JTM 45 clone and Mullard 12AX7s experiment

[casey73]

I built a JTM45 clone 6 years ago. I made a few changes to the original circuit. The screen grids are 1K 5 watt and I put 1.5K grid stoppers in directly on the output tube pins for stability. I also used carbon film or metal film resistors instead of carbon composition and silver mica caps in place of ceramic caps in the PF range. The schematic I used was the Bluesbreaker version without tremolo. PT is Heyboer, OPT is Pacific Transformer and the choke is a Hammond. Output tubes are Valve Art KT66 and original preamp tubes were a mix of current production JJ and Tung-sol 12AX7s. Originally I had F+T 32/32uf filter caps in it, but I was getting bad ghost notes. I upgraded to 50/50uf F&T filter caps, and that took care of most of the ghost note issue although there is still a hint of it when pushed real hard but not bad enough to make any changes.

Early this year I picked up an early 60s Voice of Music reel to reel tape deck at a garage sale. When I got it home I found it was loaded with 3 Mullard 12AX7s and several other useful tubes. I had a bunch of tubes tested this summer (AZ Tube Supply) including the three Mullard 12AX7s and they all tested in the hi range on a TV-7. So I thought I'd see how they sound in my JTM45 clone. One of the 3 tubes had matching triodes so I put that in V3 for the phase inverter. The one with the highest TC numbers went in V1 and the other in V2.

As soon as I started playing I could hear a microphonic ring over every note, and this was at low volume. So I swapped V1 and V2 putting the higher gain 12AX7 in V2. Turned it on and there was no more ringing. I haven't had a chance to really open it up because my playing room is the living room and it's a small house. As soon as my wife leaves for the quilt shop or the neighbor's house I'll give it a decent workout and see if that simple tube swap cured the microphonic issue.

[Roe]

picture shows many metal oxide resistors. try carbon film plate resistors for that famous plexi tone

[pdf64]

Yes, I noticed that. MO are great in power applications, where any resistor resistor noise gets decoupled, but their noise specs typically aren’t their strong point, so rarely used in the signal path, especially early stages.

[Stevem]

I find silver micas to be even more microphonic then ceramic disc caps many times.

I encase mine in clear silicone.

The distortion factor that carbon comp resistors display over metal film caps takes place only once the voltage applied to them gets up near 300, and for a common 1/2 watt resistor that’s damn close to their voltage limit anyway.

[casey73]

picture shows many metal oxide resistors. try carbon film plate resistors for that famous plexi tone

I had a large number of MO and MF resistors in my stock so I used them because I had them and because the 1/2 W carbon film leads were too short to make a full span across the turret board. I didn't use any carbon comp resistors because I from what I understand they only have an effect on tone at relatively high voltages and because they tend to drift in value with time. This is the only Marshall of this era I've ever played so I can't really ID the "plexi tone" and if changing the plate resistors to carbon film will make a difference. But I may try it in the future. I did get to crank it up today and will comment on that in another post here.

[casey73]

Yes, I noticed that. MO are great in power applications, where any resistor resistor noise gets decoupled, but their noise specs typically aren’t their strong point, so rarely used in the signal path, especially early stages.

This isn't a real noisy amp, but I will tuck that info away for possible revisions in the future. That's why I love turret board construction. Future repairs and revisions are so easy to do with them. I'm not a pro but have repaired a fair number of amps for friends over the years and no longer work on anything built on a PCB because of the labor involved and the fragility of PCBs. I'm retired so when I do make a repair it's as a favor, not to earn income.

[casey73]

I find silver micas to be even more microphonic then ceramic disc caps many times.

I encase mine in clear silicone.

The distortion factor that carbon comp resistors display over metal film caps takes place only once the voltage applied to them gets up near 300, and for a common 1/2 watt resistor that’s damn close to their voltage limit anyway.

I wasn't aware of the silver mica microphonic issue until I heard Lyle of Psionic Audio on YouTube mention it while he was working on a high end amp. I haven't experienced it. A JTM45 isn't a typical high gain amp but that doesn't mean it won't happen in the future. I assumed ceramic caps were cheaper back in the day and that's why they used them.

If you look closely at the amp, I tend toward non-conventional. I use small Davies style knobs on all my amp builds. I also prefer original Dialco pilot lights. I acquired some of them years ago and prefer the way they look. And I used Carling power switches. When I built this amp I priced after market JTM 45 cabinets and they started at $200. The original JTM45 head was needlessly wide because it was made to look good sitting on a 4-12 cab. I don't have or need a 4-12 cab so I built my own head cabinet from 7 ply baltic birch. It's shorter and a little taller than a conventional JTM 45 head cab. I also went with an abbreviated Marshall logo because it really isn't a Marshall but most folks will figure it out.

[casey73]

Today I had a chance to plug this JTM45 clone into my 2-12 cabinet. I cranked it and experienced no microphonics from the preamp tubes. I controlled the volume from my Les Paul and it was quite nice. I got typical 70s Marshall crunch, Clapton "woman tone" on the neck pickup and nice full clean tones when I rolled the volume back.