Yeti and GTP's Combo Build

Yeti

It’s time for a new amp build!

This time I’m teaming up with @guidothepimmp to build ourselves a pair of fire-breathing 1x12 combos. The broad idea at the moment is that I’ll handle the electronics, he’ll handle the wood and metal, and together we’re hoping to make something pretty special.

The first step of course is deciding what type of amp we want to build.
We’ve toyed with a few ideas, but it looks like we’ve settled on a version of the awesome Splawn Quick Rod (… or really the Street Rod since it’ll be a combo). For those of you who aren’t familiar with it, the Quick Rod is a hot-rodded Marshall circuit aiming for an 80s metal sound (think Van Halen). The clean channel is already pretty potent, but the dirty channel is where it all happens. It has three ‘gears’ - 1st gear is a hot Plexi, 2nd gear is a hot 800 and 3rd gear is a hot 800 turned to 11. It also has an OD1/OD2 switch that moves all three of the gears into overdrive and an additional Solo boost on the master… so yeah, pretty crazy stuff.

I’ve found a schematic for the Quick Rod preamp (link at the end) and am looking at putting that into a relatively standard Marshall style phase inverter/power amp with cathode biased tubes and presence control in the feedback loop. I believe that the cathode bias is a departure from the Splawn amps, but it will allow me to use VVR power scaling to keep the beast under control.

Powertube wise, we’ll probably either go with 6V6 or EL84 in AB mode. We’re thinking of putting in 4 and then giving a half power switch to disable two of them. So, it’ll probably end up with a max power of 30W but between the half power mode, power scaling and master volume control I’m hoping you’ll be able to play it at a very manageable level.

We’ll also probably look at building a foot switch while we are at it and giving it an effects loop (it looks like the Splawns don’t use buffered loops, which would make it easier)
… oh, and we won’t be bothering with a reverb tank. ?
Other than that, we’re also toying with a built-in load box for silent recording and possibly a reactive attenuator, although I don’t think that will be necessary.

So, where to from here?
Well, first step is finalizing the circuit and specifying the components. After that I want to do a proper circuit layout, with this much gain in the amp, poor component layout and lead dressing will just be noise.
Once that is done, we can design the chassis and cabinet.
Then it’s on to ordering components and starting the build.

Lots of work ahead, but I’m pretty excited about this one. ?

http://schems.com/bmampscom/splawn/Splawn%20Quickrod%20Preamp%20Schematic.pdf

Yeti

Small correction. Looks like the Splawn amps have a separate effects loop PCB... so I'm guessing they are opamp buffered.

guidothepimmp

Yeti amps.. ?

Has a nice ring to it. Colour me excited. This is cool

Yeti

So I've spent a little time looking into FX loop buffering and I think that I'm going for a FET circuit. There is a lot of hype around tube buffering but that would take space and increase cost and as far as I can see doesn't add much beyond bragging rights. Op-amps are another option but they require a dedicated power supply, which is a pain.
Beyond that, the loop will probably sit between the channel switch and the phase inverter and I intend to make it bypassable, with level controls for send and return.

I've also spent some time looking at the transformers that we'll need because these are some of the largest cost items and affect quite a lot of the design. Assuming we go for cathode biassed 4xEL84 in the power section (which I think we will) these seem to be the Hammond products that we'd be looking at:
Power Transformer - 370HX
Choke - 194F
Output Transformer - 1750NA
You can generally pick up Hammond components through https://www.trxe.com/ but they come with a bit of a price tag. There are some local suppliers who will wind custom transformers for you but I don't want to waste their time so I will hold off contacting them until we are a bit further with the design.

Beyond that, the other big decision that still needs to be made is whether we are going to include the clean channel or not. On the one hand, it is nice to have the option of the clean channel available... on the other hand, it adds quite a lot of complexity for something that is known to be fairly uninspiring... and it could be fun to build a dedicated fire spitter.

guidothepimmp

Its the never ending conundrum for me. 2 channels.. which i really like. Or single channel which i also really like ?

I think with a fire breather it is useful to have a clean channel. Unless.. channel 1 becomes a crunxh channel.. ie.. break up clean, which can go nice and clean with a roll back. And channel 2 is the balls to the wall gain channel?

Yeti

guidothepimmp Its the never ending conundrum for me. 2 channels.. which i really like. Or single channel which i also really like ?

So this 'clean' channel has both a gain and a volume knob and the second gain stage is two parallel triodes... I suspect there is plenty on crunch in there if you want it. Let's work on two channels for now, we can always drop it to one if we need to.

JGP

Sounds interesting, what speakers will you be looking at? Celestion Alnico Ruby would be a great option if budget allows, possibly Vintage 30 or M65 Creamback as a less costly choice.

guidothepimmp

We were actually chatting about speakers recently. The short answer is.. not sure yet. We are lucky enough to have a coupke of models to test and we will take it from there.

ScottyDogg

Tasty

I want one ?

Yeti

ScottyDogg Haha, let's see how they turn out first. ?

I've made some progress on getting a consolidated circuit diagram together, rev 0.1 will probably be ready to post over the weekend. After that I'll be looking at layout to see if we can squeeze everything we're planning into a box!

We've had a bit of a change of thinking around this project and may look at making it a head instead of a combo... and since it's a head we may look at bumping it up to the full 50W with two EL34s in the power section. We're still looking at the power scaling (although we obviously loose the option of the half power mode) and I'm looking at adding a post-phase inverter master, in addition to the 'master' on the gain channel... which actually sits before the tone stack, so I'm thinking of it more as a volume.

ScottyDogg

Yeti I'm liking the sound of EL34's and a head form factor even more!

guidothepimmp

Yup. This thing is gonna be awesome with el34s ?

I reckon its a sound decision going with a head..

Yeti

Right, so I've put together a first pass of the circuit (still needs a proper review so there are likely to be mistakes). The Pre-Amp is more or less a direct clone of the Quick Rod circuit, although I am tempted to tweak that clean channel a little more sparkly (possibly change the tone stack to something a little more Fendery).

The Aux page has the circuitry for the footswitch controller (using optocouplers... more reading required) and the FET FX loop. Both of these will be built on veroboard, probably with the jacks mounted straight on the board, which will fix the board against the back panel of the chassis (now that I look at this, the FET option, may not save that much space over an extra valve... but it will be cheaper). My current design has duplicate buttons on the amp and the footswitch so that channel, gear, etc. can be controlled from either... I'm not 100% sure if this is going to stay however because those switches add quite a lot of real estate and I'm worried about the wires running backwards and forwards across the amp, although this would be minimised if they were mounted on the back, near the footswitch plugs...

The Output stage is a combination of the Quick Rod inverter and a relatively generic cathode biased EL34 circuit (although, actual value of bias resistors may change). It also has a Post Phase Inverter Master Volume thrown in for good measure.

Power supply is relatively generic except for the added Power Scaling, I've included the chip number that I used for my 15W black face clone, if I remember correctly it should be able to handle the current but I will need to double check, current plan is to mount this directly onto the chassis for heat conducting. I'm aiming for the voltages specified on the QR schematic and may need to teak a few resistor values to get them but in my (limited) experience these aren't that critical for the pre-amp tubes, provided your power amps have the correct voltage. I've also included some pretty beefy capacitors and an over-sized power transformer. The reason for this is that I want a properly stiff power supply for this amp to prevent it from loading down. Incidentally, I've gone with a half wave rectifier simply because of the transformer I've found, if we get one wound, I'll probably look at halving the voltage and going bridge rectifier.

Next it's on to the layout, which will help us size the chassis and ultimately the head itself... this is quite a complex beast, so I expect it to be large.

p.s. on a side note, everything I know about grounding I learnt from the Valve Wizard. (http://www.valvewizard.co.uk/Grounding.html) For this amp, I'm planning on using the star grounding layout with chassis connections only at the AC plug and the input jack. Heater wires will be earthed via center tap on the transformer.

Yeti

Hmmm... we also need to come up with a name for this project.

guidothepimmp

Nice. Thats looking great..

Yeti

Right... starting to think layouts... this is a busy amp!

So here's what needs to go in...

Top:
Power Transformer
Choke
Power Capacitors (probably... may go inside)
4 Pre-amp Tubes
2 Output Tubes
Output Transformer

Back:
OD1/2 button
Channel button
Foot Switch Plug 1
Foot Switch Plug 2
Send Level
Send Jack
Return Jack
Return Level
Speaker Jack
Speaker Impedance Selector
Standby Switch
Power Switch
Fuse
Power Socket

Front:
Input Jack
Clean Gain
Clean Treble
Clean Bass
Clean Volume
OD Gear
OD Gain
OD Treble
OD Mids
OD Bass
OD Volume
OD Solo
Master
Presence
Power Indicator

... right... here we go

guidothepimmp

Nice. That is busy. Will be curious to see how it fits. At least we have some options if we need to reduce hardware

Yeti

First pass layout is done. There are some things that I'm not 100% happy with (e.g. the proximity of the FX loop board to the tube beneath it) but I think that it's a reasonable proof of concept. Dimensions for this chassis are 525x200x60.
Current plan is to make the build of this true point to point and use as many terminals on the tube sockets and controls as possible. Intending to use turret board for intermediate points.

I think the next step will be to lay out the two veroboards because at this point their sizes are pure thumb suck.

guidothepimmp

Ha! Thats perfect! For reference, my vox head is 470x170x65.. so thats not bad at all.. puts it at just under the big marshall heads.

So cool to see it coming together. Well done bud. Apologies for not assisting.. but my electronics knowledge is as good as my ability to speak russian.

So.. we are dumping the standby? ??

Yeti

guidothepimmp Haha, your bit will start soon.
Incidentally, this puts it almost exactly the same size as my bassman... although it will be a lot busier. ?

So yeah... standby... out of interest, what does everyone else think of a standby on a valve amp?