I've been going through a tube overdrive fetish lately and have made some pretty cool things that made me pretty excited, but then a friend asked me to build him a Rat, and I was reminded that no matter how much I like my tube pedals the Rat will always be my favorite dirt pedal.
Then I woke up yesterday with this thought in my head: why not take my favorite parts of the Rat and incorporate a tube section? So that's what I did.
I started with the stock Rat gainstage with the LM308, but I changed the Gain pot to an A50k because I almost never go past the halfway point anyway. Then I took out the infamous Filter control and put in a Baxandall/James two-band EQ. I left it with the stock 1N4148 diodes for clipping because I've tried a million different diodes on switches but always end up sticking with the 1N4148's.
Finally I removed the output buffer and stuck in a 12AX7 buffer in its place. I'm running an L7806 regulator with a diode between middle pin and ground to get me damn close to 6.3V DC for the heaters. Then I'm running a nixie power supply I bought as a kit from eBay that takes 9V DC in and spits out 180V DC, which I run to the plates (after some thorough filtering to get rid of the whine). I'm using the first triode for the output buffer, then I have the second triode set up as a switchable boost, because I like having my drive and my boost in the same box.
So anyway, here's the schematic:
(https://i.imgur.com/21FW2be.jpg)
It's still just on the breadboard right now, though I do have an unverified layout for it I'm hoping to try out soon. The board is too big for my cheap CNC machine to stay calibrated, so I'll have to see if I can use one at work on my lunch break Monday. It's a little too big for a 1590BB, but I have a nice big slanted enclosure I've been waiting to use for somehing awesome that should fit this nicely 8)
The really big reason I'm excited about this pedal is that I've designed and built a dozen different overdrives and none of them sounded bad, but none of them earned a place on my pedalboard. This one, however, is the best sounding overdrive I have ever heard. It's obviously not going to be the sound everyone is looking for, but it is definitely the sound that I want. I hope this is at the very least educational for someone, and maybe tomorrow when my kids aren't asleep I can get a quick demo recorded.
No need to use LM7806, just use appropriate resistor to drop voltage from 9 to 6.3V. Much simpler and less space!
Nice one Nate. I knew that 12AX7 PCB conversion board would come in handy!
I would say this is brilliant.
To me, the Rat is a 'exquisite' distortion (used about 15 years a Deucetone as main distortion).
The Rat's trick is in the Jfet recovery stage: the perfect "tube like" output stage.
Which better now than have a real tube there????
Quote from: rankot on September 09, 2017, 04:24:25 AM
No need to use LM7806, just use appropriate resistor to drop voltage from 9 to 6.3V. Much simpler and less space!
A 12AX7 (or anything in the 12A*7) family draws 300mA @ 6.3V, so dropping 3V across a resistor means dissipating about 1W across the resistor, whichis going to make for a HOT resistor. This means I would need at least a 2W resistor, which are pretty big and will still be getting warm. Voltage dividers are great for biasing and things like that where the ratio is more important than the actual voltage, but for tube heaters the actual voltage is a big deal. So the regulator/diode combo is what I want :)
Awesome idea! Since you have it on breadboard, have you tried it with the diodes lifted and the Ruetz mod?
My Aion Rat is my favorite distortion of all time. It has a CA3130EZ instead of the LM308 and I usually have it with diodes lifted and Sweep pot at 3:00. Somehow that combo made it the roaring voice of hell. I can only imagine how it would sound with a 12AX7!
Quote from: cnspedalbuilder on September 09, 2017, 05:13:07 PM
Awesome idea! Since you have it on breadboard, have you tried it with the diodes lifted and the Ruetz mod?
I tried the Ruetz mod and I liked it, but not enough to keep it. Obviously everyone's ears are different, and on the Rat I have on my board with the Ruetz mod wired in I usually just keep it adjusted so it sounds stock anyway :P
As for the diode lift, it was a boost in gain (obviously) so it let the tube clip a little more, but I love the sound of the 1N4148's for clipping so I put them back :P
Got a quick video thrown together to show some tones. Try not to judge the playing too harshly, we can't all be Rob or Jimi :P
Yeahh!!!
Sweet LP Studio there... 8)
Quote from: Plexi on September 09, 2017, 07:15:10 PM
Yeahh!!!
Sweet LP Studio there... 8)
Thanks :) it's not my most expensive guitar anymore, but I still love it and plan to keep it forever ;D
> a HOT resistor
SAME power (actually a hair more) in the regulator.
And heater voltage is not *that* critical. 12AX7 have been run 5.5V to 7V.
Since he is surely using a wall-wart, and probably one with good regulation, I'd expect a resistor to work fine. Maybe not cheaper. The resistor alone is smaller, but the same-heat means you need about the same air-space around it, so not really smaller.
There is a theory that tube heaters should be soft-started. Actually heater failure is very very rare. Also the UNIVAC techs with 17,000 tubes did have heater trouble, tested slow-start, and did not find much difference. However if you favor slow-start, the resistor is a little slower (due to heater cold-resistance) than a 1A regulator.
9AQ8 is a twin-triode, good Mu, 9V 0.3A heater. Change cathode resistor to 2.2K-3.3K, it will bias OK and give some less gain than 12AX7 (which is way more than you need/want). $6 at major tube depots.
Quote from: PRR on September 09, 2017, 10:22:32 PM
9AQ8 is a twin-triode, good Mu, 9V 0.3A heater. Change cathode resistor to 2.2K-3.3K, it will bias OK and give some less gain than 12AX7 (which is way more than you need/want). $6 at major tube depots.
Good to know, thanks :) I'll have to look into it, that would save a bit of trouble!
Quote from: vigilante397 on September 09, 2017, 09:35:43 PM
Quote from: Plexi on September 09, 2017, 07:15:10 PM
Yeahh!!!
Sweet LP Studio there... 8)
Thanks :) it's not my most expensive guitar anymore, but I still love it and plan to keep it forever ;D
I remember when you built it! :)
Quote from: PRR on September 09, 2017, 10:14:11 PM
However if you favor slow-start, the resistor is a little slower (due to heater cold-resistance) than a 1A regulator.
Just asking out of interest; would an LM317 wired for constant current rather than voltage be "kinder" to the heater?
Is there a mistake in the schematic?
Because V1A is not a buffer, it's a gain-stage.
To use V1A as buffer, connect plate to B+, and put 47k resister between cathode and ground.
Take the signal from the cathode using the 22n blocking-cap.
But a gain stage is probably the better thing to use here. Since you already have rather small signal (+- 0.6 volts max signal-range from the diodes) which is heavily loaded by the baxandall-tonestack.
Quote from: diydave on September 11, 2017, 04:24:56 AM
Is there a mistake in the schematic?
Because V1A is not a buffer, it's a gain-stage.
You're correct, I'm not really using it as a buffer, more as a recovery gainstage after the EQ. I tried it just as a buffer and wasn't thrilled with the tone.
> constant current rather than voltage be "kinder" to the heater?
me: "...the UNIVAC techs ..tested slow-start, and did not find much difference."
I don't coddle my tube heaters. Last bad heater I saw had been chucked 10 feet into a dumpster, no wonder.
Quote from: PRR on September 11, 2017, 09:38:50 PM
I don't coddle my tube heaters. Last bad heater I saw had been chucked 10 feet into a dumpster, no wonder.
I honestly don't know that I've ever seen a bad heater. Usually either loss of vacuum (bad seal) or an internal short killed all the tubes I've seen.
Just ordered a 9AQ8 to play with by the way, thanks again for the recommendation. 8)
Quote from: PRR on September 11, 2017, 09:38:50 PM
> constant current rather than voltage be "kinder" to the heater?
me: "...the UNIVAC techs ..tested slow-start, and did not find much difference."
I don't coddle my tube heaters. Last bad heater I saw had been chucked 10 feet into a dumpster, no wonder.
LOL. Ok, fair point. But it would be kinder to the power supply, wouldn't it?
I don't coddle power supplies either. Rectifier current rating should be much more than the transformer (not the load). If I am bored I might look at main cap ripple rating; but in audio gear we usually throw so much uFd per Amp in there that sheer bulk ensures tolerable ripple.
Ask your power company. After a summer power failure, they will just throw the breaker ON for thousands of houses. The start-up surge of the thousands of air-conditioners greatly exceeds the nominal rating of all the transformers. The system goes "HHHRng" for a second, and power is up again.
Got my 9AQ8 today and I have to admit feeling a little underwhelmed. The convenience of 9V heaters is brilliant, but it is quite a low gain tube. From my quick breadboard testing I think it would make a fine clean boost, and I may try it with 9V on the plates like a valvecaster, but with 180V on the plates it's just too clean to be useful to me.
I'm allowed to necro-bump a topic if it was my topic to begin with, right? :P
I never updated this because I was working on this before I actually knew what I was doing, so I put together a PCB and had it fabbed, then when it came back it didn't work so I just gave up on the project. Fast-forward to a couple weeks ago, I was getting ready to put in a PCB order and one of my good friends asked "what ever happened to the Lab Rat?" So I dug up the schematic (the file was long gone, I referenced the one in this thread), made a couple changes, and sent it out for fab. It came a little bit ago and I got it put together and fired it up, and was very pleased with the results.
The changes I made over the previous version:
- used SMD instead of through-hole, except for the LM308 and clipping diodes
- ditched the baxandall EQ and put the filter control back in, but wired it backwards with a C taper so it behaves like a typical tone control
- used a 6N21B tube instead of a 12AX7 so I could fit it in a 1590B
- changed the boost section a little, getting rid of the input cap in favor of a more typical grid stopper resistor
- ditched the diode on the ground pin of the regulator so the heaters run at 6V instead of 6.3V
Here's an updated schematic:
(https://i.imgur.com/5A5xnOy.jpg)
Here's a semi-organized gutshot:
(https://i.imgur.com/Bbn44Lh.jpg)
And a quick phone pic of the outside:
(https://i.imgur.com/Wr1c82m.jpg)