Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on March 18, 2014, 04:12:27 pm
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Hi guys.
A pretty darn interesting amp arrived on my bench, I thought you'd like to see it, it's quite unusual... Pics:
(http://img15.hostingpics.net/pics/390925DSC22293008x2008.jpg)
(http://img15.hostingpics.net/pics/399019DSC22333008x2008.jpg)
(http://img15.hostingpics.net/pics/488263DSC22343008x2008.jpg)
(http://img15.hostingpics.net/pics/327880DSC22373008x2008.jpg)
So it had several issues: bad V3 connection, that's solved. Bad hiss that was coming from the wiring placement, that's solved (I remember that the 1974 18w is DARN tricky with wire placement so same thing here...). The tremolo wasn't working and still isn't. I tried to take measures but pointing the red probe on V1 pins 1 and 2 makes SUCH a crackling noise it's crazy!
If you guys have any tips...
Thanks!
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... The tremolo wasn't working and still isn't. ...
The schematic (http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_1930_10w.pdf) shows the foot switch must be plugged in the jack and shorting the hot to ground to enable the trem oscillator. I'd try bridging those contacts with a jumper wire first.
If that doesn't work, you have 2 trimpots from the 12AX7 cathodes to ground. I bet one of them (maybe the 5kΩ?) is the panel Trem Depth control. The other is probably that preset trimpot shown in your pictues with what looks like a black wire to the + side of the 25uF cap. I might try bridging a new 25uF cap across the existing one (to rule out a dried up bypass cap), then tweaking that trimpot a bit to see if it brings back trem strength.
But I'm guessing lack of a footswitch is gonna be your culprit.
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... The tremolo wasn't working and still isn't. ...
The schematic (http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_1930_10w.pdf) shows the foot switch must be plugged in the jack and shorting the hot to ground to enable the trem oscillator. I'd try bridging those contacts with a jumper wire first.
If that doesn't work, you have 2 trimpots from the 12AX7 cathodes to ground. I bet one of them (maybe the 5kΩ?) is the panel Trem Depth control. The other is probably that preset trimpot shown in your pictues with what looks like a black wire to the + side of the 25uF cap. I might try bridging a new 25uF cap across the existing one (to rule out a dried up bypass cap), then tweaking that trimpot a bit to see if it brings back trem strength.
But I'm guessing lack of a footswitch is gonna be your culprit.
Hi and thanks! I have a footswitch! It's not the original, it's the footswitch I use for my Tremolux 5E9A. Even with the footswitch, still no trem... I'll do what you say!
Thanks!
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Hi there!
I have checked the 25uF cap and it's good. The trem jack was wired badly: it was grounded on the - side of the 25uF cap but also on the jack ground tab.
It's still not working though and turning the trimpot doesn't change a thing... I guess the next step is to check the three .01uF ceramic disks, right?
Thanks!
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The schematic shows that the trem oscillator shares cathode resistance with the 12AX7 gain stage. So that tells you if the oscillator is running, you should get some amount of trem.
... I guess the next step is to check the three .01uF ceramic disks, right?
Before suspecting a malfunctioning part, I recommend seeing if the existing parts are hooked up/functioning correctly. In order to oscillate the trem oscillator needs 3 series caps from plate to grid, and 3 resistors to ground (1 after each cap).
I mentioned the footswitch before; I would follow the wiring from the footswitch jack back to see if it connects to a resistor (it should, according to the schematic). If you don't have continuity from that resistor to ground (through the footswitch), the oscillator can't oscillate.
The trem Speed pot is in series with one of the resistors in order to vary trem speed. You might check that it has good connection to ground at one end.
To ensure oscillation, the 12AX7 needs to have good gain. The 25uF cap is partly to help with this, which is why I suggested checking it. You can also try swapping with a known-strong tube.
There is a 0.68uF cap from the trem oscillator plate to ground. I'm thinking the purpose of that is to shunt all signal higher than trem-frequency to ground, mainly to minimize the effect of distortion in the oscillator tube. If that cap were leaky, it would also shunt your trem signal to ground. You could try unsoldering one end to see if trem is restored.
You can try swapping the caps from plate to grid, but you might want to try one at a time, as you probably will not have a good way to determine which is the leaky cap.
You'll want to be monitoring a.c. volts on the oscillator plate to see the trem signal, in case there's something preventing you from hearing it. Depending on your meter's capabilities, you will probably have to do this by measuring d.c. volts at the plate and watching for slow fluctuation (set your amp's Speed control to minimum to help with this).
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The schematic shows that the trem oscillator shares cathode resistance with the 12AX7 gain stage. So that tells you if the oscillator is running, you should get some amount of trem.
... I guess the next step is to check the three .01uF ceramic disks, right?
Before suspecting a malfunctioning part, I recommend seeing if the existing parts are hooked up/functioning correctly. In order to oscillate the trem oscillator needs 3 series caps from plate to grid, and 3 resistors to ground (1 after each cap).
I mentioned the footswitch before; I would follow the wiring from the footswitch jack back to see if it connects to a resistor (it should, according to the schematic). If you don't have continuity from that resistor to ground (through the footswitch), the oscillator can't oscillate.
I have continuity there.
The trem Speed pot is in series with one of the resistors in order to vary trem speed. You might check that it has good connection to ground at one end.
Done, it's there.
To ensure oscillation, the 12AX7 needs to have good gain. The 25uF cap is partly to help with this, which is why I suggested checking it. You can also try swapping with a known-strong tube.
Done, doesn't change a thing...
There is a 0.68uF cap from the trem oscillator plate to ground. I'm thinking the purpose of that is to shunt all signal higher than trem-frequency to ground, mainly to minimize the effect of distortion in the oscillator tube. If that cap were leaky, it would also shunt your trem signal to ground. You could try unsoldering one end to see if trem is restored.
This cap still tests at 0.66uF with my Capacity tester.
You can try swapping the caps from plate to grid, but you might want to try one at a time, as you probably will not have a good way to determine which is the leaky cap.
You'll want to be monitoring a.c. volts on the oscillator plate to see the trem signal, in case there's something preventing you from hearing it. Depending on your meter's capabilities, you will probably have to do this by measuring d.c. volts at the plate and watching for slow fluctuation (set your amp's Speed control to minimum to help with this).
Need to do that now... The wiring seems to be good so that's the only solution left for now.
Thanks HotBlue!
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Sounds like a plan!
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Nice amp! :m11 Don't see these too often.
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Hi.
So, I have changed all the ceramic disks, one by one, from the plate to the grid and still no tremolo coming out, I replaced them anyway because they were out of specs, they were supposed to be .01uF and were about 40% lower than that, in the .0065uF ballpark...
I'd like to make sure that the jack is wired properly now if you can help me. As you can see in the third picture I posted above, the jack had the shield wire going to its ground and the live wire to the sleeve, not the tip. So I rewired it, got rid of the ground shield connection on taht end and wired the signal to the tip. On the other side of the wire, teh shield goes to the - side of the 25uF cap and the signal goes to the 680K resistor (blue-grey-yellow). I think I was right to rewire the jack connections, wasn't I? I mean, if the ground shield is used on both ends of the wire, the signal doesn't come through, right?
Another thing I've noticed is that when I touch V1 pin 6 with my MM + probe, there's a loud noise and the tremolo effect seems to turn on for about a second or two and then vanishes. If that can lead you on a nice track for a hunch, please let me know. This Tremolo is giving me a hard time and I can't find any good schematic and most of all layout out there.
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I dont know Teles from shinola about trem circuits, but I did want to throw this out. In my Marshall book, the schematic shows a notation next to the 1K trim pot below the 5K trim pot. It says 1K. I don't know if this is the preferred trim setting or what. These schematics are supposedly the most recent pulled from the factory. Again, I don't know if this has any bearing, but wanted to let you know since you are still looking for a solution.
Good luck!
Jim
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Thanks Jim!
I did what HotBlue requested (I had forgotten about that!) and lifted one leg of the 0.068uF cap: the tremolo is back! So does it mean I need to replace it? I have 0.68uF in stock, I don't think I have any 0.068uF... :BangHead:
Thanks!
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Doesn't the schematic say it is a .68?
Also, what are the cabinet dimensions? I'm curious if they used the same box for these as for the 18 watters. Would also be interested in seeing a few more pix, if you took any beside the ones above.
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I did what HotBlue requested (I had forgotten about that!) and lifted one leg of the 0.068uF cap: the tremolo is back! So does it mean I need to replace it?
Yes. It means the 3 caps already replaced may have been okay, and just the 0.68uF was leaking (which would shunt the trem signal to ground).
I also read the schematic as 0.68uF, the value you have on hand.
So, I have changed all the ceramic disks, one by one, from the plate to the grid and still no tremolo coming out, I replaced them anyway because they were out of specs, they were supposed to be .01uF and were about 40% lower than that, in the .0065uF ballpark...
You looked at the schematic I linked, right? That shows 0.005uF caps, not 0.01uF, meaning your measured right where they should.
That said, you might replace them because you want slower trem, but that's a different matter.
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HI.
Yes, I read the schematioc you gave HotBlue but it doesn't correspond to the reality of the amp I have here and to others I have found on the web. The cap is original and says 68000pF and 400V, just like on this other 1930:
(http://ivanrichards.files.wordpress.com/2012/02/jmp-trem-combo-007.jpg)
So I guess the schematic has it wrong... But if I replace it with a .68uF what will happen?
As for the ceramic disks, same thing. The schematic you gave says .005uF but the originals I have and I have found say .01uF... So you mean that installing .005uF will make the tremolo faster than with .01uF?
Thanks!
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HI.
Yes, I read the schematioc you gave HotBlue but it doesn't correspond to the reality of the amp I have here and to others I have found on the web. The cap is original and says 68000pF and 400V, just like on this other 1930: So I guess the schematic has it wrong... But if I replace it with a .68uF what will happen?
I see...
Swap with the 0.68uF to see what happens. I'm guessing you won't hear any difference. Maybe it's a typo or maybe Marshall figured out they didn't need that-big a cap.
As for the ceramic disks, same thing. The schematic you gave says .005uF but the originals I have and I have found say .01uF...
Gotcha. They say "10k" instead of "5k"? If so, yeah, they're 0.01uF.
So you mean that installing .005uF will make the tremolo faster than with .01uF?
The trem has 3 RC circuits running from plate to grid; each is a seires cap and a resistor to ground. Each section has a time constant proportional to R*C. If you make any (or all) of those RC sections with larger-value R or C, the RC product is bigger, the time constant is longer, and oscillation speed goes down. That equals slower trem.
The Speed control for all trem circuits changes one of the R's of the RC sections to control trem speed.
So if the caps aged upward to 0.005uF (from 0.01uF), then the trem would have sped up. If you want to slow the trem down beyond what it will do at its slowest setting, you should make the caps bigger (and maybe the R's bigger). Eventually, you will hit a point where the tube doesn't have enough gain at a low enough frequency to keep going; there are other tricks that can help deal with that if you run into it.
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Thanks for your input HotBlue!
Damn it, it's still not working even with a new cap... I replaced the .068uF cap with a .1uF (I thought it'd be closer to the original .068uF than a .68uF cap) and it's still not working. The tremolo works as soon as I lift one of the caps' legs and remains on even when I want to turn it off with the pedal. As soon as I solder the cap in place, the tremolo goes away.
What can it be now...?
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Then just leave that cap out. I don't know why it won't work with a good new cap, but works when it's out of circuit. You could figure out that mystery, or enjoy it working now. :laugh:
About the discrepancy between schematic & real values: I found out through an angry customer that this happens more than you'd think. I installed a Hoffman 5F6-A board into a reissue Bassman chassis for a vintage amp aficionado, who was seriously pissed when it arrived with the 0.022uF, 0.022uF, 250pF cap setup shown on the schematic. Apparently, Fender also shipped them with 0.1uF, 0.047uF, 250pF and 0.1uF, 0.022uF, 250pF setups, and the latter was the most common (according to the guy). He wanted it with the setup not shown on any Fender schematic which he claimed was the most likely circuit arrangement.
And of course, Marshall copied the 0.022uF, 0.022uF, 250pF setup from the schematic, which apparently was the least-common circuit arrangement...
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Then just leave that cap out. I don't know why it won't work with a good new cap, but works when it's out of circuit. You could figure out that mystery, or enjoy it working now. :laugh:
About the discrepancy between schematic & real values: I found out through an angry customer that this happens more than you'd think. I installed a Hoffman 5F6-A board into a reissue Bassman chassis for a vintage amp aficionado, who was seriously pissed when it arrived with the 0.022uF, 0.022uF, 250pF cap setup shown on the schematic. Apparently, Fender also shipped them with 0.1uF, 0.047uF, 250pF and 0.1uF, 0.022uF, 250pF setups, and the latter was the most common (according to the guy). He wanted it with the setup not shown on any Fender schematic which he claimed was the most likely circuit arrangement.
And of course, Marshall copied the 0.022uF, 0.022uF, 250pF setup from the schematic, which apparently was the least-common circuit arrangement...
LOL... Some customers think they know better, I've seen that too! Of course 99% don't...
I need to find a solution anyway because I can't leave it as is, I can't switch the tremolo OFF, it's always working! I can only turn it off by turning the intensity pot to 0 but the owner wants to be able to use it on stage so he'll need to turn it off with a pedal...
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... I can't switch the tremolo OFF, it's always working!
Well, you did change the footswitch jack wiring... Maybe it was correct to begin with?
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... I can't switch the tremolo OFF, it's always working!
Well, you did change the footswitch jack wiring... Maybe it was correct to begin with?
Nope, I wired it back to the original positions and it's not working either... :BangHead:
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I've seen on other Populars that there's a 470ohms resistor between V1 pin 7 and the first ceramic disk, like this, on this earlier 68 or 69 model (plexi faceplate), mine is 72 or 73:
https://plus.google.com/photos/116415517934384645566/albums/5527283465662015265/5527290791559723938?banner=pwa&pid=5527290791559723938&oid=116415517934384645566 (https://plus.google.com/photos/116415517934384645566/albums/5527283465662015265/5527290791559723938?banner=pwa&pid=5527290791559723938&oid=116415517934384645566)
Any idea what that does?
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Nope, I wired it back to the original positions and it's not working either...
Totally disconnect the shielded cable for the footswitch. Does that make it stop?
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Nope, I wired it back to the original positions and it's not working either...
Totally disconnect the shielded cable for the footswitch. Does that make it stop?
Hi Sluckey!
Now it's working... Shield is soldered on both ends and the footswitch works. The amp is now 80% functional. The only flaw left is that the tremolo is still too fast even with .01uF caps and it's also damn noisy... It's really unbearable when you're not playing... I'll try and find a 7025 V1 to check that. I have turned the trimpot back and forth and at minimum value the tremolo doesn't work anymore.
And of course when I solder the .068uF cap back in the tremolo still quits working, which isn't normal...
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And of course when I solder the .068uF cap back in the tremolo still quits working, which isn't normal...
That cap is used to smooth the LFO signal and also filter out higher frequency noise/hash that may be present. But it also reduces the amplitude of the LFO signal. It's a significant amplitude reduction, especially on the faster speeds. On the few trem circuits I've built lately I have always reduced the value of that cap. Why don't you try a .01µF. If that works, then increase to .022 and/or .047. Maybe one of those values will give you a good compromise for a strong trem with little noise.
That particular tremolo circuit is very similar to the circuit in the 1974 18 watt amp. It is very finicky and prone to noise, pops, and other undesirable effects. Gotta remember, these amps are Marshall's cheap bottom feeder line. :icon_biggrin:
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And of course when I solder the .068uF cap back in the tremolo still quits working, which isn't normal...
That cap is used to smooth the LFO signal and also filter out higher frequency noise/hash that may be present. But it also reduces the amplitude of the LFO signal. It's a significant amplitude reduction, especially on the faster speeds. On the few trem circuits I've built lately I have always reduced the value of that cap. Why don't you try a .01µF. If that works, then increase to .022 and/or .047. Maybe one of those values will give you a good compromise for a strong trem with little noise.
That particular tremolo circuit is very similar to the circuit in the 1974 18 watt amp. It is very finicky and prone to noise, pops, and other undesirable effects. Gotta remember, these amps are Marshall's cheap bottom feeder line. :icon_biggrin:
Yeah, I remember losing a lot of time solving tremolo issues on the 1974s... I have already tried a .1uF cap there Sluckey and it also cut off the tremolo effect. Something's wrong there, I currently need to have nothing there or the effect goes away..
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I have already tried a .1uF cap there Sluckey and it also cut off the tremolo effect.
Well, that's even bigger. I was suggesting use a smaller cap. Start with .01µF. The 1974 uses a .05 for that cap.
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I have already tried a .1uF cap there Sluckey and it also cut off the tremolo effect.
Well, that's even bigger. I was suggesting use a smaller cap. Start with .01µF. The 1974 uses a .05 for that cap.
Right, I have just soldered a .047uF cap. The tremolo isn't working anymore, I just get very loud pops past 5 on the intensity pot... Once one leg lift up, the tremolo comes back.
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Try a .01? maybe something even smaller, like a .002. This is where it's so nice to have a cap substitution box!
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I haven't seen any mention of swapping the tube. Grab a handful of 12AX7s and try'em all. Hopefully you'll find one that works.
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I haven't seen any mention of swapping the tube. Grab a handful of 12AX7s and try'em all. Hopefully you'll find one that works.
I have Slucky, I have. That's always the first thing I check with tube amps... I have tried 5 different 12AX7s. The worst of them all are JJ. Jeez these tubes ARE noisy!!! I have settled down on an old Ei 12AX7 that is a lot quieter than all the others. The tremolo is a tad weaker with it but at least the amp hum is tolerable.
I have also tried a 0.022uF cap instead of the 0.068uF cap. Tremolo doesn't work with it either!
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... The only flaw left is that the tremolo is ... damn noisy... It's really unbearable when you're not playing...
There's something you're not realizing regarding this trem circuit. As Sluckey mentioned, the cap to ground will help reduce noise somewhat, but the pumping when you're not playing is probably normal.
It has to do with taking a relatively small trem signal and injecting it at a preamp stage. Leo Fender got a patent on a trem circuit for the 1955 Tremolux, because it injected the trem at a shared cathode resistor of the paraphase inverter. That made the pumping noise common-mode to both sides of the push-pull output stage, which largely cancels the noise (it would perfectly cancel if the phase inverter had perfect balance.
Even that scheme didn't work 100%, and because Fender wanted to use something other than the paraphase on bigger amps, the trem was injected at the output tube grids on fixed-bias amps. And because this gets awkward with large bias voltages of high-power 6L6's, Fender took a step backward and used an optoisolator to inject trem without the benefit of noise cancellation.
All that is to tell you that injection of trem in a preamp stage is/was reserved for the cheapest amps in a company's line (for Fender, this would be the VibroChamp), because if you wanted noise-free perfect operation you should step up to the more expensive amps with the more complex circuits.
Now popping, banging, etc... that's not good. But backward pumping/woosh, that's par for this course.
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Thanks for making things clearer HotBlue!
The Tremolo works if I don't install a .068uF cap and I've tried bigger and smaller values too. I can switch it ON/OFF with the footswitch.
From 0 to 4 on the intensity knob the tremolo is weak. Between 5 and 6 it's ok. From 7 to 10 it just pops really loud and makes the amp unusable.
So I know the trem is very tricky on these amps, but there definitely is something wrong.
Thanks!
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Now that it's working, did you go back and try to adjust that trimpot? I'm still not clear on its purpose, though it looks like it could be used to insure the trem signal doesn't overwhelm the other gain stage (which sounds like what's happening when you max the trem depth).
... From 0 to 4 on the intensity knob the tremolo is weak. ...
Methinks this is normal... :l2:
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Now that it's working, did you go back and try to adjust that trimpot? I'm still not clear on its purpose, though it looks like it could be used to insure the trem signal doesn't overwhelm the other gain stage (which sounds like what's happening when you max the trem depth).
Yes I did. The trimpot actually works like an intensity pot. At the minimum setting the tremolo quits working, at the extreme too. The difficulty is to set it right where the tremolo works well but remains acceptably silent.
... From 0 to 4 on the intensity knob the tremolo is weak. ...
Methinks this is normal... :l2:
I know I may sound like a noob at times but I'm not. Don't make fun of me. I am familiar with amps that have tremolos (and that don't): Fender blackface amps, tweed tremolux and Marshall 1974s. When I say it's weak, I know it should be subtle, but here, it's unusually weak. As a comparison, with the intensity on 6 it's the equivalent of the tremolo on 2 or 3 with the tremolux I have. So I could even have said that between 0 and 4, it's not working. And there's still those loud pops when the intensity is set beyond 7. So to sum it up, the tremolo works between 5 and 6, below that you can't really notice it and above that it pops. The setting range is usable on 2 figures instead of 10...
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Not sure if you already did this, but those 3 cer. disc caps that are side by side in the picture... have you tried replacing those? I'm wondering if 1 (or more) might be leaky? And this might be totally off base, as I am not familiar with trems and how they work. My method is to keep replacing parts till I find the bad one. lol
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I know I may sound like a noob at times but I'm not. Don't make fun of me.
I'm sorry, that wasn't the intent. It just struck me funny when you mentioned the trem was weak when you turn it down. My silly side wanted to say, "Well, turn it up!"
Is the tube socket loose-fitting or something? Are the socket contacts corroded? What about wiring in a temporary replacement pot for the speed & depth?
I'm grasping at straws now, since from what you've said you have replaced everything else in the circuit and it's still not functioning properly.
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I know I may sound like a noob at times but I'm not. Don't make fun of me.
I'm sorry, that wasn't the intent. It just struck me funny when you mentioned the trem was weak when you turn it down. My silly side wanted to say, "Well, turn it up!"
No worries!
Is the tube socket loose-fitting or something? Are the socket contacts corroded? What about wiring in a temporary replacement pot for the speed & depth?
I'm grasping at straws now, since from what you've said you have replaced everything else in the circuit and it's still not functioning properly.
Yes, it's a bit loose but the tube is held on fine. Do you mean I should maybe reflow all the solder joints on the socket? They still have the original red glyptol on them.
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I'm just thinking we're running out of items that aren't 100%.
If you have replaced all 3 series caps on the oscillator, replaced/removed the 0.068uF cap to ground, replaced the 25uF cathode bypass cap, re-wired the footswitch jack... Well, there's just not much left you can mess with.
So my thoughts turn to the noise/popping. Loose or dirty tube socket contacts could do that, so I wonder about those. I wonder about the pots and trimpot, and whether they could have a wrong taper (uneven trem strength per rotation) or dirty/intermittent resistive tracks (noise, popping). I wonder if the resistors (which we normally don't mess with) have some kind of noise/broken contact/absorbed humidity/etc issue...
Normally, trem is not this much trouble. I'd suggest replacing everything, one item at a time, until you fix the thing to figure out what is not right.
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Have you replaced the two plate resistors?
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Have you replaced the two plate resistors?
Nope but I measured them and they're within specs...
I'll reflow the solder joints on V1...
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A resistor can measure the right value and still be noisy.
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I have changed the 470K resistor to no avail. I don't have any 680K at hand. I just shot a short video for you guys. I think these 40 seconds will be much more explicit that any text I can write... You'll see, something's definitely not normal.
http://youtu.be/BJ6zejtHAb0 (http://youtu.be/BJ6zejtHAb0)
Thanks for your help. At least the tremolo is working a bit, it wasn't at all before...
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I can't watch the video. It's "private".
I have changed the 470K resistor to no avail. I don't have any 680K at hand.
1MΩ is fine for the 680kΩ. Don't forget the 100kΩ oscillator plate load and the 220kΩ gain stage plate load.
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You should be able to watch it now. OK for the other resistors.
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Okay, that's a horse of a different color.
Turn the amp off. Measure resistance to ground from the 12AX7 cathode, with the trem intensity at 0. Record the value, then measure again with the intensity at full. Record the value. You know the approximate intensity setting whee the popping occurs; turn the intensity knob to this point and record the resistance value.
What were the 3 numbers you found?
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Okay, that's a hose of a different color.
Turn the amp off. Measure resistance to ground from the 12AX7 cathode, with the trem intensity at 0. Record the value, then measure again with the intensity at full. Record the value. You know the approximate intensity setting whee the popping occurs; turn the intensity knob to this point and record the resistance value.
What were the 3 numbers you found?
Intensity on 0: 1.68K
Intensity on 10: 3.1K
Intenisty on 8 (about where the amp starts purring...): 2.7K
Thanks HotBlue!
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What is the resistance end-to-end of the Intensity pot?
I was thinking this would be 5kΩ based on the schematic, but that doesn't seem possible with your numbers...
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What is the resistance end-to-end of the Intensity pot?
I was thinking this would be 5kΩ based on the schematic, but that doesn't seem possible with your numbers...
I measured on the middle lug of the intensity pot to ground: 0K on 0 to 1.38K on 10... Far from 5K!
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Looking at your pics and I can not quite see it - is that a solid wire connecting the plates on V1? Where is the 100K resistor?
Jim
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Looking at your pics and I can not quite see it - is that a solid wire connecting the plates on V1? Where is the 100K resistor?
Jim
You mean a 100K resistor connecting pins 1/6/7 like on other Marshalls? There's not any here...
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Looking at your pics and I can not quite see it - is that a solid wire connecting the plates on V1? Where is the 100K resistor?
It's on the board, connected to the 220kΩ resistor for the other 12AX7 plate.
What is the resistance end-to-end of the Intensity pot?
I was thinking this would be 5kΩ based on the schematic, but that doesn't seem possible with your numbers...
I measured on the middle lug of the intensity pot to ground: 0K on 0 to 1.38K on 10... Far from 5K!
Okay, now what is the resistance end-to-end of the on-board trimpot? Is that also ~1kΩ?
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Looking at your pics and I can not quite see it - is that a solid wire connecting the plates on V1? Where is the 100K resistor?
It's on the board, connected to the 220kΩ resistor for the other 12AX7 plate.
What is the resistance end-to-end of the Intensity pot?
I was thinking this would be 5kΩ based on the schematic, but that doesn't seem possible with your numbers...
I measured on the middle lug of the intensity pot to ground: 0K on 0 to 1.38K on 10... Far from 5K!
Okay, now what is the resistance end-to-end of the on-board trimpot? Is that also ~1kΩ?
It's 0KΩ to 4.85KΩ...
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A-ha!! I made a wrong assumption that the 5kΩ pot would be the panel control. So your measurement of the panel control at a bit over 1kΩ is correct.
Am I right that you still have nothing in place of the 0.068uF cap you found defective?
Here's what I'm thinking: the "popping" sounds like motorboating, but rather than being associated with a filter cap, it is simply the trem oscillator overpowering the 12AX7 gain stage. The 0.068uF cap would have reduced the trem strength (too much, from what you said), but "no cap" is leaving the signal strong enough to cause motorboating.
Approach 1:
Get a handful of different cap values: perhaps from 500pF to 0.047uF. Start with the smallest-value cap, and place it in the circuit where the 0.068uF cap went. Increase the cap value until the motorboating stops, but you still have trem (a cap substitution box would help speed this, but it not mandatory).
Approach 2:
The 5kΩ trimpot sets the bias of both the trem oscillator and the 12AX7 gain stage. If its resistance is increased, the oscillator gain falls somewhat, and the bias voltage of the 12AX7 gets bigger, which allows the application of a bigger (trem) signal without overloading. Turn up that onboard trimpot to eliminate the motorboating.
Most likely, you'll need a combination of the two, just as the original circuit used a combination of the two. It's up to you to strike the correct balance.
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Hey Sleepless,
How did you go.......
I have since built a clone of my own and have encounter the same problems
Marshall 1930 Popular (el34world.com) (https://el34world.com/Forum/index.php?topic=23293.0)
Getting myself up to speed with how I left it some time ago.
I do remember having some problem with the term
So I returned the circuit to as stock as I could as per schematics and photo's found on the net
And there was quite a few of different circuits.
So this is what I came up with
(https://i.imgur.com/BAicWB4.jpg?1)
(https://i.imgur.com/o7U7kgr.jpg)
When I returned it to what I believed to be stock, the trem was making the speaker flap.
The 5k trim was set about mid rotation, turning the trim in either direction reduced the flapping but also reduced any tremolo effect.
By chance when I was trying the quiet down the fizz/hum by putting a voltage divider before the phase inverter, this deleted the flap and a good tremolo effect was achieved.
The tremolo effect was fast and this seem to reduce the effect, so a .022u was slotted in and this also made it stronger and more articulate.
Again not all 12AX7 would just drop in, most were noisy and microphonic.
With the trem in a good place and some of the noise reduced by the voltage divider there was still some hum/fizz that could be reduced.
A .01u cap at the junction of the 220k and 100k plate resistors seem to do the job.
Doing these mods has had on ill effect on the overall sound of the amp.
Hope to have my stunt guitarist give it a test soon.
Hope this helps
:icon_biggrin: