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Discussion in 'The Workbench' started by Yamariv, Aug 20, 2020.
But it was derived from the 18w Marshall that was only offered about a year.
Quote from reverb :
Marshall also made a series of 18-watt combo amps around the same time in 1965. Sometimes referred to as the Bluesbreaker’s little brothers, they used much smaller EL84 power tubes. The 2x10 configuration was known as Model 1958, while the 2x12 was Model 1973 and the 1x12 was Model 1974. (Remember: Marshall
That spurred several community’s about the 18w schematic since it was readily available and was much cheaper to build. To this day I bet it is the most copied amp out there, non fender.
They were built in 18w form for 65,66 and 67. Superseded by the 20w non trem models. Always NMV. The question posed was What is the model Number of a TMB? To which I replied there is not one. As you say the TMB is a much more recent version that blends preamp gain with output gain at potentially lower volumes.
I’ve built a few based on different iterations - original spec and TMBs. They all seem to have their own thing going on.
They do for sure. I do like Mojo's take on the TMB channel. It's consistent and very old school in component selection. The tone stack is really balanced well, too. One thing I didn't get into was inquiring much about speakers, current guitar (pickups) and a lot of those externals can have bearing on where the tone stack settles with the current build. Not always something "wrong", but could be.
Ok, update time!!
Life got in the way so I was a little delayed but I finally got all the parts in and the amp circuit switched over to MojoTone's layout. I must say, I bet just completely starting the build fresh would have been faster than tracing it all component by component and swapping them out. Good thing about that though is it really forced me to use the schematic and understand exactly what was going where and why!
Ok, so I wish I had great news but I spent the day trouble shooting with no luck and need some experienced help!
The TMB Channel is giving me problems through the Master Volume and Gain knob.(Normal Channel seems to work as it should..) When they are turned up and down, I don't get the normal hiss through the speaker, I get a bunch of noise like an old ham radio being tuned in. I've filmed a quick video to show what it does and will put the link below. At the end of the day, the same noise was there but it also started a rapid soft motor boating sound as I turned the volumes way up!
I've traced every wire and the whole board twice testing continuity and everything looks right. I'm sure it's something simple but I can't see anything wrong at all. Have anyone heard this noise before and are maybe able to point me to a specific area to check out and focus on?
I've also swapped out the jack and the 3 preamp tubes with no difference..
Here's the link to the video..Thanks for any help you can provide guys!
Are you missing the 1M ground on the switching jack on that input)?
The whining when you turn the pot is the tuning of a capacitor/resistor RF circuit and in this context maybe a missing ground. In the non-used channel.
Can you post some clear pics of your input jacks, pots and wires f so that we can see where the wires are hooked up to. May need a voltage chart and some readings. Motor boating can be a phase issue and TMBs have an input to each side of the pi, which is why you cannot normally jumper the channels.
Another thing to look into is to try a temporay lift of your FX loop board up and see if the noise changes. Placement of that circuitboard over preamp tubes can be crucial, as noted in several threads by NewReligion, aka David Hopkins.
The 1M resistor on both channels are there for sure, I did rewire the jack completely to just make sure it was ok(you'll see the old temp stereo jack installed) and it still made the noise you hear in the video. The MojoTone TMB adds a 68k resistor to the input which wasn't there before.
So what can I check about the phasing of the channels? I've taken some close up pics and will put them below. On another note, could a bad cap do something like this?
I effects loop worked well in the previous design in that same spot but I'll move it around next time i'm working on it to be safe for sure!
Ok...peek at the resistor parallel the .68uF cap on stage 1...should be 2.7K. But, looks like you have a yellow band in there which indicates 270K...measure that resistor please.
Yeah, certainly off. Your key colors should be Red-Violet-RED not Yellow as the third band. I find the best way to avoid hunting mistakes is to buy yourself an inexpensive LCR meter and actually measure each component you install. Will save you a LOT of headaches as you're experiencing in this phase of a build.
Also, that 270K is large enough that you can solder a 2.7K resistor parallel to that one and still be in tolerance. That would quick-test see if that's your culprit.
Put a 68k resistor at the input grid of each channel, not sure why that’s not in your schematic.
Next, wire up the TMB channel ala JCM 800/Plexi bright channel, with a 470p/470k mixer on the right lug of the gain pot and a bright cap.
Do a 2.7k/0.68uf on the first cathode.
Bright cap can be anywhere from 470p-4700pf. Higher values are more ice-picky.
One of your volumes might not be wired correctly. When i wired up my 2203 clone into a 1959, it made similar noises like that and later saw my volumes were wired wrong.
Well, that was about 4hrs of trouble shooting this afternoon and nothing accomplished boys..so frustrating! I was all excited about the resistor you found and tackled that first! Ended up being a 2.75k resistor, I guess I ordered a close enough resistance when I first built the amp. I have other 2.7ks now and they are labelled exactly as you said they should with no yellow band. No dice so I moved on to all the things bellow..easier to do point form cause it's alot of things! (insert crying face!..)
- Pulled the Effects loop out of the circuit and tested incase..Nope
- Changed the power tubes
- Changed the preamp tubes
- Rewired the normal channel hi and low input jacks to match the mojotone circuits version just incase it was clashing with the TMB channel..nope
- Checked and resoldered every ground on the the ground rail, touched up every solder joint on the pots..
- Checked every capacitor and resistor in the board, all checked out fine
- Tested the two volume and gain potentiometers on the TMB Side giving me issues, both were functioning fine
- Followed the entire preamp and board layout tracing everything over again with continuity, all in order.
- I noticed the phase inverter pins were using opposite triodes than than the mojotone layout..swapped the PI triodes to match the layout perfectly and thought this was my problem cause of some phasing issue..NOPE!
- Last resort was chopsticking everything to find some intermittent connections..nothing
I have no idea what to check now boys..That same sound in the video is still there, nothings changed. What else could cause that? It couldn't be the Output Tranny cause the Normal Channel works fine right?
I'm going to go cry!.. Lol
this is the "fun" part. and also why sometimes u have to move onto other builds/other projects and come back with fresh eyes
this is one of the benefits of repurposing old organ chassis and building my own enclosures: often I'll get an idea while working outside. or while trying to record. or while sitting in church. sometimes i need the unconscious mind to do the figurin'
So, for whenever you get back to this. When you resume...pull that loop board completely out of there...disconnect the power taps, the IN & OUT...all of it. Also, run a solid wire to the backs of all the pots (can't count on a chassis connection to reliably get those pot shields to ground potential). Also, your B+ to Standby switch seems to be buried in a hot mess of AC wiring at the power transformer...clean that up by separating as much as you can...reroute as needed.
Then let's see where your issues are. Seems to me that you do have a decent clean channel? Correct? But, that TMB channel is the problem. I have a feeling it could be a wire dress issue, but it's hard to follow your wiring completely.
And, post up even a hand drawn schematic (pencil in changes on the schematic used...take a pic and post) so we can see exactly what you believe it to be. Else we're all trying to second guess "what" exact schematic you're trying to follow as I know you've certainly combined more than one schematic from what I'm seeing. We need a crystal clear reference.
When you pull the loop board out...pay real close attention to the master control connections...pot shape...and give us a real good look at the input jack...photos of both sides.
Still been avoiding the amp since my last afternoon of diagnosis! Not sure when I'll try to tackle this again, walking away for a bit is a must. I'm really tempted to start the board over and do the exact full mojotone layout and start over testing each component again before I put them back in. I'm thinking just doing that might be quicker than the way I switched over some parts of the layout. The board parts should be easily transferable as they are already cut and bent into shape. I know doing a grenade repair isn't the best to hone my building skills but I am contemplating drastic measures at this point! Lol
Before I completely redo it, I'll definitely pull the loop and fix any dressing issue you pointed out
Oh, on a side note..I a bought a used Scope and it's on its way now. I ended up picking up a working Tecktronix 2235! Now I just have to learn how to use it! Haha
I am afraid that it is near impossible to troubleshoot and amp remotely when there is no pattern or code to your hook up wires (at least if you are me, someone more skilled who knows the variants you used inside out might). They are all blue or brown and a lot of the components are difficult to read the values.
It is weird that the normal channel is totally normal and you will find something silly (like a grid wire shield double grounded or a misplaced link).
If you do redo it I urge you to follow a code/pattern for your wire codes (cathodes=yellow, grids=green, plates=blue, voltage supply=red, ground=black) not necessarily those but be consistent.
Please keep us posted (maybe your new toy will get you there?
Ok boys, small update and a video link with my 1st Oscilloscope effort! I started by removing the Effects loop completely and also later rewired the TMB side input with all new components after the video with no change unfortunately
So in the video, it seems to me that the noise is entering right between the the input and the first preamp tube grid which doesnt make sense to me, there's nothing there to inject a noise Am I missing something in the video? Can an issue later on down the signal path be injected backwards into the signal at the first grid or am I missing something?
Oscilloscope pros, if you can please check out the video and let me know what you think and maybe suggest a next step before I just decide to scrap the whole wiring and start the build from new. I would have done that already but if it's a simple faulty component chances are I'll re-use it and be in the same boat after the rebuild I'm also kinda hesitant to throw all the components out as they are pretty nice quality and that would be a waste..
Here's the link