#1
Recently I came by some schematics for low wattage, tube amps that run optimally with 12V being supplied to the plates. They use tubes designed for usage with low voltage car radios. The schematics can be found here
http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

So I figured I would try my hand at building a modified version of the 12AL8 version that that guy made (I incorporated a tone stack+gain and volume controls and changed a few values). I'm hoping to get a good 0.4W out of this thing once its done. I just ordered the tubes and sockets, and I'm putting out an order for the output transformer along with the rest of the passive components today or tomorrow probably.

As far as powering it I'm going to go with a mains DC power supply using a cheap radioshack transformer, a bridge rectifier, and a filter cap.

I was wondering if anyone has ever worked with space charge tubes and built one of these amps
#2
Ever measured the max dcb of that tinier terror ? I'm curious to hear about it, probably's got different dynamics than its cranked big brother.
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#3
1. You will loose a ton of gain (output) with those added controls...
2. You will need more than one filter cap
3. I am not a fan of starved plate amp designs

4. Space charge tubes are fun
#4
Well a few things here, I am ready to add filtering (I'm planning on starting off with a 2200uF 50V electrolytic. At some point I'd like to get this thing running on a lead acid battery.

The plates would be operating within their design specifications, which would be anywhere from 10-30V, with 12V being the normal operating point according to every data sheet I've seen.

I've been going through some tone stack designs and I'm gonna try out a modified marshall tone circuit before the volume. The output impedance of the last preamp stage is only about 6k rather than say 38k for many 12AX7 stages. I'm running through some values that will give me some control with minimal signal loss.
#5
i have absolutley no input for this thread seeing as i have absolutly no electrical knowledge...

however this is Polzin from AGF! sup.
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#6
Well back to this thread again, I built this thing over the past few days, and now I have a working prototype

Its pretty clean with everything cranked; only a small amount of breakup. Its quiet enough to play with people sleeping nearby, and its not as noisy as I expected it to be with my terrible wiring job

I've still got some experimenting to do with output transformer values, but hey it works. The tone stack I put in doesn't help things too much (probably needs to be re-engineered along with a cathode follower driving it possibly)

I'd recommend one of these low voltage amps for beginners. They are cheap, simple, and for the most part a lot safer than a high voltage circuit; as long as they aren't chewing on the wall voltage lines.

I'll throw up some pics tomorrow, I would now but my computer is malfunctioning like crazy in the 100+ degree weather here
#9
Man, that's insane - I love it! I don't know anything about tube amp circuits, But you should definatly do a tutorial on this or something - one of the things that puts me off messing about with amps is the voltage,

Edit: Just saw the link,
How much did it cost you?
Last edited by jimRH7 at Aug 17, 2008,
#10
You could get by with under $100 (heck 50 if you really cheap out) of parts and basic tools really. I haven't really thrown together any good figures, but if you have a drill, a soldering iron, and some wire strippers the parts aren't that much.

I'm using a $10 radioshack 12V@3A power transformer (parts express has a 25V 2A t-former for $3 and it might work for a '30V' version), a parts express $5 70V PA line matching transformer for the OT, the power tube is $3, 12U7 cost me $5, chassis is 15, various passive components are under $15 (hell probably under 10) if you buy em from a place like mouser, I have $8-9 worth of pots, and a few dollars in other mounting hardware. Shipping is what really sucks about ordering online.

Once I get a nice design up and running I'm planning on selling a few to some friends and then maybe I'll do some tutorials to encourage the DIY spirit, and maybe even design a PCB for it, which would make the build much easier for first timers.
#11
that would Be Wicke if you could do all said above stuff
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#12
Space charge tubes ARE a lot of fun. I've had good results using 12U7s for the preamp and tone shaping, and 12K5s in push-pull configuration for the output. You can actually achieve some pretty sweet and creamy tones by pushing those 12k5s. I've been playing with space charge tubes for a couple years. I even had a guy design and build a couple custom OTs to work better with the 12K5 impedance and tiny power levels. I found the Hammond 146N also works well with the 12K5.

I definitely recommend messing with space charge tubes to someone interested in learning about tubes.

As far as the power supply goes, I find these to fit the bill: http://www.mpja.com/prodinfo.asp?number=17554+PS
#14
Quote by Losenger
Space charge tubes ARE a lot of fun. I've had good results using 12U7s for the preamp and tone shaping, and 12K5s in push-pull configuration for the output. You can actually achieve some pretty sweet and creamy tones by pushing those 12k5s. I've been playing with space charge tubes for a couple years. I even had a guy design and build a couple custom OTs to work better with the 12K5 impedance and tiny power levels. I found the Hammond 146N also works well with the 12K5.

I definitely recommend messing with space charge tubes to someone interested in learning about tubes.

As far as the power supply goes, I find these to fit the bill: http://www.mpja.com/prodinfo.asp?number=17554+PS
D'you have any sort of recordings of a PP 12K5 amp? How would you run it? That's an idea I was looking at... Can the tones compare to bigger amps?
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#15
Quote by MrCarrot
D'you have any sort of recordings of a PP 12K5 amp? How would you run it? That's an idea I was looking at... Can the tones compare to bigger amps?



I have some vids of my experiments, yes.

How would I run it? What do you mean?

Yeah, the tones are pretty good, I think. The output is just a lot quieter. Sounds close to push-pull 6BQ5/EL84.
#16
any chance we might see a schematic for this??? amp buildin' seems to be somthin i want to get into after my guitar build (if the f***in body and neck ever show up....) and this seems like a good place to start
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Last edited by AngusJimiKeith at Aug 19, 2008,
#17
Quote by Losenger
I have some vids of my experiments, yes.

How would I run it? What do you mean?

Yeah, the tones are pretty good, I think. The output is just a lot quieter. Sounds close to push-pull 6BQ5/EL84.
I mean bias points, operating voltages, preamp tubes and bias, resistor values on PI etc. If yours has worked out well and you can show with some clips... You might find me wanting to make one also, if you aren't sworn to secrecy

And really? Wow, awesome, I just happen to love EL84 tone... Don't suppose you have any of those videos online? I could potentially make an awesome, crankable baby AC30...

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#18
I'm not sure yet whether I want to disclose the particulars of my circuits. I've spent hours and hours experimenting and tweaking. I'm one who believes you learn more by doing the work.

I may post a video soon, if I get a chance to upload. I might even record a small one later today. The videos I have now are hundreds of MB.
#19
okay...fair enough

besides, its probably more fun if i experiment and figure the stuff out myself

nice job though, i demand sound clips when its done!!!


EDIT: oh and by the way, i kinda meant that question about schems for TS, too...again, no big deal if u dont wanna share 'em- its ur work, not mine
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Last edited by AngusJimiKeith at Aug 24, 2008,
#20
Alright, I'm putting myself out there. Please read the descriptions. It was late, I was drunk, and hopefully I brought some joy into your life. Both guitars are completely stock. The sound you hear in the background is the water softener regenerating. I am planning on recording another demo this weekend with an all tube design, without the SS. If you have any questions, don't hesitate to ask. Space charge tube are the way to go if you're just getting into tubes and don't want the HV hassle.

http://video.google.com/videoplay?docid=-2796947051832308831

http://video.google.com/videoplay?docid=7548349589032525553

http://video.google.com/videoplay?docid=-5130154707029567328

I forgot to mention that the master pot between the 12U7 gain stage and phase inverter was maxed the entire time. The only variables were the guitar knobs, the clipper pots, and the pot in series with the 12U7 bypass cap.
Last edited by Losenger at Aug 26, 2008,
#21
your playin doesnt suck.....

and thanks, ive decided not to copy ur design but it helps to know as much about this stuff before i start as possible....
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#22
I have a funeral to attend today, but I hope to be able to construct an all tube circuit and have a video up by tomorrow. The 12K5 sounds different when driven to saturation, which I didn't accomplish in the previous videos.

Edit: Another edit. This circuit doesn't sound as good as others I've constructed in the past. I merely recorded what I had set up on the breadboard at the time. I'll post a new one soon. Experimentation is the real purpose. Tweak until you hear the sound you want to hear.
Last edited by Losenger at Aug 27, 2008,
#23
I constructed a 100% space charge tube circuit today. Here's a closeup video, including some function generator manipulation, etc.

http://video.google.com/videoplay?docid=7120828950057422531

Here's the schematic:

Then, I replaced the 1M gain pot with a 2M to increase the gain. I used the Schecter Damien stock. It breaks up a bit using the bridge pickup. If you want more gain, add a couple more 12U7 gain stages.

http://video.google.com/videoplay?docid=-61079272005287205
#25
Just a little presence control to boost the gain of high frequencies. During the demo, I start with the pot rolled back, then slowly increase the conduction through the 6.8nF cap. You should hear an increase in brightness.
#26
Hi all! I just joined this forum, after finding this thread via a google search. I've been itching to build one of the amps from the sophtamps.ca site. I've got the OT, a chassis, sockets, and the 12AL8, but that 12U7 is one scarce tube, these days. Was almost tempted to just go ahead and sub in a 12AU7 (it supposedly works), but I'd rather not compromise the sound.

Axecutioner, any sound clips yet?

Losenger, I like the way you were able to make a PP amp, using the 12K5s. Could you recommend any good reading for a novice?

I'm still baffled at the task of figuring out what size OT to use for a given tube circuit (when trying to design my own). I've got quite a few 12CX6s around, and was figuring they could be well used in PP circuits, maybe even as quads. Hoping to make something loud enough for busking on busy streets (and hoping it will sound better than a Pignose)
Last edited by Puguglybonehead at Oct 7, 2008,
#27
Sweet amp Axe. This thread was recommended to me here http://ultimate-guitar.com/forum/showthread.php?p=16941607#post16941607 . I am designing a battery powered tube amp with a goal of around 1W output for 30 minutes. A circuit something like this seems optimal. I am kind of scared of building a switching supply or inverter circuit to power a regular power tube. These space charge tubes sound more appealing to me. Care to throw some feedback on my thread?
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#28
12U7: http://store.tubedepot.com/nos-12u7.html

Reading material: http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

As far as the PP design, I just used conventional tube designs and tweaked while powered up. That's the advantage to low voltage circuits: being able to replace component values without shocking yourself or destroying a tube. And if you do wreck a tube, who cares? They're $6.

Match the OT to the recommended tube load.
#29
Quote by Losenger
12U7: http://store.tubedepot.com/nos-12u7.html

Reading material: http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

As far as the PP design, I just used conventional tube designs and tweaked while powered up. That's the advantage to low voltage circuits: being able to replace component values without shocking yourself or destroying a tube. And if you do wreck a tube, who cares? They're $6.

Match the OT to the recommended tube load.


Thanks for that! I really need to buy some books, I think.

I have a whole bunch of 12CX6 pentodes that I'd like to use. (they were going dirt-cheap this summer) They're not intended as an audio amplifier though, so the only available specs load-wise, are plate resistance, transconductance, etc.

As far as I understand it, am I just using Rl = (0.1) Rp to calculate my load resistance? So, say a 12CX6, which has a plate resistance of 40K would have a load resistance of 4000 ohms?

Am I then just multiplying that times 2 for a PP amp? I would look for a transformer with an 8K primary? (and hopefully an 8ohm secondary) Am I missing something? This almost seems too simple.

Sorry for all the newb questions.
#30
The rule of thumb is that the load impedance of a power tube is roughly 1/10th that of the plate resistance. The data sheets regarding that tube type don't specify a load resistance. I would say an OT with a primary of 4Kohms would be a good start. If you're going PP, use a transformer that has a primary impedance of 8K center tapped. Each output tube should see its respective appropriate load. In other words, each tube should only be presented with the appropriate amount of current, which is limited by the output transformer, along with any plate resistors, which I don't use with space charge tubes.

Edit: Bah! I meant CATHODE resistors! Don't drink and type, kids!
Last edited by Losenger at Feb 1, 2009,
#31
As a fellow tube amp builder, I applaud your efforts on this one. I've never had the time to mess around with space charge tubes (I have a weird desire to work with small signal pentodes and 7-pin tubes, most of my experiments go that way).

Good call on the prescence control BTW. if you want a bass control, you can do something similar...a 1uF cap that always bypasses the cathode resistor, then a 22uF cap in series w/ a pot. Works pretty well...may have to tweak those values slightly to get the effects at the frequencies you want...

One thing for you to try if you want more growl, put a pot in front of ONE of the tubes after the cathodyne. Basically, so that you're intentionally unevenly driving the power tubes (unbalanced that is). Should thicken up the tone some, depending on the degree you do it to. What's ideal in that circumstance is if you can get the signal levels so that you're overdriving one tube but not the other one...

Cool stuff, keep it up!
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