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 » Mon Mar 11, 2019 12:45 am

Progress note:
I measured all the voltages, everything seemed to come out in reasonable ranges, so I think I’m on the right track. In general, I think I’m measuring a little higher than expected in some areas, but it think I’m ok.

My problem right now is with the bias adjustment. I am measuring 54mV and 46mV on V4 and V5 respectively. Too high (shooting for 40-45 in the documentation for KT66. I even waited for about an hour to see if things would settle down. They didn’t. Unchanged.

As was pointed out previously the 1 ohm resistances can screw up bias current measurements. I was never able to verify the true resistance (1 ohm is at the fringe of most meters because it’s on order of other resistances in the measurement circuit), but now I think I have to, before I consider changing a bias adjustment resistor or something. The solution is a 4 wire resistance measurement. Fun.

For anyone interested, here are my recorded measurements thru the testing phase. V1-V3 have 2 numbers, the top number is before KT66 tubes were added. These valves are expected to be high without output tubes in. As I indicated earlier, this is with the Mercury Magnetics power transformer. The line voltage was tested at 120vac. Sorry, I wish I kept track of which tube I put where.
https://ibb.co/FBQQ4Sy
https://ibb.co/KFXtkSF

Annnnnnnd,for fun, I mounted it all up to see what it would look like. (Still not ready to rock until I get the bias worked out and test the output transformer wiring). I like the look of my Lunchbox JTM. Head cab is from Mojotone. Bottom is a reissue of the circuit with tremolo from Marshall (head version of the famous combo). Gut shots in a previous post in this thread. Looks like my amp had a baby.
https://ibb.co/zX1TZ3Y

Cheers!

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

Post by neikeel » Mon Mar 11, 2019 6:42 pm

:rock:
Neil

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

Post by Tcaradonna » Wed Mar 13, 2019 12:22 am

Progress note:
Amp was sounding good as is, with bias voltage @ V4=54 and @ V5=46. (Yes I’m saying voltage because the without having accurately measured the R of the so-called 1 ohm resistor, I can’t estimate the current). So I played it for a few hours. Sweet sounding!

I then pulled the chassis out again and checked the bias voltage again. It seemed to have drifted downward and become more “matched” at somewhere in the mid to upper 30s. Interesting, just as I’d hoped! I then set the bias voltage to V4=43 and V5=41. Again I played it for a few hours and checked again. Same! Alright!

And for the record, (to any other builders using a Mercury OT) the primary wiring hook up is:
Red to V4 pin 3
Black to V5 pin 4
This is consistent with other Mercury JTM45 examples I have seen on the web, and should be the correct wiring for all future Metroamp/valvesform JTM45’s using Mercury OT. Mercury says they will never change the wire color convention.

My only issue I’ve noticed is there is a little noise with the High Treble Loudness 1 knob all the way off(at zero). When bringing up that volume a few degrees, the noise abruptly vanishes and the amp becomes dead quiet. The noise is only in then at/near zero position. I suspect a faulty CTS pot. I’m probably not gonna mess with it, and just keep the volume up!

Cheers!

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

Post by neikeel » Wed Mar 13, 2019 4:24 pm

:champ:
Neil

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

Post by Tcaradonna » Thu Mar 14, 2019 2:53 am

Ok a bit of troubleshooting...

In between playing the amp and enjoying it, I did a bit of chopsticking around to track down this noise problem I have.

The symptoms:
Only when High Treble Loudness 1 is all the way off, there is a buzzing. It seems to be increased with turning of the middle tone pot in various degrees (not necessarily proportional to the amount of middle tone). Once the volumeof loudness 1 turns up to be even minimally audible, it vanishes, completely...and the amp functions as I’d hope. I figured there was something going on with the pot in the zero position. It turns out that doesn’t seem to be the case. I was fiddling with leads to the loudness 1 pot and found the middle (?wiper) wire to be very positionally sensitive to the buzz. Touching the wire with a bare hand or even being close to it and not actually touching it would make the noise go away (like a theremin). Interesting. I wonder if in my pursuit of have cool looking wire routing, I inadvertently created an antenna that picks up noise. It’s weird how it goes away when the volume starts to amplify the input. I discovered a wire routing for which the noise is completely gone (not a single buzz at all) and the amp in beautiful. It’s unfortunate won’t be something that I can just keep because it’s just hanging in the air. I took some photos at different angles of one noise free position. Perhaps there are some ideas why this wire is so sensitive to noise, and how I could permanently fix it? Perhaps having a wire with grounded shield would be a solution?

Pics:
https://ibb.co/31pGZCf
https://ibb.co/K71FL02
https://ibb.co/RztdhpW

Cheers!

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

Post by neikeel » Thu Mar 14, 2019 5:44 am

That behaviour sounds like a suboptimal ground to your control bus.
Suggest implement full Larry grounding.

Oh and also remember the amp is supposed to be in its Faraday cage (aka headbox with aluminium shield on the bottom.
Neil

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

Post by Tcaradonna » Thu Mar 14, 2019 9:30 pm

I think you may be right about the grounding. It shouldn’t be too hard to add Larry ground.

I plan to line the bottom of the cabinet with appropriately wide aluminum tape, probably in 2 strips.

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

Post by SPeter » Mon Apr 15, 2019 1:45 pm

Any ghost notes at full volume on the 12 fret area of G and B? I just built one and it drives me nuts. I have the classictone transformer set and much higher B+ voltage (460V) and I start to wonder if that might be related to the problem.

Great looking build!

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