Effectrode LA-1A tear-down and trace adventure

[vigilante397]

I tend to get wordy when talking about awesome gear, so I'll apologize in advance Feel free to skip down to pictures if you like.

So a while ago I decided that my niche in stompbox building was going to be submini tube preamps and clones of other tube pedals with submini tubes. This has been a neat novelty that has gotten me a little attention, but nowhere more so than the bass community, who devoured my 6N21B powered Alembic clone. I was informed that bassists largely feel ignored in the stompbox world, and all the really cool stuff goes to guitarists. So I've started doing more designs geared toward bassists, and one of the things they ask about a lot is a tube compressor. One of the more renowned models I've been hearing about is the Effectrode LA-1A, based on the legendary Teletronix LA-2A Leveling Amplifier.

Well fast forward a couple months and I was somehow talked into getting one to reverse engineer and see if I could make a tiny one with submini tubes. Well it showed up last Friday, and I promised on another thread that I would document my adventure. Naturally before I opened it up I had to plug it in and see what the fuss was about. I mean it's just a compressor, right? Well my mind has been blown. I have a decent bass that I run through a mediocre rig, and this thing made it sound like a million bucks. I couldn't find a bad sounding setting on it, which is uncommon for compressors in my opinion. I tried it on guitar as well, and very much the same experience. The highs were crisper, the lows were richer, the mids were mid-er (?). Everything just sounded "enhanced" for lack of a better word. So yeah, I see what the fuss is about.

Well 5 minutes later I had the thing disassembled and sitting on my desk. It has quite a bit going on, as you can see. This is going to take some work Obviously the goal would be to non-destructively figure out what's going on, but non-destructive in this case may mean gently removing some things and gently re-connecting them later. So I haven't started tracing yet (I have a bunch of orders to build), but I wanted to at least get this thread started so I have a place to put things as they progress. And now for the glorious pictures:







Wish me luck!

[Ben N]

Good luck, Nathan! I eagerly await word of your adventure.

[Ripthorn]

I love a good tube leveling amplifier on bass! I have a particular fondness of the Summit TLA-50 that I used in the studio years ago, but I hear the LA-2A is supposed to be a similar story. Will love to see how this goes!

[anotherjim]

Mind bending task!
I see LDR's bent over SMD LED's, but what are the x3 black boxes that look like slider pots but aren't?

[garcho]

^ power resistors?

cool project, tube compressors (understandably) often live in the $5,000+ 19" studio world

[vigilante397]

I see LDR's bent over SMD LED's, but what are the x3 black boxes that look like slider pots but aren't?

Those are actually NSL-7053 vactrols, the LED is encased in the plastic package with the LDR. As for the little black boxes I had to look them up, they're apparently reed relays, which I'm not familiar with https://www.mouser.com/datasheet/2/89/9007_series_reed_relay_datasheet-1667047.pdf

For simplicity I'm planning to just look at the audio portion and ignore the power supply and switching for now. Anything I make will likely use my typical power supply and just be true bypass anyway.

[garcho]

they're apparently reed relays

that explains their accompanying flyback diodes

[Plexi]

Good luck, mate!

[marcelomd]

For simplicity I'm planning to just look at the audio portion and ignore the power supply and switching for now. Anything I make will likely use my typical power supply and just be true bypass anyway.

Aw... I'd love to see the power section =)
That transformer seems to be an isolation transformer. And I'm not seeing nothing like an inductor fot a SMPS. Unless it's under that metal shield.

[vigilante397]

Aw... I'd love to see the power section =)
That transformer seems to be an isolation transformer. And I'm not seeing nothing like an inductor fot a SMPS. Unless it's under that metal shield.

I mean it looks to be a fairly standard power section. They have an IRF740 MOSFET, same one I use, and the inductor is actually hiding under a metal heatsink in the bottom left corner. I don't see an NE555 so they must be using something else for the switch timing. But it takes a 12V input, so that's obviously what they're using for tube heaters in series. I see a PIC16 which no doubt controls the relay switching, and there's a 78L05 to power the PIC. It's not that I don't care what they're doing, it's just that I don't think there's anything revolutionary that warrants me finding out exactly what they're doing. I'll probe it to see what voltage they're running on the plates, that's probably it.

The transformer is actually a D.I. transformer for balanced output.

EDIT: it looks like they actually use a different fast rectifier diode than I do for the HV supply, but mine is a little cheaper so I'm going to stick with it

[marcelomd]

Aw... I'd love to see the power section =)
That transformer seems to be an isolation transformer. And I'm not seeing nothing like an inductor fot a SMPS. Unless it's under that metal shield.

I mean it looks to be a fairly standard power section. They have an IRF740 MOSFET, same one I use, and the inductor is actually hiding under a metal heatsink in the bottom left corner. I don't see an NE555 so they must be using something else for the switch timing. But it takes a 12V input, so that's obviously what they're using for tube heaters in series. I see a PIC16 which no doubt controls the relay switching, and there's a 78L05 to power the PIC. It's not that I don't care what they're doing, it's just that I don't think there's anything revolutionary that warrants me finding out exactly what they're doing. I'll probe it to see what voltage they're running on the plates, that's probably it.

The transformer is actually a D.I. transformer for balanced output.

EDIT: it looks like they actually use a different fast rectifier diode than I do for the HV supply, but mine is a little cheaper so I'm going to stick with it

Ah, that clears it up. Thanks! =)

[11-90-an]

I think this schem will be kinda helpful  :

Teletronix LA-2A

http://www.proreplicas.com/teletronix_la-2a.html

After tracing, a JFET variation...? 

Anyway, good luck, Nathan!

[vigilante397]

Yup, I have an LA-2A schematic printed out and ready to have on hand for when I'm ready to trace.

And have you seen any of my builds? I don't do JFETs, there's going to be a submini tube variation

[Ben N]

As for the little black boxes I had to look them up, they're apparently reed relays.

They have better tone for saxophones, clarinets and oboes.

[Ripthorn]

After tracing, a JFET variation...? 

I think bajaman at the other forum has already done this, if I'm not mistaken.

[vigilante397]

Alright, I got started tracing last night, and things got weird fast. The pedal uses (2) 12AU7 and (1) 12AT7, from a quick glance it looks like the 12AU7s are used for the compression and the 12AT7 is just used for the boost, so I started with the 12AU7s, which is also where the input signal starts. So the weird thing (at least to me in my limited experience) is that all 4 tube stages connect directly to the input with their own grid resistor, they all have separate cathode resistors and bypass caps, and they all have their own plate resistor (sort of, they're all in parallel), but they all share a coupling capacitor



That's as far as I've gotten so far. That 47n 400V cap on the far right is the coupling cap from input to the Boost section, so that will tie in there. C29 and D6 look like they're in the feedback loop of an op-amp, so that will be the next place to trace, but that actually brings another weirdness: the op-amps are labeled as MCP6021, which looks like it should be a single op-amp (MCP6022 is the dual version) but they are very much routed as though they are dual. Every pin is connected, including pin 8 to a voltage rail and pin 4 to ground, both sides seem to have their own stage with their own feedback loops and everything. I stopped last night because this threw me off, I was unable to find a part labeled as MCP6021 that was a dual op-amp. Any thoughts on that one?

[willienillie]

Wow, 4 triodes paralleled.  I've never seen that before.  Already quite different than the LA-2A, but I guess it had to be in the front end.

[vigilante397]

Wow, 4 triodes paralleled.  I've never seen that before.

Right? Me neither! But I've also never seen parallelled triodes share a coupling capacitor and nothing else. That was a first for me.

[willienillie]

Yeah they could've saved some parts.  But maybe there's some drawback to sharing cathode resistors etc, some distortion that wouldn't normally be noticed in most guitar gear.  I know that is a somewhat popular mod to the 5F6-A/JTM-45 preamp circuit, separating the V1 cathodes.  Honestly I don't know why though.

[Ben N]

Wow, 4 triodes paralleled.  I've never seen that before.

Right? Me neither! But I've also never seen parallelled triodes share a coupling capacitor and nothing else. That was a first for me.

Hmmm, yeah. You would expect to see shared cathode R/Cs, like on a tweed Fender. right?