Installing Variable Voltage Regulator (VVR) in a valve amp

As I mentioned in an earlier post, just for noodling about at home without disturbing my family and neighbours, the 5 watt SE-5a valve amp is just a bit loud.

This is not a problem when I’m playing clean tones, the SE-5a is just a perfect amp for that. It took a little experimentation and I found that at low levels on the master volume the sound was a bit thin and lacking in warmth, even on a neck humbucker, if you whack the master volume right up and control the level at the guitar, you can get a fantastic tone even at low volumes.

The problem comes when you’re after that crunch – just as the power valves are starting to lose the battle with what you’re asking them to do – is the sound I love and the SE-5a delivers this in spades. Trouble is to get that sweet spot you need to gain and the master volume both up around 7 or 8, with the guitar volume up full. This is just too loud for “normal” home use.

I investigated a number of options for solving this:

  • an attenuator which sits between amp and speaker and soaks up the power;
  • switching to a less powerful output valve like the 6n1p which will deliver around 1 watt, or even using half the valve to give around 0.5 watt;
  • variable voltage regulation (aka VVR) which reduces the DC voltage powering the valves, allowing them to overload at lower volume.

I know a lot of guitarists like the sound when the speakers too are right at the limit of their capabilities. Obviously none of the solutions I talked about above would solve this. In my circumstances it is not an issue because, with two 75 watt Eminence Legends, the amp, even at full whack, never comes close to pushing the speakers.

After a lot of research, and advice from Barry at ampmaker.com, I decided to go with the last of these options and Barry recommended I contact Dana Hall at hallamplification.com.

I ordered a VVR kit from Dana and, because the SE-5a is a cathode biased amp, I was able to buy the simple (and cheaper) version.

Fitting the kit is relatively simple, but I did put a lot of effort into looking at the schematic of the SE-5a and the schematics and instructions supplied by Dana beforehand, and planned out in detail exactly what I was going to do.

Here are the key install decisions I made:

  • I was going to apply the VVR to the whole amp. It is possible to apply it just to the power valves, leaving the pre-amp untouched. There are pros and cons of each approach and not being sure which would suit me best I chose the easier install;
  • I wasn’t going to mount the MOSFET on to the VVR PCB, because it would limit the placement options. The MOSFET can get warm and needs a good contact to the chassis, or a heat sink, to dissipate this. By running three wires from PCB to MOSFET it gave me a wider choice about where to put it;
  • I was going to install the VVR in place of the master volume. As I mentioned above, the best sound is when it is turned up full anyway. Once I can use the VVR to control the overall amp volume, the master volume is essentially obsolete. I did think about drilling a new hole in the front panel or getting a combined on/off/standby switch to replace the two current switches, leaving a spare hole for the VVR, but again, in the pursuit of simplicity, decided against either of these options (which would also have involved re-labelling the front panel too).

The install steps were:

  • Prepped my work area and grabbed all the relevant tools and prints of all the instructions, schematics, sketches and my original build log;
  • Took the amp out of the combo enclosure and tested the voltage across all of the key exposed parts to check I wasn’t about to kill myself;
  • Soldered five wires on to the board. Three for the MOSFET, one for the input to the VVR and one for the ground connection, then soldered the MOSFET to the three wires, then quickly unsoldered the MOSFET, turned it round and put it on the right way (NB: the picture below is the wrong way round). I kept to the red and black standard colours for the input and ground, but must have had a reggae themed mindset as I went for red, gold and green for the MOSFET;

    VVR

  • I took the master volume control out of the front panel, turned it up to max, and cable tied it into a small plastic bag. Long term the pot will get removed/bypassed but I didn’t want to do this until I had the VVR working, and the plastic bag was to stop it shorting out on anything whilst it wasn’t fastened down;

    VVR

  • Desoldered the output from the standby switch and attached this wire to the output of the VVR;
  • Attached the new wire from the VVR input to the recently vacated solder tag on the standby switch;
  • Soldered the VVR’s ground wire to the ground tag right next to the master volume;
  • There’s an unused 3mm hole right next to the power transformer which coincidentally matches the 3mm mounting hole in the MOSFET, so I bolted it down to this making sure that the thin plastic insulator sits between MOSFET and chassis;
  • Mounted the VVR pot and PCB into the vacated master volume hole;

    VVR

  • Turned the VVR up to full and, with the valves out, went through the test measurements from the original SE-5a build guide and compared the new readings to my original record;
  • Turned the VVR down to min to observe the reduction in voltages across the big capacitors and… bollocks… they were exactly the same as when it was turned up to max. Back to the drawing board…
  • With my updated sketch of the schematic, I used a highlighter pen to trace the actual connections against the logical picture of the schematic (i.e. comparing how it was wired with how it should be wired);
  • I quickly discovered that I’d taken the wrong wire off the standby switch for the input to the VVR. It took a matter of seconds to switch them over and retesting showed it was all working as I’d expected;
  • Valves back in, another quick test to see nothing had changed and plug the guitar in.
  • I played for about an hour and kept checking to see that the MOSFET wasn’t overheating

So how does it work? To be frank I’m astounded by how successful it is. I can now get exactly the right tone and level of crunch that I’m after, using the gain/TMB controls and then with the VVR, pick the volume I want. The tone stays remarkably consistent from 10 down to about 3. Any lower than 3 and, whilst the sound itself stays true, you can start to hear the unamplified guitar (particularly in the treble register) over the amp itself. I’d read that a couple of people had a noise problem after installing the VVR, but I had no such issue, and the amp remains very quiet and hum free.

As I lay in bed, after completing this I couldn’t help but wonder why all valve amps don’t have this. It really is so simple and so effective I can’t, as yet, see a downside.

I left two jobs outstanding which finished up over the weekend:

  1. Removed the master volume pot, which was as simple as taking the wire that came from the centre lug of the master volume and attaching it to the centre lug of the treble pot, and snipping the other wires;
  2. Some slight changes to the pre-amp circuit because at low levels on the VVR it pushes a little bit of DC back through to the guitar which makes the guitar’s volume control slightly scratchy. It takes a couple of extra capacitors and an extra 1M resistor to sort this out. Dana recommended 0.1uF and this works fine for me. So the last steps to finish the job off are;
    • Desolder resistor and centre of shielded cable from lug 1 of input jack
    • Solder 0.1uF capacitor to lug 1 of input jack
    • Solder resistor from lug 4 and centre of shielded cable to other end of capacitor
    • Insert 0.1uF capacitor between lug 2 of the gain pot and the existing wire from lug 2, and add 1M resistor between this wire and lug 3 of gain.

    Here’s a before/after sketch, which I scribbled down, to plan out what I was going to do.

    Pre amp changes
    [click to enlarge]

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4 thoughts on “Installing Variable Voltage Regulator (VVR) in a valve amp

  1. I have studied your wiring for a long time and it looks as if the red and green wires to the MOSFET should be the other way around. The connections on the following circuit diagram seem to show gate and source the other way around:

    http://yeomansinstruments.blogspot.co.uk/2008/07/vvr-greatest-thing-ever.html

    We wired up a VVR with the connections differently from yours and it worked.
    Are you sure your connections are correct, or have I missed something?

    Robert

    • Hey Rob, Thanks for posting. You’re both right and wrong! Yes they were wired the wrong way round, and no, you didn’t read it properly! Did you see the bit of the picture description where I said “NB: the picture below is the wrong way round”?

  2. Hi, I am looking to do much the same with a 5W Champ build, and this is one of the better installation examples I’ve seen. I was just curious, since it’s been 2 years since you posted this, are you still just as pleased with it now as you were when you wrote this? Also, did the extra mods you had to do to the pre-amp make any difference to the sound of the amp when using it on “full”? Thanks.

    • Thanks for the feedback, and yes, I’m still absolutely delighted with the modifications. When turned up full there is no audible difference and that is supported by my measurements. The before and after B+ voltages are identical.

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