Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on March 03, 2014, 10:49:56 am
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Hey guys! I'm in the process of building a PA-26, and I was thinking I'd throw a VVR in it. It seems like scaling the power amp and PI tends to be the preferred the method. My problem is, I'm having trouble visualizing how I would do that with this build (I've linked the schematic and layout).
I also remember another method of "attenuation" that was discussed at length around here. I think it used switchable bypass caps or something, but I can't find it again?
What are your preferences? If VVR, PA, PA+PI, or whole amp?
Thanks as always!
(http://i.imgur.com/4J75ZMc.jpg)
(http://i.imgur.com/LpEJ2NK.jpg)
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Install a Hall VVR , they are cheap and work well , changing the voltage to the PI and the powe amp circuit while leaving the preamp and the normal voltage may work but the amps tone will suffer.
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Don't VVR the preamp, maybe the PI, if you don't VVR the PI use a PPIMV to limit the drive to the power tubes.
Brad :icon_biggrin:
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The Hall VVR reduces the voltage to the entire amp.
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Since you have a vol pot just before the PI I recommend that you scale both PI and the power tubes.
The volume can be used for setting the "right" signal level for the scaled power amp.
I have successfully used this circuit for VVR. At least IRFP450PBF power mosfet can be used in the circuit
but I think most of the power mosfets having enough voltage range can be used as well.
http://1.bp.blogspot.com/-6NVucFSTFl4/T-I2QAOX9CI/AAAAAAAABi4/RMkF33olPzY/s1600/vvr1.jpg (http://1.bp.blogspot.com/-6NVucFSTFl4/T-I2QAOX9CI/AAAAAAAABi4/RMkF33olPzY/s1600/vvr1.jpg)
/Leevi
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Thanks for the replies. Leevi, the schematic you posted is one I've been referencing from here. I'm having trouble wrapping my head around where I would insert, so that it does not effect the preamp? Would I basically be inserting it between the standby switch and the first filter cap, but then have the third filter cap bypass it?
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The Hall VVR reduces the voltage to the entire amp.
Depends how you set up the power supply B+ rail. The easy way is to power scale the whole amp but there's a few ways to use a VVR/Power Scaling circuit and they will sound different.
Since you have a vol pot just before the PI I recommend that you scale both PI and the power tubes.
The volume can be used for setting the "right" signal level for the scaled power amp.
Some people want the distortion from the PI.
Brad :icon_biggrin:
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I'm having trouble wrapping my head around where I would insert, so that it does not effect the preamp? Would I basically be inserting it between the standby switch and the first filter cap, but then have the third filter cap bypass it?
You have to - Y - off (parallel) the B+ for the preamp before the VVR/Power Scaling circuit.
Tubenit has a number of schematics here somewhere that show how to do this. You have to use a single pair of 1N4007 diode to branch/- Y - off for the preamp to isolate it from the VVR/Power Scaling circuit and add some more B+ filtering for the preamp.
Brad :icon_biggrin:
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Did you look in ARCHIVES? Lots info, schematics and layouts.
:think1:
http://www.el34world.com/Forum/index.php?topic=6899.0 (http://www.el34world.com/Forum/index.php?topic=6899.0)
With respect, Tubenit
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No. :laugh: I always forget about the archives, I thought it was in schematics. :BangHead:
Brad :icon_biggrin:
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There's 1 more thing to think about with VVR/Power Scaling (old style) and that is the control pot it self.
Alpha and similar types of pots are only rated for 350v resistive track and 500v to case arch over. PEC 2w military type pots are rated 500v resistive track and 900v to case arch over.
So depending on the B+dcv rail in a given amp you might want to go with the PEC.
Brad :icon_biggrin:
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I'm having trouble wrapping my head around where I would insert, so that it does not effect the preamp?
You can add it after the standby where you split the supply into two circuits, one
for preamp (unregulated) and second one to the PI and power amp (regulated).
You can keep the existing filtering as it is and use it for preamp. Just "copy" A and B after the VVR power supply. But as stated here there are schematics in the archive.
/Leevi
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Thanks all! Leevi, that definitely clears it up for me. I also found the archived thread, and downloaded all the schematics. Here goes nuthin'!