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12v 12U7/12AL8 amp, have a few Q's

Started by joecool85, August 11, 2007, 12:30:04 PM

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theres the slight possibility of killing the transformer, but isnt very likely, theres difference in the sound...
well, found a better awnser:
from geofex.com - http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch


So for a line out jack I might want to run an ~8ohm resistor from tip to sleeve?  It's such a low voltage (12v) low wattage (40mw) setup that I doubt it would reck the transformer anyway, but I'd rather be on the safe side just incase.
Life is what you make it.
Still rockin' the Dean Markley K-20X


it will only hurt if you push it really hard, in nomal home use its ok i think  :)
saw other advantage of having it matched, was just above that link:
"Q: Why do I have to match speakers to the output impedance of the amp?
A: You'll get the most power out of the amp if the load is matched. "


The pot would have to be in front of r5 , r5 is your bias , with a pot there when you reduce the resistance the tube will go into runaway or you  can put a pot in series with R2 making it R3.

                                      Rock On


This does look like a pretty cool amp to build. The sound clips on Sopht Amps sounded nice. I liked the Marshall-ish tone of the 12AL8 version the most. I'm also new to tubes, and wondering if a build like this is really as straight-forward as it looks?  Have you gotten it built yet?


It is really a straight forward build.  I built it and have since sold it.
Life is what you make it.
Still rockin' the Dean Markley K-20X


Quote from: joecool85 on August 11, 2007, 12:30:04 PM
I'd like to build this tube amp using a 12U7 and a 12AL8 tube.

More info here:

Now, I've never built a tube amp before and I've always wanted to.  This little fella seems pretty cool but I have some questions.  What do I use for a PSU?  Could I basically just use a 12v regulated source and a 5000uF cap for filtering?

Where does the volume pot go on this thing?  Could I have a gain control on this?

I've priced out parts and I can get everything minus an enclosure and a speaker for around $50.  I'm going to do a lot more reading on tube stuff before setting out to build it, as I always research heavily before getting into a new project.

Also, what about biasing the tubes?  Hows that work?

Yes, I am a tube newb.

I think i´m going to build this amp, too  :tu:
I´m still new to this topic, too, but aren´t the screen and the control gitter of the 12AL8 swaped in this schematic? (hope the designations of the gitters are corrrcet as english is a foreign language for me^^)

---> The 12k5 has a space charged gitter, that´s why the gitters seem to be swaped :)  :o


If my memory serves me well,,,Then with a HT voltage of only 12 it is unlikely that biasing is needed at all.
And also not much chance of blowing anything up by mismatching transformer outputs with only a 12 volts HT. [but don't do it with normal tube outputs]

Very unlikely you wil get a clean sound [but Hey,, I could be wrong ::)]
As these valves are no longer working in there linier region and hence all the standard values will be very different. [ heck 1 meg or 10 meg  grid R suck it and see]
The transformer will never be able to saturate [if your looking for that overdriven OTr fat sound].
IMHO I think there are far better thing to do with valves.
There was a circuit by Anderton "Stack in a box" [I think] and that ran 40 volts HT with no OTr,,, again not really worth it.
The "Mesa Vtwin pedal" would give far better results than most of these things.
but yes it uses normal HT 200 VDC.
Cheers,, Phil.