JTM45 Build with 6P3P output tubes and GZ34

[Brian Roth]

Might be motorboating.

https://en.wikipedia.org/wiki/Motorboating_(electronics)

ri

 

[Spencerleehorton]

Some notes start the motorboating and some notes stop it!? With the amp upside down it shakes so I don’t know if I turn it the other way whether it will do it?

 

[Spencerleehorton]

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?

 

[Spencerleehorton]

Tested with the amp upside down proper way and does motor boat when bass over 4 and ch2 up over half way!!
Also the mains txf is warm to the touch, this ok? I’ve got a heat meter to get exact temp!?

 

[dai h.]

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?

actually I think one of the main reasons for installing a post- (as opposed to a pre-) phase inverter master is to not lose the PI's contribution so you can get enough distortion (assuming the player wants the amp to do that). (As I vaguely understand) I think the more modern ones are set up so there is adequate distortion before the PI (and don't rely on the PI distortion). I found a Youtube video somewhat recently where you can hear the difference as the signal is tapped into and heard going through the successive stages from input to speaker output (the amp circuit is the bit later version with split cathodes, etc. but basically the same thing) :



p.s. perhaps try the version of the PPIMV (which Dave Friedman apparently likes (and which might also be found on the latest Suhr versions of a Plexi) where the dual pot (wired as a variable resistor and not a potentiometer) is wired over the existing grid load/bias return Rs (so the grid load/bias return Rs go down as the pot is turned down). (I think the total R is about 100k ea. side (assuming 220k fixed Rs and 220k or 250k dual pot)).

 

[StarTrucker]

I've also always heard post PI is the way to go on Marshalls since PI distortion is an essential part of the cranked plexi sound.

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.

After the black flags came the JMP amps, which are the classic 1959 super lead and 1992 superbass circuits.


https://reverb.com/news/a-history-of-marshall-amps-the-early-years



As for motor boating, if you have a wiring issue that pulls too much current from the power transformer it can mechanically vibrate. Maybe you're picking that up on a microphonic tube. Perhaps the bass notes require more current and it starts the PT vibrating.

 

[StarTrucker]

How does the triode switch work? Doesn't that just lower the screen voltage by taking it from the B+ supply instead of the OT primary? Wouldn't that still be Tetrode mode.

Is the tap off the OT primary sort of emulating an ultra linear set up?

 

[Matador]

The switch is hard to make out but it looks correct: when the bottom two contacts are engaged, then the screens are connected to the respective plates (triode). When the top contacts are connected, then the two screen nodes are connected after the choke through the 1K resistor (tetrode).

 

[Spencerleehorton]

I have a few mods to now try, V1 split/shared cathode, NFB 3 way switch, V2 cathode bypass switch, tonestack bypass, presence cap switch (0.68uf/0.1uf), PPIMV and FX loop.
Also adding in repacing the 16uf for another 2 x 32uf seems to help.
may look at a SS rectifier switch but i already have that in my plexi so pondering this one.

 

[Spencerleehorton]

all mods seem to have gone well but i still seem to have this odd motor boating and when i turn up the presence control i get a squeal?
the squeal has been there from he start i was just having the presence down?

any ideas how i can say goodbye to the squeal and the motor boating?

 

[merlin]

all mods seem to have gone well but i still seem to have this odd motor boating

Motorboating is usually caused by insufficient power supply decoupling. Does it change when you turn the gain control?

and when i turn up the presence control i get a squeal?

Is your output transformer phasing correct? What happens if you swap over the power tube anode connections?
I would remove the 47pF between the phase inverter anodes, as it tends to make loop stability worse. In fact, it would probably be better placed in parallel with the 27k feedback resistor.

Your diagram still has a drawing error in it around the bias trimmers -see where the 220k grid leaks connect.

 

[Spencerleehorton]

with the OT Primaries i had swapped them over as there was the horrible squeal when i first turned on!!
the motorboating stops when i turn down bass or CH2 i turn down if i have both channels linked.

so remove the 47pf and put it in parallel with 27k NFB resistor, i'll try that.

cant see the mistake sorry, i thought thats how i have it at the moment? if you mean the lines are not connected they are meant to be like that, just imagine they are connected.

 

[Spencerleehorton]

i thought i read something about it could be the 1R resistors on the cothode of the OT?

have moved the 47pf and the motorboating is still there, dont know if the sound is any different, doesnt seem to be.

 

[Spencerleehorton]

Ah yes I haven’t done the gnd connections correctly on the bias trimmers!! Oops!!
how does this look?

 

[StarTrucker]

I would remove the 47pF between the phase inverter anodes

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.

how does this look?

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.

If your amp works in tetrode mode, I would think that your problem is likely not in the tone stack, coupling caps or another part of the preamp. Maybe check NFB wiring too.

 

[Spencerleehorton]

I will update schematic properly as I’ve put in several mods now which all sound really good.
I’ve got a funny feeling something’s not right with the input jacks as I noticed when they were not linked the ch2 volume did have an effect when I turned it up half way!!?
I’ve put in a 3 way switch for the NFB now, the 27k is still there leading to the switch which has 100R, 68k and 33k on it, this then goes off to the wiper on the impedance switch.
Again I’ll add this as it’s more like the JTM46 now by Ceriatone.

https://ceriatone.com/wp-content/uploads/2024/06/JTM46-All-Access-25-June-2023.pdf

 

[Spencerleehorton]

Also just noticed on that Jtm46 layout the presence and tone stack and inputs are all grounded at the 32 + 32 uf cap gnd.
I’ve also read this makes a difference so will move that gnd, at the moment I’ve got a single 22uf in there but have a 2 x 32uf on order.

 

[dai h.]

(dunno if this will help, but) the one time I experienced motorboating, I forgot to connect one side of a cathode bypass cap (I forget which side--cathode end??).

re: the unused vol. pot having a slight effect: that is normal. I think it had something to do with the passive mixing with no buffering. One side of the unused pot (top lug) is closer to AC ground and the other is DC ground. On (Marshall) PA versions of that circuit (more channels) people who use them for guitar might unhook the other channels for more gain.

 

[merlin]

have moved the 47pf and the motorboating is still there, dont know if the sound is any different, doesnt seem to be.

That change was for the sqeal with presence dimed. Did that fix it?

Motorboating is a separate issue. You seem to be missing a power supply smoothing filter: this may be your problem!

 

[Spencerleehorton]

Yes no squeal now, so that did work, I’ll have a closer look at the B+ supply and update.

Pretty sure that 10k is there but like I said I’ll double check all values