New amp build finally started. JTM45 with KT66.

SG John

SG John

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After spending the last several months sourcing stuff as my wallet allowed me, I have finally received the last little packet of goodies and started on my new project. As I was starting to collect stuff, Ivan H and Plexi have been really, really nice in offering me advice to purchase particular resistors, capacitors, and whatnot. Plexi has been very good at sending me assorted photos and quizzing me on what I should do. I am really grateful, as it has been very valuable in me feeling more comfortable jumping into this.

The basic design is the second generation of the JTM45, circa 1963-1964 with KT66 power tubes. I'm using the schematic drawn by Mark Huss and a MetroAmp build sheet for reference. I now have the Merren Audio early style Drake transformers. Here they are, nicely mounted on the ValveStorm aluminum chassis along with some NOS McMurdo 8 pin valve sockets.

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I have early NOS Piher resistors along with Philips Mustard capacitors, and some other nice old "Made in England" stuff that Ivan sent me.

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Unfortunately, one of the .022 uF Mustard Caps did not survive the trip from Germany. I'm actually surprised that they made it at all. I was relieved to see them in the mail this week.


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Currently, I have the heaters all wired up, as well as the HT fuse. Started on the choke and output transformer. Realized I'm unclear on a couple of things, as the Metro Amp sheet is far from perfect. Taking a short break for now.


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When I'm ready for the power transformer voltage selector strip, I have this nice rail that Ivan made for me, along with a more period correct turret board. Can't wait to dive into those. The selector rail will get mounted on the side of the chassis near the big can capacitor to the left of the power transformer in the above photo.


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And, all the work is being done with my new Hakko soldering station. It's my first real soldering station, and I must admit, I never want to use a regular soldering iron ever again.


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When it's ready to power up, I'll be using an RCA branded UK Mullard GZ34 rectifier tube, some various old RCA, Sylvania, Westinghouse, and other old 7025 and 12ax7 tubes, and I'll break it in with a pair of Genelex Gold Lion KT66s. After that, I have a pair of 1950s GEC KT66s to go in it.


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If we need to I'll take mine apart and send you a circuit pic or whatever you need ? It's unmolested inside n like new, but doesn't have the RS hammer finished OP tranny. Sounds heavenly thou somewhat dated to when it was made while you can play it on 10 all day and your ears aren't even close to ringing, so no aural damage at all!

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Vox AC30 said:
If we need to I'll take mine apart and send you a circuit pic or whatever you need ? It's unmolested inside n like new, but doesn't have the RS hammer finished OP tranny. Sounds heavenly thou somewhat dated to when it was made while you can play it on 10 all day and your ears aren't even close to ringing, so no aural damage at all!
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Thanks for the offer. I'll let you know if I need to take a peak. So far I have four different things for reference that don't match.
 
Well Plex I lived through a period in time in which $400 was what perfectly original Marshall cabinets went for while the speakers were already broken in. I managed to get four so I had each version of Celestions in em. 20's, 25's, 30's but the 55hz ones which are the bass cones on the 30H speakers. This stuff was all over the place back then. The JTM45 … I had to let go of an killer sounding 69' metal front 50w to finance, That was hard to do. It went to a rehearsal studio in Chinatown, NYC. Year later they wanted another since it was the most requested head in their studio. The JTM45 Power iron was tapped @ 105V, not 120v so, and I can't remember the math now just the concept. I unhooked the center tap and created a different artificial center tap …. took off the bell cover and snaked a tap thru the laminations down out the bottom of the tranny and used a higher existing tap to trick the iron to be expecting exactly 120 v. The head was very happy after that though it took longer for me to feel I didn't f^*k up.
 
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