"Black Flag" JTM100 - the build has begun!

[shakti]

After lots of talk, much sourcing of parts, and even more lack of time to get started, I have finally begun building a replica of an early/mid '67 "black flag" JTM100. I'm going to build this as a straight-up clone with NOS mustards and RS silver micas, some Iskra resistors, Marstran 1203-80 PT, Marstran 1202-119 OT and Metro 352-114 choke. Dual rectifier with stock filtering.

Only differences from the original will be Larry style grounding throughout, even power tube cathodes! I'm going to run them directly to grounding point 5.

I'm done with the heaters, so now the fun begins.



Before I go on, I have two questions:

1) I was planning to run the OT secondaries (blue, green, yellow) between V6 and V7 since that gives me the shortest path. However, they will be running closer to the PT that way. Is that a problem? Could it induce hum somehow? I was unsure whether to have the OT primary or secondary side closest to the PT, but opted to have it as pictures, as I could then run the OT primaries directly between V5 and V6, plus I could run the secondaries a short distance to the impedance selector.

2) As I said, I am running the power tube cathodes directly to grounding point 5. However, if I can, I would still like to install 1 ohm bias reading resistors. So I take it I still have to link pins 8 and 1, and have the 1 ohm resistor between pin 1 and ground, or could I have the 1 ohm resistor off of pin 8, and then link pin 8 and 1?

[Roe]

looks great sofar. tell me if you need some parts. I want to send a couple of piher 100Ks for that build.

[shakti]

Sweet! You're a good guy, Roe!

I'll try to update this thread regularly, could be a nice reference for others who want to build one.

I had to drill new mounting holes for the Marstran 1202-119. In the process I moved the OT a little further away from the PT and even closer to the front end of the chassis, per photos of the originals.

[Roe]

btw, these amps are found both with iskras and pihers. also some values are often CCs. my 67 sounds the best with pihers, CCs and alpha pots

[VintageCharlie]

Shakti, good luck for the build - looks neat already. I'm very interested in how it will turn out and how it will compare to your 45/100 build.

[shakti]

Checked my old JTM45 instructions as I remember the biasing resistors were installed slightly differently on that one. Seems like I can simply put the resistor between pin 8 and 1, and run the ground wire from pin 1. I assume I can also run the ground wire to pin 8, but just so I don't mess up...does that make any difference?

[Roe]

putting the resistor between the pins make a slight difference in bias readings. I'd just put one leg of the resistor through pin 1 and 8 and the other connected to the ground wire. I've done this in two amps with great results. twist the wires from the resistors. dont just use one wire but 4 (or at least two - one per pair)

[shakti]

Any opinions on where to route the OT primary vs secondary wires?

[frenchie]

don't care about routing if you wrap them wires with a layer of aluminium tape and heatshrink over it to avoid possible shorts , good trick to reduce noise ,and when shielded this way , wires don't have much influence on one to another

[Roe]

I guess I would just root the OT wires like they are in your picture

[shakti]

A little progress today:



Back panel installed, impedance switch with OT secondaries connected. Inserted a piece of violet wire on the 8 ohm tap for negative feedback. AFAIK, black flag amps typically had the NFB wire on the 8 ohm tap.
Ground wire for the heaters run all the way across the chassis to grounding lug #1. I'll probably tape this wire flush with the chassis.



Ground wires for each power tube socket run from pin 1 directly to grounding lug #5.
Screen grid resistors (1k5W) installed between pins 4 and 6 on each output tube socket.



Detail showing how I attached the 1 ohm bias reading resistors. You'll get the most accurate reading by using one leg of the resistor to bridge pin 8 and pin 1, then have the body of the resistor dangling off of pin 1, attaching the other leg to the ground wire. I opted for the slightly less accurate method where the resistor goes between pin 1 and 8, and the ground wire soldered to pin 1. I feel the ground connection will be more secure this way, with the wire soldered to a pin and not to a resistor leg.



The boards (beautiful work by Brian H!) will conceal all these extra ground wires when the amp is finished. I like my amps to look neat!

[shakti]

BTW, I had to drill two extra holes for the board mounting posts on the far right side of the filter boards, otherwise they ended up right on one of the turrets. I decided to position the mounting posts right in the middle of each rectifier "diamond" - looks very neat, and easy access to the mounting washer.

[Roe]

you need to twist the wires from the 1 ohm resistors

[Roe]

also, the mains grounding should not be that close to the PT - too close to the PT will lead to quite noticeable hum.

[shakti]

you need to twist the wires from the 1 ohm resistors

Why? They are just ground return paths, would I gain any noise cancellation by twisting them?

Also, I put the mains ground (#5) in that same spot on my 45/100, which is dead quiet.