Fender Twin with 12AT7 in the preamp

Day 3 with the Twin Reverb and I managed to get a couple of hours to explore further. I tried swapping the 12AX7 tube in V2 (the Vibrato channel) for a 12AT7. It’s a fairly subtle change. It means that the preamp doesn’t slip into overdrive as easily. I’ll leave it in for now but no doubt I’ll be going back to the 12AX7 before too long.

I also got chance to experiment with my collection of pedals. As many of others have already documented, the Twin Reverb is a great amp with pedals. I had most fun with this stripped down setup. Perfect for rockabilly twang. The gem of these is Alfie’s Blue Alpaca. It just makes the amp sing.

twin60

Right to left they are: Behringer DC9 Compressor, Nocturne Brain Seltzer, Alfalfasprout Custom Shop Blue Alpaca (inspired by the Way Huge Red Llama) and finally a DigiTech DigiDelay for a bit of slapback.

All of this was with my Telecaster. I’m really looking forward to 12th Jan when I’ll have my Gretsch 6120DSV back home.

A DIY valve overdrive pedal – Goldie

In an amongst the guitar building I decided to break out the soldering iron to build an overdrive pedal – as light relief. I had stumbled across the “Valvecaster” schematic and layout at Beavis Audio.

This is a very simple circuit that uses a 12AU7 valve, running at a low voltage. Because it is running at such low voltage it is very easy to overdrive.

The schematic specifies an operating voltage of 9v but I decided to bump it up to 12v to give a fraction more headroom. I chose one from the tangle of old wall-warts I have tucked away in a drawer. It could use a battery but I suspect that the current draw for heating the valve would suck a PP9 dry in a matter of minutes.

If you search for “valvecaster” at Youtube, you’ll find plenty of examples of the pedal in action. One of the common comments is that this is a naturally “dark” sounding pedal. This was not exactly what I was after and so I was going to experiment with different types of tone stacks. As laid out in the schematic it includes a simple treble cut tone control. Even turned up full this would only make the pedal darker, allowing treble frequencies to escape to earth. I decided, initially, to build it with no EQ, and then add it later. One of the results of this was that it is not a dark pedal at all. It is wonderfully balanced and punchy just as it is, and it’ll not be getting any additional EQ added. To remove the tone control I just eliminated the 10nF capacitor (C2) and the A100k potentiometer (VR2).

I reused an old enclosure I had laying around, which I shot with a coat of black nitro-cellulose and a very light top coat of gold (both left over from my Shaftesbury restoration). The gold coat is thin enough to allow just a hint of the black to show through.

The chemistry geeks amongst you may, by now, have worked out where the inspiration for the name and colour came from.

I have yet to print up and apply the decals for the pedal but there’s the standard 1/4″ input and output jacks round the back. The top has the valve, a true-bypass footswitch and the power toggle switch. On the front, left to right, are the gain, a dummy pot (filling up a surplus hole in the old enclosure) and the output volume.

When I was buying the parts for this I also got hold of a 12AT7 valve. This works well too. It is more subtle and has less gain, but in some ways is all the better for it; smoother, warmer and just a bit less wild.

You can listen to a quick demo of Goldie with the 12AU7, that I recorded for my friend Alfie Lanos, who was really helpful in helping me plan this one out.

It was recorded on my mobile phone so the sound quality is not the best, but gives you an idea of what the pedal does.

Demo of the amp with VVR installed

Just recorded a short-ish snippet on a digital camera, using the in-built mic, while I was testing the VVR installation. Not one for the audiophiles!

It’s the Benford Lestercaster straight into the amp. The amp’s plugged into the 12″ speaker in my Roland Cube 100 combo. The volume is fairly low and, if you listen carefully you can hear the TV from the next room.

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]

Installing new Eminence speakers

My initial choice of speaker for my combo, the Celestion G10D-25 turned out to be fairly disappointing. They were great for the quieter clean tones, and were fine when the amp was turned up beyond 8, but were very farty and flappy in the zone in between. Unfortunately this coincided with reasonable home practice volume with just the slight crunch I was after. After a search for alternatives and advice on Twitter from Steve Busby of BBZGuitars I settled on a pair of Eminence Legend 105’s. These are an American made 75 watt, 10″ speaker that would fit right in as replacements.

After a bit of searching round I found that the best price available was from Blue Aran. If you are looking for speaker or PA hardware then I’d recommend that you check them out. They have a great range of products and, certainly for the items I investigated, about the best value available in the UK. As promised these fitted straight into the combo without a hitch.

I was going to use a length of fairly high spec hi-fi speaker cable that I had available. With two speakers you have two potential wiring options; either in series or parallel.

To calculate the resulting overall impedance you use the following formulae:

  • When wired in series,
    OverallImpedance = ImpedanceOfSpeaker1 + ImpedanceOfSpeaker2;
  • When wired in parallel,
    OverallImpedance = 1/((1/ImpedanceOfSpeaker1)+(1/ImpedanceOfSpeaker2));

The amp has 8 ohm and 16 ohm outputs. The speakers are each 8 ohm and were replacing the two Celestion G10D-25 speakers which were 16 ohm each. Two 16 ohm speakers were wired in parallel and were therefore 8 ohm when combined. The two new 8 ohm speakers would need to be wired in series to give 16 ohm.

I didn’t have any examples of series wiring to follow and nor could I find any advice on the internet. I found plenty of schematics, but no suggestion for the neatest way to actually wire them up, so I put a little thought into it myself and this is what I came up with.


click to enlarge

The cores of the cable run side-by-side together from the jack plug. As they pass the first speaker, one of the cables is cut and the end nearest the jack plug is soldered to the negative terminal. Thother side of the cut is soldered to the +ve terminal. Then on to the second speaker, where the cable from the +ve terminal of the first speaker is soldered to the -ve terminal of the second speaker. The uncut side of the cable is soldered to the +ve terminal of the second speaker and runs straight back to the jack plug. This creates the circuit with the speakers wired in series, with a minimal length of cable.

Note: The polarity of the whole thing is not relevant. The main thing to make sure is that the connection between the speakers goes from the +ve of one to the -ve of the other. If you don’t then the speakers will be wired out of phase (i.e. one will be moving forward at the same time as the other is moving back) which will result in a very thin sound, because the speakers are trying to cancel each other out.

Building my amp kit step-by-step

So the day has finally arrived… time to build me a valve guitar amplifier. I’d bought a combo cabinet from Chris (see earlier posts) and the kit had arrived from Ampmaker. Now I’m not lucky enough to have a workshop so it was going to be happening on the dining room table. Since dinner was due to be at 19:00 I had a deadline ahead. After a quick coffee and breakfast I got cracking at 09:30.

Kit

Here’s the kit. It was packed better than this. I removed most of the bubble wrap during the initial check.

Step 1 – Check the contents

I went through every single component and checked it against the kit contents list. This included using a multimeter to test the value of all the resistors. There was one omission (more of which later).

Step 2 – Gather all the tools and materials and prepare the work area

I’d previously made a list of all the tools I’d need for this job. So I wandered round the various rooms in the house and sheds gathering everything together and then prepared and laid out the work area.

Tools

Step 3 – Fitting parts to chassis

Not much to say other than I followed the instructions and fitted all the parts to the chassis, trimmed the volume pot posts and screwed on the chicken-head knobs. If I was doing this again, I’d trim the pot posts before mounting them into the chassis.

Chassis

Chassis

Chassis

Step 4 – Label all components

This was an additional step that was recommended by a friend, Ian Chadwick, who is also working on an Ampmaker kit. I stuck a piece of masking tape on to each component and wrote the value, and reference number (relating back to the instructions) and laid them out in order. It was amazing the difference this step made to the build. I was able to build up a really nice rhythm when soldering instead of having to go rooting around for each piece when I needed it.

Labelled components

Step 5 – Mount components

Working left to right across the turret board I would dry mount each section, check the measurements, trim the component leads and then solder them in place. After each small section I would go back across and use the multimeter to test every solder joint.

Turret board

Turret board

Step 6 – Wire up board

Kept following Ian’s advice and labeled each of the 21 turret board wires with where it was going to (i.e. VR2-3 meaning 3rd lug of variable resistor 2). Now if labeling the components had simplified the mounting on the turret board, with the wiring it was absolutely vital because when mounted into the chassis you can’t see exactly where the wire comes from (they’re soldered to the underside).

Wired board

Step 7 – Chassis mounts

The items missing from the kit were the spacers to hold the turret board away from the chassis when it is mounted. This point neatly coincided with lunch so I grabbed a quick sandwich and headed off for B&Q. I bought six M3x25mm machine screws and twelve extra M3 nuts for them and improvised the following;

Mounting

Step 8 – Wiring it all up

This was probably the most time consuming aspect of the build. You mount the chassis into the kit and then slowly an methodically work your way through the detailed steps outlined in the kit instructions. Once the turret board is mounted, it can get a little tight getting in to solder in some areas of the chassis, such as the earth lug, near the input socket. But I took my time and aside from a couple of unfortunate soldering iron/finger interfaces I emerged relatively unscathed.

Valve sockets

Wired and done

Step 9 – Testing

There are four main stages of testing the amp. Firstly working across the chassis testing some of the key safety items, such as making sure all of the relevant parts are connected to earth. Secondly is to power it up without the valves and check some key voltages across the amp, and then again with the valves installed. The last step was the scariest and best bit – plugging in a guitar and speaker and hearing the first noise come out. Oh yes – my baby worked first time!

Step 10 – Mount in enclosure

The amp neatly mounted into the cab supplied by Chris and I tidied up the speaker wiring. The original messy cabling had been put in quickly to test the speakers after buying them on eBay and needed stripping out and redoing.

Speaker wiring

Step 11 – Crack out the Telecaster and play!

Finished amp

Lessons learned

  • Labeling all of the parts before starting seemed a bit anal, but saved so much time and confusion and, at the point where you’ve got a soldering iron in hand, just allows to to concentrate on what you’re doing.
  • Test after EVERY step and don’t move on until it works.
  • Take the time to work methodically.
  • Get a highlighter pen and work through the instructions word by word and mark it through only when you’ve tested that the step has been finished.
  • I almost got the polarity wrong when soldering in a couple of the capacitors. It would have been better if I had used the highlighter pen to mark all of the components where polarity mattered (some capacitors and both of the diodes).

Initial impressions

  • Great dynamics and very responsive to the volume of guitar.
  • The sound has lovely character and warmth and yet, especially with the Telecaster, you can get a superb razor sharp clean tone.
  • Breaks up very controllably and still keeps definition in each note even with the gain wound up to 10.
  • Usable tone controls right through their range. On most amps I’ve ever owned, if you get to any of the extremes of the treble, mid, bass, it sounds awful and the “sweet” spot is very narrow. With the SE-5A, the tone controls give very usable sounds at every extreme.

When I get my music room back I’ll record and post some sound samples. In the meantime my (long-suffering) wife seems to be under the misapprehension that it is a guest room for our current visitor.

And before you ask, yes the table was set for dinner at 7:00pm, but only just! All in, excluding tea breaks, lunch and the trip to B&Q, it took about 8.5 hours.