Author Topic: adding a tube to a RAT 2 salvage (now a ROT box)  (Read 1984 times)

apotheodaimon

adding a tube to a RAT 2 salvage (now a ROT box)
« on: October 28, 2014, 03:46:40 AM »
so I'm having some difficulty with this idea. It might sound insane but I just want to give it a shot.

Simply, I have a RAT 2 which I had gotten in nonworking condition, got it back to RAT spec and working nicely. I added a mighty mouse switch and clipped R7 replacing it simply with some bits of wire to form a jumper. Now its a dirty boost pedal.

I also added a socket for the LM308 I put in.

My question is: if I wanted to replace the LM 308 with a tube (just for shiggles) a) is there something which has an 8pin if mimic for a plug connected to a tube socket I could buy or b) which posts would I wire to each other between the IC socket and a 12ax7 or some other tube? I'm no electrical engineer.

I looked all over the web and can't seem to finy ANY info about even beginning to comprehend how to do this. Someone please drop some knowledge on me or tell me I'm a fool and that a tube simply can't be wired to an IC socket in a RAT circuit.

craigmillard

Re: adding a tube to a RAT 2 salvage (now a ROT box)
« Reply #1 on: October 28, 2014, 05:14:35 AM »
Welcome to the board apotheodaimon

Quote
tell me I'm a fool and that a tube simply can't be wired to an IC socket in a RAT circuit.
Your a fool :icon_biggrin:

Its not possible to do this as to start with the tube needs the heater to run... they can be used as clippers in a diode formation but again this is quite major circuit changes..

anchovie

Re: adding a tube to a RAT 2 salvage (now a ROT box)
« Reply #2 on: October 28, 2014, 06:32:40 AM »
Its not possible to do this as to start with the tube needs the heater to run...

...and on top of that, a triode and an opamp are so different that you'd need to change all the surrounding circuitry, so you may as well build a new tube-based pedal if you really want one.

Have a look at the million-page Valvecaster thread.
Bringing you yesterday's technology tomorrow.

apotheodaimon

Re: adding a tube to a RAT 2 salvage (now a ROT box)
« Reply #3 on: October 28, 2014, 09:41:41 AM »
I had my suspicions since I could find no info on replacing an opamp with a tube and also I figured 8 pins on an IC and 8pins on a tube was too simplistic a way of thinking.

"they can be used as clippers in a diode formation but again this is quite major circuit changes.."

Call me simple minded, it was just a wild hair up the ass idea I had so I was just wondering if it was even possible to replace the LM 308 chip with a tube. I knew it was possible to use a 12au7 as a clipping diode set, but I didnt think it was a major circuit change. I thought I could just wire pin 4 to the power and five to the ground, then wire pin 1 to the first anode spot, three to the final cathode, six to the first cathode and eight to the second anode... but maybe that'd sound crummy in a RAT.
Thank you for clearing that up though its too bad I can't do it.  

On a slightly different note on a similar topic for the same box: any suggestions for cap values in replacing C3 and C13? Id like more volume and bass. Is it already optimized as is or should I just go for broke and stick some tropical fish in those spots? From what I can understand in my research it isnt.
« Last Edit: October 28, 2014, 10:57:57 AM by apotheodaimon »

PRR

Re: adding a tube to a RAT 2 salvage (now a ROT box)
« Reply #4 on: October 28, 2014, 04:45:40 PM »
A '308 is a semi-complete amplifer. Transistors AND resistors (and a cap or two).

A tube is only a part of an amplifier. Many more parts needed to make it amplify.

A '308 has gain over 100 at the top of the guitar band, way over 1,000 for the lower guitar zone.

One unit of 12AX7 can barely do gain of 50. Two units can give gain like a '308, but that's a lot of parts; more to get the declining gain of a '308.

That's assuming "normal" tube voltages, a 200+ Volt supply. With 12V for the amplification pins the gain is very much less.

12AX7 needs power at pins 4,5,9, *and* higher-voltage power *through* resistors to the several electrodes.

Finally: the way the '308 is used means the actual amplifier "hardly matters", as long at it has a LOT of gain. The external feedback (including diodes) determines the actual audio gain. If the '308 were perfect, the 12AX7 amp would sound very much the same. (In fact the '308 runs shy on gain at the top of the guitar band with maximum boost, which is part of the "sound". So a 12AX7 would sound different. Good, bad, mod-able, who knows?)

This isn't like dropping a GMC engine into your Chevy truck. More like a Ferarri crank in your Firebird... you need other parts like block and heads.
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Bill Mountain

Re: adding a tube to a RAT 2 salvage (now a ROT box)
« Reply #5 on: October 29, 2014, 09:56:49 AM »
I experimented with a Tube Rat by designing a two stage (1 ea. 12AX7) gain section tweaked to imitate the frequency response of the op amp section of the Rat and then followed it with clipping diodes, a filter (modified to work with the high output impedance of the tube section), into a buffer, and a level control.

It sounded.....





OK. 

It wasn't really worth the effort.  I spent so much time wondering if I could do it that I never bothered to ask if I should.

I still think it's a novel idea worthy of further investigation but on a crowded stage no one can tell I'm using a lowly Turbo Rat.