Weber 5E3 Kit

Okay, so I just placed my order for a 5E3 kit from Weber. If I could've swung the extra couple hundred I would've pieced it together myself from various suppliers, and that's what I'd do if I were building this just for sale. I'll probably gig with this for a while and keep an open sale/trade offer up on craigslist. If there's a demand for these maybe I'll crank 'em out, who knows.

It'll be a few weeks before it gets here, but I'll post build pics as I go. Since there's a possibility I'll sell it, I'm going to keep it stock, though I'll probably use a grounding scheme similar to Ceriatone's.

Here's a clip from the soundtrack that finally sold me on making one of these (though Neil's is decently far from stock):

Mmm, sweet blocking distortion.

Bogen Update

I converted the Bogen to cathode bias and the voltages came up a little higher than predicted but once I have the preamp and PI wired up they should fall right back in line. Cathode voltage? Spot on at 18V. The 7591 plates are at 480V and the screens are at 430V (w.r.t ground, not the cathode.) Even with the preamp tubes in I'll probably have to tweak the power supply's dropping resistors slightly because DAAAMN.

Dead silent so far - as damn well it should be! We'll see how that holds up as I get the preamp and controls wired up, but man am I excited.

Morley Little Alligator - Reverse Audio Taper Mod

I bought one of these volume pedals a while ago - actually I bought two, to the delight of the clerks ("haha, that's gonna get loud!") - because they were cheap, I needed a volume pedal, and I had plans to make a stereo pedalboard.

tl;dr - do this.

After a while, the taper on these started to bother me. I tend to play weird shit(tm) and these weren't so great for volume swells. Morley is happy to explain that these are linear taper volume pedals, but in practice they function more like audio taper pedals - not much change over the first half of the pedal travel, then a sudden jump near the end of it. Overall I'm happy using these - they're completely noisefree and don't seem to load down the signal at all (no "tone sucking.") If I were playing guitar for a living, I probably would've bought something else, but it's just a hobby/passion so I've just dealt with it.

Fast forward, oh, a year probably, and today I find this thread about Ernie Ball volume pedal tapers. About halfway down the page there's a comparison of "old" vs. "new" potentiometers used in these things. You'll notice a distinct lack of a linear taper, only audio on the new ones and reverse audio on the old ones.

Well, shoot! RG Keen shows how easy it is to change the taper of a linear pot! The only problem is, of course, that the Morley Little Alligator doesn't use a potentiometer, it uses a clever arrangement of photoresistors to form a voltage divider, which is how a pot is used as a volume control. Check the schematic here.

This is where it got a little trickier. To add a taper resistor you need to know (roughly) the maximum resistance of the pot/voltage divider you're using. Try as I might, I can't get an accurate read of the Morley photoresistors. It doesn't help that their maximum resistance happens when they're dark. Okay, I could've hooked up a wheatstone bridge, but accuracy isn't important here.

Seriously, that's it.

With a multimeter, the highest value I could get consistently out of either photoresistor was in the ballpark of 5M. This means the total resistance is somewhere in the ballpark of 10M (or higher).

LDR 1 is hiding under L3.

Referencing the schematic and RG Keen's site, for a reverse audio taper we need to solder a taper resistor across LDR1. For a decent taper we might choose something in the ballpark of 2M, but since this was more of a "proof of concept" mod I just grabbed a 1M and soldered it in there.
 

I suppose I should mention that I did try to take the board out but in addition to the screws and jacks it seems to be glued somewhere in the upper half. Maybe it just needs more convincing.

Putting the resistor over the LED works better than under.

An added bonus to having a pair of these to mess with is someone already modded the other one - they put another 12k resistor in parallel with R3 which makes the range of the "minimum volume" control a little more useful, so I did that too.

I didn't have a 12k so I used a 10k. Again, this isn't the Manhattan Project.

 And that's it!

Deja vu!

The taper is much more useful now, though the 1M value isn't quite ideal. The volume used to start increasing about halfway through the pedal's travel, and now I can get it around the first quarter, which is nice because the "mute" function of a volume pedal is super handy. The bulk of the volume change now happens over the middle half of the pedal's travel, so you can get nice fade in, swell, and even tremolo if your ankle can take it.

1M isn't quite perfect though; there does seem to be a bit of a corner at about halfway through the pedal travel. Now that I know it works, I'll probably try a few values between 1M-3M.

I haven't contacted Morley about the specification of these LDRs - the 400 - 850 listed on the schematic seems to be the range of wavelengths over which the LDR is effective. It doesn't matter too much though, the tolerance on LDRs seems to be massive and every player is going to have their own preference anyway. To that end, if you wind up with a sharp "on and off" response you might want to try values lower than 1M. It should be relatively independent of what guitar you're using - that's what the minimum volume control is for - but if you're using active pickups or hitting this pedal with a really hot signal you might not want to bother with paralleling R3.

One last point is this will work on any volume pedal (or wah pedal, or volume/tone pot) with a linear taper. Read this RG Keen link again. This will actually be easier to dial in when you're using actual potentiometers because you'll know the total resistance you're starting with. Small Bear Electronics is a good source of replacement wah pots and they sell a few linear taper ones as well.

Alright, well, I was excited so I had to post this. I don't think anyone has posted on the webternets about how to do this, so I guess I could've done what that Ernie Ball guy did and tried to turn this into money, but what the hell. I get the feeling I won't make it far in this business if I don't believe in intellectual property, but really who'd go to a gear swap and say "oh, you have a Keeley modified compressor and a BillM Blues Jr? Well I've got a PaulP modded volume pedal!"

They'd probably say PP, which is fine. You like the 'brown sound,' just wait'll you hear the yellow sound! Okay, I have to go to bed.

There's a tube post coming soon.

Recommended Reading

It can be hard to find decent information which is specific to guitar amps. There are plenty of sites where you'll see advice like "You have a Bender QuintupleRec JCM ABC 3.14 Green? Change R14 to 1M, C2 to 1uF... (etc) - Instant Page/Hendrix/Young*!"

...but if you ask "why that component? why that value?" then your options are a lot more limited. Fortunately there are a number of online communities now, many of which feature People Who Actually Know What They're Doing - both people with electrical engineering degrees and other people who've been amp techs/builders for decades.

To that end, I've been updating the "links" list as I remember more sources I've used. If I had to pick one, hands down, it would be Merlin Blencowe's Preamp Book. The first edition is phenomenal and he just released a second edition that I want really badly. It's a perfect balance of technical and practical - possibly not the best for absolute beginners, but he's not afraid to simplify when appropriate.

Tubes have been obsolete technology for a while now (w.r.t. consumer electronics) so the classic books are old and focused on minimizing distortion - reproducing music, where you want the amp to stay out of the way, instead of producing music where the amp is part of the sound. They're also generally more technical than needed for guitar amps, but still a valuable resource for fundamentals. I'm still working through Amplifiers (The Why and How of Good Amplification), G. A. Briggs, 1952 which has been interesting and is well written with occasional dry jokes - it's not that nerds don't have a sense of humor, it's just that they generally have a bad sense of humor.

Somewhere in between are Richard Kuehnel's books - very technical, but very powerful tools. I have his power amp book, and if you're interested in understanding blocking distortion, this is the best treatment of the topic I've found. Some topics he goes into considerable depth, but others he doesn't write quite enough. Really math heavy, but that's a good thing. Great calculators on his site too.

Across the various boards the people are mixed, but generally if someone is way off someone else will step in. If you see Randall Aiken, Merlin, or RG Keen start talking, pay attention. My favorite forums are the AX84 BBS and Music Electronics Forum. The former is hard to search and the latter is frequently hacked so I usually search both through Google.

*Your choice: Angus, Neil, etc. I've never seen "instant La Monte Young"... oh man, that would be awesome.

MXR Distortion+

I snagged a General Guitar Gadgets MXR Distortion+ kit (partially assembled) off of the Pittsburgh Music Gear Forum for only $10! Never been a huge fan of this circuit but I couldn't pass it up. Also the GGG PCB has extra pads for mods, so I'll finally get to spend some time playing with these distortion mods instead of just building another Big Muff.

Also, this pre-made printed circuit board idea? Brilliant. Why the hell have I been etching all my circuit boards myself? Spending two hours to save $10 is just not worth it.

I gotta say, I really like Madbean Pedal Projects. Very dedicated fellow, cloning some unique pedals, and his Cave Dweller delay is absolutely phenomenal.

Bogen CHB-35A Conversion

Okay, so I've gone through a ton of revisions on this thing. It's been quite an undertaking to design an amp from scratch, but once it's built it's going to sound fantastic and the next one will be much easier.

Six knobs? Plenty.

I got this amp for $30 off of Craigslist. I don't know if it was turned on without a load or what, but the OT was fried. Shouldn't be a huge problem finding a 6.6k primary OT that can handle a pair of 7868s, right? Turns out I found an 8.4k primary OT  (from Musical Power Supplies) which was much cheaper and looking at the datasheets that just means it'll be a hair louder and a hair more distorted. Oh no!
But what about the increase in screen grid current at overdrive? Bigger screen grid stoppers, of course. It did take me a while to wrap my head around sliding screen operation, but it turns out 1k screen grid stoppers should be fine. From my limited experience frankensteining a single ended EL84 amp, I know I'm not particularly fond of the change in overdriven tone from excessively large screen grid stoppers, but I think these will be reasonable. 

Original guts.

Oh, if only I'd listened.

There are a handful of oddities already in the stock schematic. Well, not "oddities" so much as "deviates from Fender/Marshall." The rectifier is a voltage doubler, the bias supply is capacitor-coupled, there's a lone 7-pin triode as a cathodyne phase inverter and the choice of output tubes is unconventional.

The PT really calls for a voltage doubler and that hasn't been unreasonable to work with. I changed the novar sockets for the 7868s (9-pin the size of an octal socket) to octal sockets for 7591s. Same tube, just with a different base. The biggest difference is I have a pair of UOS 7591s.
Old stock 7591/7868 tubes are rare and expensive and there are a handful of urban legends about why. Some say there were whole warehouses just dumped because these tubes were developed late in the tube era and transistors were taking over. Others say speculators bought up every tube they could find for the Japanese audiophile community.
It doesn't matter, really; what does matter is that these bad boy beam tetrodes can put out up to 40W a pair yet only require about -20V bias. They're often described as a cross between a 6V6 and a 6L6, and apart from some Ampeg amps they're largely overlooked in the guitar amp world.

The bias supply has been giving me trouble, so I decided to switch to cathode-biased, at least for the time being. When they say capacitor-coupled bias supplies can turn into charge pumps if they're not sufficiently loaded, they aren't kidding! God bless lightbulb limiters.

The latest version. Pretty sure there have been more than 7 of these.

For a while I was thinking about making this a two-channel amp, but I decided I'd rather just have one great channel instead of two compromised ones. Also I haven't messed around with relays and optoisolators yet, so all channel switching would have to be done externally. If I could start with a different PT, maybe add a couple more triodes for the dirty channel and mixing the two channels, I'd consider it.

I find a whole lot about this design exciting (as well I should, considering I'm designing the damn thing.)

• So far I have an irrational love of the cathodyne phase inverter. It's in the 5E3 Tweed Deluxe, the old Orange graphic amps, the Fender Princeton - anywhere this PI goes, awesome seems to follow. Super easy to design, and Merlin's warned to stuff it (and the output tubes it pushes) with huge grid stoppers, so that's what I've done. Now that I've got an oscilloscope I might jumper them to see some nipple distortion, but I'll probably just take his word for it. 
• I'm totally sold on directly-coupled cathode followers when it comes to overdrive. I'm somewhat afraid that this bootstrapped arrangement might make way too much gain, but I can always put that on a switch later. 
• The tone stack is a tweaked FMV with no mid pot because I've only got six knobs and that thing is useless. It's got much less mid cut, and the "shift" control moves the mid notch to a lower frequency as well as making the Q wider. This stack won't get the blackface Fender tone (who'd want it?) but it should be very useful for both clean and overdriven tones. 
• Parallel input stage! Smaller grid and plate resistors for (theoretically) less noise right where the S/N ratio is at its worst! Two different bias points (warm & cold) for "complex" harmonics! Two different bypass caps for a multi-level mid & treble boost! Below ~100 Hz the gain is lowest, slightly higher between 100 Hz and 300 Hz, and highest above 300 Hz. Should be pretty awesome. Why not just give each triode its own input and add a switch to bridge them, you say. Well, I'm building this amp for myself and my pedalboard output is stereo so I want two identical inputs if need be.

Heater wiring!

I've used 22 AWG teflon-insulated 630V silver-plated wire (from Apex Jr.) for everything.  The math says this gauge is actually more than sufficient for my heater draw, and the heaters were originally wired with 24AWG anyway. Teflon is a bit of a pain to strip and it doesn't like to hold a bend very well. I intended the hot glue to be a stopgap but I think most of it will stay put even when the amp gets hot so I'll probably leave it.

The heaters work!

 

Progress so far.

 Gotta love point-to-point - with only the power supply wired up it's already looking like a rat king. I've actually abandoned this "true star" grounding system in favor of a "local star" system which makes a whole lot more sense and cleared out a lot of wire.

Since I took this picutre, I got the power tubes, bias supply, and phase inverter completely wired but I'm going to take the bias supply out (or at least disable it) in favor of cathode biasing. I went through the trouble of load lines and, y'know, ALGEBRA to figure out what the bias resistor should be and it's actually the same as what the data sheet calls for.

Now I'm just waiting for parts. I still need to order a bunch - all the pots, the bias resistor, the heater elevating resistors - I should check that cathode follower again; I might be okay with the 21V of elevation I'll get from the bias resistor. And then, of course, I have to finish building this thing... and then debug it... and then tweak it. GONNA BE A WHILE.