Help needed with JTM45/ 50W homebrew!

Aaron0602 · Post by **Aaron0602** » Wed Aug 18, 2004 4:38 am

Hey guys, any help on this would be appreciated. George and I have been trying to figure this out for a couple days, but I don't want to tie up his phone all the time, so I'm coming here.

So I built a hybrid between a JTM45 and a 50W plexi. It uses George's JTM45 board, but has MM 50W OT and PT, a tube rectifier, and EL34's. It's similar to the earliest 50 watters that marshall made. Anyway, it works relatively well, but there are a few things that unsettle me. First off, when I crank the high treble loudness (the normal loudness is at 0), and no guitar or cable is plugged in, I get a very raspy, intermittent hum/static type of sound. What could be the possible cause? Also, just a few hours ago, i lightly tapped the chassis while it was on, and now there's a distinct hum, even when both volumes are at 0. If I plug a guitar in, you can faintly hear it through the amp, despite the volumes being at 0. What did i bump out of place? Now the oddest thing is the voltage throughout the amp. I'll include my voltage chart with this, take special note of the voltages in V3. They're very odd, especially the difference between pins 1 and 6. I checked all the connections on my board with a multimeter, they're all correct. All the resistors test as correct. What could this be attributed to? Does anyone have any solutions or recommendations?

Thanks a million,
Aaron

Aaron0602 · Post by **Aaron0602** » Wed Aug 18, 2004 10:39 pm

hey,
so my voltages are closer to normal, i just changed some resistors, but i'm still getting strange noises. A recap: with both volumes at 0, there's a pretty quiet hum. if you plug into a Volume 1 input, turn down volume 1, and turn up volume 2, you can hear the guitar through the amp at a noticeable volume. The same works vice versa (plugged into input 2, turn down volume 2, turn up volume 1). if i turn up volume 1 all the way with no guitar plugged in, i get an assortment of very loud hums, squeals, buzzes, pops, and intermittent static. Come on people, I need your help! 31 views and not a single post? one of you has got to be able to help me.

rjgtr · Post by **rjgtr** » Thu Aug 19, 2004 10:05 am

My guess would be either a ground issue or a wire dress issue. Maybe a picture would help. Try moving some of the wires around and also try swapping some tubes.

Aaron0602 · Post by **Aaron0602** » Thu Aug 19, 2004 7:13 pm

I redid all my grounds, and have determined that one tube is faulty. However, even with a different tube, I'm getting crossover from channel 1 to channel 2. to spare you from sorting through the above posts, i'll retell. With nothing plugged in, and both volumes at 0, there's a pretty soft hum. with channel 1 cranked, there's a pretty loud hiss/hum. With channel 2 cranked, same thing. With both volumes at 0, If i plug in my guitar, I can hear it through the amp. If i plug into channel 1, turn down volume 1, and turn up volume 2, i can hear my guitar through the second channel. The same works
vice versa. any ideas as to what the cause of the problem is? Why are my channels crossing, and why can't my amp be silent with both volumes at 0?

Flames1950 · Post by **Flames1950** » Thu Aug 19, 2004 9:01 pm

It's fairly typical for Marshalls to cross over into the other channel. You've maybe not noticed, but adjusting the other channel's volume, even without jumpering the inputs, can and will affect your tone as the signal can bleed into the other channel slightly at the mixer.
Also, if you have a bright cap of any kind on a volume pot, that cap is allowing some frequencies around the volume control, thus you will get some sound even on zero volume on that channel.
It's only a defect in the circuit design, not your workmanship. My '75 and '78 heads both do this; I just confirmed it on the '78 so I wouldn't be relying on my memory alone.

Aaron0602 · Post by **Aaron0602** » Thu Aug 19, 2004 9:05 pm

well then, in that case, you've put my mind somewhat more at ease. Thanks for the clarification. It's just that I don't think It had done it prior to a few days ago, I could be mistaken though. Also, Input one of channel 1 squeals. Why? It starts when the volume hits 4-5, and it's really annoying. I'm pretty sure that one of the 12ax7's, but even when it's in V3, the channel still squeals.

Flames1950 · Post by **Flames1950** » Thu Aug 19, 2004 10:09 pm

The squealing may be an oscillation, though, especially if swapping tubes doesn't help. Usually it comes down to going over your wire routing very carefully (work with a good pic of an original Marshall or one of George's finished builds or something as a guide) or you may have to use shielded wire on your inputs.

Aaron0602 · Post by **Aaron0602** » Sat Aug 21, 2004 9:25 pm

So after receiving some ANOS mullard EL34's and working preamp tubes, my amp is working much better. I compared it to my guitar teacher's jtm45, and mine is actually quieter, thankfully. However, if both the high treble volume and presence are at 8 or above, it still squeals on input 1, volume 1. If everything is cranked, and i turn the presence below 8, the squealing stops. Could it be due to a faulty cap on the presence pot? Or should i still get shielded wire for the inputs? Also, how does shielded wire work?

jerrydyer · Post by **jerrydyer** » Fri Oct 15, 2004 6:36 pm

I notice that the negative feedback is the most sensetive area of this amp. i had a .022 here by mistake one time and it caused the probs your describing. Also another tie I had a resistor that was in place nice and snug so everything worked good, but it wasnt soldered at all. When I tap the amp all kinds of noises popped up, rightfully so. Check and recheck my friend.

Guest · Post by **Guest** » Fri Oct 15, 2004 10:37 pm

jerry, i'm pretty sure it's not a solder joint, because the oscillation is constant at certain levels, even through a dummy load (you can see it through the scope). However, i noticed a few interesting things. When i tested the plates of every single tube, with the scope probe to the plate pin, and the ground clip to ground, there was no oscillation through the scope. However, if i hooked up the scope to the dummy load, (scope probe to hot lead, ground clip to ground lead), there was oscillation at certain settings. This makes me very very nervous, because if i'm not mistaken, the only thing between the tubes and the speaker/load is the output transformer....

George, any ideas? Also, can you tell me a good time to call you, because reaching you is about as easy for me as curing the oscillation in my amp

thanks guys
aaron

Aaron0602 · Post by **Aaron0602** » Sat Oct 16, 2004 12:54 pm

sorry, forgot to log in for the above post

Post by **VelvetGeorge** » Sat Oct 16, 2004 1:13 pm

Hey Aaron, sorry about the trouble reaching me.

To see if the occillation is in your output section, put a resistor on each output tube grid (pin 5). Try something between 1-10K. Just unsolder the wire from pin 5 and put the resistor between the wire and the terminal.

Let me know the results.

George

Guest · Post by **Guest** » Sat Oct 16, 2004 2:07 pm

Hey george,
I actually already have 5.6k swamp resistors in place. I talked to Lord Valve, and he recommended that i set the amp to the oscillation point, and spot solder 1pf snubber caps between the plate and the cathode of the preamp tubes, one at a time. He said that if i find one of those areas where the oscillation stops, just leave that one snubber cap and be done with it. I still need to buy those caps, but what do you think?

Aaron0602 · Post by **Aaron0602** » Sat Oct 16, 2004 2:08 pm

Dammit, that was me again, and I meant 10pf

Aaron0602 · Post by **Aaron0602** » Mon Oct 18, 2004 6:18 pm

George, what are your ideas on the 10pf snubber cap? Also, did you get that email i sent you a week or 2 ago? It's still pretty much accurate of what's going on in my amp. Jon, your input is also welcome.