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Author Topic: How Hard Would It Be?  (Read 5262 times)

skynyrd

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How Hard Would It Be?
« on: August 15, 2010, 10:36:16 AM »
How hard would it be to put a single 12ax7 preamp in place of a solid state pre amp?

DJPhil

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Re: How Hard Would It Be?
« Reply #1 on: August 15, 2010, 11:09:41 AM »
I'd guess that the primary complication would be the power supply, followed by inter-stage coupling. I know Vox does something like this, but I haven't dug up schematics yet to see how they did it. I believe they run a dual triode like the 12ax7 at low B+ voltage in an amp with high supply rails to begin with. Come to think of it, I haven't managed to find VT50 schematics anywhere yet as Vox is tightfisted with their in-production schematics. I'm pretty sure Teemuk's book has a bit on this in the preamp section.

I'd imagine that you'd have to do a lot of work to make it worth the effort too. From what I've read it's not as simple as just wiring in a tube, it takes some careful design to make good use of the non-linearity inherent in a tube without undesirable side effects. There are a lot of 'tube preamp' amplifiers out there where the only thing the tube is really doing is increasing the manufacturing cost by 20% and the retail price by 300%.

Just a noob here, but hope that helps some. :)

tonyharker

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Re: How Hard Would It Be?
« Reply #2 on: August 15, 2010, 02:00:47 PM »
There's always this < http://cgi.ebay.co.uk/New-product-MKII-12AU7-buffer-amp-Tube-/260644578435?pt=Vintage_Electronics_R2  >  this shouldnt be too difficult to adapt.  This runs off 12v boosted on the PCB to about 45-55v to power the tube.

J M Fahey

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Re: How Hard Would It Be?
« Reply #3 on: August 15, 2010, 03:02:33 PM »
Easy if you can supply at least +160VDC (250V would be even better) plus 6.3V @300mA.
You can build the "normal" Twin channel preamp or if you vcan use 2 tubes, either a Plexi or JCM800 preamp. You will need to increase filament current to 600mA.
Those would be *real, guitar amp* preamps, doing what's expected from them.
Most low voltage solutions work on the "Mojo" area instead.

skynyrd

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Re: How Hard Would It Be?
« Reply #4 on: August 15, 2010, 07:14:47 PM »
Thanks for the replies guys, Tony thats what got me wondering  was browsing ebay and finding several different 12ax7 add on boards, found 1 with 3 12ax7`s for the same price as the one you shown. May give my local tech a call and see what his thoughts are, he builds Hiwatt clones and sell out of his store and know 2 guys that bought them and had zero trouble to date out of them. So he must know what he`s doing.

bry melvin

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Re: How Hard Would It Be?
« Reply #5 on: August 15, 2010, 07:51:14 PM »
If one is just looking for good sound...not a pedal....take a look at the schematic for a Peavey TG Raxx TB Raxx

(full 2 12ax7 preamp)



phatt

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Re: How Hard Would It Be?
« Reply #6 on: August 17, 2010, 09:43:13 AM »
Hi skynyrd,
               Judging by some of your comments and your handle ,,you seek a rock to bluesy sound.
If so? Then do it the other way round if anything.
i.e. SSpreamp and Valve Power Stage.
Hi gain metal sound will benifit from lotsa preamp scream but probably not what you seek.

Triode distortion is just lots of fizz,,whereas full bore pentodes SING (partly due to the OT)
Remember that Pentodes where invented to get around the pathetic perfomance of triodes.

If you absolutly can't help yourself and you wish to go in that direction then download the Mesa Vtwin pedal ,, might give you some ideas to work with.

Have fun, Phil

phatt

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Re: How Hard Would It Be?
« Reply #7 on: August 17, 2010, 10:22:55 AM »
Oh forgot to mention,
                             I built a cut down version of the Vtwin > into SS poweramp, so I know how well it works. Good but no match for a SSpreamp into a decent, well thought out Valve power stage.
Triodes can't do the Square wave trick (which is what you need).
Even when triodes are used as PP output through a transformer,, still no Cigar. :'(
Phil.