Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bigwan on November 30, 2012, 06:57:43 am
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Hi Guys,
Long time lurker, first time poster!
I've been reading a bit on the forum with a view to my next amp build and have settled on the Tweed Overdrive Special.
I have a Marshall JCM900 sl-X 50 watt that I would like to use the chassis and transformers from. My question is, do you guys think it would be a suitable basis for a TOS conversion? The plate voltage of the JCM is about 430v and the OT has a 3.4k primary.
I'm unsure of whether to convert the amp to cathode bias (amp uses a capacitive bias method as there is no bias tap on the PT and no center tap on the PT secondary), or even what output tubes to use for the build (would prefer to use the EL34s that are in the amp), but these are things I'll decide as I go along.
Cheers from Northern Ireland!
Ian
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Welcome Bigwan!
Calling Dr's tubenit and Geezer! Marshall JCM900 sl-X 50 to TOS conversion stat! :l2:
They'll help you Ian, it's their mad scientist brain child and they love it when their brain child has offspring.
Cheers from Northern Ireland!
OK, since you live in Ireland do you have any good stories about Rory Gallagher, Thin Lizzy and Van the Man Morrison you could tell use? Hmmmm? :undecided:
If you would be so kind to start a new thread in other topics on any or all of them it would be interesting.
Thanks, Brad :icon_biggrin:
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Yes, I certainly do think the JCM900 would be an appropriate base for a TOS. Ironically, I did eventually put cathode biased into my friend's JCM900 to Plexi 50w conversion. So, I don't see any reason why it wouldn't work with the TOS. I think I used a 47uf/430R-10w approach, IIRC?
The one thing I think you'll need to watch is the layout because of the tubes being in the middle of the chassis on the JCM 900, you have very little room to put a layout board in. I actually used terminal strips on the JCM900 to Plexi 50w conversion. I would recommend a paralleled turret or terminal approach vs. the Hoffman board because of limited space ............ UNLESS you are willing to move the preamp tubes location. You could consider relocating all the preamp and power tubes more towards the back of the chassis and then capping the current hole locations & putting your layout board there?
http://s28.beta.photobucket.com/user/tubenit/media/CPlexi50chassisinteriordescription.jpg.html?sort=3&o=51#/user/tubenit/media/CPlexi50chassisinteriordescription.jpg.html?sort=3&o=51&_suid=135418905940006262767463437295 (http://s28.beta.photobucket.com/user/tubenit/media/CPlexi50chassisinteriordescription.jpg.html?sort=3&o=51#/user/tubenit/media/CPlexi50chassisinteriordescription.jpg.html?sort=3&o=51&_suid=135418905940006262767463437295)
A poor layout could result in some oscillation issues given this is a high gain amp.
It also will make it very challenging adding foot switchable relays. However, I think you could at least find a way to have one footswitchable relay DPDT for the clean/overdrive.
The other thing is you'll need to consider adding full size pots to the JCM900 chassis instead of the PCB pots (which I don't like). Look carefully at the inside of the chassis, cause there is one pot hole that would be challenging given how the chassis faceplate attaches. Hopefully, that comment makes sense and you can figure it out.
I did build a TOS with PT & OT on opposite ends .......... so I know it's possible to make that piece work out OK.
I personally would attempt this, but be prepared to troubleshoot oscillation concerns or noise concerns at idle.
With respect, Tubenit
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Thanks for that T. It's funny that you could be stuck for space in a chassis this size! I'll strip out the chassis and see how much i actually have. Already have alpha pots at the ready and was thinking of a multi section JJ cap for most of the filtering.
I'll keep you posted on my (likely slow) progress...