Metropoulos Forum

My JTM50 breaks up with the volume on 2 with my Les Paul. Is this normal? I always thought it should be fairly clean up until 5. This is with the tone controls all set to 5.

Its basically a JTM45 but with 470K mixers and the NFB connected to the 8Ohm Tap with a 27K resistor. Its also got the additional filtering for the preamp that the later amps had. 100pF Bright Cap on the volume control as well which I've tried changing.

Normally 27k NFB on 16ohm tap (i.e. more NFB).

What is the rest of the amp set up like (and what cab).

What OT are you using (3.4k for EL34s or 8k for KT66s)?

I would expect it to start breaking up around 4-5 (at least my original '67 does, although it has 270k mixers)

Strange thing about the NFB is I did at one point have a 47K resistor on there and when I changed to 27K it didn't make a mass of difference. Not on where the amp breaks up anyway.

I'm using a 2x12 with Celestion Heritage G12M, rest of the amp is basically stock JTM45, only differences are the NFB, 470K mixers, additional pre amp filter node, 32uF on every node and EL34s with a 3.4K OT. It also has a PPIMV but I never use it. Not at band practice anyway.

Maybe my guitar has a high output as I did put a scope on it, sent a 100mV 1Khz signal into the high channel 1 input and it didn't break up until 5.5 on the volume control. Odd though as my Gibson Les Paul has a 57 Classic Plus in the bridge which isn't a hot pickup by any means.

I am using Alpha pots though which have a 15% taper on them. I suspect the original Marshall pots have a 10% taper. I was going to look at some CTS or Bourns pots to get a better taper.

Change the mixers to 270k. There's your cleans.

coldengray wrote:Change the mixers to 270k. There's your cleans.

Yes and 27k on 16ohm tap = more NFB than 27k on 8ohm.

Sorry I haven't replied guys. I didn't get an email saying you had responded!

If I need 270K Mixers and more NFB then I'll leave it. I like the amp as it is, I was just wondering why it had a lack of headroom.

JTM50s actually quite often had 27K NFB on 8Ohm Tap . Some had it on 16Ohm, depends on how Marshall felt that day!

I changed the EL34s from TAD EL34B-STRs to JJ EL34 IIs and this made a huge improvement in tone. The amp sounds amazing now. The headroom is the same when playing with guitar volume full on and using the amp volume to clean up but....if I run it with my normal settings I can clean up with the guitar volume a bit easier. Not sure how that works but ah well.

My normal settings - P - 10, B - 0, M - 10, T - 7/8, HT V - 8/10 and N V - 0.

Apparently the TAD EL34B-STRs are actually Beam Tetrodes and not Pentodes, so they tend to have a bigger bottom end. Not good on a bass heavy amp