Results 1 to 30 of 30

Thread: JTM45 build presence oscillation

  1. #1
    Member
    Join Date
    Jun 2006
    Location
    Southwest U.S.A.
    Posts
    261
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Rep Power
    14

    JTM45 build presence oscillation

    I just finished building a JTM45 clone. I got the chassis from AES because they had the chassis and front/back faceplates for a great price. I got most of the parts I didn't already have on hand from AES, including a Hammond OPT and choke. But I had to get the power tranny from Mojo because it was the only one that fit the chassis. I took my time and it fired up first try. I've tried it with VA KT66 and VA EL34 power tubes. It's my first JTM45 experience and it lives up to the reputation as far as tone. I went with the low voltage power tranny (300-0-300) and get around 390 VDC on the plates. Playing a Les Paul with Dimarzio 36th Anniversary PAFs it starts breaking up before noon.

    There are two issues I'm having. One is when I turn the presence up past around 7, I can hear a faint high pitched oscillation depending on where the tone controls are set. The other issue is when cranked up I can hear some ghost notes in the background. I went with original specs and I know modern amps have more filtering than the JTM45 did. I didn't scrimp and used the best parts I could buy, Cliff, CTS pots, F&T filter caps, Sozo, Carling switches etc. The presence feedback resistor is connected from the board via a short jumper from the turret to the 16 ohm tap on the OPT impedance switch.

    Any tips on layout and how to address the ghosting issues?

    1 Not allowed! Not allowed!

  2. #2
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,797
    Thumbs Up/Down
    Received: 2,659/4
    Given: 3,100/0
    Rep Power
    30
    Regarding the oscillation. Lead dress is CRITICAL with this style of amp. Make sure the presence circuit lead from the impedance switch is routed away from the preamp leads. Make sure grid leads are not longer than they need to be. Make sure OT leads (primary AND secondary) are also not longer than necessary, twisted together and routed away from the preamp.

    Regarding the ghost notes. Double check your wiring for the power supply. If the power supply is wired correctly with good working caps it shouldn't ghost. You may have a wiring error that is preventing the necessary filtering and/or decoupling.

    1 Not allowed! Not allowed!
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  3. #3
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    3,282
    Thumbs Up/Down
    Received: 747/6
    Given: 1,488/10
    Rep Power
    18
    All I can say is, switched/reversed plate leads on an OT are not "guaranteed" to cause wild howling or screeching. The bad effects can sometimes be subtle. Hey, it's free, quick, and easy to check...

    Justin

    1 Not allowed! Not allowed!
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  4. #4
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,909
    Thumbs Up/Down
    Received: 77/0
    Given: 0/0
    Rep Power
    18
    assuming that you've signal traced the amp, what's the frequency of the oscillation and which stages have it?

    1 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  5. #5
    Senior Member
    Join Date
    Jan 2014
    Location
    Chiraq
    Posts
    1,002
    Thumbs Up/Down
    Received: 213/5
    Given: 250/1
    Rep Power
    8
    I was gonna say can you remove V1, then V2, then V3 to start honing in on what stage is oscillating. Seems like it would be visible on a scope, or if you don't have one just set the amp up to the edge of oscillation and listen through a speaker (probably easier anyway) and start chop sticking some wires around to see if moving any of them makes it better or worse.

    1 Not allowed! Not allowed!

  6. #6
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,816
    Thumbs Up/Down
    Received: 555/1
    Given: 484/2
    Rep Power
    18
    There are a few schematics described as JTM45; please provide a link to the exact one used.
    Some of these don't have power tube grid stoppers; if that's the case with yours, suggest that something in the range 1k5 to 10k is fitted to each power tube control grid socket terminal, 5k6 being the classic value with these.
    As noted, good practice with balanced systems such as heaters and power amp signals is to tightly twist the wires carrying the balanced signals.
    Some detailed photos of the chassis internals may help with remote diagnosis.

    1 Not allowed! Not allowed!
    Last edited by pdf64; 08-30-2017 at 11:06 AM.

  7. #7
    Member
    Join Date
    Oct 2015
    Posts
    160
    Thumbs Up/Down
    Received: 21/0
    Given: 0/0
    Rep Power
    5
    you must have done something right because that JTM 45 is famous for the Marshall Squeal.

    maybe not turn the Presence up so much?

    is the amp in the cabinet? does it have a steel bottom plate?

    how far from the amp were you when testing for squeal?

    get some chopsticks and move OPT leads around while squealing.

    maybe raise the ends of the grid stoppers up in the air like they do on a lot of marshall amps.

    try changing resistor values in the Presence circuit, sometimes you see two different values printed on the schematic, this could mean they ran into the same thing at the factory and revved the value.

    try a lead from the speaker frame to the amp chassis.


    OT: want the Blues Breaker sound? sport down for some brown base Genelex tubes for insane midrange detail like you have never heard in your life! yeah there $$$ but they are the dope. just a question of how bad you want good sound.


    Click image for larger version. 

Name:	kt88.JPG 
Views:	87 
Size:	212.7 KB 
ID:	44705

    1 Not allowed! Not allowed!
    Last edited by cjenrick; 08-30-2017 at 09:58 AM.

  8. #8
    Supporting Member mozz's Avatar
    Join Date
    Oct 2015
    Location
    NEPA
    Posts
    840
    Thumbs Up/Down
    Received: 197/1
    Given: 138/0
    Rep Power
    5
    As was said, these are known to squeal. One schematic it puts DC on the presence pot, there is a mod for that. Just built one last year and it squealed above 7-8.

    1 Not allowed! Not allowed!

  9. #9
    Member
    Join Date
    Nov 2013
    Posts
    320
    Thumbs Up/Down
    Received: 125/11
    Given: 40/0
    Rep Power
    7
    i tried recently a replica with KT66 and mercury Magnetics irons,stock circuit,and sounded perfect at any volume,no squeal no noise.
    For sure you have to check out the feedback wire from the board to the presence pot,keep it further away from the preamp,but having so much feedback,27k on a 16 ohm tap,is also a problem ,it could lead to oscillations,i'd try a 100k and maybe even a lower tap just in case.
    Post pics of the build.

    0 Not allowed! Not allowed!

  10. #10
    Senior Member
    Join Date
    Jan 2014
    Location
    Chiraq
    Posts
    1,002
    Thumbs Up/Down
    Received: 213/5
    Given: 250/1
    Rep Power
    8
    Quote Originally Posted by pdf64 View Post
    As noted, good practice with balanced systems such as heaters and power amp signals is to tightly twist the wires carrying the balanced signals.
    When you say power amp signals do you mean primary and secondary wires of output transformer or something else??

    0 Not allowed! Not allowed!

  11. #11
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,816
    Thumbs Up/Down
    Received: 555/1
    Given: 484/2
    Rep Power
    18
    I meant that any wires carrying balanced signals, ie identical signals of opposing polarity https://en.wikipedia.org/wiki/Balanced_audio should be twisted, in order to minimise their susceptibility to interference and their radiated field.
    So in a regular p-p tube amp, the wires between phase splitter outputs and power tube control grids, and those between the power tube plates and OT primary; and the heaters.
    Hope that makes sense now, apologies for phrasing it poorly before.

    1 Not allowed! Not allowed!

  12. #12
    Senior Member
    Join Date
    Jan 2014
    Location
    Chiraq
    Posts
    1,002
    Thumbs Up/Down
    Received: 213/5
    Given: 250/1
    Rep Power
    8
    No worries thanks for the answer. I never thought to twist the PI --> screen grid wires. So now I'm a little smarter

    I also wondered if you possibly meant the HV power supply node wire and the ground return wire from cathode of gain stage to cathode of power supply cap

    1 Not allowed! Not allowed!

  13. #13
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,797
    Thumbs Up/Down
    Received: 2,659/4
    Given: 3,100/0
    Rep Power
    30
    Quote Originally Posted by nsubulysses View Post
    I never thought to twist the PI --> screen grid wires.
    Me either. I always connect the PI outputs to the control grids

    I don't imagine twisting the screen wires is necessary. And I've never twisted my PI output leads, but it makes sense. A lot of guitar amp desgns include a snubber cap across the PI outputs, but some don't. I think the twisted leads could contribute more stability than the cap with less HF attenuation. Just the thing if your non snubber clone or design is misbehaving

    1 Not allowed! Not allowed!
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  14. #14
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,816
    Thumbs Up/Down
    Received: 555/1
    Given: 484/2
    Rep Power
    18
    Can the ghost notes be described in more detail (eg akin to a ring modulator) or better, clips of the issue be recorded and made available?
    Is the issue worse/same/better with EL34 (compared to KT66)?

    1 Not allowed! Not allowed!

  15. #15
    Senior Member
    Join Date
    May 2006
    Location
    Cornelius, Oregon
    Posts
    1,420
    Thumbs Up/Down
    Received: 114/0
    Given: 208/2
    Rep Power
    15
    Every amp I've ever seen that was a Marshall or a similar clone such as an early Traynor will oscillate like that when the presence is turned up. Some do it when the treble is turned up too. I couldn't see anything on a scope the last time I worked on an early Traynor for someone and it was doing that. I just followed best practices for lead routing and was able to minimize it. Each circuit is different though so if could link to the schematic that you used, that would certainly help in troubleshooting.

    Greg

    1 Not allowed! Not allowed!

  16. #16
    Member
    Join Date
    Jun 2006
    Location
    Southwest U.S.A.
    Posts
    261
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Rep Power
    14
    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.

    1 Not allowed! Not allowed!

  17. #17
    Member
    Join Date
    Jun 2006
    Location
    Southwest U.S.A.
    Posts
    261
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Rep Power
    14

    JTM45 build pics

    Click image for larger version. 

Name:	DSC_0021.JPG 
Views:	270 
Size:	726.0 KB 
ID:	44796Click image for larger version. 

Name:	DSC_0022.JPG 
Views:	386 
Size:	731.6 KB 
ID:	44797Click image for larger version. 

Name:	DSC_0023.JPG 
Views:	321 
Size:	709.0 KB 
ID:	44798

    1 Not allowed! Not allowed!

  18. #18
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,909
    Thumbs Up/Down
    Received: 77/0
    Given: 0/0
    Rep Power
    18
    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.

    1 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  19. #19
    Member
    Join Date
    Jun 2006
    Location
    Southwest U.S.A.
    Posts
    261
    Thumbs Up/Down
    Received: 3/0
    Given: 0/0
    Rep Power
    14
    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

    0 Not allowed! Not allowed!

  20. #20
    Senior Member Pedro Vecino's Avatar
    Join Date
    Jul 2006
    Location
    Spain
    Posts
    685
    Thumbs Up/Down
    Received: 213/2
    Given: 104/0
    Rep Power
    14
    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. Itīs 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.

    1 Not allowed! Not allowed!

  21. #21
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,909
    Thumbs Up/Down
    Received: 77/0
    Given: 0/0
    Rep Power
    18
    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.

    1 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  22. #22
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,060
    Thumbs Up/Down
    Received: 1,830/24
    Given: 4,684/11
    Rep Power
    23
    Quote Originally Posted by bob p View Post
    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.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  23. #23
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,454
    Thumbs Up/Down
    Received: 164/0
    Given: 2,485/0
    Rep Power
    4
    Quote Originally Posted by Pedro Vecino View Post
    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. Itīs 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

    0 Not allowed! Not allowed!

  24. #24
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,454
    Thumbs Up/Down
    Received: 164/0
    Given: 2,485/0
    Rep Power
    4
    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.

    Click image for larger version. 

Name:	Interior1a.jpg 
Views:	1675 
Size:	623.2 KB 
ID:	44808

    0 Not allowed! Not allowed!

  25. #25
    Senior Member
    Join Date
    May 2006
    Posts
    616
    Thumbs Up/Down
    Received: 37/0
    Given: 0/0
    Rep Power
    14
    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.

    1 Not allowed! Not allowed!

  26. #26
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,909
    Thumbs Up/Down
    Received: 77/0
    Given: 0/0
    Rep Power
    18
    Quote Originally Posted by g1 View Post
    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.

    1 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  27. #27
    Senior Member
    Join Date
    May 2006
    Location
    Cornelius, Oregon
    Posts
    1,420
    Thumbs Up/Down
    Received: 114/0
    Given: 208/2
    Rep Power
    15
    Quote Originally Posted by bob p View Post
    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

    0 Not allowed! Not allowed!

  28. #28
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    32,546
    Thumbs Up/Down
    Received: 2,068/7
    Given: 0/0
    Rep Power
    55
    There is a customer. I want it to work, but don't change anything.

    Sheesh.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  29. #29
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,797
    Thumbs Up/Down
    Received: 2,659/4
    Given: 3,100/0
    Rep Power
    30
    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

    1 Not allowed! Not allowed!
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  30. #30
    Senior Member
    Join Date
    Mar 2008
    Posts
    832
    Thumbs Up/Down
    Received: 101/0
    Given: 229/0
    Rep Power
    12
    or do one of this:

    Click image for larger version. 

Name:	bus.jpg 
Views:	131 
Size:	29.6 KB 
ID:	44817

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

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. JTM45 build, static
    By ColeC in forum Maintenance, Troubleshooting & Repair
    Replies: 27
    Last Post: 09-13-2016, 01:26 AM
  2. JTM45 build advice
    By carryonplease in forum Build Your Amp
    Replies: 4
    Last Post: 11-19-2014, 11:22 PM
  3. I did something stupid to my JTM45 build
    By mort in forum Guitar Amps
    Replies: 55
    Last Post: 09-25-2013, 03:11 AM
  4. JTM45 squealing with presence controll wide open.
    By Bluefinger in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 06-20-2011, 06:51 PM
  5. Hiwatt DR-504 oscillation; presence control
    By Chevy in forum Maintenance, Troubleshooting & Repair
    Replies: 6
    Last Post: 06-05-2010, 02:31 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •