Hello,
I got a few parts in one of the members excellent grab bags that included a small preamp tube. I've seen a few very simple tube distortion units and am trying to figure out a good recipe for a point to point build. Any directions I should or shouldn't go? Thanks C
Knowing what tube would help...
Yes, I guess it would, I thought I'd included it... 12AX7EH. Thanks Merlinb
Oh, that one? Man, hard to find!! What can we make with THAT thing????
No, just kidding! That is a very useful tube; the most common preamp tube in the world. It is a dual triode, which is similar to say, 2 JFETs in one package. So you can make a cascaded gain stage booster/overdrive type thang.
Look up "Valvecaster"! Great one to start with.
A tube with lower internal plate impedance and higher current amplifying capacity, such as 12AU7, would perform better in "starved plate" circuits than a 12AX7 but if goal is basically to generate distortion then I wouldn't worry too much what tube you actually stick to the socket. Getting that achieved shoudln'y be much of a problem. If it works too well just misbias it intentionally and call it a day.
I'm in the same situation, but I'll ask a slightly different question. What if I'm looking for something mostly in the sweet and rich, slightly outside of clean, sound area with one of these tubes? Possibilities?
I found this Valvecater build with a couple of mods, there's even a point to point version (my 16 year old daughter is obsessed with that idea ever since we replaced a bad cap on an old pedal with no PCB). So is 12volts absolutely necessary for a tube/valve pedal or just this variant? Thanks
https://cdn.instructables.com/ORIG/FXM/9B6H/IK1FLK5B/FXM9B6HIK1FLK5B.pdf
After some research, I think I might try to build a Westbury W-20 (at 266V). Schematic from R.G.:
http://www.geofex.com/FX_images/westbury.gif
Quote from: Chrisfromiowa on October 14, 2017, 08:59:06 AM
So is 12volts absolutely necessary for a tube/valve pedal or just this variant?
The heaters (filaments) in the 12A_7 tubes are designed to run on 12.6v (series), or 6.3v (parallel). So you need one of those voltages (or close) in your circuit. People sometimes run the plates (anodes) on 12v along with the heaters to keep the power supply simple.
(parentheses)
imho if ya want any kinda possibility of tone, go with the highest voltage ya can. you need to run the heaters with 6.3 or 12.6v but the higher the voltage to the plates, the more headroom and less garbage noise ya can get. if all ya want is distortion, 12v is fine i guess, but i found on my builds i couldn't find a happy medium below about 48v on the plates, and even that was kinda gritchy sounding, but less noise and sputter and HISSSSSSSSS when it was cranked up.
if ya experiment with higher voltages, remember to over rate the voltage handling of your caps by at least 50%, if ya use low volt stompbox stuff it can cause some interesting results usually including letting the magick smoke outta some caps. smells terrible and quite a surprise when they pop, just how much crud is packed in there ;)
This is a kind of curse we're getting ourselves into, isn't it?
Just like "dabbling" in modular synth circuits or building a simple fuzz circuit so you can then build the "next" better one ad infinitum. :icon_eek:
Isn't there something about copious amounts of feedback for tubes because they're quite non-linear? Couldn't the sound we want be achieved by just removing feedback? More gain, more distortion.
http://www.theaudioarchive.com/TAA_Resources_Tubes_versus_Solid_State.htm
Nevermind. I guess transistors are highly nonlinear without "Complex circuits and considerable negative feedback required for low distortion".
I would've thought tubes are like FETs in terms of non-linearity, but maybe that's only outside of 'small signal' range.
I think the very first circuit I tried to build for guitar was something much like what OP has in mind, one triode of a 12AX7 cascading into the other triode to make distortion. This was over 20 years ago so I don't remember what I did well enough to know what I did wrong. It oscillated, like a bad tremolo. Maybe I didn't decouple properly, I don't know.
Quote from: thermionix on October 15, 2017, 06:06:14 AM
I think the very first circuit I tried to build for guitar was something much like what OP has in mind, one triode of a 12AX7 cascading into the other triode to make distortion. This was over 20 years ago so I don't remember what I did well enough to know what I did wrong. It oscillated, like a bad tremolo. Maybe I didn't decouple properly, I don't know.
LOL, that can be one fun result of playing with gain stages! Power supply issue?
For the OP: a triode gain stage is just that...a small amplifying stage (small at low voltages - at high voltages, where things like 12AX7s are designed to operate, really...you can be amplifying quite a big more). If you run one gain stage into another, you are multiplying the amount of gain you apply to the original signal...so, a gain of 10x in the 1st stage, 10x in the 2nd = 100x gain, theoretically. Til you reach the limits of the power supply, at least. You can control how much gain you feed the 2nd stage with a pot or resistor divider so that you're not running b@lls to the wall all the time....tweaking the amount of signal going to each successive stage is part of 'voicing' these things properly. Eventually, you'll want 4 or more stages ;)
The 'magic' of tube circuits is in the way you tailor the frequency response of each stage, as well as in between stages...where you operate on the load line (and yes, tubes ARE quite linear until you hit the 'bounds' of the power supply/biasing, often on purpose), and what you use for a supply voltage. One look at a load line will show you that you get best performance when you use a higher voltage - but you CAN run on 12V with some pretty good results, to get up and running in the tube game. Then later, you make the nice 300V tube preamp.
Merlin has a lot of info on his "Valve Wizard" site, I recommend you go read some! (Google him) But for now, a valvecaster type circuit will get you going, safely, without the high voltage risks until you learn some more. And will probably get you more curious, which is a good thing!
Quote from: GibsonGM on October 15, 2017, 08:40:47 AM
Power supply issue?
Maybe. What I do remember about the project...I had purchased an old military tube commo *something* box from a mil-surp shop. It had an appropriate PT, with HT and filament secondaries. I don't remember the rectifier situation, but I'm sure I used whatever was in there, pretty sure I even tried using the original filter caps, tube socket, and maybe some coupling caps. I was pretty clueless for sure. I thought it would be cool to have a "real tube distortion!" in a big heavy duty military-looking box. I probably trashed the whole thing after my experiment failed. Well, I think I saved some parts. Pretty sure the speaker binding posts on my stereo came from that thing. I might even still have the PT in a box somewhere. Hmm...maybe I should look!
Sometimes the 2nd set is better, y'know?? LOL, you may find it was just a bad cap or something!! Or being young maybe didn't have your wire runs clean enough...coupling....the things we do when we don't know better, ha ha!
I threw something like 30V into a transistor base (oh, that was some biasing!) with no base resistor when I was starting out. Glad I wear glasses :)
Well that PT is long gone, but digging around I found a cool OT that I forgot I had. Allen/Heyboer TO35MT. Like a Pro Reverb OT. When did I buy that? Hey I've got a PT and a choke too...
Also found a roll of 60/40 solder. It's a good day!
So, after going through .000001% of the information on valve casters on this site alone, it seems like I can achieve Big Muffish distortion with as little as 12v, so I'm going to give it a try... And I'll be sure to name my first born after y'all..... Thanks again, regards CfI
Nothing better to build than the 'Bajaman Real Tube'. or keep it for a valve replacement spare for your real valve amp. :icon_wink:
A 12AX7 on low voltage is a waste of time. Go to a thrift store and see if you can find a wallwart that puts out 12-13V with a transformer in rather than a switching power supply. And as much as I don't like the idea, get another one, could be low current, and put the low voltage winding on the 12V of the other one and rectify the voltage that comes off what would normally be the primary.
http://www.charlestonarea.com/mctube/real_mctube.htm
For higher voltage circuits, are nixie tube supplies any good?
Im looking at this one: https://www.amazon.com/dp/B074N5LVFH/ref=cm_sw_r_tw_awdb_x_Xb87zbH8JKN4X
Otherwise, I had been looking at this supply:
http://diy-tubes.com/index.php?route=product/product&path=59&product_id=565
Chris, I hope you don't mind my continued barging in on your thread....
Arent those like deathly high voltage? Trying to thin the herd?
Quote from: Chrisfromiowa on October 25, 2017, 11:11:21 AM
Arent those like deathly high voltage? Trying to thin the herd?
About 230VDC. Potentially lethal, yes.
I'm old and collect power supplies so I've got a whole box of them, including a 12 point something volt dc I figured I'll use. One of the valvecaster layouts on the PDF uses 12v so I thought I'd just wire it directly into the unit since it has its own fuse built in. I've seen all of these versions using 120/240 with step up/step down transformers and if I was going to do all that I might as just build a new amp..... Now that does sound kind of cool, an old style point to point wired tube amp... find a couple of those big transformers to make it look brutal... OKOKOK, stop, I've got to get back to work.
Quote from: printer2 on October 24, 2017, 06:18:00 PM
A 12AX7 on low voltage is a waste of time. Go to a thrift store and see if you can find a wallwart that puts out 12-13V with a transformer in rather than a switching power supply. And as much as I don't like the idea, get another one, could be low current, and put the low voltage winding on the 12V of the other one and rectify the voltage that comes off what would normally be the primary.
http://www.charlestonarea.com/mctube/real_mctube.htm
Damn, that looks very cool and the land o lakes woman always gets me going... I've never searched Land ' lakes girl but this image was the very mildest of the results.
(https://s1.postimg.org/620hnrquzf/do-land-olakes-indian-butter-boob-trick.1280x600.jpg) (https://postimg.org/image/620hnrquzf/)
Quote from: EBK on October 24, 2017, 06:38:28 PM
For higher voltage circuits, are nixie tube supplies any good?
Im looking at this one: https://www.amazon.com/dp/B074N5LVFH/ref=cm_sw_r_tw_awdb_x_Xb87zbH8JKN4X
Otherwise, I had been looking at this supply:
http://diy-tubes.com/index.php?route=product/product&path=59&product_id=565
Chris, I hope you don't mind my continued barging in on your thread....
These are pretty nifty.
http://www.ebay.ca/itm/DC-DC-8-32V-to-45-390V-High-Voltage-Boost-Converter-ZVS-Step-up-Booster-Module-/322374615632
Quote from: printer2 on October 25, 2017, 06:31:11 PM
Quote from: EBK on October 24, 2017, 06:38:28 PM
For higher voltage circuits, are nixie tube supplies any good?
Im looking at this one: https://www.amazon.com/dp/B074N5LVFH/ref=cm_sw_r_tw_awdb_x_Xb87zbH8JKN4X
Otherwise, I had been looking at this supply:
http://diy-tubes.com/index.php?route=product/product&path=59&product_id=565
Chris, I hope you don't mind my continued barging in on your thread....
These are pretty nifty.
http://www.ebay.ca/itm/DC-DC-8-32V-to-45-390V-High-Voltage-Boost-Converter-ZVS-Step-up-Booster-Module-/322374615632
(https://s1.postimg.org/4jxfu2o7vv/5_F48_FB58-223_D-4_F59-_BFAE-4_A11_F1_D0_F93_B.jpg) (https://postimg.org/image/4jxfu2o7vv/)
Btw, not at all, I do it all the time, I consider forum threads like conversations at a party, if you have something to add or a question you should be able to join in, at least in my opinion. Case in point, I would have never considered higher voltages, but that little guy is pretty cool. So, you could take a point off your daisy chain and step it up pretty easily with this, right? And the little set screw would keep it at a constant level without having to rewire a transformer... right?
I have a tube preamp circuit from an electronics mag so probably a no-no to post it? Anyway, it uses a switchmode/switching regulator (mc34063), inductor, cap, mosfet, etc. to boost 12.6 V up to about 265 V. I'm sure the gurus here can whip out an example circuit should they care to do so. Might be a decent alternative to using step up transformers, though that booster module does make things easy and isn't expensive.
Quote from: Chrisfromiowa on October 26, 2017, 10:58:10 AM
So, you could take a point off your daisy chain and step it up pretty easily with this, right?
I'd want a three-prong AC plug going into the box that holds this, so I can attach a safety ground.
I feel like I need to do some math first to figure out what amount of current I need.
Quote from: MaxPower on October 26, 2017, 12:26:59 PM
I have a tube preamp circuit from an electronics mag so probably a no-no to post it? Anyway, it uses a switchmode/switching regulator (mc34063), inductor, cap, mosfet, etc. to boost 12.6 V up to about 265 V. I'm sure the gurus here can whip out an example circuit should they care to do so. Might be a decent alternative to using step up transformers, though that booster module does make things easy and isn't expensive.
How old is the mag, is it new enough I could find it in the library? Thanks
those circuits are often called "nixie" supplies. they very often have a trimpot to allow the output voltage to be adjusted - the circuits are also very easy to hack for lower voltages, run 60V, 90V, 134V if you like.
Quote from: Chrisfromiowa on October 26, 2017, 05:07:37 PM
Quote from: MaxPower on October 26, 2017, 12:26:59 PM
I have a tube preamp circuit from an electronics mag so probably a no-no to post it? Anyway, it uses a switchmode/switching regulator (mc34063), inductor, cap, mosfet, etc. to boost 12.6 V up to about 265 V. I'm sure the gurus here can whip out an example circuit should they care to do so. Might be a decent alternative to using step up transformers, though that booster module does make things easy and isn't expensive.
How old is the mag, is it new enough I could find it in the library? Thanks
Hopefully this isn't considered spamming. Anyway, it's a two part project in the Jan and Feb 2017 issues of Everyday Practical Electronics. It's a UK mag. There is a digital version which costs $2 US. The schematic and parts list are in the Jan issue.
I've ordered a copy of Merlon's book, Designing Tube Preamps for Guitar and Bass. Loved the info on the Valve Wizard site, and I'm eager to learn more. Besides, a preamp seems to be where I'm heading for the sound I want.
Lately I have come to the point where the whole concept of tube tone is far from preamp tubes in high gain use. Of course running on low voltage suits distortion circuits better. High gain on the other hand is not where tubes shine the most. As mentioned triodes are quite linear with a hunch of even order harmonic content. It may be non mainstream yet a booster could be nice.
This site:
http://diyaudioprojects.com/mirror/members.aol.com/sbench101/
has distortion curves for 12AX7 and other tubes with operation with a 250 VDC supply voltage plotted against RMS voltage output. Other parts indicate how well various tubes do with starved filament current. This will give you hours of reading until you get to the point where you wonder why you asked the question.