MOJOTONE Marshall British Style 18W TMB BUZZING!! Cant find the gremlins!

[rud85ny]

 
Built a British Style 18W TMB Guitar from MOJOTONE. The amp technically works but the gain channel has issues with distortion and hiss when the Gain and Volume are turned up past 5. Gave it to my local amp shop, they told me they cant pin-point the issue and it will cost more to repair than the amp is worth. 

Was also informed that the sound is some oscillation from the pre-gain stage. Pinned out all of the connections and component values, verified all connections are identical to the ones as per MOJOTONE layout.
D-Labs has a great video on inductance noise coming from the output transformer leads and there's a good mod to rout them outside of the chassis away from interference. I will defiantly try that.

If any one had similar experiences with these 18W marshal TMB builds, Ill gladly take any info.

[pdf64]

The layout looks messed up - OT primary close to the input, gain and master are bizarrely swapped over.

As a first step, does using screened cable between input socket and input stage grid socket lug help?

[glass54]

+1 with pdf64
Am I hearing oscillation when you wind up the Volume? Would be good to get an Oscilloscope onto the output while injecting a tone on inputs (say 30mV at 400Hz) As pdf said "screened cable between input socket and input stage grid" plus some earthing Mods might get you out of jail.
Mojotone's Layout uses a horrible grounding scheme, Cathode Bias grounding, decoupling for Preamp and Inputs all linked https://cdn.shopify.com/s/files/1/0604/9615/0624/files/British_18W_TMB_WD.pdf?v=1750265870
All the best rud, I'm sure you'll get there
Regards
Mirek

[rud85ny]

@PDF64 & @glass54, Thanks for the info!
I will place the OT closer to the power tubes, with its laminations oriented at a 90° angle to the Power Transformer. I did double up on the grounding for all components including the POTs because yes the MOJOTONE kit definitely is  poorly grounded. Il pull the wiring and re wire everything tighter with as less wire material as possible. I will also redo the heating grid wires to Fender over the top/elevated style routing, I think this is the most efficient way to run heaters As per advice, will defiantly swap all the input wires to shielded as recommended.

What about the gain and volume "clipping" sound at high volumes? can that be attributed to bad preamp tubes or some faulty components ?

[pdf64]

I think the PT and OT orientation is good, the issue is that the OT especially its primary wires are insanely close to the input.
Under the board, I suspect the unscreened input wire may be extremely close to the primary wires.
If moving the OT, I suggest to use the secondary wire chassis hole as a pivot point, flip the OT 180° so that the primary wires have a new chassis hole that will be much closer to the PT than the input.
New OT mounting holes will be required.
Then route the primary wires so the all twisted and clear of the preamp and LTP circuitry.

[rud85ny]

Will defiantly do that.
Also looked into running a NEGATIVE FEEDBACK LOOP from the inverter circuit 47kΩ resistor and inserting a new 100Ω to ground, then junction between 47kΩ and the new 100Ω Resistor to my 8 ohm speaker output. Read good things on this mod.

lastly maybe also installing a Snubber Cap on the EL84s.
Hopefully i can knock this out next week and report back.

https://www.18watt.com/viewtopic.php?t=2226

[acheld]

Make one change at a time -- makes debugging so much easier.

[pdf64]

Your call but I suggest not to rush and not to apply bodges / stocking plaster, 'there I fixed it' solutions in an attempt to resolve fundamental issues.

[AlNewman]

I can't open the mojotone layout, but with oscillations, sometimes the best thing to do is move wires around with a chopstick under signal load, and see where the best position is.
This includes your input wires and any sensitive signal wires.  The OT wires can be especially sensitive when near signal wires.
Obviously a shielded input wire should be a standard, and perhaps other shielded signal wires as well, but you can learn a lot by just moving things around.

[shaun]

+1 with acheld. Relocate the OT and do new lead dress first, and maybe the problem will disappear. If not, you know it wasn't that. If it works, you'll know that was the problem rather than some other mod you did at the same time. Also, mods don't always go as planned.

Looks like you're doing nice work though, and it'll be a decent amp once sorted out.