Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on April 02, 2019, 05:33:11 am
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Hi guys, I haven't tinkered with the VVR for awhile.
I have a PT when rectified should be around 360v.
I want to use a couple of EL91 in push pull, these like about 260v on the plates, so I need to drop 100v.
I know the VVR will do this.
Can I just drop the voltages to the plates and screen.
(https://i.imgur.com/sfD541x.jpg)
Nodes C-D-E are PI and preamp.
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That'll work.
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One of the amps I built had 6AQ5's in it & I did that very thing dropping the voltage on those (& maybe the LTPI?) but kept the preamp volts up higher. I always played the amp that way & IIRC, I think I used a chicken head knob and screwed a small screw in the chassis (to block the knob from turning too far) to make sure I couldn't put too many volts on the 6AQ5 plates.
With respect, Tubenit
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I think I used a chicken head knob and screwed a small screw in the chassis (to block the knob from turning too far) to make sure I couldn't put too many volts on the 6AQ5 plates.
There's also a simple high tech solution. Just put a resistor on the top side of the pot. Notice that Timbo has a resistor on the bottom of the pot that prevents being able to adjust the voltage down to zero. Putting a similar resistor on top of the pot will prevent being able to adjust the voltage up to full B+.
For example, you are feeding a VVR with 400V but you don't want the output to be any more than 200V. (Nice round numbers so I can do the math in my head) Use a 1M pot with a 1M resistor on top. Now the pot can only adjust to 1/2 the input voltage, ie, 200V.
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So school me guys because I'm confused. Is there a technical reason Like noise or something that the the voltage on the B+ rails can't connect to the preamp section sooner and then get knocked down by resistors at a different point in the rails after that for the output section.
That's exactly what Timbo is doing. You just need to provide adequate filtering because the ripple is higher the closer you get to the rectifier.
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Ok. so it could be done with passive components just like we normally do but power the preamp section first and be fine also as long as the filtering was accomplished then power the output tubes after reducing the voltage. Just the opposite order from what I normally see on schematics.
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Nothing is normal once you throw a VVR circuit into the mix. :l2:
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Thanks guys :icon_biggrin: