From what I've found on the forums, it's set at a lower primary impedance than the data sheet would suggest, and the OT gets hot as a result, but the better sound is worth it (hope I didn't mis-represent anything in this brief summary).
If nobody has measured one, is there any way to estimate the primary z from the schematic (the Marshall one in this thread that combines the JTM and JCM circuits on one schematic)?
Thanks in advance for any help. If I don't find out any more, I'll probably wing it with a 12k primary z and see what happens
Sorry, one more question - would the Hammond 125A push-pull 3W OT be a decent choice? It can be set to a bunch of primary and secondary impedances depending on which pins you connect.
Any other better candidates? I don't imagine the original Dagnall TXOP 00039 OT that Marshall used is available as a replacement part, since I'm not an authorized Marshall repair dude.
Ken,
I don't know enough to compute or back solve for the primary z from the schematic. A while back, though, I built an amp with a pair of 6SN7 wired parallel (one tube = 2 triodes wired PP, the second tube wired parallel to the first, basically a 4 tube amp in 2 bottles.) The 6SN7 is close to an octal equivalent to the 12AU7. I used a 10K primary z. Somehow, I had reasoned that one 6SN7 wants to see about 22K. I had an OT that was 10K and that played a part in the build decision. I figured, close enough.
Quite a few years ago, I had a conversation with an aerospace engineer who was very knowledgeable about tube amps (and a decent guitar player, too.) My recollection is that he told me triodes are very tolerant of the primary z. I think if you choose anything from about 5K to 25K the amp will work OK. The real question will be what primary z gives you the tone you like best. You may need to experiment with it. 12K seems like a very reasonable place to start.
One 12AU7 (both halves) is good for maybe 4.5W max. It seems to me the way to knock this down to 1W is to use the tube in a less efficient circuit. I suppose tinkering with the primary z is a way to do that?
As for a hot OT, I hope we all realize too hot to touch starts at around 105F (depending on the person), but for real too hot is probably 180F or higher. It may be that you need not be concerned about too hot?
In recent times, Postimage, where I have stored the images of this topic, has been experiencing problems. I'm slowly fixing this, but it's still not all right. I hope you understand.
Sure, here's a screenshot of a simulation I used for the amp. Ignore the additional 3.3K resistor I've got on my HT line as this was simply added because I couldn't find a PT with a low enough secondary. At the time anyway. I found a perfect PT afterwards but that's always the way!!!!
Very happy with the amp, it has a real nice smooth drive and I can't get a bad sound out of it, no matter where the controls are set. I have all three tone controls on my amp, not just the treble control. Has very little headroom so you can forget a clean tone. It does clean up a bit with the guitar volume control (very responsive) but the tone is never completely clean if you get what I mean.
I think the amp could benefit from a presence control but in it's current state I can't see how you'd add this. One idea I had was to put a .1uF bypass cap on the 3rd gain stage cathode but have a pot on it to control it and call that presence. I've noticed that in the JTM145 they have that cap permanently attached.
I play mine through a 2x12 with G12M Heritage speakers and it works really well with those.
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Could someone explain how to calculate the appropriate specs for an OP for the JMP?
I'm still new at this. I see 200v b+ at 470ohms = approx. 43ma DC current.
Is there more to add to this calculation?
Most 3 watt OP I see are far below this in max DC. If I go up to a 5-10 watt OP, will I lose some of the necessary character of the circuit?
Thanks
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Hey guys, ive been trying to build this thing, but it just doesnt want to do anything, ive already fixed some mistakes but im at a point where i just have no clue what my problem could be anymore, can someone of you who also build one glance over my layout if he finds something that i missed? I would be incredibly thankful... Ive also tried injecting a signal and traced it thanks to a suggestion in another thread, but ive hit a brickwall again.
The signal goes into V1a, V1b, and V2a no problem and seems to get amplified, but at V2b it goes in as a 24~VAC signal and the output on the plate is cycling between 35, 7, near 0, OL and starting again on the upper end...
here is Matecs schematic i followed and the layout i used:
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Not sure why you switched threads, but whatever...
It's possible that you've built an oscillator. That requires positive feedback, which can present itself in some surprising ways.
Typically, low frequency oscillations are caused by coupling through the power supply. Do you see the B+ bouncing in the same way?
Disconnect the NFB. That's the 1M + 22nF coming off the OT and going back to the grid of V2B. If that fixes it, then either swap the wires from the OT to the 12AU7 plates or swap the wires to the grids of the 12AU7. (I doubt that's the problem, but we need to make sure.)
Try pulling the 12AU7. What AC do you measure on either side of the coupling caps C4 and C10?
Tony Bones wrote: ↑Fri Feb 08, 2019 10:36 pm
Not sure why you switched threads, but whatever...
It's possible that you've built an oscillator. That requires positive feedback, which can present itself in some surprising ways.
Typically, low frequency oscillations are caused by coupling through the power supply. Do you see the B+ bouncing in the same way?
Disconnect the NFB. That's the 1M + 22nF coming off the OT and going back to the grid of V2B. If that fixes it, then either swap the wires from the OT to the 12AU7 plates or swap the wires to the grids of the 12AU7. (I doubt that's the problem, but we need to make sure.)
Try pulling the 12AU7. What AC do you measure on either side of the coupling caps C4 and C10?
Sorry for switching, i thought it might help to have a little context since most of what ive done came out of this thread, also hoped that someone out of here who build it might have some insight. Ill see if i can delete the other thread.
I do however see that most of my problems just simply come from not knowing enough about this stuff and i really should have chosen a simpler build, but i also dont want to scrap everything since i think im quite close to finishing...
I fixed some grounding issues and i do have guitar sound on the speakers now and even volume and tone seem to do its jobs. At low volumes it sounds normal now and just has a slight 150hz hum, at louder volumes it crackles and goes silent if i play more strings and the sound comes back 2-3 seconds later, but i guess its much better than no sound at all! Ill keep looking for problems since they all seem selfmade instead of schematics related. Big thanks to you for trying to help though
Well, you've been given rule #1 when trouble shooting: start at the input and move along the signal chain until you find where things go from right to wrong. Once you've isolated where the trouble happens it's a lot easier to figure out why.
Sometimes I start at the output and move backwards, but when bring up a new build that might have multiple issues it's usually easier to start at the input and keep fixing things as you move towards the output. Keep at it; you will eventually win.
When checking over wiring visually I often overlook the same mistake over and over. It might help to have someone else compare your wiring to the layout. That's one of the reasons we ask for photos.
You also have a NFB circuit int he amp (I too am completely confused as to why you didn't just stick to the original thread you started?)
You may need to swap the leads of the output transformer, or disconnect the NFB for a test at any rate. Otherwise, I'd keep this back to your thread so you don't mix up this thread about someone elses build.
