New Build - 35w 15" Single-Channel Black Panel w/ KT66s

ElliotKnapp

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I just officially embarked on my 19th amp build and I thought it'd be fun to document the process a bit more than usual and note some observations and preferences developed over the last few years and builds.

The project: a ~35 watt "black panel" Fender combo based on a Princeton Reverb-sized Weber 6A14HP chassis and higher-powered transformers, with a 15" Weber 15F150 speaker. Weber's 6A14HP circuit is basically a 6L6 Princeton Reverb, but I decided to go with a few other particularities: foregoing any kind of tremolo and using the spare 12AX7 triode for a long tail pair phase inverter; using the spare trem potentiometer holes for a mid control and a master volume bleed control before the PI. So, really, it's probably more similar to a single-channel AB763 Vibroverb (er, except without the "verb" part), but so much has been removed that it's kind of neither here nor there.

I'm so far from "needing" another amp at this point that I have to come up with esoteric excuses for embarking on a new build--this time I've strung a few together: 1) the fact that I have a spare Weber 404248 output transformer on hand, 2) I've got a 15" alnico Blue Dog that I use and love with my 5F6-A, but I haven't loved it with my black panel circuits, so I "needed" a new cab with the black panel "preferred" 15F150 to see if I like it better...if I'm going to have another speaker cab, why not shove another amp chassis in there?, and 3) What would a high power black panel with a 15" sound like with KT66 output tubes instead of the specified 6L6s? The KT66s are the riskiest component, since I'm still not sure if there's even room--I attempted some math based on Weber's chassis drawings and things seemed to add up, but I won't know until the chassis and power transformer arrive and I can fit check, so it might be a 6L6 build instead! Bottom line: it's just fun to have a new project.

To develop and verify my layout I used the Weber 6A14HP layout and power section, the AB763 PI, and this handy tremolo-free layout with a master and mid control from the ever-generous Steve Luckey over at the Hoffman forum. Attaching my rough-and-ready DIYLC layout here, which is a few generations removed from one available on Robrob's site. Not exact for what I'm doing in terms of all of the components or the precision of the layout (and there are some vestigial voltages and other info that's not relevant); I'm using it as a reference with everything in one place.

A couple days ago I started the build. With almost all of my builds except tweeds ≤5E3, I favor turret board construction and I make 'em by hand, so I started with laying out the board via template. The first few boards I built years back, I laid out by punching through the existing holes of a fiber board for the amp--didn't work great, since the vertical turret offset makes a significant difference in how the components can coexist, so now I do it by hand on paper using actual components--works for me and it's nice to have the template available for future builds.

I usually buy my blank 3mm G-10 boards from Tubedepot, but Amplified Parts now carries the material and I was able to get everything else I needed in one order from them, so I gave theirs a try. What I don't love is that their board pieces are 11 7/8" x 7 1/16" (whereas Tubedepot's are 3" by whatever length you want--they cut it). I had to get it down to 11" x 3" myself, for which I don't have the best tools (a Dremel). So, the edges are a bit "rustic."

Tape the template on top, center punch the holes, and it's off to the drill press. Amplified Parts' 3mm turrets need a ever-so-slightly-larger-than 2.5mm hole. I used to just use my 2.5mm bit and call it good. If you do that, it's pretty tough to line up the turret with the hole and if you're like me you WILL get some bent, destroyed, or even broken-off turrets trying to line them up and hammer them in with the anvil and staking tool. So nowadays I drill 2.5mm holes and then kiss the top of each hole with a 3mm bit. It makes a huge difference in ease and speed of installing the turrets. While it's still a somewhat tedious endeavor pounding each turret and then flipping the board to press the ends, it's always satisfying to see the finished product.

FedEx delayed my cab and chassis packages until today, so I spent yesterday adding leads and components to the board...more in the next post.
 

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schmee

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Sounds cool. I've built a 6L6 based in a Weber Princeton Rev chassis before. I love the 15" idea and wish I still had a 15F150 around, although I love my EV15 and my Celestion 15.... I wonder years later now how I would like the Weber up against either...
Not sure I understand the need for the 9 pin socket for the LT PI though....? You mean it's going to be switchable from one PI type to another?
 

ElliotKnapp

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Sounds cool. I've built a 6L6 based in a Weber Princeton Rev chassis before. I love the 15" idea and wish I still had a 15F150 around, although I love my EV15 and my Celestion 15.... I wonder years later now how I would like the Weber up against either...
Not sure I understand the need for the 9 pin socket for the LT PI though....? You mean it's going to be switchable from one PI type to another?
The Long Tail Pair PI takes two triodes, so it needs a whole 12A_7 tube--the base Princeton Reverb design's V4A is the tremolo oscillator and V4B is the single-triode cathodyne phase inverter, so eliminating the tremolo frees up half of the tube to go for the LTP instead.

I had thought about going for a switchable PI but didn't want to over-complicate things. The way the layout works, you can pretty easily convert things back to a cathodyne by only moving resistors around...but then you're sitting with half of an unused tube, so I'm guessing I won't be making that move anytime soon.
 

ElliotKnapp

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Got caught up with work and building this amp so I haven't had a lot of opportunities to post here but the build has progressed.

Day 2:
Populated my turret board (pic 1). For most of my past non-tweed builds I've typically installed the jumpers, then plopped the board into the chassis and done all of the wiring in the chassis. Since the chassis, transformers, etc. were still en route, I went ahead and populated and installed most of the leads on the workbench. Another occasion for building this amp is the fact that Sozo's blue caps are back in stock after a long absence from the market--however, a keen eye might notice that the third .1uf cap in from the left is different from the others! I had that one from before the supply dried up, and they are definitely different now from before. Not only is the blue color different, the new ones are a few mm longer. Pretty interesting, and I'd hazard a guess that they've switched manufacturers.

I like to install a bus bar on my turret board and take care of preamp grounds there--I don't really mess around with the back-of-the-pot grounding scheme or the brass plate that Fender and Weber used, and instead I land a single ground point under one corner of the turret board's standoffs. Never had noise issues with this setup and it's handy to have the bus available for mods or switchable pots. Also of note is the reverb driver cathode--my most recent Princeton Reverb builds using Hammond reverb driver transformers have required a lot of tweaking and introduced blocking distortion into the signal when using the stock Fender component values (unless the dwell pot is cranked down), so here I started with where I ended on my last Princeton: no cathode bypass cap and a slightly hotter 1.6k bias resistor (removing the bypass cap really drops the output, and lowering the bias resistor brings it back up a hair). Additionally, I started with a .001uf cap going into the LTP grid--this value has worked the best with my 12" Princeton Reverb builds, which can get a bit boomy with a larger cap. More on these two items later.

Day 3:
The chassis kit, transformers, and hardware have finally arrived from Weber! I was very eager to fit check the power transformers and tube placement with the big bottle KT66s, and was very happy to find that they fit with room to spare (pic 2)! I had mocked up a paper template using the dims provided in Weber's chassis engineering drawing, but you never know how close the tolerances will actually be. The tubes are fairly close together but I'm not too worried about it in the open-back config. Onto assembling the chassis and power section in the next post...
 

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ElliotKnapp

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Late Day 3: Preparing the chassis, installing hardware, etc., and wiring up the power section.

Weber has been supplying amp kits for a long time now, and you can find a lot of forum posts out there from years back containing feedback on the quality of their kits and, often, the need to modify the chassis because holes are missing, are the wrong size, or are in the wrong place. Not a big deal for me now, since pretty much all of my builds include some chassis modification, but if it were one of my first builds I would have been screwed--no drill press, no step bits, no reamer, no nothing. So, I was curious to see how much modification is really needed on a Weber chassis in 2022. The answer is some, but not too much.

The holes you'd have to drill if you were building a stock 6A14HP are one for the bias board, and one for one leg of the output transformer--this is the same chassis they sell with their standard PR 6A14 kit, so the larger 30+watt OT fits a bigger footprint. I was pleasantly surprised that no other modification was required--you could get away with a hand drill and some patience if you needed to. For my build's needs, I also drilled four holes to mount my turret board, widened the vibrato RCA jack hole to accept a 3/8" bushing pot for a Dwell control (Weber's RCA jacks are kind of weird and smaller than the 3/8" ones I'm used to seeing), and added a hole to the right of the RCA holes for a mini toggle switch. Not too bad, compared to builds where I've had to line up and add another potentiometer hole on the face plate, switches, or additional tube holes (which definitely stretch the limits of my equipment).

Other impressions of the chassis and kit:

-It's interesting that all of Weber's kits are stainless steel; I assume it's more expensive so it's kind of odd (especially with the brutal price hikes on steel products) that that's what they go for, even for their non-tweed kits. I didn't have any real problems drilling it, though.

-The front and back plates are plastic. I don't mind as this one was kind of a "budget" build for me, but the extra thickness made it tough to add a 3/8" lock washer on some of the pots.

-The classic Fender AC "courtesy" outlet next to the power inlet is way too long. While it fit the hole just fine (i had read past feedback that the component fit the hole really poorly), if you look at the pic, the plastic is encroaching on the rectifier socket--the metal terminals extended so far that I just pulled them out and didn't include the socket in my build. I don't really have a need for it, and the need to fuss with the rectifier wiring and/or insulate things that are touching or almost touching was more trouble than it was worth. Definitely a miss, but pretty low impact. Happy that the component fits so I can at least have hole plugged.

-Weber's pilot lamp fixtures are weird. It's pretty well-known that most of the components they provide come from China and some are better than others; I've built two kits partially using their components and each had a different lamp fixture, both of which were different than what you'd get if you bought one from Tubedepot or Amplified Parts/AES. The one that came with my 5E5-A kit was pretty bad and the entire jewel component was plastic, including the threads. This one is much better and resembles what you'd get from the aforementioned suppliers, but the jewel threads don't line up with aftermarket Fender-style jewels, so you can only use the red one that comes with the fixture. I'm using a different color, so I had to swap in my own fixture (saving the aesthetics reveal for a later post).

-The tube sockets aren't great; I'm typically not a huge snob about this, but I wouldn't use or recommend using the 9-pin sockets; I had some spare 8-pins so I used my own there too (Belton). One weird thing about the holes for the 8-pins on Weber chassis is that they're not quite big enough for Belton mounting plates to sit flush to the chassis; luckily Weber supplies nylon standoffs to place between the chassis and plate, which I used this time (not the case for an earlier build of mine, which didn't come out quite so pretty).

-The chassis layout is a little weird. The control pots are really close together compared to a Fender/reproduction, I think the front plate is probably at a steeper angle, and the area for the bias board is very cramped. This last piece is probably due to the fact that Weber specs the same power transformer for several amps; it has different primary choices that make it appropriate for different amps. I appreciate the design efficiency, but I could see folks who are really keen on authenticity being upset that their Princeton Reverb doesn't have the correct undersized Champ power transformer. This build is bastardized by nature and higher-power so it's a moot point.

-The bias board is ridiculously small and the hole's right in the middle. This made it really tough for me to use the Sprague electrolytic cap I supplied myself (I had to place an extra piece of board under the cap so it doesn't sit on the screw top and leave extra lead length on the cap so it could fit in the first place, and so the screw could be tightened and the cap pressed down afterward). I think it would probably still be pretty tough even with the smaller-footprint cap supplied by Weber. One of the more irritating parts of this build for sure.

-I was disappointed to find that the chassis hole for the capacitor can isn't wide enough to accommodate the terminals for a JJ cap can and only fits the Weber-supplied can. This was the #1 pain in the ass for me for this build--the can mounts on a fiberboard wafer (you fold down the ground tabs after inserting them through the board), then the board mounts on the chassis with screws. However, you also have to ground all of the tabs...and the can is a 20uf x 4, and the circuit needs one 40 uf node, so you have to wire up a parallel 20uf cap with one can terminal and ground (see why I was trying to just use the JJ 40x20x20x20 in the first place?!). Got it all wired up, including the inter-nodal resistor...and, surprise! Realized the chassis hole is too small to pass the can housing, so you have to insert the cap into the hole, then place the wafer and screw it onto the chassis...so I had to desolder all of the grounds, unfold the tabs, and do it all over again. The extra cap on top of the can looks janky any way you cut it, and the fact that I had to re-do the wiring and thread a ground all the way around the cap perimeter makes it one of the least pretty parts of this build. If I have to replace the cap at a future date I'll be either widening the hole or creating a solution to elevate the JJ can safely away from the hole edges, or maybe place it on its side or something.

-I really like that Weber chassis include a #8 threaded bolt installed in the chassis next to the power transformer for your power ground lug. Super convenient and better than doing it through a hole.

Other random build notes:

-I like wiring up as much of the PT and OT wiring as possible right away since it cleans up all of those floppy leads for the rest of the build.

- Try as I might (including emailing JJ about their spec sheets), I couldn't resolve my questions about whether or not the KT66 pin 1 might have internal connections with other components that make it unsuitable for landing one end of a grid stopper, so I chose to tie the pin 1s to 8 and ground; my grid stoppers go from pin 4-5, and I used terminal strips for screen resistors instead of accomplishing all of this using tube socket terminals.

-For my last few builds I've been landing the 6.3v filament leads from the PT directly on the power tubes and running 22awg wire from the first power tube back to the pilot lamp, 20awg between the power tubes, and then 22 awg between the preamp tubes. Picked up this idea from YouTube (D-Lab's Terry). We all have our idiosyncratic build preferences that don't really functionally impact the final product and Terry's got a lot of them that I don't really agree with, but this one makes wiring up those 9-pin sockets much easier. I can hear the green cloth 18 awg solid core purists gnashing their teeth!

-Usually I'd go for Cliff jacks for the inputs, but like I said, this is a "budget" and "lazy" build so I didn't feel like messing with the stepper bit on those holes. I don't have a problem with the jacks Weber supplies, and in my experience I've had more jack switch issues with genuine Switchcraft switches than I've had with these jacks.

Next up, wiring in the board....
 

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ElliotKnapp

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Day 4: Wire it up, fire it up:

This is where it all comes together, but there's less to say than earlier posts. I really like working in these 60's Fender chassis compared to the smaller tweed ones--so much more room to breathe. As you can see in the pic, I like to color code my leads--for the tubes it's blue for cathodes, orange for plates, and black for grids. It makes it so much easier to mind the lead dress and visualize the circuit. Another trick I've come to prefer is landing a couple turrets on the edge of the board for the V1 grid stopper resistors--since they work best attached directly to the tube socket and I really avoid terminal strips whenever possible, I find this is far and away my preferred solution. You probably don't even need shrink tubing on the leads from the board, but why not?

You'll also see in the layout that there's a switch for removing one of the three gain stages--I first tried this on one of my Princeton builds to be able to effectively switch between the AA964 non-reverb and AA1164 versions of the amp. It's a great pretty dramatic and usable way to alter the amp's headroom and somewhat its tone (eliminating the reverb mixing portion of the circuit), although the output changes, of course. I've never had any noise issues with the more complicated plate wiring.

After wiring it all up, checking all of the sensitive areas for continuity (or lack thereof) with the ol' DMM, I moved into the startup procedure. I don't really use one quite as complicated as, say RobRob's--hook it up to my light bulb current limiter, verify the AC by switching on/off with the pilot light. Plug in the rectifier, on again for some quick "voltage present" checks, then install the rest of the tubes and verify the limiter's not flashing before moving to the standard AC outlet to dial in the bias. For tubes, this amp's got: V1 black plate Raytheon 12AX7, V3 a Wurlitzer-branded RCA 12AX7, V2 and V4 JAN Philips 12AT7s, a pair of Shuguang KT66s in the power section, and a Shuguang 5AR4/GZ34.

So, I hook up the "bias setup of a thousand multimeters"--I have a couple Eurotubes socket bias probes plugged into a pair of cheap AstroAi DMMs and my Fluke measuring plate voltage. Right away, I can see the plate current is way too high even with the 50k pot set at minimum. No redplating, but I definitely shoot for the 70% target. It took a couple resistor swaps on the bias board to meet the KT66s cooler-than-6L6 requirements--instead of the 15k resistor on Weber's schematic, I ended up with 82k. I think I'm probably stretching the upper limit of the power transformer's 45v bias tap to get my -52.5v rectified bias voltage. I should also note that one of the coolest features of Weber's power transformers is that there are 2 primary options (one for 120v and one for 125v), so you can compensate for today's higher wall voltages and get closer to the "classic" internal voltages specified on Fender's layouts if needed. For this build, the hotter brown lead works the best, giving me 430v on the KT66 plates, which is maybe a hair low but certainly within the "happy" range.

Once the bias is good, I check the output transformer polarity by switching the negative feedback off/on--got this one right on the first try! I'll also note here that I went with the standard values for the reverb driver cathode--didn't have any issues with Weber's reverb transformer being too hot or causing blocking distortion elsewhere in the circuit as I've had more recently with other transformers, so that one's stock!

I've spent the last couple weeks playing and tweaking the amp--there were a few component parameters that really needed adjustment to fit my tastes. I'll cover these in a final post and share the finished package. In the meantime, enjoy these pics of the more or less final wired-up chassis. Weber's stainless chassis is so reflective it makes for chaotic viewing from some angles, sorry!
 

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ElliotKnapp

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Looks great!
Curious how the KT66 are stacking up vs 6L6 for sound and EQ.
I wish I had enough 6L6 experience to provide a meaningful impression! My 6L6 experience is limited to 5881s in my 5F6A and a 5E5A build that I didn't have for long. I will say, though, that they sound great with the 15--the bass is big and natural but not overpowering and when you really crank the gain (I'm typically using attenuators) the distortion is really something--like something in the amp is being destroyed, in good way (almost like a fuzz). I'd say this is characteristic of the KT66s, since it's similar to the Trinity Triwatt Hiwatt clone I built that also has the big bottles. There's definitely a midrange honk to the Shuguangs and for whatever reason they're still pretty easy to come by despite the factory's closure.
 

ElliotKnapp

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Day 5 and beyond: Tweaking

With the amp up and running, it's of course time to mess around and see how it sounds! I've spent the last two weeks tweaking, probably more than I usually would, so I'll summarize where I landed on a few key components and share some pics of the final purp-tastic aesthetics.

The first thing that was immediately apparent is that there's a lot of gain on tap--more than I expected or want. Being a AB763 circuit reduced down by multiple 12AX7 stages, the fact that there's no optical tremolo or Normal channel sapping the signal to ground means things are HOT. The 100k audio "Master" pot is there to address this, but I found I was running it between 15k and 20k to get the headroom I'd expect for a black panel Fender on the Volume knob (breakup starting around 4 with my Firebird). I'm most typically inhabiting the area between completely clean and solidly "overdriven," so I ultimately dropped the Master pot value to 25k audio; even with this value it starts to break up healthily above 3 and there's oh-so-much gain as you continue cranking. It's kind of a 6 to one, half a dozen to another situation between running the Volume low and the Master full or vice versa, but with a 25k pot I find the most I'm shaving off is down to 8 or 7, which is down to 13.5k or all the way to 7k. It's kind of shocking that it would be that low but it sounds good so I'm not worrying too much about it. Maybe it's just my brain playing tricks on me, but I think it sounds better to run the Volume at 4 and get to clean via the Master instead of running the Volume at 1, which for some reason doesn't seem opened up enough to me. I guess it's happening a lot later in the signal processing using the Master.

I started with a .001uf cap after the Master pot based on past Princeton Reverb builds, which seem to get a bit boomy without a smaller cap. Surprise again here, as I wanted more bass and it sounds so good and natural with the 15" in the open-back cab. So I upped to .01 and am very happy there--cranking the bass on this amp is really satisfying. Instead of purely upping low-mid mud, it fills in more and more of a deep bottom foundation that doesn't start crowding the fundamentals (@Lowerleftcoast not sure how much of that can be attributed to the KT66s working in harmony with everything else!).

Having a variable Mid pot with this circuit is really fun--25k definitely leans toward straight-up boost territory but is still usable (unlike a "Raw" complete tone stack defeat switch, in my opinion). My only functional issue here was the pot taper--I wanted to be able to easily recall the stock 6.8k value, and with an audio pot that's around 7, which means most of the taper is less mids than your standard Fender. I found I prefer more control over the amount of boost, so I switched to a linear pot and calibrated the knob so I could precisely hit 6.8k with the spine of the 4 on the knob. 5 is 10k (the max of the classic black panel Fender mid control), and just under 7 is 15k, which is the fixed mid boost value I've typically put on a switch for these types of amps. Really fun having this control on the front of the amp and I like tremolo but don't really miss it.

Finally, for fun I ultimately auditioned several dropping resistor values in the power filtering and ended up landing on the values you'd find in an AB763 Vibroverb--1k before the PI and 4.7k before the preamp nodes instead of the dual 18k Princeton Reverb values. This brings the 12AX7 plate voltages up from the mid-100's you'd find in a Princeton to Vibroverb/Super Reverb territory--around 255v on the V1 plates, for example. Can't say it made a huge audible difference to my ears but I thought it'd be fun to further differentiate it from my similar circuits.

I'm ultimately surprised at how much I love this amp--it started as an excuse for another project, but I ultimately like it even more than my single-channel Deluxe Reverb and it fills a more distinct niche. It sounds great, even with the MiC Weber transformers. Weber's kits have always had a bit of a "lesser than" reputation, but I have to say, you'd be hard pressed to find a better deal for a kit--$761 all-in (except tubes) including a nicely finished cab and a great speaker is a screaming deal, and you can address most of the perceived component deficiencies with a few bucks' worth of caps and resistors (assuming you don't need Sozos!). The value alone is definitely attractive for future builds.

With that, there's not much else to do but keep on picking (I may eventually muster the effort for a sound clip or two) and share the pics of the finished combo in its purple glory. I've typically gone for more classic Fender aesthetics on past builds, so decided to go with some thing equally as loud on the eyes as the ears here. Started with an aqua jewel thinking it would highlight the turquoise grill cloth, but purple all the way ended up looking the best. Final preference item is that I often install a jack plate on the back panel since I'm using attenuators most of the time. I prefer having the speaker wired up to a fixed point, and it's much easier to use the 15" as an extension for other amps this way.
 

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