Marshall 18watt + VVR

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Here one of my latest guitar amps. Marshall 18Watt /w VVR. To be honest I hated the original layout of this amp. Placing the phase splitter tube socket between both first stage preamp tubes doesn’t make sense to me, and can lead to several oscillation problems in the tremolo channel if lead dress is not done properly. This amp produces a very specific sound, turn the volume up and you’ll get really good overdriven tones!!

Some time ago I decided to add a VVR to the amp, so this is a small guide on how I installed a variable voltage regulator, and a couple of things to bare in mind.
Before talking about VVR, a couple more images of the amp and some general specs.

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                                                                                    Ig Präzisions-Wickeltechnik Power & Output Transformers.
Celestion G12H30 Heritage speaker

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                                                          Mullard Blackburn (Labelled Miniwatt) EL84, Mullard Blackburn EZ81
Norm Channel & PI : Mullard Blackburn ECC83
Trem Channel : Tesla ECC83
LCR 32+32uF Capacitor

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Talking back to VVR, I decided to regulate only the Power amp and Phase splitter.It was for a couple of reasons. First of all this amp is quite loud for home use, and installing a VVR seemed the best option to control the overall volume of the amp, while still getting nice overdriven tones and preserving the original taste of the amp.

Secondly, I read such good comments about it, specially when not regulating the preamp tubes, that  couldn’t wait to hear it myself. In addition, I had one spare LCR 32+32uF capacitor laying around, that could be used when splitting the power rail, and some power resistors in various values and wattages to get the voltages I wanted.

I just needed to decide whether to build my own VVR circuit or buy a PCB version of it. To be honest, one could save some bucks if building it oneself, but  then you can get a PCB version, ready to assemble from hallamplification.com  for a fairly reasonable price. The design of the board is really clean, and you get the few components needed including the potentiometer soldered directly to a tiny PCB board.

For those who’d like to build their own board, here goes the schematic taken from the drawings from Conger at 18watt.com

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I needed one extra hole on the amp’s faceplate to install the new potentiometer, so instead I decided to get rid of the standby switch (pretty useless, and even harmful for the ez81, in this and almost all guitar amps around) , and install  the PCB board there. The chassis of the amp is made from aluminum, so bolted the NTE2973 (and its mica washer) directly to it applying a generous amount of thermal paste.

DSC05466Now came the hardest part, splitting the power rail.  I took special care when performing this last step, as I wanted to get roughly the same preamp voltage that was present prior to installing the VVR.
You should bare in mind that unregulated voltage will increase to some extent when you turn down the VVR pot. For that reason I had to choose the values of dropping resistors to find a compromise between the voltages I wanted both at full and lowest VVR settings.

To make a long story short ,this is how the original power rail was, and some initial voltages.

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And now the final power rail, how it was split and some final voltages.

vvrThe sound of the amp after splitting the power rail was fine at full VVR, but it turned out quite harsh and fizzy when the VVR pot was turned down at high volume settings. To fix this issue I increased the value of the EL84 grid stoppers to 82K and installed a couple of 8K2 resistors as grid stoppers for the phase splitter tube.

After all this all I can say is that I strongly recommend adding a VRR to this amp!! Tone is maintained quite well if the VVR pot is not turned too low, and it does really help if you want to get high overdriven tones without blowing up your ears.
Finally, the guts of the amp turned out this way, a little messy buy what the heck…

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~ by fperezroig (22nanofarads.com) on March 14, 2013.

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