JTM 50 Planning Stage Questions

[Monsieur Lebeaux]

Hi! Me again. I've sourced and ordered the majority of the parts for my build and hope to get started on it next weekend or the following week. Anyway, me being a novice, I still have a bunch of questions that I need to have answered before then. I have used the search feature to try to find answers on my own as I am sure most of my concerns have been previously addressed. Unfortunately, sometimes it's hard to find exactly what I'm looking for, not to mention it's a lot less time consuming to just go ahead and ask.

1) The layouts that I have seen do not mention a wattage for the majority of the resistors on the board. A few are labeled as 2 watt, but I'm not certain if the rest should be 1 watt or 1/2 watt. To add to the confusion, sites like Valvestorm for instance have the largest variety of CF resistor values with a 1 watt rating, a few 2 watts, and no 1/2 watt. The CC's on the other hand are mostly 1/2 watts and a couple 1 watts, but no 2 watt values. Why is this? Does it even really matter? I think I came across a thread somewhere that mentioned 1/2 watts are acceptable in most locations and 1 watts can be used where it specifies 2 watts. Is this correct?

2) Faceplates. It's hard to find a Black Flag faceplate. Valvestorm is out of the Sparky faceplates and I don't really want to order one from Malaysia (Ceriatone) mostly because I'm impatient and I want to get started ASAP. Does anyone know of any other sources? I found an older thread where someone made his own layout and had a plexi panel custom laser engraved. What program would I need to use to do that and does anyone already have any files for a JTM50 faceplate?

3) Chassis. Valvestorm is also currently out of the galvannealed 50 watt chassis. Does anyone know the process they used to achieve that finish? I work in a machine shop and I was considering media blasting the chassis to do away with the gold finish and possibly baking it for a period if blasting isn't enough to get it to where I want it. I also don't want to ruin it. Does anyone know what grade of steel they use for the Metroamp chassis that Valvestorm sells? I'm assuming it's cold rolled.

I'm sure I'll have more questions as time goes by and I'll add to this thread as they surface. It's getting late and I've got to get up in a few hours. That is all for now. Thanks.

[neikeel]

If you are using NOS PIhers (sound best) or Iskras then nearly all the resistors are 0.5w, the exceptions are the voltage droppers (B+ line) which are the big 8k2 and 10k. These are 1w.
The best source for NOS PIhers is here:
If you look you need 'short' (they are a lot cheaper and you will only cut them down) 220k x3, 68k x4, 56k x1, 100k x3, 82k x1, 470k x2
http://www.ebay.co.uk/itm/No-5-1-2W-Pih ... xyOMdS9SYL
You need one of these:
http://www.ebay.co.uk/itm/1-rare-8-2K-2 ... Swe-FU2eNc
Two of these:
(looks like he is out of 10k 1w (the 10k 2w Pihers are a bit big for the mid board droppers Marshall often used AB 1w 10k in carbon comp here too).
From these you need x2 820R (cathodes) , x2 100k (cathode and on V2 socket), x1 82k (for bias resistor) to be full lengths. You also need x4 1Meg , x1 470R and x1 10k.
http://www.ebay.co.uk/itm/5-Piher-1-2W- ... SwNSxU4kgp.
Of course if you are going new production you will need resistors that span the turrets. The ones that Robert sells are 1w as these are the same physical size as the old 0.5w. Similarly the larger 10k and 8k2 droppers will need to be 2W
I use mustard caps as they are smooth and require little breaking in but YMMV.
Shame about Sparky faceplates they are very good but the Ceriatone ones are good too and I use those too.
I have a galvanised chassis and it is very good replica for a 67-71 type build. You can soften the galvanised look by buying some zinc galvanising paint spray (matt finish) and giving the chassis a single light spray which is also very effective.
Good to see you are planning it properly, like many things a good JTM50 is like the 45/100, they are very simple amps but they are truly the sum of their parts, if you use the right parts you get all the tones, nuances and subtlety when you play old school techniques with a good guitar
This old scot pro demonstrates this very well:

Guts here:http://s477.photobucket.com/user/dunara ... t=3&page=1

[Monsieur Lebeaux]

So does the physical size of the resistors matter? If the new production resistors are smaller than the vintage ones with the same ratings, is it just for aesthetics or does size actually play a role? I've always heard that a larger physical sized cap has more depth or detail than a smaller one with the same rating. Does the same apply to resistors or were the old ones just better made?

[neikeel]

The reason Valvestorm specs 1 watt Xicon new production resistors (like the ones sold in the old metro kits) is so that they fit across the turrets of a Marshall board without having to extend the wires or use a smaller board - nothing special. As far as technical performance there is no problem, but yes the older Piher resistors are of better quality and IMO sound better (but don't hold me to a blind testing, there is probably a NOS bias!!).

[ValveStorm]

The Sparky panels are back in stock. I may be able to get more 50w galvannealed chassis if you really want one. The only one I have was punched for an IEC, and the punch went thru a bit rotated.

[danman]

I have purchased from the seller that Neikeel linked to many times and I have also used this seller...http://www.ebay.com/itm/181269373504?_t ... AIT...also. he seems to have a pretty good assortment of Pihers at fairly decent prices. If you are not going the vintage route and just want to purchase quality modern resistors, the 1 and 2watt from Valvestorm will work just fine. Good luck with your build and be sure to post plenty of pics along the way

[Monsieur Lebeaux]

I finally thoroughly checked over Neil's list of resistors and compared them with the layout that I've been referencing (triode store). That layout also shows a 27k negative feedback resistor, a 15k in between the negative ends of the 10uf resistors, and a 56k bias resistor. I'm thinking I have seen different bias resistor values on different layouts, so thats not an issue.

Anyway, does the resistor wattage affect the tone of the amp? I've read opinions that half watt resistors will be stressed in these amps and therefore provide more "bloom". Conversely, 1 watt resistors supposedly offer a firmer response. Any truth to this?

I was going to do a mostly new production amp and never considered sourcing NOS resistors, though I originally intended to get a set of mustards. Now I'm second guessing that. I'm still waiting on my transformers so I still have time to order different parts. Hell, by the time they get here, I'm liable to have enough parts for two amps. Might have to order a set of ClassicTones and a few more caps and then I can build a 1987 too.

[neikeel]

As per my previous post for the small extra and particularly if you are getting NOS mustards I owuld get the 0.5w Pihers. I do not know who said that 0.5w resistors are stressed in these. The Pihers work very well and are very reliable.

Save the other bits and later build a 1987 or a 2204, or even better a 2 in 1

[danman]

I started my first 1987 build several years back with the intention of using all new parts. Once I started looking and reading about all of the great builds here throughout the years, it wasn't long before I had sourced a full set of Pihers, Mustards and Lemcos for the build. I agree with Neikeel, if you can afford the Pihers then I would certainly consider it. I warn you though...this hobby does get addictive very quickly...lol!