Calling all jtm45 kit experts..cuz I ain’t one...

Get support and show off your MetroAmp JTM 45 kit builds.

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Tcaradonna
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Thu Feb 28, 2019 10:32 pm

Thank you for the suggestion. That’s seems contrary to what the metroamp instructions call for, but looking at some examples of George’s actual GPM45, he seems to do it from both above and below.

In fact, I see at least 3 different ways done by George. They can be seen on various websites in google images and George’s Facebook page.

1. Serial 45041 (Feb 2, 2014): Routed pretty close to mine (basically by metro instructions), twisted going to tube from below
2. Serial 45044 (Feb 4, 2014): green wires, twisted coming from above (very few of this example)
3. Serial 45045(Mar 24, 2014), 45047, 45048, 45051, 45055 and 45056 black and red wires (not green), UNTWISTED go to the far right and the up to the left, down to output tube

I’ve seen another, serial # unknown with green lines from PT twisted going up to the left and down to output tube. These lines are arching up off the chassis to the output tube, rather than laying flat.

What route do you suggest? It seems after March of 2014 George heavily favored red/black untwisted from above. Interesting.

Cheers

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Fri Mar 01, 2019 2:11 am

Progress note:
I’m considering changing the green heater wiring based on some observations of changes to George’s GPM45 design. See above post for some examples. Meanwhile I’m moving ahead where I can. Today the high voltage to rectifier wiring was placed.

https://ibb.co/MBtcvLk

Question:
I’m getting ready to wire up the fuse, when I noticed something interesting. The schematic (as shown in the metroamp pdf) shows the fuse should go to neutral but the metroamp instructions seem to bring it to ground. In other words neutral and ground are tied together in this amp, correct?

Cheers!

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neikeel
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Fri Mar 01, 2019 2:11 pm

HT centre tap fused to ground

Fuse in live line to switch and to chosen primary

Neutral to common of PT

Dont put neutral to ground = unhappy ending
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Fri Mar 01, 2019 10:06 pm

Thank you, that sounds more reasonable. The schematic in the instructions pdf, is clearly not matching which is frustrating to those of us who actually know how to read one.

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Sat Mar 02, 2019 4:19 am

Progress note:
Today I found time to do the heater wires. Fun! I think it went pretty well, just taking a slow step at a time seems to do the trick. I think it looks about as good as the examples I’ve seen. I do take issue with the instructions calling for 6 inch pieces of red and black wire. No way. It was more like 7 inches for red and about 8 inches for black to be safe. (Black has the longer route with the loop to pin 9.)

I enjoyed watching George’s YouTube (along with groovy music) showing how he does his heater wiring. Interestingly he seems to solder each individual wire on V1-V3 before adding the second wire and soldering again. I don’t know if his sockets have dual lugs, but it seems more secure to dry fit both wires and solder them together. Just a personal preference, I can’t knock his work. I did find painters tape to be indispensable for this part of the build. It holds down the wires like a third hand and makes soldering a lot easier. I keep reusing the tape as I move along.

I’m still not sure about what to do with the PT to tube heater routing. I feel going above the tubes seems the right choice based on George’s own work. I reached out to him, to see if he could provide any of his insight into that design change for him, but alas he is a busy man, and I’m just a forum newbie. If he does respond I’ll post his reply. There is still time for me to rewire it. I’m so puzzled why he chose to do it straight, and take a long route at the same time (all the way to the left, up to edge of chassis the over to the right along the chassis before dropping down to V5.

Pics to follow...
Cheers!

https://ibb.co/5T4nwSv
https://ibb.co/xDcLNbx
https://ibb.co/wJ6MGSw

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neikeel
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Sat Mar 02, 2019 4:37 am

Good progress
I personally don't leave those black wires looped as long as you have them, but some people put them like that so that when the wires (green grid and blue plate etc) head to the socket they cross at exactly 90 degrees - you want to keep any signal wires from running parallel and close with another wire as you get capacitive coupling.

Other thing is that you make the blue plate wires run straight down fron the board, angled and on to the socket along the chassi floor. The green grid wires are to be kept up and away from the heaters and the plate wires.
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Sat Mar 02, 2019 3:13 pm

Progress note:
Thank you for comments and suggestions!

It’s raining today, so extra thinking time.

I took a step back and replaned some wiring choices. Indeed I decided I did had some excess black loop on the V1 heater so I trimmed off about 5mm/0.25”. Now it looks more even with the others. The length in all could probably be shortened, but given it looks pretty on par with the wiring examples I’ve seen including multiple examples of George’s, and the possibility of screwing up a socket I decided to keep these as is. I’m consciously deviating from the directions based on changes George has made in later versions of his GPM45 and recommendations above: I decided to change the green heater supply wiring from the PT to come from above rather than below. The length is only slightly longer, remains twisted (why does George not do this???), and so far doesn’t cross and signal lines (only crosses ground, no problem there). I also changed the color to red/black to maintain consistency (my Mercury PT has lugs rather than wire leads, so I could easily take this liberty).

New rework pic...

https://ibb.co/SmhLYKN

Cheers!

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neikeel
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Sat Mar 02, 2019 6:56 pm

Yes, looking good. :thumbsup:
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Sun Mar 03, 2019 2:28 am

Progress note:
Complete thru Step #7.

Bias resistors shipping with kit were 1 watt although the instructions call for 2 watt. 2 watt is certainly overkill for these, but I think I see why 2 watt are preferred. You need a long lead to reach the ground lug. I actually have some 2 watt 1 ohm resistors and even using those, and bending the ground lug over it was barely making it. (But it did for me.).

After some futzing around with wire routing, the next few steps have been pretty straight forward. I got my mercury magnetics O45JT-16 wired for KT66 operation, so only the 8 and 16 ohm (green and blue respectively) are used. There is no 4 ohm operation. There is also a white 100v (what the heck is that for???) lead, which in addition to the grey (used with EL34) I terminated with shrink tube and bundled with more shrink tube. I think this might just live under the turret board eventually where it won’t bother anyone, and if by some bizarre turn of fate I should decide to run EL34s, I can make the modification.

There is some confusion about the primary winding of the Mercury OT and which lead should go to which output tube. The metroamp instructions only go so far as to say, one goes to the right and one goes to the left. A fairly comedic instruction. Searching the forum shows others with similar confusion and without any definitive answer the only recommendation has been flip a coin and keep leads long enough to switch if it doesn’t work. That advice does not bring the warm fuzzies...or perhaps it will...so I’ll be sure to goggle up and and wear a flak jacket on initial power up. I inquired to Mercury, but I’m not holding my breath.

Pics time!
https://ibb.co/4t9dqw5
https://ibb.co/zSG0n2R
https://ibb.co/gRh44hR
https://ibb.co/bWW4Thj

Cheers

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neikeel
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Sun Mar 03, 2019 4:52 am

The original 784-103 Drake transformer was made with 8, 16 and 100v taps which went to a paxolin selector, either on the side bell of the OT (actually first ones like that were 784-73 but...)
Some people at Mercury felt that having that 100v (used to drive PA systems) was crucial to the tone so Mercury still supplied it, how that jives with having an extra tap to get 3.4k grey wire tap is lost on me).
So yes fold back, shrink wrap and bundle neatly away.

The primaries are a different matter (the back story is omitted from the instructions):
There is a start and end of the primary wind and a centre tap. This arrangement affects the current direction of flow in the secondaries, such that if you take the negative feedback from the OT as usual and it is in the wrong direction of the feedback that is put back into the power amp and you get a howling scream through the speakers on first fire up with tubes (oddly sometimes just motorboating).
The OT manufacturers seem to place fast and loose with which is start and finish colour and AFAIK there is no way of checking before hand. Heyboer (who supply Marstran and Metro) and MM and Magnetics Corp (Classic tone) all seem to have been affected at some time. As I said before my fave OT is the Marstran/Metro or ****** but I am sure you can make MM work for you.
So strategy is leave both primary wires long. Get amp running. If it howls lift the NFB resistor. If howling stops you have reversed primaries, swap them over and shorten and lay them in correct arrangement once it is working properly.
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Sun Mar 03, 2019 5:13 am

Oh and if you are going to use the 1ohm cathode resistor method you should really use +/- 1% tolerance (you used 5%) if the ones you use are at 5% in each direction you potentially have 10% measuring error

eg Plate voltage 450v, desired bias current 60-70% of 25w max at idle so lets say 17w = 17/450 = 38 mA

if you extrapolate 1mA= 1mV measured then you may measure 38mV but actually have 40mA or 36mA, which is not disastrous even if the numbers are correct but if your tubes are different by a few mA it all adds up.

Also factor in the screen current which will be several mA at idle. This means if you shoot for 70% you will always be in the safe < 70% zone.

ps the joys of waiting for empty emergency operating theatre - got to be here poised to go twiddling thumbs- perfect on line time-killing!
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Sun Mar 03, 2019 2:37 pm

That is an excellent point about the tolerances and bias.

I actually blew over this when I checked the resistance on 2 decent meters (a Fluke and my trusty HP). Both meters have difficulty below 5 ohms, so I was reading 1.1ohm on all my 1 ohm resistors (I have a lot) on both meters. So I assumed I had just reached the limits of the tester since I was seeing the same consistency and they couldn’t all be off (10% on gold bandin outta spec). I didn’t want to go to the 4 wire method of testing small resistance because I was being lazy, but perhaps I should reconsider. Fortunately, the resistor is easy to check in situ.

If I can measure 1 ohm on these metal oxides I should be fine. I’m not gonna worry about temp coefficients... :)

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Mon Mar 04, 2019 2:19 am

Progress note:

PT, switches and fuse wired up. Having the Mercury transformer and very short lead on the lamp once again forced me to deviate from the instructions. I choose to route the lamp to transformer common (over the top of the transformer) and from common taking a white lead to the fuse which will be going to white neutral from the power cord (US color convention). Black line will eventually go to switch. Some of these colors are very confusing and I now wished I used green for ground (like the chassis floor fuse terminal and the CT of the heaters (also going to ground). Oh well, thank god this circuit is so well known or someone working on it would be very confused.

I bundled up and shrink wrapped the unused transformer primary taps for 220, 230 and 240v (those are the brown, red and yellow wires respectively). Doubtful they will ever be used but they are neatly out of the way.

The can cap mounting bracket needed some modifications as a tab was sticking out above the bottom of the chassis. I checked with Valvesform to see if maybe they machined the holes in the wrong place. Of course they wouldn’t admit it, and they told me to drill new holes or break the tab off. Neither a very appealing option. I whipped out my Dremel tool and ground off the tab smooth. Moving on...

Control pots are mounted, starting to look like an amp!

The 0.1uf capacitor on the presence control is a synergy (the old sozo, new name) which is substantially larger than the stock Mallory. You can see from the picture, it comes quite close to the chassis and PT mounting bolt. That kind of worries me a bit (although if I read the schematic correct, this side is grounded anyway) and also the capacitor is a bit wobblely (only mounted on one side as the lead makes a U-turn). So I injected some clear electrical silicon beteeen the capacitor and the pot to stabilize it.

Pics time! (A bit more to cover different angles and to capture some covered components)
https://ibb.co/PFmWN5W
https://ibb.co/khcYXMN
https://ibb.co/v4Y8vhm
https://ibb.co/5K70ZP5
https://ibb.co/yVsVX8R
https://ibb.co/RhzjVJy
https://ibb.co/XY5jqBp
https://ibb.co/P6QXSRw
https://ibb.co/JQXdRZr
https://ibb.co/n08dwvx
https://ibb.co/3Rzq7S6

Cheers!

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Mon Mar 04, 2019 6:46 am

The other way to wire the presence cap is longitudinally/obliquely with the ground wire soldered to the ground bus near the bass pot - but looks fine as is.
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Tue Mar 05, 2019 2:11 am

Progress note:
Silicon cured. Cap secure!

Today is all about getting jacked! It looked difficult to wire up the jacks in place, so I copied the spacing on a rigid template and manufactured 2 sets. I gave extra leads just in case (I feel metroamp directions skimp on them, they can always be cut later anyway). My 1M ohm Allen Bradley NOS Carbon comp resistors are a bit on the big side, but I was able to shoe horn them into the space. I cleaned the leads with some steel wool. I also dry fitted all the parts and wires before soldering up (directions call for it in stages, but I think that’s asking for trouble). I use bits of old cardboard boxes and painters tape to help wedge things into my desired alignment before bringing the iron in. I think it all looks ok.

No more time today. Tomorrow working late, so no amp plans for 2 days...

Pics!
https://ibb.co/5RbX0V1
https://ibb.co/GQZ8jZT
https://ibb.co/GvkvxPK
https://ibb.co/xmrzY77

Cheers!

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