The Grail is in Switzerland

Hi all,
 
It's been a long time!  But this week, the Grail amplifier prototype (AKA the Fletchinator) finally made its long voyage across the Atlantic from Richmond, Virginia to Uster, Switzerland!  My good friend Chris Fletcher and I just spent an hour or so on Skype putting it through its paces, and I'm pleased to report that it arrived in one piece, is fully functional, and sounding great.   A big thank you to Douglas Nickels for taking it with him on his flight!  I evidently also got the wiring conversion from 120VAC / 60 Hz to 230V / 50 Hz worked out properly, which is frankly a bit of a relief, since after rewiring it for the trip I had no way to fully test it here in Richmond.

In any case, there were a number of tweaks to the circuit -- some of them important for proper operation, and others that greatly improved tone, that were made since the last schematic was published here.  I will work on producing an updated schematic that reflects the final as-shipped amp, and will post here in due time.  Unfortunately, I had a hard-drive crash a couple of months ago from which I still haven't recovered most of my data.  I may need to redraw the schematic - but it'll get done eventually.  In the meantime, please regard the last posted schematics here as what they are -- draughts.


Finally, Chris has promised to record some stuff with the amp -- I'll be sure to post the results here!


Cheers,

Paul

 

  The Fletchinator delivered, and safe and sound in its new home.

The Grail is in its cab!

Been awhile since I've posted an update, but I've been busy.

The Grail is coming along nicely.  There's still some tweaking to do to get it just so  -- but everything except the "Watts" variable voltage control is up and running.   Version 1 of the VVR circuit ran into a bit of trouble with the small signal transistors getting reversed biased, but I think a fix is in hand... just haven't had a chance yet to redo that yet.  In the meantime, the basic amp runs fine without VVR.

I had it out for its first full-volume test drive with talented Richmond musician Marshall Costan last week.  First impressions were very favourable! Nice cleans, and overdrive that goes from fairly subtle all the way to that full-on sustain-forever buttery drive that I was hoping for.  Still the session pointed the way to a few areas for further attention.

Since then, the main changes to the "build version" of the schematic involve alterations to the reverb circuitry. I increased the drive slightly.  But in particular, I had to significantly change the ratio of dry to verb signal in the reverb recovery mixer in order to get the verb wet enough at max.  In fact, I totally eliminated the verb mix resistor, and am now just using the output impedance of the first recovery stage as the series resistance in the VE mixer.  The dry resistance was bumped up to 750K.  Finally, the 1Meg reverb control was replaced with a 500K linear pot, in combination with a 470K fixed resistance in the ground leg. I may tweak these values a bit more yet, but it's pretty good as is.  I'm probably going to drop the first recovery plate resistor to 100K.  An LTSpice sim suggests that any gain realized from the larger 150K plate resistor is immediately neutralized by the increased output impedance of the stage... which interacts with the VE mixer to drop effective wet level... so there's no point in adding the noise that comes along with the larger plate resistor.

But the big news is that I slapped on the faceplate and dropped it into its case for the first time today.. and it looks gorgeous. It's always a relief when the custom faceplate actually fits perfectly!

What do you think?  I'm taking it out for its second spin on Sunday with my friend Andy.

Grail amp cabinetry

I'm back from vacation now, and I've got some pictures of the cabinetry for the Grail amp that I brought back with me from British Columbia. As with the Wombat Mk I amps, the cabs are custom made for me by Loren Mazereeuw of Maz Woodworking in Nelson, BC, Canada.  As you can see, the quality is just astonishing:

The cabs are constructed with solid birch, and completed with a lustrous post-catalyzed finish:

Incredible precision dovetailing:

 Louvers for heat dissipation.  A bit of a sports car look:

I can't wait to see how the whole unit will look assembled. Won't be long now.

Grail amp wiring is complete

I think I'm done with the wiring for the Merlin and Dumble-inspired Grail amp.  Just a couple of small things to clean up with the power transformer wiring.  It's time now to spend a couple of hours to verify the circuit to make sure I've actually built what I think I've built.  But barring any unforeseen issues, I'll be doing an initial fireup later today.  Here's the current schematic. In the meantime, here's some gutshot pictures with everything all harnessed up, and a couple of snapshots as well of the faceplate:

 As you can see, there's just a couple of tack-soldered leads in the power supply that need to be trimmed.  The toroidal Antek AN-3T325 power transformer I chose I think will do a good job here, but a little bit of investigation was necessary to make sure I had the phasing relationships correct on the various coils.

 

Everything about this amp is big, including the faceplate. I'm really pleased with the way it turned out.  Normally, I use anodized metal for my faceplates, but the material didn't come in the right dimensions for this job.  So this time, I used a gloss black plastic material with back engraving. Jeanne Kangas at BNP lasers does a great job on front and back panels for amplifiers, and have no hesitation in recommending them.

Grail amp main board and board fitting

The Grail amp is progressing rapidly now.  This morning, I finished the main board -- the last of six turret boards that this amp requires.  At 21.5" x 3", it's easily the biggest turret board I've ever layed out and constructed.  But with only one exception, everything went smoothly.  The one glitch that I discovered only after having mounted the turrets was that the pinout on the vactrols wasn't quite what I thought it was.  So the vactrols are mounted in somewhat of a spaghetti monster fashion.  It should work just fine, but version 2 of the board will definitely correct this.

As this was the last of the boards, I also had a chance to do an initial test fit of all the boards within the chassis. It's looking good!

Left closeup:

Middle closeup:

Right closeup:

Now all that's left to do is start wiring this puppy up. It won't be long now until the Fletchinator roars to life!

The Grail is coming together

I've been too busy drawing, drilling, and soldering to post too many pictures here lately.  The Grail amp project has been coming together rapidly.  But tonight my soldering iron died, so I took the time to snap a few pictures.

Gutshot showing the power tube wiring and the boards that have already been completed: 

Note that the whole area to the left will be occupied by the main board. There are no less than six boards on this amp.

Here's a closeup of the what's going on in the power section. On the side wall left is the DC heater board,  on the back wall is the bias supply board, and visible right behind the heater transformer is the variable voltage regulation and rectifier board:

Here's the preamp voltage regulator board that's visible in the first picture, before it was installed:

 

This picture gives you a good idea of the size of the chassis, which is a huge 27" x 10" x 3".  Big as it is, nearly every inch will be filled with circuitry by the time it's finished:

Finally, here's a snapshot of the top deck, with all ten tube sockets visible.  Note the cool-looking Antek toroidal power transformer:

Introducing the Wombat Amps "Grail"

Here's a couple of schematic and layout sketches for the shortly-upcoming Wombat amps "Grail" amplifier, A.K.A the "Fletchinator". This amp will be a no-compromises monster suitable for everything from playing on the big stage to recording in the studio, to practicing in the attic.  It's a full-featured 10 tube, 100W-class amp with two channels, built in reverb, high quality parallel effects loop, 12V DC heaters for the preamp to ensure whisper quiet operation, foot-switching (for overdrive, tonestack lift boost, and reverb), and variable voltage regulation so that the amp can be dialed in anywhere from about 5W to somewhere in the 80-100W range.  The power supply is regulated and semi-stabilized with a with a capacitor multiplier replacing the choke, and a zener follower at the beginning of the preamp chain.  The amp is inspired by the Dumble amplifiers popularized by Stevie Ray Vaughn, Eric Johnson, Larry Carleton, Robben Ford, and Carlos Santana. However, its design is also very much informed by the principles of modern musical instrument amplifier design promoted by the Valve Wizard,  Merlin Blencowe, in his excellent books and website.  It is not a clone.  The chassis, made by BrianS for me at Dirty Dawg, is a humongous 27" x 10" x 3", and every inch is needed.

Follow along with me over the next month or so as the amplifier begins to come together. Many of the components have already arrived.

The schematic and layout here are very much preliminary draughts -- there are known errors and the drawings are currently incomplete -- however, the final amp will be constructed substantially along these grounds. Please stay tuned!

The "Fletchinator" power supply

Here's a first sketch for the power supply for the amp I'm doing for my "Brother-From-Another-Mother" Chris Fletcher, proprietor of The Canadian in Uznach, Switzerland, and a damn fine guitar player.

This amp is going to be by far the most ambitious I've done to date.  If it works as planned, it's gonna be insane. A power scaling 100W head with Dumble-inspired topology, but with built in reverb and a high quality effects loop.  If it doesn't work as planned, well then, it'll be a noble experiment and expensive bust.

The intention here is to do a power supply that is totally modern. You won't find this in Grandpa's Fender. It implements variable voltage regulation (AKA power scaling) for the power tubes and phase inverter, uses a capacitor multiplier instead of a choke for filtering the power tube screens, and amplified Zener regulation of the preamp tubes.  This is obviously inspired by Merlin's latest book.

As I said, this is just a sketch. I'll need to replace the MOSFET shown on the VVR with something substantially beefier.  I'll probably also want to reduce some of the voltages across the board, and also tweak some of the values when I get better estimates of the power tube plate and screen currents. The preamp currents should be pretty accurate though.  Some of the PSU caps will be pretty close to their maximum rating, so I'll have to give some thought to that as well. I'll have to take a look at how well it'll do with start up transients.  Let me know if you have any other comments.