Metropoulos Forum

Hey all, Im pretty new here so forgive the newbie talk....I recently picked up a JTM45/50 off of the classifieds here, and as much as I am impressed with it, I do have a feeling I should have gone with a more aggressive circuit like the 1986 Bass circuit.

Now the JTM45/50 was built with a Mauradian JTM45 circuit board with NOS Mustards, and the circuit was slightly altered with parts from a Ceriatone JTM50 kit. The builder installed EL34's as opposed the the KT66's. The transformers are JTM45 Marstran replicas I believe. Im buying a SS Rectifier to tighten it up a bit. Im wondering what value changes I would have to make to turn it into a JMP 1986 curcuit. I can take pics of the circuit if that helps.

I think this covers it unless I missed something.

Board changes from JTM 45 to 1986:
- change mixer resistors from 270k to 470k
- change NFB from 27k to 47k
- change first voltage dropper from 8.2k to 8.2k + 10k (or two 10k's)
- change first bias resistor from 220k to 150k (far right of board)

Based on the Metro layouts and schematics I found, it looks like the preamp filter cap changes from 16uF to dual 32uF or dual 50uF and there is an additional 10k dropper before the V1 plates on the 1986.

Below is the 50w bass spec board from the Metro kit instructions.

The original JTM45 Model 1986 was the bass version of the JTM45 Model 1987 , which in turn was the head version of the JTM45 Model 1961 combo amp. Is this 1986 the amp you're asking about ? If so, I would suspect that the only differences would be that the 1986 might not have the bright-cap on the bright channel volume control. I can't find a specific schematic for the JTM45/50 model 1986, but here is the schematic of the 1961:

http://www.drtube.com/schematics/marshall/jtm45tr.gif" onclick="window.open(this.href);return false;

Now if you are looking for the later JMP Model 1986, here is a schematic of that:

http://www.drtube.com/schematics/marshall/1986u.gif" onclick="window.open(this.href);return false;

No im looking for the 1986 Plexi circuit. Is there a big difference between the 10,000 series spec and the later 12,000 spec in the superbass models?
I think I wanna go with the 1968 12,000 if its going to be more aggressive. If anyone can help me out with what changes I would need from JTM45 to 12,000 series 1986 50W that'd be great.

floydfanatic wrote:No im looking for the 1986 Plexi circuit. Is there a big difference between the 10,000 series spec and the later 12,000 spec in the superbass models?
I think I wanna go with the 1968 12,000 if its going to be more aggressive. If anyone can help me out with what changes I would need from JTM45 to 12,000 series 1986 50W that'd be great.

The 10 and 12 series specs refers to the 100watters (early Superbasses and Superleads in the 10 series 100watters were identical except for the 5nF bright cap on the volume I pot).

The 50 watters are a bit different. Both bits of information you were given above were correct. Before recommending changes we need to know what spec your amp is built to now (you have described some changes from basic spec already).

If your amp has a JTM50 board already.....

I think that an SS rectifier will help, smaller PI output couplers help (0.047/0.039/0.033uF are easily available as mustard 400v). I would make sure my mains filter was a dual 50uF can in parallel for 100uF but I would keep my screens at 16 and PI at 32 with dual 32 in the preamp.
If that does not work a 5000pF bright cap on V1 may help, next after that you will need to split V1 cathode by adding a 0.68uF/2k7 as per the 1987 diagram.

Wow! Thanks for the great information Neil! I have no idea what is in the amp right now for filtering...and I have no knowledge on this subject really. If I were to take a high quality picture, you think some of the guys here could figure out what was going on?