M Fowler wrote:TriodeStore spec the JTM45 with KT66 using 8k OT
Mojotone spec the JTM45 with EL34 using 3k4 OT
Hope this helps.
Mark
This is what I'm talking about with the screens - running KT66's with 400 to 450VDC on the plates AND screens into 8K will kill them. I've seen them arc running into 6K6.
Honestly, if you're looking for landmines, the output section is deceptively simple in these amps, at least if you want to not murder your tubes.
Life is a tale told by an idiot -- full of sound and fury, signifying nothing.
My JTM45 build didn't really rock my boat for strat playing. I ended up doing the following:
- Dropped the V1 cathode cap to 5uf to remove the boomy bass.
- Changed the slope resistor to 33k and upped the treble cap to 500pf. More mids.
- Increased the 16uf power supply caps to 32uf. Got rid of ghosting and tighter bass.
- Swapped the kt66's for el34s.
So I guess I ended up with something similar to a 50 w plexi, except with tube rectifier.
Of course, at the time I built mine, I think there was only one chinese factory making kt66's, and they didn't impress me.
M Fowler wrote:TriodeStore spec the JTM45 with KT66 using 8k OT
Mojotone spec the JTM45 with EL34 using 3k4 OT
Hope this helps.
Mark
This is what I'm talking about with the screens - running KT66's with 400 to 450VDC on the plates AND screens into 8K will kill them
The first Marshall that I worked on was a 1965 JTM45 with 460V on plates, RS OT with the 8K tap connected. It's a fine combo with the right tubes. I run the 8K, GT Russian KT66, 440 on the plates and a few volts lower on the screens with no issues.
JTM45's never had the 3.4K OT and EL34 combo. The JTM50 ran this combo and the majority of the those had silicon rectifers.
Steve no use in reinventing the wheel if your guy wants to go this route.
Power 45 Watt@50Hz 1,0 Tesla
primary unduction
230Volt@50Hz 260 H
leakage induction
1V@10kHz 9,34mH
pimary impendance 8 kOhm
secondary impendancies 4,8 und 16 Ohm
The classic JTM 45 Output-Transformer. Manufactured by the British transformer factory Drake Ltd, this Output Transformer was used in Marshall® JTM 45 Amplififiers with KT66 power valves.
As with all other replicas, an original output transformer was available, which of course was carried out meticulously and carefully. The rebuild became an additional 4 Ohm tap, the 100 Volt winding is burried and not attached by a white cable like the original.
Exact layer winding, the use of original materials and carefully stacking of the transformer laminations are only part of the basic prerequisites for an exact copy of the original. The output transformer has the original dimensions and can be easily replaced. Delivery is as shown with the silver powder coated bell ends for vertical mounting.
Steve H. Here's what I did. I built a 5F6-A clone. 5881 tubes, Weber X-formers. Great sounding amp; I really liked it when it started to growl, but it was pretty loud by then. I tried splitting V1cathodes like the JMP50, and used a 12AX7 to get a little more preamp gain. Didn't sound so hot. A bit harsh.
I then built a JCM800 clone for my son. This is a real beast.
I eventually cannibalized the 5F6-A to build a JMP50 clone, only with JJ 6V6S power tubes. Tossed the bright cap on the bright volume and went with the 1986 (bass amp) tonestack. That is my ticket. I also added a pre-PI master volume. I had tried the LarMar (post PI) Master volume and prefer the pre better. I jumper both channels and blend with the Bright and Normal Vol controls. SS rectifier. 450 volts on the plates. Originally into a 4 x 10" cab with Emi Legends, and now Emi Ramrods. This is a real rock&roll amp that won't overpower the neighborhood. Not sure how suitable this would be for church services.
Got me wondering now what the 5F6-A would have sounded like w/6V6 output section. Annnt. (Game show buzzer). Going to try a 6V6 Express next.
I'll tell you what. This may turn out to be a very rewarding hobby. Two guys from church came down and tried various amps and cabs, and that 5F6A blew everything away. We changed tubes and talked about what we could do to get the perfect sound for their needs. I hope I get to build a few of these things.
The only big landmine I can think of and happened to me too is that you'll build one of the greatest sounding marshalls ever, and the customer tells you that it's too loud and needs to changed/crippled. I mean we compared it to a orig JTM100 and even the drummer heard it.... Oh well.
I mean we compared it to a orig JTM100 and even the drummer heard it.... Oh well. 
