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Old 8th February 2007, 03:03 PM   #1
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Default DIY : SWR Interstellar OverDrive (bass preamp)

hi, i'm thinking of building an overdrive pedal, a bass overdrive pedal actually. I've run into this 2 schematics:

SWR IOD bass preamp (which is pretty weel known in the bass communitty and has very good reviews).
Click the image to open in full size.

and the Real McTube II:
Click the image to open in full size.


Now i have some question:

Referin gto the first schematic.
I understand pretty much the whole schematic, but i don't knowwhich voltage transformers to use for the power supply.
I also want to know, if there will be any difference in performance adding a load to the power output on the power tube, or there will be no difference sound wise.

Referring the second schematic.

Would it be posible to use this schematic as the input for the EQ section of the first schematic? and not using the tube gain stage of the first schematic?

thanks! i'm an engineering student and i have basic electronics knowledge. I'm reading a few tube books. Do you recomend any?
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Old 8th February 2007, 04:17 PM   #2
Merlinb is offline Merlinb  United Kingdom
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Well the first schematic isn't a pedal- it's a whole amp! (And there's more solid state in it that I care for!!)

The Real McTube is, unfortunately, p*ss poor. You can get it sounding half decent if you totally redeisgn it, but then it's not really a Real McTube anymore. It is however, a good place to start. It's pretty easy to get it going and then tweak to taste afterwards.
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Old 9th February 2007, 12:49 AM   #3
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weel i'm actually thinking of going with the first one. The fiorst one is a whole amp, you're rigth. A 10W amp. Even though it's used as a preamp with a power tube to give it that all tum amp overdrive sound...or at least that's what SWR intended. It has an output after the gain stage and another output after the power tube.
My biggest question is, if the output taken after the power stage would vary sound wise if i plug a load to the output power of the power stage or it will sound exactly the same.
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Old 9th February 2007, 11:35 AM   #4
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Quote:
Originally posted by Merlinb
The Real McTube is, unfortunately, p*ss poor.
Yours is the first negative comment I've heard about it. Can you give detail to your comment?


Most tube OD's are not meant for bass and sound crappy with them. As a bass player, look at running the tubes at *very* low current (I have 1Meg on the 12A_7 anodes with 250 to 300V supply and the anode set at B+/2 bias). Follow with a cathode follower or varieties of.
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Old 11th February 2007, 07:36 PM   #5
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thanks Geek, i'll experiment first with something you propose. But i wanna build the whole fisrt schematic sooner or later. I need to know what the 53VAC from AFX1 means....i guess it's 53 volts of AC ?!?! so i would need a stepdown to 53V? but homw many A's it needs to handle.
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Old 12th February 2007, 05:15 AM   #6
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It's being regulated to a 15V split supply. Any transformer with a 30VCT and higher secondary @ 500mA will work I'd say. Rather than Zeners and resistors, you can use a 7815 and 7915 for simplicity.

Cheers!
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Old 12th February 2007, 11:32 AM   #7
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oh yeah i see, so i just find a setpdown to 30V with two 15V outputs and a pair of voltage regulator (one for +15V and one for -15v) and then i don't need the whole power supply part of the schematic!!?!?!

that's great then!


thanks!!!!!

by the way can you recomend me any books ? i been going into this web page : http://www.pmillett.com/tecnical_books_online.htm
which has several old books about tubes on PDF.
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Old 13th February 2007, 12:26 AM   #8
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You're welcome!

Quote:
Originally posted by miguel567_98
by the way can you recomend me any books ?

Valve Amplifiers

RDH4
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Old 13th February 2007, 12:45 AM   #9
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great! thanksss so much!!!!
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Old 14th February 2007, 02:18 PM   #10
engels is offline engels  Israel
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Default bass pre

Have you seen the Alembic bass preamp scheme? It's called F-something... Dave Gilmor used one, try to google it. I think they've copied some Fender Twin preamp with TMB. It's a bit more complicated than McTube but less solid-state than SWR.

If you wish to experiment I may give you an idea: recently I've built an amp with space charge tubes (12V plates and filaments) and it works very good with my bass. I'm planning to make me a little bass preamp with spacecharge tubes. The main advantage is the power supply: 12V wall wart - and you're there!

Take two spacecharge tubes, a 12V regulated power supply (1A might be not enough, but 2A should be okay). The tubes are all grid-biased (grounded cathoded and 1M or 2.2M grid resistors, look at the datasheets). 0.1uF coupling caps to pass the basses, 1M volume between the tubes (and if you want a guitar overdrive - take 0.01 - 0.02uF coupling caps).

I dont' know how much overdrive you're looking for, but two 12AJ6 -s will deliver lots of distortion and 12FM6 stays much cleaner. I also tried 12DV7 which is also relatively clean. The plate load resistors are 1M for 12FM6 and 12AJ6 and 100K for 12DV7.

Antique Electronics sell those tubes for dirt cheap.
www.tubesandmore.com
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