Octal Fatness In A Box?!

[Paul Marossy]

OK, so I have an idea, I need a second opinion or two.

Here we go: Take the Octal Fatness preamp section (6SJ7 & 12AX7) including tone stack and master volume and adapt that to a stompbox format. I was thinking that I could use a 48V transformer that I have lying around, full wave rectify it and have around 65VDC strarting B+ voltage to work with. Lower voltage means physically smaller filter caps, smaller transformers, etc. I think all of this could fit into a sloped front enclosure that I have which measures approximately 6"x3"x2.5" high.

Any opinions? Am I crazy, or would this have potential? I'm thinking that it could sound pretty good if designed right....

[Ben N]

Why not?  It wouldn't be the first AC powered all-tube preamp in a stompbox (Matchless Hotbox, Real McTube, e.g.).

Or, add a Firefly output section and an switchable attenuater for a mini-amp/od.  (Would definitely need a bigger/open enclosure).

[Paul Marossy]

Yeah, I know about the Matchless Hotbox, Real Mctube, Chandler Tube Driver, Shaka Tube, etc. These are all stompboxes that use one or more 12AX7 preamp tubes. I'm just wondering how that 6SJ7 (pentode) would like having low plate voltage...

I was also thinking of maybe using a voltage doubler for around 95V B+ voltage. It seems like I could halve some of the appropriate resistors since the B+ voltage would be roughly half of what I have in my Octal Fatness amp and get roughly the same result...

[Joe Davisson]

For high voltages, you can do a voltage quadrupler to get around 200V, but the filtering is harder than with full-wave rectifiers. I would look into transistor regulators, which should end up smaller than the huge filter caps required for the voltage quadrupler. The quadrupler more or less outputs pulsed DC which requires more smoothing than full-wave rectification.

With the full-wave rectifier I would say just use filter caps, but in this case it might turn out smaller by sticking a transistor/heatsink in there. along with a smaller filter cap. Something like a TIP31 or 2N3055 should do the trick. (TO220 case, not the huge TO3 things).

You may not need the zener if the transistor base is filtered with it's own cap, if zeners aren't available in the voltage you need.

regulation:
http://www.tpub.com/neets/book7/27k.htm

voltage multipliers:
http://www.tpub.com/neets/book7/27m.htm

quadrupler:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/voldoub.html#c3

-Joe

[Paul Marossy]

Thanks Joe! It looks like I have some reading to do...

[Aharon]

Or use a couple of 115/12V transformers back to back like the McTube.
I recently changed my Hotbox to back to back transformers + voltage doubler to free up the other iron I used to build the original for another project since it was way overkill for the HB.
I have to say that it works better than before,even less noise,don't ask why,maybe the new layout.I did use a separate heater tranformer tho"...I don't like running heaters at 12V.Anyway,with that arangement I got 280V.I could've used it at around 150 or so without the doubler too if I was building another type of pre.
Aharon

[Paul Marossy]

Yeah, I remember that the Real McTube uses two back to back transformers. I don't understand how you get that kind of high voltage with two 12V transformers, though...

BTW, why don't you like to run the tube heaters at 12.6V? Is it noisier that way? I did seem to make a difference in the noise floor in my Firefly when I changed tube heaters from 12.6V to 6.3V (had it wired incorrectly the first time...)

[Aharon]

Yeah, I remember that the Real McTube uses two back to back transformers. I don't understand how you get that kind of high voltage with two 12V transformers, though...

BTW, why don't you like to run the tube heaters at 12.6V? Is it noisier that way? I did seem to make a difference in the noise floor in my Firefly when I changed tube heaters from 12.6V to 6.3V (had it wired incorrectly the first time...)

Hi Paul.Yeah,I don't like 12.6V heaters,noisier I think.
How do you get 280V from 2 12V trannies?,connect one to the wall(115V to 12V) then the resulting 12V to the other 12V winding.That second trannie will output 115V from it's primary winding now.
After that just use a voltage doubler.Rectification will give another  *1.41 and you end up with close to 280V.
You have to be carefull with current ratings cause all that back-to-back and voltage doubling reduces the current capability of the PS.
Hope that helps
Aharon
WARNING: Never connect a 12V winding to 115V wall socket and don't play with electricity unless you have knowledge of what you are doing.Seek help from a tech first.

[Alpha579]

Paul, sounds like a great idea! If you used a voltage doubler(or quadruapler) , does it mess with the current you can draw?

[Paul Marossy]

OK, now I get how that works.  :oops:  
That's actually pretty simple!:idea:
I guess someone had to explain it to me, 'cause I've never seen that done before. Well, come to think of it, I guess that's done all the time around me on the power grid that I plug my whatever into, huh? Just never thought about it really.

The only drawback about the back to back transformers is the amount of room they take up physically. But, it is a convenient way to get a good B+ voltage. Hmm...

[Michael Allen]

Someone have a link to this Voltage Doubler?

Thanks...

[Aharon]

The only drawback about the back to back transformers is the amount of room they take up physically. But, it is a convenient way to get a good B+ voltage. Hmm...[/quote]


Well......not really.It could be less or comparable to a single trannie realestate wise and the price is right,
for example the RS trannies I used cost me $0.99 ea on sale,how can you say no to that.Plus of course a couple of bucks worth of caps for the doubler and voila.They are the 500mA 12.6 types.

I've been thinking for a while to build a all purpose power amp,let's say something like the Tone Brewery but with OT and everything ,then just build preamps.I think in the long run it makes so much more sense at least moneywise,and workwise too,most power amps are very similar.
Aharon

[Paul Marossy]

Yeah, I was just thinking about buying another 12.6V RS transformer to compliment the one I already have. A pair of those would be perfect for my proposed project. I looked at the inside of the Real McTube, and I guess you can get them pretty compact if you lay them out properly. Only one problem, though. I don't think you can run a 6SJ7 on a 12V heater supply.  
That would mean three transformers would be required? Or could I use a voltage regulator to get ~6.3V?

I remember reading in a thread somewhere that you don't want to use a voltage divider to halve the voltage on a heater circuit. Is this true, and why is it a bad idea?

[Aharon]

You could use regulated DC,but that's an area that I don't go too much into.


Here's a link to the doublers and other multipliers.

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/voldoub.html

Aharon

[Michael Allen]

What are appropriate values for Caps, and are the diodes just regular 1N4001?

So you have the first transformer and connect the secondary to the secondary of the next trafo, then connect those primaries to the voltage doubler, fullwave rectify it, and there you have the B+?

[Paul Marossy]

"So you have the first transformer and connect the secondary to the secondary of the next trafo, then connect those primaries to the voltage doubler, fullwave rectify it, and there you have the B+?"

Yep, that's basically it. Here's how it's done on the "Real McTube II":
http://www.dogstar.dantimax.dk/tubestuf/graphics/mctube2s.gif
Except that it doesn't use a voltage doubler... I think I would be happy with ~150V to work with...

[Aharon]

What are appropriate values for Caps, and are the diodes just regular 1N4001?

I can tell you what I used:1N4007 diodes (don't skimp on those,there's surges at start-up that can blow them)and used 100uF/350V caps.I've seen 200uF/250V caps used too.The thing is that the caps should be rated for the full doubled voltage plus some.I do not know if the caps in series "uF halve/voltage adds" applies here.
Aharon

[Aharon]

"So you have the first transformer and connect the secondary to the secondary of the next trafo, then connect those primaries to the voltage doubler, fullwave rectify it, and there you have the B+?"

Yep, that's basically it. Here's how it's done on the "Real McTube II":
http://www.dogstar.dantimax.dk/tubestuf/graphics/mctube2s.gif
Except that it doesn't use a voltage doubler... I think I would be happy with ~150V to work with...

WATCH OUT cause the circuit for a voltage doubler used in conjunction with a bridge rectifier is to my knowledge different.
Aharon
WARNING:Exercise caution when working with electricity,it can harm or kill you.Unless you are absolutely sure of what you are doing.....DON'T DO IT.

[Paul Marossy]

Yeah, I didn't think about that. Most voltage doubler circuits use diodes to double the voltage. Maybe an isolation transformer would help? I am not totally sure, I haven't really messed with voltage doubler circuits, except for the power supply on the Shaka Tube circuit.

I still like the back to back 12V transformers. ~160V is a good voltage to work with, IMO. And that way, you can get 12V for the tube heaters. I think that is how I am going to approach my new experiment. I just need to verify how I get 6V for the tube heaters instead of 12V. Anyone have any suggestions?

[Aharon]

I still like the back to back 12V transformers. ~160V is a good voltage to work with, IMO. And that way, you can get 12V for the tube heaters. I think that is how I am going to approach my new experiment. I just need to verify how I get 6V for the tube heaters instead of 12V. Anyone have any suggestions?[/quote]


Yeah,you are right,150V is good for a number of circuits out there.
6V instead of 12V?,that's a good one....due to the current involved I think zeners are out of the question......
If you are only using one 12Ax7 there's a 115V/6.3 trannie out there (at 500mA) that will do the trick for a couple of dollars.I wish I could remember where I got it tho'.......it wasn't mail order so even if I did remember it would not do you any good,but if you can get one around your area....but Paul,I would not discount 12.6V just yet,maybe go for that and see.
Aharon
PS: I heard you sound clips,awesome!!!.