Guitar Gear Forum

As a quick aside from my rack preamp goofings off I'm converting a busted bugera 1960 a friend game me years ago into a dedicated power amp.
The OT is shot and quite convincingly failed the neon short test so it's a paper weight at best unless I ever fancy a go at rewinding it, which I don't. Thankfully I have a spare 100W 4K primary OT in my spare parts pile and the PT is good for somewhere between 480-500VDC which is premium KT88 territory for a single rail.

Design wise I'm plumping for a parallel input buffer and long tailed pair PI. It'll have a very SLO looking set of power amp controls. I may swap the presence for a 5k scratchy type and the depth for a CCV type. There is also a pot to set the amount of NFB which will control the power amp feel from tight to loose. I may also put a pot in line with the 470r LTPI bias resistor as an additional sort of MV which will allow the PI to break up at a lower volumes but this might be better suited to a switch.



The power supply will be slightly unconventional as I want to try out a regulated screen supply, rather than use the original choke feed. This has the advantage that running the screens at a rock solid 400VDC will put way less stress on the screens while still delivering full power and punch. I can also feed the LTPI straight off this node too as well as implement current limiting if there is a screen short. You'd only really notice the difference between this and the choke at super high volumes and I want a big solid bottom end which is better achieved with regulation.



Here is a quick test of the PT to check it's doing what I expect. 508VDC unloaded. More on this tonight.

Cool... Did you finish it? If so, howd it go?

I went on vacation : lol:
It's the first thing in doing when I get back though.

Back from vacation, time to get on this. I have too many ideas to work on that need a dedicated power amp to test...

Zozobra wrote:As a quick aside from my rack preamp goofings off I'm converting a busted bugera 1960 a friend game me years ago into a dedicated power amp.
The OT is shot and quite convincingly failed the neon short test so it's a paper weight at best unless I ever fancy a go at rewinding it, which I don't. Thankfully I have a spare 100W 4K primary OT in my spare parts pile and the PT is good for somewhere between 480-500VDC which is premium KT88 territory for a single rail.

Design wise I'm plumping for a parallel input buffer and long tailed pair PI. It'll have a very SLO looking set of power amp controls. I may swap the presence for a 5k scratchy type and the depth for a CCV type. There is also a pot to set the amount of NFB which will control the power amp feel from tight to loose. I may also put a pot in line with the 470r LTPI bias resistor as an additional sort of MV which will allow the PI to break up at a lower volumes but this might be better suited to a switch.



The power supply will be slightly unconventional as I want to try out a regulated screen supply, rather than use the original choke feed. This has the advantage that running the screens at a rock solid 400VDC will put way less stress on the screens while still delivering full power and punch. I can also feed the LTPI straight off this node too as well as implement current limiting if there is a screen short. You'd only really notice the difference between this and the choke at super high volumes and I want a big solid bottom end which is better achieved with regulation.



Here is a quick test of the PT to check it's doing what I expect. 508VDC unloaded. More on this tonight.

Congratulations on intimidating the shit out of me!!

Looking forward to watching the conversion - your build threads are awesome!

Planning the bias supply and regulated screen supply layout. I just going to use the half wave bias supply that was in the Bugera and pretty much every other Marshall clone with it's own bias winding on the Px but I've decided why half arse it and will go with a full bridge.

I have somehow lost the 200V Zener diodes for the regulator though Yet another order to RS it is...

I do keep scratching my head at this chassis and wondering if I should just use another and forget about the headshell. It's not like it's going anywhere

Waiting on some pretty boring components to arrive. I've seemingly exhausted all of my supply of machine screws and hardware to mount the filter caps safely

I've made the executive decision to can the conversion and to move the Px over to a spare chassis I have drilled the ever living shit out of as it's just going to be too much of a ballache to make things work cleanly in this 1960 chassis. Ah well, I'll find a use for it with some smaller iron I have.

Initial plan is to build something based loosely on the BE deluxe, using an LND150 boost stage in place of the HBE boost. I'll add a loop so this can become a general use power amp for the rack stuff once I've got the thing working. First things first - time to draw up a schematic and then lay it out.

Initial schem:


I've not shown any of the switched parts. I'm not relay switching anything either as it'll be home/studio use only.

What's the M1 LND150 thingy on the schematic?

It's a depletion mode MOSFET. In this case, he's using it like an extra tube gain stage up front.

What they said.

It's a high voltage mosfet that can be more or less substituted for a tub gain stage. Their characteristics are more akin to pentodes than triodes but for this application I just want a 4Vpp or so kick in the pants for the rest of the amp. The LND150 is commonly found in high voltage mosfet fx loops as the return stage. Some loops use them as the send stage but it's better to use an enhancement mode device like an irf820 for that as the LND150 is super high gain and can be noisy/pick up shit you don't want and in any case you're buffering the signal down so the gain isn't really needed.

With a bit more effort they can be made to behave like a 12ax7 which is something I want to experiment with but I need to work on my PCB design chops before doing that.

Layout. I need to check this over before getting the iron hot. I'm using two rows of tag board to prototype on as it's quick and dirty.

jgreenwd wrote:It's a depletion mode MOSFET. In this case, he's using it like an extra tube gain stage up front.

Zozobra wrote:What they said.

It's a high voltage mosfet that can be more or less substituted for a tub gain stage. Their characteristics are more akin to pentodes than triodes but for this application I just want a 4Vpp or so kick in the pants for the rest of the amp. The LND150 is commonly found in high voltage mosfet fx loops as the return stage. Some loops use them as the send stage but it's better to use an enhancement mode device like an irf820 for that as the LND150 is super high gain and can be noisy/pick up shit you don't want and in any case you're buffering the signal down so the gain isn't really needed.

With a bit more effort they can be made to behave like a 12ax7 which is something I want to experiment with but I need to work on my PCB design chops before doing that.

Thanks!

Test fitting the reclaimed iron in an old project chassis.

Tonights dicking around. Working with a blank chassis is so much slower than slapping things in a pre-cut one...

Layout for the power supply. This is a regulated 400VDC screen supply, 70 heater elevation supply and a full wave bias supply on one tag board.

I've also done a few bit of hardware mounting but now it's time to go to the pub so no more for tonight.

Your drain appears to be floating.

Good spot, thanks

Zozobra wrote:Good spot, thanks

I’d love to see an updated layout when you get it sorted. I’ve been toying with the idea of regulating the screens on a Fender’ish build.

What is your pass device? Are you sinking it to the chassis or something more substantial?

Sure thing. It's probably just a dumb copy error from flipping between documents and not thinking about it properly. I was considering knocking together a mini PCB for screen regulation as it's a pretty simple circuit. I'd add foldback current limiting to protect the MOSFET too. Could be a nice thing to have available with current production el34s having a hard time with screens over 450V.

I can't remember the exact device as I'm on my phone atm. It's bolted to a heatsink which is kinda overkill really.

Device is a 14nk60, good for up to 600VDC. The one I'm using is in an insulated package so I don't have to mess around with insulating spacers. I'm sure a suitably rated irf would be fine too.

Edit: 600V, not 60V

Zozobra wrote:Device is a 14nk60, good for up to 60VDC. The one I'm using is in an insulated package so I don't have to mess around with insulating spacers. I'm sure a suitably rated irf would be fine too.