Converting a Bugera 1960 into a 2xKT88 high gainer

[Zozobra]

Hi Folks,

I'm converting a Bugera 1960 that a friend gave me with a blown OT and some other gremlins lurking in the PCB that local techs couldn't diagnose or decreed wasn't worth the effort/cost of doing so.
The chassis is a bit of a pig to work in and I'm currently working out the best plan of attack as while it's long, it's rather narrow and the existing holes are annoyingly placed. Still, I want to see what I can pull out of it using stuff in my spares pile as I can potentially piece together a nice amp in a headshell for very little investment



Here is my best guess at a layout using Merlin's Dev PCBs (the PCBs are still free to move and I'm not adverse to rotating them 90°). The OT and choke are underneath the terminal board to the right. The OT will be located to the far left, with the laminations running vertically to be orthogonal to the PT laminations.

Using the old octal sockets is problematic unless I use huge standoffs for the PCBs to clear sockets and in any case I'd have to skip a socket (i.e., use 1st and 3rd, or 1st and 4th) as they're too narrowly spaced to get 2 KT88s right next to each other. My concern with placing the OT and power tube sockets is that I then am faced with a rather long heater and B+ run to the OT and tubes, and this is going to be a high gain preamp so I want to avoid injecting noise in to the preamp, although that looks to be unavoidable.

Does anyone have any suggestions?


A more extreme option is to cut a big section out of the top and to slap a piece of ali sheet on so I'm not limited by the current holes. I could probably get away with siting the OT and power tubes to the right/PT side then. I do kinda want to avoid doing that if I can as the chassis is rather heavy duty steel by the looks of it so it will be an arse to drill out/cut with my tools.


Here is a shot of the chassis with nothing in to show the holes I'm working with/against.

[pdf64]

From what I can see, I think new holes for the octals at least would be the best starting point; there looks to be room for a couple between the existing holes and the back panel?
Not a pleasant/easy job but perhaps a price worth paying to avoid big compromises on layout.
Shouldn't be too bad with a step drill bit, or ideally a punch.

[Zozobra]

Yeah that is also a consideration I have some hole punches so I can go the route of sticking new holes between the inner and outer pair without too much pain. It's so annoying having such a huge chassis with so little easy to use space. I'm currently leaning towards whipping a big piece off the top and sticking some fresh aluminium to work as I see fit down. I may also see if the PT can be rotated 90° to I can orientate the OT the other way as that will help compact the layout no end come to think of it.

[olddawg]

Can you simply just buy or make a new chassis?

[Zozobra]

I thought about that but the chassis isn't a standard Marshall size. While I have folded a very simple chassis to test out preamps, something like this is beyond my skillset unfortunately.

[olddawg]

I thought about that but the chassis isn't a standard Marshall size. While I have folded a very simple chassis to test out preamps, something like this is beyond my skillset unfortunately.

If it's just wider you might just be able to trim the end(s) off, especially if it's an inexpensive blank. Putting a plate over the top is a good idea. I'm not sure if I would even cut out the old chassis. Just secure it over the top with sheet metal screws and cut your holes. Looks like a steel chassis... I wouldn't use aluminum. Dissimilar metals can cause electrolysis, ground loops, other gremlins. If there's a metal recycler close you might get lucky. Once upon a time a HS could bend you a chassis in 5 minutes. But metal shops are gone and I'm showing my age.

[bob p]

I'm familiar with the odd layout of those Marshall type Bugs because I've been thinking about modding a perfectly good 1990 as soon as the warranty expires. The box and the chassis are actually pretty nice, so I can see why you'd be interested in keeping it... especially if you want to keep the build cost down.

Bug located the holes where they did because they PCB mounted all of the tubes. The locations are OK if you want to fix the original PCB but if you're going to go with an eyelet board setup or Merlin's PCBs then the holes are definitely in the wrong places. If you're committed to use the Merlin boards then I think your best bet will be to cover the original holes and punch new ones. You might not need a steel sheet. You might be able to get away with patch discs that screw into the existing holes, add some bracing on the underside, and just punch new holes where needed. The sheet could be more sturdy, but it won't look all that good unless you mounted it from below and painted the area under the holes.

Another option if you want to go with 2xKT88 would be to use 2 of the non-adjacent existing holes (as KT88 require 4" mounting centers) and cover the other 2. If I were scratch building a circuit into that that chassis I'd probably take a different tack -- I'm thinking that I'd use the existing holes and wire PTP using terminal strips instead of the PCB. With some creative thinking about layout I think you could shoehorn a PTP layout in there without having to move any holes.

edit: Forgot to mention: is the chassis deep enough to allow you to mount Merlin's PCB's vertically?

[Zozobra]

If it's just wider you might just be able to trim the end(s) off, especially if it's an inexpensive blank. Putting a plate over the top is a good idea. I'm not sure if I would even cut out the old chassis. Just secure it over the top with sheet metal screws and cut your holes. Looks like a steel chassis... I wouldn't use aluminum. Dissimilar metals can cause electrolysis, ground loops, other gremlins. If there's a metal recycler close you might get lucky. Once upon a time a HS could bend you a chassis in 5 minutes. But metal shops are gone and I'm showing my age.

I'll go with a plate on top I think. The chassis is totally non-standard dimensions so I'm stuck with working with it if I want to keep the headshell. Good call on the dissimilar metals. I do have an ideal sized piece of aluminium but I will look for some steel to be safe.

I'm familiar with the odd layout of those Marshall type Bugs because I've been thinking about modding a perfectly good 1990 as soon as the warranty expires. The box and the chassis are actually pretty nice, so I can see why you'd be interested in keeping it... especially if you want to keep the build cost down.

Bug located the holes where they did because they PCB mounted all of the tubes. The locations are OK if you want to fix the original PCB but if you're going to go with an eyelet board setup or Merlin's PCBs then the holes are definitely in the wrong places. If you're committed to use the Merlin boards then I think your best bet will be to cover the original holes and punch new ones. You might not need a steel sheet. You might be able to get away with patch discs that screw into the existing holes, add some bracing on the underside, and just punch new holes where needed. The sheet could be more sturdy, but it won't look all that good unless you mounted it from below and painted the area under the holes.

Another option if you want to go with 2xKT88 would be to use 2 of the non-adjacent existing holes (as KT88 require 4" mounting centers) and cover the other 2. If I were scratch building a circuit into that that chassis I'd probably take a different tack -- I'm thinking that I'd use the existing holes and wire PTP using terminal strips instead of the PCB. With some creative thinking about layout I think you could shoehorn a PTP layout in there without having to move any holes.

edit: Forgot to mention: is the chassis deep enough to allow you to mount Merlin's PCB's vertically?

If I was going with something lower gain I'd totally go with tag strip construction. I have some great shots of the hexe amps model-t clone using this method so I'd have a great template to work from. Alas I've just finished up a superbass style amp so another would be kind of redundant.

The chassis is 75mm tall (from memory) and merlin's PCBs are 60x100mm so vertical mount on a piece of angle could be an option to consider. Another option I'm seriously considering is to just build this as a full featured poweramp in a headshell. I do have a 2U rack chassis and a suitable toroidal PT for a preamp that I could use and that way I can squeeze 2 or 3 channels in. I'm mainly using this to record with so I don't care that it's not terribly practical to transport.

[olddawg]

I'm mainly using this to record with so I don't care that it's not terribly practical to transport.

Famous last words. "Wow.. this sounds great, to bad I can't take it to the club!" Lol! Been there...

[Zozobra]

Yeah I'm having a rethink. Considering converting this into something Ampeg'ish as I have nothing like that in the stable and they aren't terribly common over here.

I'm currently working on the rack pre and Ill throw up a build thread when I have a bit more done on it. I have enough space and spare bits to add an ecc99 pp poweramp and dummy load so I can always run it into a big solid state power amp if I want to play out with it.

[bob p]

Famous last words. "Wow.. this sounds great, to bad I can't take it to the club!" Lol! Been there...

As it comes from the factory that head is damned heavy -- the chassis isn't heavy but the iron and the MDF box end up adding a lot of weight. The only way to get around that MDF problem would be to re-cab the head. But that isn't practical because one of the main reasons to salvage one of these amps is because the cabs look nice. If you were to ditch the cabinet then you might as well just pull the transformers and throw everything else away to go with a standard Marshall sized chassis / cab.