JTM45 Build with issues

[Tone_Chaser]

I have recently completed my JTM45 build. While going through the turn up process, my voltages looked okay but when I got to the point of adding the power tubes and turning on the standby I had a very loud buzzing or motor boating sound. I installed the grid stopper resistors and that had no affect at all.

I will post some progress and current pictures along with a partial voltage chart to show where I am in the build.

There were some issues discussed here before with squeal and motor boating going on with the standby switches. Per the notes in that thread, I can say that I am using SOZO caps, JJ KT66 tubes. The bias circuit is grounded on the pot bus. The 32 & 16 uf caps are grounded to the same point. I am using the F&T cap on the preamp board and the OT wires are running paralell to the grid wires, although they are fairly short and close so they don't run very far together.

I had also planned on using the star ground method but I had made some ground connections using the step by step instructions before checking the layout for the star ground so I didn't end up doing it exactly like the layout. I don't know if that could cause an issue or not.

Ground points:
1. Inputs and V1 cathode
2. Volume POTS, V2 cathode and preamp filter
3. Mid and prescence POTS, output jacks
4. Bias circuit
5. Screen/main/PI filter cap ground, HV CT (via fuse), Heater CT
6. Power cord ground

V4/V5 are grounded to the mounting lug on the power tubes.

More information to come.

[bill bokey]

I sounds like what happened to me a few years ago : the OT was wired the wrong way around. You might want to try that

I see Neil suggested the same on the other topic...

[Tone_Chaser]

I've been really busy and am just getting back to my build. I bought a blank alumimium chassis and laid out all of the components and pretty much followed the 45 build directions from there.

[Tone_Chaser]

Transformers installed but I can't seem to find the rest of the progress pictures. I did try both speaker jacks and am now getting a crackle with all knobs at zero.

There was no wiring diagram with the output selector but I wired it as I have seen in other pictures and I think I checked it with a meter before making any connections. It should be right I'll take some more pics and you can tell me what you think.

[Tone_Chaser]

This is the impedance selector switch. There was no wiring diagram and I couldn't find one. I did find some pictures and that is how I wired it. I think I used a meter to verify but I can't remember for sure. Please take a look at it and let me know if you think there is something wrong with it.

[Tone_Chaser]

Here are the rest of the gut shots. Please look them over and see if you see any errors that would cause this issue. I unplugged the speaker and tried to quickly finish the rest of the voltage readings with no load on it and it started buzzing in a very high pitched hum so that makes me think maybe something is wrong with the OT wiring.

[Tone_Chaser]

The files for the transformer wiring diagrams were too large so this is the link to them on the site.

http://www.classictone.net/Marshall-Sty ... rmers.html

I used 40-18033 & 40-18039

[danman]

Did you switch the OT wires around on pin 3 of the power tubes like it was mentioned above? You mentioned that you disconnected the speaker to finish your voltage checks. It's never good to disconnect the speaker load when you have the power tubes installed. That in itself can damage the OT. Much better to use a resistive load during testing to protect the OT from damage. I sometimes use my attenuator set at full attenuation to test when a speaker isn't possible.

[danman]

I have never used that impedance selector myself but I am wondering what the black lead coming off the switch is for? It looks like you have the 4, 8 and 16ohm leads wired correctly with the yellow(common) lead going to the sleeve connection of the output jack but I am not sure what the black lead is for. You should have a ground lead attached with the yellow at the output jack but it appears that your is going to the selector switch instead of a ground point.

[Tone_Chaser]

Thank you for the reply. Sometimes I need to be hit over the head for things to sink in. I was only thinking about the impedance selector side of the OT when he mentioned it might be backwards and over focused on that. I'll check that tonight!

The black wire goes to the tip of the output jacks. It didn't have a center lug like the Marshall style does.

Thanks again!

[Tone_Chaser]

I just checked it and the black wire goes to V5 and the red wire goes to V4 which is how it is pictured in Elad E's thread using the same OT. I was going to try to switch them anyway but neither of my soldering irons are here. I'll try it tomorrow.

[Tone_Chaser]

I swapped the OT wires on the power tubes and the howling is gone but it sounds like it's making popcorn. Any ideas on that one?

[danman]

I was checking your pics to see if I noticed anything. Do you have your board jumper wires mounted underneath?

[neikeel]

Too late for now but much better if builds are made post 69 style with jumpers above and wires looped up through board and wrapped around turrets.
Those turrets look great for big and multiple leads but seem a bit empty of solder are you sure about them? Crackling sounds like an intermittent or arcing connection (dark room and look for flashes is a method that has been advocated, as does chopsticking.

Sorry but with the leads all bundled and some loopy wires of non-stock colours I cannot tell what is what. Having said that danman has been doing a good job of trouble shooting recently

[danman]

Thank you for the kind words Neikeel! After years of reading and learning here on the forum, I hope that I can give back a little to help the up and coming builders.