know. I have approximate knowledge of many things.
I improved the situation by changing tubes -it's definitely louder than a cell phone now, but until I can find some new tubes all I have is a stack of different used tubes. I found the "shiniest" ones I could. I don't know how to tell using my eyes, and I don't have a tester.
I improved the situation by changing tubes -it's definitely louder than a cell phone now, but until I can find some new tubes all I have is a stack of different used tubes. I found the "shiniest" ones I could. I don't know how to tell using my eyes, and I don't have a tester.
you could use diodes paralleled with 1 ohm resistors on the cathodes of of your output tubes to measure current draw per tube but that would take the fun outta guessing which tubes are best!
Don't worry about different resistances. When they wind the transformer they start at the core and when they get to the center, they bring out the center tap. Then the other half is wound on top of the first half. It has the same number of turns so the output is equal but since it is wound on top of of the first half, it has a longer circumference, so the winding is longer than the first half and had more DC resistance.
As for tubes, you would do well to get some new ones for your new build so you can eliminate tubes as the source of the problem.
As for tubes, you would do well to get some new ones for your new build so you can eliminate tubes as the source of the problem.
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Thank you for the explanation boobtube.
I tried all of my preamp tubes today and it is clear that there is one particular Raytheon brand tube that is noticeably better than all the others. Then there's a Sylvania tube that is second best and when I use them together I actually get a bit of distortion and volume. I definitely was on the right path with buying new tubes.
I tried all of my preamp tubes today and it is clear that there is one particular Raytheon brand tube that is noticeably better than all the others. Then there's a Sylvania tube that is second best and when I use them together I actually get a bit of distortion and volume. I definitely was on the right path with buying new tubes.
With tubes installed, your B+ is probably around 300 volts. You should check. You can use the resistance of each half of the primary of the OT to figure out the correct Bias point.
With 300v B+, you can push 100ma per tube and be within dissipation , but that is a bit much. I would go for somewhere between 50ma to 70ma per output tube.
So, using ohms law, and the resistance of each half of the OT, for 50ma per tube you would want to measure 5.5 volts across the 110 ohm winding and 6.25 volts across the 125 ohm winding.
For 70ma per tube, you'd want to measure 7.7 volts across the 110 ohm winding and 8.75 volts across the 125 ohm winding.
Hope this helps.
With 300v B+, you can push 100ma per tube and be within dissipation , but that is a bit much. I would go for somewhere between 50ma to 70ma per output tube.
So, using ohms law, and the resistance of each half of the OT, for 50ma per tube you would want to measure 5.5 volts across the 110 ohm winding and 6.25 volts across the 125 ohm winding.
For 70ma per tube, you'd want to measure 7.7 volts across the 110 ohm winding and 8.75 volts across the 125 ohm winding.
Hope this helps.
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No.
Amp plugged in, Power on, standby on.
Volumes on "0" and no guitar plugged in. You always take bias measurements with no signal going into the amp.
Make sure you have a speaker plugged in also. Don't get shocked.
Amp plugged in, Power on, standby on.
Volumes on "0" and no guitar plugged in. You always take bias measurements with no signal going into the amp.
Make sure you have a speaker plugged in also. Don't get shocked.
Update on bias checks and a new question.
I got voltage values that didn't make sense at first, so I read about bias checks and discovered that I would be able to do a shunt current check pretty easily.
I got 12 mA and 3 mA. B+ of 312VDC
Since that was obviously not a good result I threw an old Sylvania tube into the socket that was reading 3mA.
This time I got 12 mA and 18 mA.
I've got common cathode bias with a 240 ohm resistor and 47 uF cathode cap.
Is it safe to assume that I need a good matched pair of output tubes before I can go any farther?
I got voltage values that didn't make sense at first, so I read about bias checks and discovered that I would be able to do a shunt current check pretty easily.
I got 12 mA and 3 mA. B+ of 312VDC
Since that was obviously not a good result I threw an old Sylvania tube into the socket that was reading 3mA.
This time I got 12 mA and 18 mA.
I've got common cathode bias with a 240 ohm resistor and 47 uF cathode cap.
Is it safe to assume that I need a good matched pair of output tubes before I can go any farther?
I think I'm getting my footing a little. I adjusted the cathode resistance down to 214 ohms and I have a 100uF cap in parallel. New readings are 20 and 13mA at 312vdc b+.
I've got 295 volts on pin 4. Is that what we call screen voltage?
I've got a little over 7 volts across the cathode resistor.
I've got 295 volts on pin 4. Is that what we call screen voltage?
I've got a little over 7 volts across the cathode resistor.
I'm having a lovely little conversation with myself now. Unless I'm missing an edit button somewhere? Sorry.
I found a pair that measured 42 and 42mA. Problem solved. Amp is loud now. Sounds pretty good too. MWAHAHAHAHAHAHAHAHAHAHAHAH. (excited about this)
I found a pair that measured 42 and 42mA. Problem solved. Amp is loud now. Sounds pretty good too. MWAHAHAHAHAHAHAHAHAHAHAHAH. (excited about this)
well thankfully it wasn't problems with the transformer or your build now you can take the guitar world by storm mwhahahaha!
Yes. Now I'm going to take the amp to a place where it can be loud, and then I'm going to make it be loud. This is exciting.
So many times it comes down to the tubes. When changing resistors you described in post # 11 and still only getting 20 and 13mA with those voltages, that should tell you it's tubes.
I got really lucky too. NOS GE 6L6GC. I thought they were bad after my first build didn't work. I nearly threw them away. They're matched and everything.
Are those the fat boys with the two side getters? If so, great tubes. Don't ever throw away tubes unless they are known to be bad such as a short, leak or gassy. Or just plain worn out. Even if the guide pin is broke. Look at the bottom and see where the guide used to be. Then with some sort of high temp marking or tape to write on, put an arrow on the side of the base to show where it used to be and be VERY careful when installing the tube to match up to the notch in the socket.
Yes they are.
I did something screwy today. I decided that I wanted to see what the circuit would be like if I cascaded the first stage into the second, rather than the parallel stages that I had at first. My output is basically gone now. I can hear some volume, barely.
I borrowed from a 18w TMB Superlite schematic, adding a bunch of components in an attempt to make it work.
Then I think I might have burned up my heater xfmr as well. I was getting 6.3 volts with all the tubes removed, but it was 0 volts with the tubes installed. Then I found a spot on the return wire that was bare.
(yes I have a separate heater xfmr, yes I'm thinking it may be time to buy a new PT that can handle alla dat)
I did something screwy today. I decided that I wanted to see what the circuit would be like if I cascaded the first stage into the second, rather than the parallel stages that I had at first. My output is basically gone now. I can hear some volume, barely.
I borrowed from a 18w TMB Superlite schematic, adding a bunch of components in an attempt to make it work.
Then I think I might have burned up my heater xfmr as well. I was getting 6.3 volts with all the tubes removed, but it was 0 volts with the tubes installed. Then I found a spot on the return wire that was bare.
(yes I have a separate heater xfmr, yes I'm thinking it may be time to buy a new PT that can handle alla dat)
New question:
That's what I built at first. Worked fine.
I made the following changes. Temporary changes, but anyways...
*I disconnected the tone pot and connected it in between the first and second gain stages as a gain control. The 500pf and .01uf caps are still in the circuit, but not connected to anything. (dead end)
*I put a small (22nF) coupling cap between the first and second stage.
*I removed the 120k plate resistor and installed parallel 100k plate resistors to separate the stages. (noobish mistake ahead)
*I also separated the cathodes. I put a 820ohm cathode resistor and a 10uF cathode cap on the second stage.
*I added a 100 ohm resistor in series with the ground side of the "gain" pot. (I wanted to always have some minimum signal being passed to the second stage.)
I was getting very quiet output (this is not related to the filament xfmr failure btw, in case anyone read my other thread)
I eventually realized that I'd screwed up the voltage divider. I added 70k resistor in series with the parallel 100k resistors to get back up to 120k and it worked, but I want to make sure I didn't overlook anything. Is that the correct answer?
I also had strange operation with the new gain control. At minimum gain it sounds clean-ish. As I turn the gain knob up, the amp's output actually gets quieter for a moment before it starts to get loud. I get the impression that there might be some parallel path that I don't understand. The quiet spot (at about 1 or 2 on the dial) feels like some kind of a balanced condition - as if competing signals are defeating each other?
And the last - I think I heard some swirling. I was getting a little bit lazy about the lead dress so it wouldn't surprise me if that's just signal interference, but I'm worried that maybe I created a loop or something like that.
Any help would be so handy to me. 🙂
That's what I built at first. Worked fine.
I made the following changes. Temporary changes, but anyways...
*I disconnected the tone pot and connected it in between the first and second gain stages as a gain control. The 500pf and .01uf caps are still in the circuit, but not connected to anything. (dead end)
*I put a small (22nF) coupling cap between the first and second stage.
*I removed the 120k plate resistor and installed parallel 100k plate resistors to separate the stages. (noobish mistake ahead)
*I also separated the cathodes. I put a 820ohm cathode resistor and a 10uF cathode cap on the second stage.
*I added a 100 ohm resistor in series with the ground side of the "gain" pot. (I wanted to always have some minimum signal being passed to the second stage.)
I was getting very quiet output (this is not related to the filament xfmr failure btw, in case anyone read my other thread)
I eventually realized that I'd screwed up the voltage divider. I added 70k resistor in series with the parallel 100k resistors to get back up to 120k and it worked, but I want to make sure I didn't overlook anything. Is that the correct answer?
I also had strange operation with the new gain control. At minimum gain it sounds clean-ish. As I turn the gain knob up, the amp's output actually gets quieter for a moment before it starts to get loud. I get the impression that there might be some parallel path that I don't understand. The quiet spot (at about 1 or 2 on the dial) feels like some kind of a balanced condition - as if competing signals are defeating each other?
And the last - I think I heard some swirling. I was getting a little bit lazy about the lead dress so it wouldn't surprise me if that's just signal interference, but I'm worried that maybe I created a loop or something like that.
Any help would be so handy to me. 🙂
Edit: Transformer looks OK, just make sure your B+ isn't too high wired this way. These transformers are designed to be wired with the CT to ground and 2 diodes from the red leads becoming your B+.
Your Guitar input connections are wired wrong. Look at any old Fender Or Marshall schematic to see how it should be wired. You need 68K resistors from each input "hot" going to first grid of first tube. Only 1 Meg resistor going to bottom jack "hot" to ground. Upper jack doesn't need 1Meg res.
Everything else looks OK.
Your Guitar input connections are wired wrong. Look at any old Fender Or Marshall schematic to see how it should be wired. You need 68K resistors from each input "hot" going to first grid of first tube. Only 1 Meg resistor going to bottom jack "hot" to ground. Upper jack doesn't need 1Meg res.
Everything else looks OK.
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