Thanks, Martin, I was not aware that was an option. That would be way easier!
Which way is the best way to orient a sidewall-mounted auxillary transformer: plate facing the chassis "flat" or the chassis sidewall? The Dumble below has a laydown PT and the aux tranny above it. Because I have a standup PT, if I rotate this relationship 90 degrees longways to the chassis, it seems the sidewall-mounted transformer ought to be in the same orientation--essentially parallel to the OT.
My problem is my chassis is only 2.5 inches deep. [searching Mouser] Looks like this could do it:
I was thinking mount it with the U bracket parallel to the top of the chassis. This side wall location is pretty far from the sensitive parts of the circuit and from the OT, so it may not make much difference.
If you want to experiment, temporarily attach the PT (securely taping off all exposed leads), and the OT, and connect AC power to the PT and the aux transformer (with its leads taped off). To be really safe, you could attach and wire in the IEC inlet, mains fuse, and the power switch so that everything carrying high voltage is secured. With headphones connected to the OT secondary you will be able to hear any induced hum, or you can connect your DMM and measure it in mV. Position the aux transformer on the chassis wall and rotate it to see if there is an optimum orientation. You can then drill the mounting holes for the aux transformer.
No other components or connections needed, just have the OT sitting in its planned location. The magnetic field produced by the energized PT's will couple with the OT and produce an induced voltage. Connect the highest impedance output tap to your phones or meter to measure it.
Anyone know where I can find a voltage spec chart for a 100W Silver Jubilee? I found a dead link on the old Metro forum that SDM had posted for one, but I can't find it in search. I emailed Marshall for one, we'll see where that goes. Thank you!
Funny thing about the Silver Jubilee circuit is that even the official schematic isn't necessarily accurate - what I mean by that is that my amp could be an earlier revision, but I compared the official schem to my original '87 2550 (left the factory in September or October) and there are quite a few things that are off. Most values are correct (but not all), it's mostly the component numbers that are slightly off. For example R25 might be a plate resistor on the schem, but in the amp R26 is the plate resistor etc. The strangest thing by far though is that C5 is supposed to contain the bright cap, but mine has a RESISTOR in that spot. What's even more strange is that the gain pot clearly acts like there is a bright cap there, but I can't actually find it on the PCB. Can't really say that it keeps me up at night as the amp sounds absolutely killer, might be my favourite (stock) Marshall of all time even. Given the choice between a 2550 and a (stock) 2204 I'd take the 2550 every time, BUT there's a plethora of modified 2204-based circuits that give the 2550 a pretty good run for its money (literally, as the originals are overpriced these days imo).
As far as doing a build from scratch, the only disadvantage of building a 2550 is that you are sort of putting all your eggs in one basket as there is just one mod to these amps that I'm aware of - which isn't really a mod, it's the Wizard Modern Classic circuit that requires a different OT, a certain plate voltage (high) and more filtering which is a bit of a pain to do. If you build a 2204 there's a LOT of mods you can do, with little to moderate effort. It ultimately comes down to the sound you're going for, if you really need a clean channel etc.
That’s interesting, I’ve noticed several errors as well. The last amp I built was a legit 2204 JCM 800 and it sounds great. I found myself setting it very differently for high and low gain tones, so I decided on a Jubilee based build. I’m building it now, slowed down a bit by the panels. I’ve been impressed by the Jube’s sounds, so I’m excited to see how it turns out. More to come...
ViperDoc wrote: ↑Wed May 13, 2020 1:01 am
That’s interesting, I’ve noticed several errors as well. The last amp I built was a legit 2204 JCM 800 and it sounds great. I found myself setting it very differently for high and low gain tones, so I decided on a Jubilee based build. I’m building it now, slowed down a bit by the panels. I’ve been impressed by the Jube’s sounds, so I’m excited to see how it turns out. More to come...
Nice one, if you already have a 2204 it's a no brainer really. The most significant difference to me is that the Jube sounds killer at any volume (when using the lead master), while the 2204 needs quite a bit of volume. EQ is also very different (which is to be expected with CF vs plate driven), don't think I've ever played an amp with a more responsive EQ than the Jube. They both have their place though, I'm a bit of a gain whore so a stock 2204 is a little lean until it's really cranked (MV 6-8)
