Power scaling on 18w TMB Question

[Governator]

Hi, just finished an 18w TMB build with plexi style pre amp. I’ve also installed a VVR which power scales the power amp and p.i only, pre-amp is fixed/unscaled (although I did need to drop the pre-amp volts via a 18K dropping resistor to match the voltage it had before I installed the VVR - around 260v) it all seems to works great.

Just one question - when checking my voltages, I noticed that as I decrease the b+ via VVR, the pre amp voltage increases (i suspect from the decreased loading on the power amp) from 260v up to 278v with the VVR all the way down. Is this ok? Just wanted to check before i take it off my bench and box it up!

[Stevem]

Yes, your right , that's a normal accurence as the current demands on the outout stage go down.

I guess one way to combat the gain and slight tone difference you will get out of the preamp section would be to add another power supply dropping resistor in series straddled by a switch so you can take it out of the circuit as needed.

Now that I think about it a 4 tap rotary switch would let you install 3 different value resistors and have 3 different levels of preamp attenuation to play with as you Crank down the output stage.

[Guy77]

Hi Governator. Just curious, who makes the VVR you are using or did you build it yourself from schematics on the web.
Are you using a large heat sink to keep the chip cool?

Cheers!
Guy

[Governator]

Thanks for the replies, I built the circuit myself on some tag board, based on below diagram, I believe Trinity use the same circuitry.

A rotary switch would work, or some sort of voltage regulator on the pa supply, but then again the only time the VVR is is used is at home as courtesy for my neighbours and my ears so probably is ok as is. I’m just not too sure what effect the higher pa voltage has on the tone.

[Guy77]

Thanks for the replies, I built the circuit myself on some tag board, based on below diagram, I believe Trinity use the same circuitry.

A rotary switch would work, or some sort of voltage regulator on the pa supply, but then again the only time the VVR is is used is at home as courtesy for my neighbours and my ears so probably is ok as is. I’m just not too sure what effect the higher pa voltage has on the tone.

Great work! I have built that one in the past too on several amps. I installed it once in a 50 watt amp but the components were getting too hot and the mosfet I was using overheated and so I removed it. I should have had a heat sink on it. Congrats on our diy VVR !

Cheers

Guy

[Masco]

I know Kevin O'connor has a VCK Voltage Clamping Kit, sounds like exactly what you need. The kit has more components than your VVR though.