JTM45 Build - questions before I commit...

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Explorerman
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JTM45 Build - questions before I commit...

Post by Explorerman » Tue Nov 13, 2018 10:42 pm

Hi, I’m new to amp building however I have build about a dozen effect pedals (including neo vibe, germ fuzz and tonebender, etc), I also have an electrical trade of 16 years so I’m sure this project will be well within my capabilities hopefully.

Just looking at the instructions from the Valvestorm site to make sure I know what I’m up for, and noticed the following:

- the schematic doesn’t show the valve socket pinout numbers, is there a schematic that shows this at all?

- the instructions show connecting a 1 ohm / 2 watt bias resistor to the output valve pins 1 & 8, however there are no 1ohm/2w resistors in the equipment list? They are also not on the schematic.

- the 2 and 1 watt resistor values and qualities dont seem to be the same between the instructions parts list and the VS parts list?

- the OT and impedance selector switch on the schematic only shows 8, 16 and 100v - the instructions refer to 4, 8 and 16 ohm transformer taps? Is “8” on the schematic OT meant to be 4, is “16” meant to be 8 and is “100 volt” meant to be 16? If so, where the schematic shows “to tap 16” from 27k resistor, does this still connect to what is shown as “16” (or 8 ) on the OT?

- any tonal or quality differences between the Mallory and Synergy royal mustard caps? Or between the alpha or CTS pots?

- the colour coding of the OT wiring to the two output valves in the instructions doesn’t line up with the schematic.

- the instructions say to connect the 470 ohm ohmite 5w resistors across pins 4 and 6 on the output valves but the schematic doesn’t reflect this (schematic also says 1w)?The connection of screens grids and choke in the instructions slso doesn’t seem to line up with the schematic either?

Cheers

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neikeel
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Re: JTM45 Build - questions before I commit...

Post by neikeel » Wed Nov 14, 2018 3:37 pm

Not sure you have the best schematic.

This one is correct and gives different specs.

Image
Last edited by neikeel on Wed Nov 14, 2018 5:08 pm, edited 1 time in total.
Neil

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Re: JTM45 Build - questions before I commit...

Post by neikeel » Wed Nov 14, 2018 3:37 pm

Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- the schematic doesn’t show the valve socket pinout numbers, is there a schematic that shows this at all?
The octal location slot goes towards the back of the chassis (pins are clockwise) and the B9 gap points to front left (both with the chassis with guts up towards you and controls towards you.
Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- the instructions show connecting a 1 ohm / 2 watt bias resistor to the output valve pins 1 & 8, however there are no 1ohm/2w resistors in the equipment list? They are also not on the schematic.
They are an addition for DIYers to adjust the bias easily with just a voltmeter (1mV =1mA across the resistor) you can use a bias probe or the shunt method or primary resistance and voltage method (need good meter for that).
Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- the 2 and 1 watt resistor values and qualities dont seem to be the same between the instructions parts list and the VS parts list?
The kits use 1w Xicon carbon film which are decent resistors and span most of the turret distances. 2w are two types, some CF just to reach across turrets and the Metal Oxide are dropping resistors in the B+ line.
Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- the OT and impedance selector switch on the schematic only shows 8, 16 and 100v - the instructions refer to 4, 8 and 16 ohm transformer taps? Is “8” on the schematic OT meant to be 4, is “16” meant to be 8 and is “100 volt” meant to be 16? If so, where the schematic shows “to tap 16” from 27k resistor, does this still connect to what is shown as “16” (or 8 ) on the OT?
The transformers come with 4, 8 and 16 ohm taps (the original schems are for PA amps that use 100v to drive old PA cab cable runs).
Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- any tonal or quality differences between the Mallory and Synergy royal mustard caps? Or between the alpha or CTS pots?
I cannot comment - people tell me th Synergys are v like original mustards which is what I use
Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- the colour coding of the OT wiring to the two output valves in the instructions doesn’t line up with the schematic.
leave the primary wires (to pin 3 on the octals) a little long as the wiring of stop-start on the OT can be unreliable and you get feedback issues that are apparent only on testing with a speaker.
Explorerman wrote:
Tue Nov 13, 2018 10:42 pm
- the instructions say to connect the 470 ohm ohmite 5w resistors across pins 4 and 6 on the output valves but the schematic doesn’t reflect this (schematic also says 1w)?The connection of screens grids and choke in the instructions also doesn’t seem to line up with the schematic either?
The correct resistors are 470R between 4 and 6 fed by yellow wire before the choke (one end of the 8k2 2w resistor on the board. Early amps also had a 1k resistor feeding these two individual resistors (search flying 1k resistor).

Look at this schematic (read Brian's comments in the foot note) :
Image
http://www.marstran.com/JTM45.gif
Neil

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Re: JTM45 Build - questions before I commit...

Post by Explorerman » Fri Nov 16, 2018 9:09 pm

Sweet, thanks for the reply, with my electrical experience I'm wire from schematics with terminal numbers and not descriptive step by step instructions, but I guess the instructions are catered for the less experienced diy'ers.

Anyhow, I stumbled across a very handy schematic vie a simple google search that linked to another forum. After going through the instructions step by step, wire by wire, component by component and compared to the schematic (this took me some time) and it is spot on. It even has the earthing points numbered to suit the Larry star grounding method!

https://music-electronics-forum.com/att ... 1376363636

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neikeel
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Re: JTM45 Build - questions before I commit...

Post by neikeel » Sun Nov 18, 2018 12:21 pm

The schematic you link to looks good.
Only bit I disagree with is the 500pF cap on vol 1 pot (I use 100pF after much experimentation.
Also I would tend to use carbon films throughout (except the screen resistors) I too like having a schem to hand and it is ideal to use the schematic for wiring and and amp layout to help you with optimum wire dress and layout. The build instructions also help with the order in which you plan your build.
Neil

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Re: JTM45 Build - questions before I commit...

Post by Explorerman » Wed Nov 21, 2018 3:42 am

Ok, what difference does swapping the 500p make? The BS/Metro instructions use a 500p here as well.

Also, which is the best PPIMV mod to use Lar Mar or Rich mod?

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neikeel
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Re: JTM45 Build - questions before I commit...

Post by neikeel » Wed Nov 21, 2018 9:43 am

IMO 500pF is a bit harsh, 100pF sweetens the clean end, try a few values (100/250/500 and 1000) see what you like, think of it as a bypass cap rather than the generic 'bright cap' and experiment for your amp and guitar choices.

The Rich mod is quite nice, it actually adds a little distortion that you may or may not like (George uses it in his builds (albeit with a high quality PEC pot) so that should tell you something. The Lar-Mar is pretty transparent and when up at max is effectively out of circuit. I have used both more than once but as I tend to do more restoration or vintage clones I tend not to install them (I have three different attenuators to choose from instead).
Neil

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Re: JTM45 Build - questions before I commit...

Post by Explorerman » Thu Nov 22, 2018 4:19 pm

Thanks for the info! After doing a lot of reading and watching YouTube videos I’m thinking a 50w 1987 lead plexi might be more of what I’m after (play mainly rock - AC/DC, zeppelin, gnr) but really like the Hendrix bold as love and little wing cleans. Is it possible to voice the JTM45 to have a tighter bass and more aggressive by adding a ss rectifier with a switch? Or do I just go for a 50w plexi kit? Does the ppimv work on the 50w plexi as well?
Cheers

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