JTM45 Build In Progress, All Done! Thanks For The Help!

[NickC]

Building a JTM45 with KT66 output tubes, Mercury Magnetics iron (their Radio Spares version PT, 6.6k primary OT, 7H choke).

Question: The MM PT has separate secondaries for 5v rectifier heater, and two 6.3V 2.5A center-tapped feeds for other tube heaters.

Since KT66 draw 1.3a, and preamp tunes draw 0.3a, is it okay to feed the KT66s from one heater tap (drawing 2.6a from the 2.5a feed) and the remaining three preamp tubes and 6V pilot light (1.2a) on the other tap .... or should I split it up like this:

1@KT66 (1.3a) + 2@12AX7 (0.6a) = 1.9a on one tap


1@KT66 (1.3a) + 1@12AX7 (0.3a) + 1@6V pilot light (0.3a) = 1.9a on the other tap

What is best??? Thanks in advance for any advice.

[Phil_S]

Put like tubes on the same heater circuit. Keep the KT66's together. You need 2.54A for those two tubes. They should be fine on a 2.5A rated filament supply. I wouldn't sweat the 0.04A.

[NickC]

Put like tubes on the same heater circuit. Keep the KT66's together. You need 2.54A for those two tubes. They should be fine on a 2.5A rated filament supply. I wouldn't sweat the 0.04A.

Thanks Phil! Much appreciated!

That will certainly make the heater wiring simpler and cleaner.

[Firestorm]

Since the windings have the same voltage, you should be able to parallel them and get 6.3VAC @ 5A. Gotta make sure the phase is correct (!!!!).

[Phil_S]

Good call, Firestorm. I should have said that!

[NickC]

I'm adopting this "Larry" grounding scheme, as recommended by ToneMerc:

[img:400:255]https://tubeamparchive.com/files/thumbs ... me_143.jpg[/img]


Chassis Prep-in-process:

[Structo]

A little inspiration for you.

[bandm]

Very nice work Structo! Is that waxed spot tie that you used for tying the wire bundles. That's what we used when I worked for a major Aerospace company, building prototype radar test equipment. It works great. I have a couple of spools of it left. A black and white version. It is a lot less expensive than the plastic zip ties.

[tubeswell]

Very tidy Structo. How much did you get stung for the .1uF mustards?

[NickC]

That is indeed a neat-and-clean build Structo! I'm going to be embarrassed to post pictures of mine.

I have primary PT taps (for various "world" voltages) that I didn't cut. I heat-shrinked the ends, wound them up and tucked them between the can caps and the stand-up PT. There's no voltage through them so no coil issue. If I ever need to wire the amp for Euro volts, it'll be relatively easy.

Another issue with my build; the PT has separate 6.3V heater taps. I used one for the two KT66 sockets and the other for the 12AX7s. It makes the heater strings a bit messier looking than having only one.

I was mindful throughout the build to make solid mechanical connections of wires, and not use solder as glue, thanks to a recent thread on the subject here.

I have to solder the three noval sockets to finish it up. I'll post some pictures when all the wires are in situ.

[ToneMerc]

A little inspiration for you.

I don't think this is Tom's actual build. IIRC, a member on Metro's forum built that JTM45 clone maybe 3-4 yrs ago. I think I have pictures of the JTM50 that this person did as well. His is some of the best vintage Marshall clone builds around. Funny thing is, if you look at many vintage Marshall's most are sloppy when it comes to eye candy.

TM

[Reeltarded]

I'll never make an amp that beautiful.

I have run out of .1 and few .68s left. .68 are spendy, $40 NOS, but a complete set with .1s for JTM 45 are no more expensive than a terrible distortion unit.

[ToneMerc]

I'll never make an amp that beautiful.

I have run out of .1 and few .68s left. .68 are spendy, $40 NOS, but a complete set with .1s for JTM 45 are no more expensive than a terrible distortion unit.

I'm down to my last two complete NOS Mustard sets. I could use a .68

TM

[Reeltarded]

Go build an engine and a firearm...



Then you can drive to where they are and have them for free.

[NickC]

The building part is done. Next comes the testing.