View attachment 136231
this is what i believe to be correct!!
ive also read that putting in a PPIMV wont help get the amp sounding good when cranking it as it wont drive the output tubes which is the key to get this amp sounding good, chaning the preamp to 2204 ive read is the mod which will improve this?
any other mods which could help?
is it worth doing the "Blackflag" version" or is this the same mod?
Check this article about Marshall history. Black flag refers to the reverse printed logo that was on the front of a small number of amps in late 1967. After the JTM45 was upgraded to the JTM45/100 by adding two output transformers, Marshall released a dedicated 100 watt amp with a single big drake OT. I believe at the beginning of this run (mid 67?) the letters JTM were printed in black next to the power switches. For a couple months at the end of the year there was a black box with the letters J.T.M. reverse printed in this same spot. The letters were gold b/c the faceplate shown through, hence the name black flag. It's not a mod but a specific run of 100w JTM100s with the big Drake OT.is it worth doing the "Blackflag" version" or is this the same mod?
Motorboating is usually caused by insufficient power supply decoupling. Does it change when you turn the gain control?all mods seem to have gone well but i still seem to have this odd motor boating
Is your output transformer phasing correct? What happens if you swap over the power tube anode connections?and when i turn up the presence control i get a squeal?
This - I read somewhere that it was a factory part added to decrease oscillation in some amps when they did large production runs. Not necessary in one-off amps and for your case with loop like Merlin said.I would remove the 47pF between the phase inverter anodes
220k should go on the junction of the 50k rheostat and 56k resistor. One leg of the rheostat connects to the 56k/220k grid resistor and the other leg goes to gnd. Adjust the rheostat to give you lower resistance between that junction and ground.how does this look?
That change was for the sqeal with presence dimed. Did that fix it?have moved the 47pf and the motorboating is still there, dont know if the sound is any different, doesnt seem to be.
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