Guitar Gear Forum

So I've been building up a prototyping rig now for a while in between work and doing the occasional repair/revalve/mod for people. Premise is pretty simple. 2xKT88 power amp pushing about 70/80W using an AC coupled cathodyne phase inverter. The poweramp looks suspiciously like that of a VHT D60 as I wanted something loud and clean to let the preamp and pedal choice do the talking. No loop as its not needed for testing purposes and I need the real estate. I've started this thread to motivate me to do some damn work on it

Its not exactly using the best components in some respect as its really just for dicking about with some ideas. Amps that come of it will have more money thrown at them. I'm using an antek toidal PT and a multiprimary 100W OT which has okay bandwidth but isn't too pretty and may well struggle at higher power output. I started with a blank chassis and had to do all my own drilling/punching which is a bit tricky when you don't always know what is going to go in there as I have at least 3 different preamp designs I want to try out.

A few pictures of work to date.



Test fit of the transformers and sockets


Heater wiring


Main filter board


Cathodyne PI board and bias supply. I switched to using PTFE to build on as its much nicer to work with. The turrets are reusable and are mounted with M2.5 nuts rather than being flared to the board to cut down in waste.



The first design I want to try is going to be based loosely on the low gain input of the 8 knob AOR-100 as I love that amp. There is a magic to it which comes from the local NFB recovery stage found after the tone stack which makes it growl in an unholy fashion

Expect this thread to be updated infrequently

And the 1st infrequent update:



PI and bias board partially fitted. Missing a few 47k resistors and some 10µF 100V caps for the bias supply.

Pretty cool! What type amp is your build influenced by or similar to ?

The idea is to have a flexible platform to drop different preamp boards into to try out different ideas I have kicking about inside my head. I've opted for a very clean poweramp with presence and depth NFB controls although the depth might not be needed in some of the final circuits. The poweramp is pretty much the same as a D60. I wish the chassis was a little bigger but they're quite hard to come by in the UK. I suppose I could have gotten one custom made but it wasn't really worth the effort for this exercise.

Schematic:


Nothing too magical going on. AC coupled cathodyne PI with the usual bells and whistles.

The first preamp I'm working on will be a variation of the AOR 8 knob low gain input as that thing is one of the best preamps I've used for taking pedals for sludge/doom tones which is what I'm mainly about. I have a few high gain circuits I want to try too, one of them is similar to the splawn nitro and I'm sure it'll need tweaking to work with this particular power amp/PI as that uses an LTPI. I also have a mid gain design using an EF86 that I want to try out as you don't really see those used outside of vox'y circuits and you can do all sorts of odd things with them.

Nice! I'm never done with a project.. always goes from "custom amp build" to "prototype rig".

FFUUUUUuuuu

Anyways, good work!

As a side note that has been on my mind - I have found that amps with a 50k-100k NFB tend to take pedals MUCH better than amps withtout an NFB. When I had built an AC30 circuit, I found that adding the 100k NFB made it much more usable with various pedals. However, it didn't quite sound like an AC30 anymore.

I'm quite keen to leave the poweramp/PI alone in this build if I can other than playing with the amount of NFB.

I'm going to use the least amount of NFB as I can get away with, Same with the filtering, as I feel too many amps go a little too excessive here and it makes the amp to stiff and sterile. I'm not playing tight metal anyway so I don't need it. The AC30 is an odd beast and the lack of NFB is a large part of is character but as you say this really ruins its capacity to take pedals up front without overdriving the poweramp almost immediately.

Wow, that's silly pretty and clean!!

Thanks. I still have plenty time to turn it into a rats nest! I'm amazed that no one noticed that I hadn't deburred the holes for the pots yet in that photo...

That's some beautiful work.

I'd someday like to build a Laney Supergroup clone. (It'd be basically like building a Marshall though. They are VERY similar, aside some of the layout, and the partidge trannys. Finding the right tolex/cloth & knobs will be tricky as well.

Nice !

With a heavy heart I realise that this just isnt (quite) going to do what I want it to do as fitting everything I want to do into a fairly mundane sized guitar amp chassis is going to be a pain in the ass and compromise optimum layout. So... I'm still going to build this but only as a poweramp. The rest of the chassis will be empty and I'll pull the unused preamp sockets. Sort of wasteful but if I were to do this project again I'd either build it in a huge chassis (which would be a dick ache to move) or I'd split the pre and power amp into separate entities.

So whats next you may ask? I'm going to buy a 2-3U rack chassis for building preamps in. I have priced up a custom toroidal PT with the following specs:
Primary: 230VAC
Secondary 1: 6.3VAC @ 1.8A
Secondary 2: 285VAC @ 90mA
Secondary 3: 15VAC @ 70mA

This will only run about £40 shipped from toroidy.pl and there is enough juice for 6 preamp tubes there which is more than enough. 15VAC is for switching and other low voltage supplies (LEDs and that kinda shit) if I want to include those in the build. 285VAC will give me about 400VDC so I have options for trying lots of different plate voltages by dicking with the dropping resistor value from the reservoir cap, this could also be VVR scaled too but its probably not worth the effort and heat build up in the chassis. The PT is also quite small (85mm diameter, 34mm tall) and there is bags of space in a rack chassis.

As it is I need to reserve one of the tubes for the output buffer. I may replace this with a MOSFET buffer in the future if I need the bulb back. Still 5 pres is a lot to play with! The other advantage of this system is I can try it with any amp with an FX loop. Let the fun and games commence!

EDIT: The PT and rack chassis are in the mail now

I have a long train journey to go to a conference today so I think I'll work on a turret layout for a splawn nitro lead channel. Doesnt look too complicated in the scheme of things.

So my 2U chassis has finally arrived. No thanks to FedEx though, who delivered it to someone who wasn't me and who doesn't live at my address. Oh well, its here now. Waiting on the hardware and the PT now

PT arrived

So not much has changed on this project yet unfortunately. I've just moved house and I'm in the final stages of getting a paper through peer review which has sucked up all of my time. the good news is that I have myself a cool workshop/man cave in my new place so things should pick up quickly once I get it set up

The preamp stuff looks awesome. I'm currently building an Ampeg B15 preamp into a rack unit. How are you going to mount everything on the inside? I've been looking into options myself but I'm wondering what the best way to go will be to keep everything flush with the top and bottom.

I'm going to make a long L-bracket/rail to mount the preamp tubes on as I did something similar years ago and it worked well.



I'll make this one using a push plate from a door and a sheet metal folding brake.

The board will just be mounted on 15mm hex spacers.

The whole thing probably wont look too flush in the end as its only a cheap rack chassis and I'll be drilling holes in at at will anyway so I'm not bothered by the aesthetics of it as its really just a test platform.

Zozobra wrote:The idea is to have a flexible platform to drop different preamp boards into to try out different ideas I have kicking about inside my head. I've opted for a very clean poweramp with presence and depth NFB controls although the depth might not be needed in some of the final circuits. The poweramp is pretty much the same as a D60. I wish the chassis was a little bigger but they're quite hard to come by in the UK. I suppose I could have gotten one custom made but it wasn't really worth the effort for this exercise.

Schematic:


Nothing too magical going on. AC coupled cathodyne PI with the usual bells and whistles.

The first preamp I'm working on will be a variation of the AOR 8 knob low gain input as that thing is one of the best preamps I've used for taking pedals for sludge/doom tones which is what I'm mainly about. I have a few high gain circuits I want to try too, one of them is similar to the splawn nitro and I'm sure it'll need tweaking to work with this particular power amp/PI as that uses an LTPI. I also have a mid gain design using an EF86 that I want to try out as you don't really see those used outside of vox'y circuits and you can do all sorts of odd things with them.

WoW! This looks terrific! Your design seems very well thought out and well drawn, kudos. I was surprised that your power amp doesn't incorporate a post PI master volume control though. Do you have a drawing of your pre amp design too?

Finding the right size chassis for a prototype project can certianly be a challenge. It might, in the future, be worth the trouble to have a custom chassis made for you.

Good luck and keep us posted on your progress.

Thanks. I do tend to spend way much time drawing things and not enough time with a soldering iron in my hand

Never been a huge fan of the PPIMV to be honest. Much prefer a clean and loud power amp for the tones I go for and PPIMVs screw with the NFB loop and I like the tone shaping that can be doe there. I may add one at some point down the line though.

Yeah, we're really limited on chassis options over in the UK. I do have a few contacts who will bend me a chassis as I need it and I could possibly even get it laser cut if I can produce a DXF file for it!

I'm going to aim to have my workshop set up and ready to go by the weekend. The actual poweramp section doesn't need a great deal of work. Only the PI board installing and the sockets wiring up so assuming I have everything I need it shouldn't take too long. It does need an input stage adding too but that will just be a paralleled 12ax7 which is center biased, possibly with an LED if I have one that is suitable. It may need an additional pad/divider too but we'll see how it stacks when its all put together.

I haven't got any fully drawn up preamp designs yet. The first thing I'm trying is the clean channel from a Laney Klipp as they are great amps for the sort of music I make an they take pedals well. I'll add a master volume to it and probably a few other tricks too. Nothing too fancy, mainly stuff like interstage treble shunting on switches and what not. Next up is something based on the Splawn Nitro as I can see that being a fun circuit to tinker with as it has a lot of interstage tone shaping that can be altered to taste.

I think the stuff you are doing with the NFB looks relly interesting. I like NFB as well but have limited my exploration to variations of the presence control but I can see you have taken things a bit further.

Yeah I've added depth so I can add bass into the power amp as certainly some of the higher gain pres I want to try will need the bass restricting to avoid blocking distortion. I may well make R19 switchable to reduce the NFB too.

Zozobra wrote:Yeah I've added depth so I can add bass into the power amp as certainly some of the higher gain pres I want to try will need the bass restricting to avoid blocking distortion. I may well make R19 switchable to reduce the NFB too.

It looks really well thought out I hope it works out as planned, but the tweaking process is half the fun of building a prototype design.

This looks great so far. Keep it up. I'll keep an eye on this thread.

Regarding the chassis there is a company quite local who does the bending, laser cutting and silkscreening. I'll see if i can remember their name. Based in Huddersfield.

This is the place.

http://www.prl.co.uk/

They used to make transformers for matamp! I asked them for a quote for some pretty hefty OTs and they were pretty reasonable.

Where are you based? I'm from Wakefield originally but I escaped to Manchester 13 years ago.