Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TerryD on February 02, 2016, 08:19:57 am
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OK. I got this Fender PA 100 up and running. V1 (or the first half of v1) is stock, v2 (or 2nd half) is 64 twin preamp. With the second tube, the only one left for the remaining channels 3 & 4, I wanted to run three into four to make extra gain...just a little more grittiness.
I'm using Dan Torres book on "extra gain stage" and just can't get it happening. On the first stage I just have a volume control and the second has a volume and all tone controls. Once I had the first volume control working, but nothing taking place with the second volume or tone controls. Now both channels are just on and no volumes or tone controls work.
Is there some description how to do this somewhere? I just want a little extra gain. I think I got it how to keep it down but the main thing I'm not getting.
Thanks, Terry
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TerryD, Could you post a pic of the guts or a diagram of how you have it hooked up? Also, what style of gain are you looking for (i.e. plexi-vintage, mesa-hi gain)?
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I assume from your description you are trying, in essence, to place two preamp sections (ordinary R-coupled amplifier stage) in series. This should work, although you may have uncontrollable gain (which can be fun!) You have to, you realize, take any given output stage's gain from underneath the plate resistor and couple it via a cap to the next grid. This also means that wherever the input #3 and input #4 preamps went to in the stock amp is disconnected. Just checking that we're on the same thought-page.
When you do stuff like this, you should be checking the cathodes of the four input stages. We want to see 1.5 or so volts there, that is simply the best indication that the tube is working, conducting....that it is passing current through its cathode resistor and thereby producing a voltage drop across same.
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You might look at the D-style 4 gain stages as an example of successful 4 gain stage preamp.
With respect, Tubenit
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Is that 4-stage preamp known to work and suspected of being correctly drawn as shown, Tubenit? Honest question.
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Came straight out of the Dumble files: http://ampgarage.com/forum/viewforum.php?f=5 (http://ampgarage.com/forum/viewforum.php?f=5)
Reportedly, this is #183 Dumblish preamp.
My understanding is that it is a proven design. I simply removed the DPDT relay switching between clean/OD in the drawing, removed a couple of switches in the tone stack (PAB & rock/jazz) & removed the passive effects loop.
With respect, Tubenit
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Tubenit Thanks a lot. The layout above is kind of the opposite of what I did, so I'm looking forward to trying it. I had a volume only with the next stage all the tone stuff.
I can't seem to figure out what I'm supposed to grab from the link.
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I can't seem to figure out what I'm supposed to grab from the link
It's OK, you're not supposed to grab anything. I was just responding to the previous question about whether this was a "real" schematic and citing the source of it.
With respect, Tubenit