JTM45 build presence oscillation

[casey73]

Thanks for all the replies. Too many to reply to individually.

I did leave the plate leads longer than necessary so I would have enough lead left to flip them if I got the reverse lead howl, which has happened a few times over the last 25 years. So I shortened them to the shortest needed once I got it running. Grid leads run shortest path under turret board to output tubes.

I went with the traditional early 60's something version except I added 5.6K grid resistors and opted for1K screen resistors in place of 470 ohm so I can run KT66 or EL34. I also chose the lower voltage 300-0-300 power tranny from Mojotone giving me around 390 volts on the plates.

As for lead dress, there isn't much space in the JTM45 chassis but all my leads are neat and as short as possible as with all amps I've built. The fact that some JTM45 owners said this has happened to them tells me is goes with the territory in this style amp.

The squeal isn't awful, but I will probably play with the feedback resistor till I can tame it. The ghost notes aren't as pronounced as when I first turned it on last week. Very subtle now. The filter caps are new F&T caps, a 50/50 and 16/16. I'm thinking maybe they aren't fully formed yet and maybe that's why I was getting the ghosting. Everything is hooked up right. I only experienced ghost notes like that one other time and it was with an Alessandro Bloodhound a friend let me play with years ago.

Don't have much playing time on it yet as it's still on the bench in my garage. I need to buy or build a head cabinet. Haven't decided which yet. I live in Arizona so it takes a while this time of year for my little AC to get the temp down to a tolerable temperature. I works good as long as it's blowing right on me. Any woodworking has to be done early in the morning.

Thanks again for all the feedback.

[casey73]

JTM45 build pics

[bob p]

Saying that all marshalls do it is a cop out.

Merlin's book on preamps has an excellent chapter on stabilizing amps that use NFB. it covers calculation of the poles, zero points, open-loop gain, closed-loop gain, dB of feedback and stability margin -- everything you need to know to attack the problem without shooting in the dark.

Mercury iron isn't the solution, lmao.

As was previously requested, a link to the exact schematic is essential.

[casey73]

Bob - I'm not copping out. Just being pragmatic. As you can see from the photos I'm pretty anal about layout and have built quite a few amps over the years. Nobody said all Marshalls have this issue. I didn't go back re-read all the comments but a couple commented that their JTM45s have similar issues with squeal on high presence settings when using the treble channel. A friend who plays in 3 tribute bands told me the same thing. So I'd say there may be something to it. I think Mercury Magnetics transformers are overpriced. I used a Heyboer power tranny because that's the only one that would fit and a Hammond OPT and choke.

This schematic is pretty close to what I did, except I changed the first bias supply resistor off the rectifier to a 100K and used a 50k trimpot to adjust the bias voltage. It gives me the range to use EL34 or KT66 outputs. As noted earlier I added output tube grid resistors and 1K screen resistors instead of 470 ohm.

http://milas.spb.ru/~kmg/files/schem...0amp/JTM45.gif

[Pedro Vecino]

From my point of view, in my English to translate songs (sorry):

- The output of the wires towards the equalization and presence is too short. They should be longer to stick to the chassis and then ascend vertically towards the potentiometers. This allows them to be reoriented.
- Same with the connections to anodes/grid/cathodes in preamp tubes. They should be longer so that the anodes and cathodes go down to the chassis and then can be oriented in the best way. Grid wires should be kept high, away from the others.
- The orientation of the preamp sockets is not the best. It is advisable to install the open part of the pins circle facing the circuit.
- Feedback connection is too short and does not allow any orientation. It´s better to be longer and circulate near to the chassis and away from high voltage directed in perpendicular form to the circuit (27K resistor).
- Finally, output line of the driver towards power tubes can be done aerial with shielded cable. The mesh can be connected to the bias voltage (on the other side of the 220K resistors). This can save many problems and once done you can go back to the unshielded lines in the traditional way if you want to compare.

[bob p]

Pedro, that's an interesting idea to use the bias supply as a ground for the shield on the PI outputs. I can't remember seeing that done before... maybe I just haven't been paying attention.

Casey, I wasn't referring to you copping out, I was referring to the people telling you that all Marshalls do it. They are definitely copping out.

It's not the case that all marshalls do it, but the circuits were never designed with stability margin in mind. They were just copies of Fender circuits, with mistakes incorporated along the way. The result is that lead dress ends up causing more problems than it should.

Marshall's failure to design a stable circuit is what leaves us with stability problems. It's these circuit design faults that cause so many home builders have problems; the circuits aren't built to address the problem and that makes construction techniques matter more than they should. If the circuit were designed with stability in mind it would be a lot easier for DIY builders to build these amps without problems.

I think what we have going on here is that the original circuits ignore the problem and the DIY builders who faithfully copy a layout diagram don't know enough to solve it. The result is sort of a "perfect storm" for getting oscillation. I think this would be a much more uncommon problem if people would address this as part of the circuit's design. The problem is that most builders have two impediments in their road to success: 1) the obsessive desire to build an exact clone an original vintage circuit, complete with all it's defects, and 2) an inability to solve the oscillation problem from a design engineering perspective.

I don't mean to beat a dead horse, but for anyone who is interested in the subject of designing an amp with a good stability margin, Merlin's book provides some interesting reading. And best of all, it's written in a fashion that you don't have to be an EE to understand it.

[g1]

The problem is that most builders have two impediments in their road to success: 1) the obsessive desire to build an exact clone an original vintage circuit, complete with all it's defects, and 2) an inability to solve the oscillation problem from a design engineering perspective.

Further to this (and not directed at Casey at all), I've noticed a very strong trend among DIY builders that puts looks above all else, no regard for functionality or serviceability. Maybe it can be traced back to the Joyce HiWatts, but I guess it's a lot easier for folks who know nothing about electronics to get all OCD about the looks of the guts, rather than learn about the circuits and functional layout.
Some of these builds I'd rather decline repair, than worry about ruining someone's 'art'. I might get a component lead bend 2 degrees off or something.

[mikepukmel]

From my point of view, in my English to translate songs (sorry):

- The output of the wires towards the equalization and presence is too short. They should be longer to stick to the chassis and then ascend vertically towards the potentiometers. This allows them to be reoriented.
- Same with the connections to anodes/grid/cathodes in preamp tubes. They should be longer so that the anodes and cathodes go down to the chassis and then can be oriented in the best way. Grid wires should be kept high, away from the others.
- The orientation of the preamp sockets is not the best. It is advisable to install the open part of the pins circle facing the circuit.
- Feedback connection is too short and does not allow any orientation. It´s better to be longer and circulate near to the chassis and away from high voltage directed in perpendicular form to the circuit (27K resistor).
- Finally, output line of the driver towards power tubes can be done aerial with shielded cable. The mesh can be connected to the bias voltage (on the other side of the 220K resistors). This can save many problems and once done you can go back to the unshielded lines in the traditional way if you want to compare.

Awesome comments/suggestions, Pedro. Ive been wanting to make a post asking about two of the suggestions you brought up.
1) For vintage amp chassis Ive seen (Ive been collecting lots of images from teh web of 60's-70's era fender and marshall chassis for build info) - the wires leading from the main eyelet board to teh tube sockets all looked conspiculously like the builder pushed the wire down to the board, then bent the wires so that they hit the chassis as soon as possible, then ran the wire right along the chassis to the tube pin connection.
2) The other side of the board, to the pots and jacks, they always ran the wires in twisted bundles. One bundle to the vol/tone and another one to the reverb/tremolo. The only two wires I noticed running alone were from the input jacks and those wires were far from HV side.

Question: does running the wires along the chassis prevent picking up induced signals from other nearby wiring? Is that the only reason for doing that? Re twisted bundles, similar question.

So, the absolute length of the wire is less important, from a parasitic oscillation point of view, than the routing, i.e. along the chassis, at right angles, and or in twisted bundles? Ive seen quite a few suggestions, but not many explanations.
I can understand running two wires in parallel, one induces a signal on the other. Not so sure I can see that a little longer wire, a couple of inches, would add much capacitance, compared to the vacuum tube internal capacitance and other circuit component capacitance. Tell me if Im wrong, but is the problem with long (too long) wires is that they are more likely to pick up induced signals from other wiring or circuit elements?

And, Ive never seen a vintage amp build chassis with lots of wires bundled parallel and tied together, like with some kind of cable connector/wrap.



Thanks!
MP

[mikepukmel]

To my untrained eye, *very* untrained, I dug up a few "vintage JTM45" chassis images picked off the web, the major difference I can see between your chassis and the chassis I have images of, is that your heater wiring isn't twisted tight all the way to the tube sockets, and at the other end (where the green 6.3 v heater leads come from the transformer to the first power tube) the green heater wires are not twisted tight together. I don't know if that makes "the" difference though, re sound, oscillation etc.

[Gregg]

I would check the TS wire (from the SM cap) going to the Treble control. The CF is radiating very strong signal around an my suspicion is it's feeding back to the previous stage/s.

[bob p]

I've noticed a very strong trend among DIY builders that puts looks above all else, no regard for functionality or serviceability. Maybe it can be traced back to the Joyce HiWatts, but I guess it's a lot easier for folks who know nothing about electronics to get all OCD about the looks of the guts, rather than learn about the circuits and functional layout.
Some of these builds I'd rather decline repair, than worry about ruining someone's 'art'. I might get a component lead bend 2 degrees off or something.

I'm with you. I've commented before that some people build their amps with the intent to take pictures of them when they're done to show them off online. I'd rather play my guitar.

Not that there's anything wrong with it, but that "artsy" sort of build thing only works well when someone has bought a professionally manufactured kit that has been through several iterations of design, so that every turret, every hole, every socket has had it's position refined. That's just not possible with a one-off design or a prototype.

Personally, I don't have the patience to go through successive iterations in a one-off design that's never going to go into mass production, so I won't even bother trying. When I see a perfect work of art, I recognize it for what it is.

Anyone can built a perfect looking kit but almost nobody can build a perfect looking scratch-build.

[soundmasterg]

Saying that all marshalls do it is a cop out.

Bob P, I didn't mean with my comment that all Marshalls do it, and if you read my comment above you'll notice I said that all Marshalls or their clones that I have seen squeal when the presence and/or treble control are turned up. It seems to be a trend with this circuit/layout design but since I haven't seen all amps ever made of this type then I can only comment on the ones I have seen. The ones I have seen exhibit this problem, and good lead dress and adding shielded cables in appropriate places seems to minimize the issue, but in the ones that I have seen, it wasn't possible to grossly change the layout as the customers wouldn't allow it. If the layout was redesigned then improvements could be made and this problem might not occur anymore in these amps. I would suspect that the cathode follower circuitry and lead dress may play a part as from what I have seen, cathode followers can be very susceptible to interference and oscillation, especially if the lead dress and layout in that section of the circuit isn't optimized.

Greg

[Enzo]

There is a customer. I want it to work, but don't change anything.

Sheesh.

[Chuck H]

Ok, I only browsed what's been posted since my entry above and some of this may have already been covered, but...

I would at least try snipping the ground buss wire on the back of the pots between the bass and presence controls. Ground the buss section on the preamp and tone stack near the input. Ground the presence pot away from the input. I can't see where the PI is grounded, but ground that and it's filter node with the presence pot away from the preamp ground buss. Then lengthen the tone stack leads just enough to push them up against the face of the chassis and lengthen the presence lead enough to route it up and away from the preamp.

It seems like a PITA, but it shouldn't take long really once you're at it.

JM2C

[catalin gramada]

or do one of this:



(massive bus from a plexy clone made from scratch few years ago- standoffs isolated from chassis with nylon washers)