Category Archives: Valve Amp Designs

An in depth review of the Lamington amp series

I recently received this in depth review from Colin P who has built just about all of my Lamington amp designs!

It makes an interesting read as Colin has learnt a great deal about valve amps along the journey of building Lamington amps.

Hope you find it interesting as well!

“First Lamington build: 5/15W
As a first build the Lamington 5/15 watt amp is not something you want to be attempting with no help at all, especially if you have no experience with valve amps construction. I tried using only the schematic as a guide, and three knock-down re-builds later I bought the construction manual from Grant – So I can honestly say this is money well spent.

Although the finished amp has a little background hum (because of my sub-optimal tube placement) the result of following the manual’s wiring practices is a great clean sounding amp with loads of headroom and excellent overdrive tones. I opted for the 5/15 switch and it’s loud through a 1×12 cab with a Celestion vintage 30 – even at half volume on 5 watts. Tone control is smooth, and with the gain up high this amp is a screamer that compares very well to some of the big brands at a fraction of the cost.

I built the chassis in to a 9mm marine ply box using GutterGuard for the front grille, added red and green pilot lights to show the 5w and 10w options and added a blue LED strip for some bling.
I also had a local machine shop knock up the chassis out of a sheet of 1.5mm aluminium so it’s not a true Lamington because I didn’t source the chassis from K-Mart – And 150 coats of shellac always looks good on a decent slice of timber – Here’s a photo.:

colin-lamington

Second Lamington build: Lamington II (40w)
I found Grant’s Lamington II schematic on the OZVALVEAMPS web site, and it became the basis of my second tube amp project. For this build I wanted to test out some newly acquired turret board knowledge so I tracked down another legend of Oz valve-amp construction – Lindsay Wallace. His input with regard to power tube, input tube and transformer placement, and his original turret board layout allowed things work out a lot smoother with the build.  I was also fortunate enough to source two of the original 27-0-27 power transformers from Lindsay so the build is all original, with only one power transformer.. Although there was no construction manual for this build I used a lot of the knowledge gained from my first project to build a monster that is grossly over-powered for my small music room. Played through a home-built Eminence Legend 1×12 cab this amp starts to hurt your ears at 3 on the volume dial. It is a good option for gigs, and I have used it successfully at an open mic night in a room with over 300 people.

Initially the amp did not work, attributable to a faulty output transformer which is highly unusual. I also damaged one of the rectifier diodes somehow and was not getting clean power once the OT had been replaced, so sound was a challenge. After replacing the diode all I got was screaming that sounded like feedback.. Any amp builder who knows their stuff could have told me to swap the OT input around to fix the issue, and luckily I knew Lindsay Wallace.

One of the mods I included in the build to make this powerhouse more manageable at lower volumes was a Post-Phase Inverter Master Volume (PPIMV) that works really well. Although I have not compared the sound to how it sounded prior to the mod is still sounds amazing.. I also built the power supply on a PCB using the free version of Eagle for the layout and etching in a plastic tray in my kitchen, and the chassis came from the same source as my earlier amp (Simon at Northlight Engineering in Brookvale is a really good guy).

This chassis was powder coated and built in to a box made of recycled marine ply – so I stained it darker to hide some of the water marks. The (by now customary) bling remained – Hence the red and green glow.. I think it needs some blue LEDs too..

colin-lamington-ii

Third Lamington build: Lamington Junior (2w)
I already had two Lamington amps done, and was looking for something smaller to use in my 3×3 music room when I saw the Lamington Junior design which became the subject of my 3rd Lamington build.

For this project I chose Grant’s point-to-point methodology which is much lower in cost, and it went without saying that I bought the build guide- Just to make sure everything was done right.
For the chassis I used a spare piece of 1.5mm aluminium sheet the same size as an A4 page, with a couple of extra pieces pop-riveted on to make the front and back panels – So it’s not strictly a true Lamington Junior either, but I will take the hit on that one..
Although it is a small amp it is also surprisingly loud, and sounds great through both my 1X12 Eminence and Celestion cabs. Tone control is also very precise, and cranked up this amp sounds fantastic with my Les Paul and my Strat. It’s a bit small for gigs but it’s an excellent practice and studio amp and it’s easy to build – even if you do the metalwork from scratch like I did.
This one has the power switch to drop the output to 1w – and from my experience there is not a whole lot of difference – They are both loud…

After construction and checking of all the wiring, component values etc the amp came up first time with no issues and I can honestly associate that with Grant’s build guide which is very easy to work with.
In case I haven’t said it before: “Be sure to buy the build guide” – It’s money well spent and it takes the hassle factor out of the equation when you are trying to figure out why things are not working, especially if you are new to tube amp construction..

Here is my Lamington Junior – Bling and all…

colin-lamington-jnr
Fourth Lamington build: Lamington Lite (10w)
I knew I was going to enjoy the Junior but I also wanted something a little larger that I could use for small gatherings where a PA was not necessary, so I started a Lamington Lite build just as I was finishing off the Junior. Using the same point-to-point technique as Grant’s other designs this amp also worked off the bat and I have to say that it has become one of my all-time favorites.
It is simple to set up, the big clean tones are magical, and the overdriven sound is way too good for an amp that costs just a couple of hundred dollars to build. I bought a second set of tubes just in case, so if anyone has spare pair of Halogen transformers I want them !!

The chassis is made out of another piece of scrap aluminium with sides riveted on, and the halogen transformers came out of a friend’s roof – So it cost almost nothing to construct, and it’s not much bigger than the junior.

The thing I like most about this amp is the clean headroom which is crystal clear– it needs to be well up on the volume dial before it starts to break up so if you are looking for a great little amp that doesn’t cost a bomb this is the one to go for.

I also built mine in to a marine ply box with a billion coats of shellac for a mirror finish (about standard these days), but with this one I opted for a quilted maple veneer on the front panel instead of the LED bling. There is also a panel on the back but I have allowed for airflow so it does not get too hot inside..

colin-lamington-lite

Fifth Lamington build: Lamington III
My latest build is the Lamington III. This is a little more complex because it has these unusual things called Anode Caps (which I had no clue about) and on the power side I also made a PCB in my kitchen sink.. For this build I also brought the Bright and Presence switches on to the front panel , and of course I bought Grant’s build guide to ensure the point to point wiring was done correctly.

Since I had a spare 27-0-27 transformer I only needed one PT, so space was not much of an issue in the full size chassis that was bent for me by my old mate Simon at Northlight Engineering.

I mounted the output transformer on the underside of the chassis to keep it away from the PT – Not sure whether or not this is bad practice but the amp was working fine and sounding fantastic at both low and high volumes with the right tones dialed in. Speaking about tone it is very responsive on the treble side but mine also has a lot of bass gain that starts to break up when the control is at the 2pm position so I need to check and see if I put an incorrect component value in the tone stack.

When I get round to it the box will be my usual – Marine ply with a million coats of shellac, GutterGuard in front and loads of blue and red bling (which of course is for aesthetics only and totally useless in the sound department).

colin-lamington-iii

For reference here’s a gaggle of Lamingtons  on top of an Eminence Legend 1×12″

colin-lamingtons

The Lamington Reverb – a better valve reverb unit

I have been kicking around the idea of a “Lamington” stand alone spring reverb unit for a while, and recently began a build.
The inspiration for the design came from a Channel Road Amplification tech paper found here: http://www.channelroadamps.com/articles/reverb_driver/

Driving a reverb spring tank presents some challenges. The commonly used driver circuit drives a low impedance tank via a small transformer. While this works, it provides constant voltage drive which is not the best way to drive a spring tank. It results in poor high frequency response due to the inductive nature of the drive coil. A much better way to drive a tank is with a constant current drive circuit. This results in a wide band response from the tank.

Inspired by the Channel Road paper, I looked at building a stand alone reverb unit. Using a high impedance tank and a constant current pentode driver that eliminates the drive transformer, it is possible to build a simpler (and superior) spring reverb unit . In addition, a plugpack power supply similar to the one used in the Lamington Junior amp can be used to simplify the power supply and make it a cheap build.

More details to come!

Some progress – the chassis was prepared with holes for the controls, valves and spring tank. As mentioned, a 12V plugpack supplies power to the valve heaters and a toroidal transformer steps up the 12V AC to generate the HT for the valves.

An Accutronics 8EB2C1B spring tank was at hand and is ideal for this build with an 800 ohm drive impedance. These tanks are readily available as a spare for the Fender Blues Junior amp both here in Australia and O/S.

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Have now completed the Lamington Reverb and pretty happy with it.  Firstly, a photo of the finished reverb unit.
From left to right: input, dwell, mix controls and output.
Also visible is the 6BX6 tank drive valve and a 12AX7 preamp and recovery valve.

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And an underside photo. Not a lot to it – a 12V to 240V toroidal transformer, the reverb tank, and some components clustered around the valve sockets. You can click on the image for more detail.

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Here is the schematic for the Lamington Reverb. You can click on the image for more detail.

the-lamington-reverb

It is a simple circuit, but works well.

The input signal is fed to the first gain stage with the dry signal fed straight to the MIX control from the cathode. This first stage drives the current source pentode via the DWELL control. This determines how hard the tank is driven and changes the reverb tone as a result. As discussed, the tank drive circuit is taken from the Channel Road paper and provides current drive directly to the tank eliminating a drive transformer. The tank used in the prototype is an Accutronics 8EB2C1B which is used in the Fender Blues Jnr and is readily available here in Australia and overseas. Its drive coil impedance is 800ohm and is ideal for the constant current driver which is a commonly available 6BX6 (EF80). The output of the spring tank is fed to the other half of the 12AX7 input triode and the output of this valve is fed to the mix control and output socket.

The power supply is a simple arrangement using a 12V 1A plugpack which feeds the heaters and a second transformer which steps up the 12V to a high voltage for the valve HT.

Some construction details to come.

Some comments on constructing the Lamington Reverb.

In building a spring reverb unit, it is very easy to couple unwanted mains magnetic fields into the tank pickup coil causing hum in the finished unit. My approach to have a hum free unit was to use a toroidal step up transformer which has less external magnetic field than an ordinary EI lamination transformer. Also, I positioned the tank with the pickup coil at the other end of the chassis from the transformer. In addition, I found if I inverted the tank further hum reduction occurred.
If you are using an EI step up transformer, I suggest waiting before mounting it. With some extension leads on the transformer, move it around to find the position that induces the least amount of hum and mount it there.
You may note that I have used fairly large values of HT filter caps for the prototype. This was necessary to eliminate any HT hum from being introduced into the unit.Re suitable valves for the Lamington Reverb, the 12AX7 is readily available. The 6BX6 is no longer manufactured, but was used by the squillions in the day in B/W TV sets. It can be substituted with about any RF pentode such as the 6AU6, 6EJ7, 6ET6, 6CB6, EF94 etc
The Accutronics 8EB2C1B reverb tank is readily available from Evatco here in Australia or from Ebay. You may wish to use a longer tank which would work fine in this unit as long as it has a high impedance (600 or 800ohm) drive coil.Re the transformers, the plugpack is a 240V to 12V1A AC unit. The stepup transformer I used is a 10VA 240V to 12V toroidal transformer wired backwards. Just about any low power 240V to 12V transformer could be used – just check its location with respect to the reverb tank as mentioned to minimise hum.

So overall, I have been very happy with the Lamington Reverb – it sounds very lush with no unwanted noise and hum. The ability to vary the drive to the tank with the dwell control is an added bonus to change the character of the reverb tone.

A 6CM5 hi-fi amplifier

I regularly receive emails with questions about a Hi-Fi valve amplifier design I developed many years ago. The design has been posted on a few websites and I thought it would be a good idea to re-post it here and to add some updated information.6CM5 amp3

Here is the schematic for the amplifier – click on it for full size.6CM5 Hi-fi Schematic

And here is the text that accompanied the original design:

“This is a design for my 15W/channel ultralinear hi-fi stereo amplifier. The front end of the circuit is conventional. The Mullard 5-10 amplifier uses the same phase splitter as I have, and the text refers to it as “a cathode-coupled phase splitter”. The output stage initially used 6L6 valves with a supply of 300V as again I had some on hand, but after quite a bit of experimentation with other output valves I landed on 6CM5 valves. 

As is well known, they are an odd valve to be used for audio applications, and I had several unsuccessful attempts to use them. I thought initially that I’d feed the screens with 1/2 B+ as that is how data sheets suggest you should do (and incidentally how similar line output valves such as 6DQ6 are configured in old guitar amplifiers). However, any attempt to use them in this way caused fairly violent oscillation at ultrasonic frequencies. In addition, I wanted to use them in ultralinear configuration and tying the screens at 1/2 supply did not permit this. The ratings indicate a maximum screen supply of 200v and so I hesitated to use the ultralinear mode. However, after trying several configurations which either oscillated, distorted or otherwise misbehaved, I tried them in ultralinear mode with a fairly high amount of bias (-50v), and they worked really well. They were by far the most linear of any valve I tried and worked well with a fairly low standing current (approx 25ma each) and put out the maximum power (17w). This seemed to justify their rather high heater power requirement. So it seems that it is fine to run these valves with higher screen voltages.

The power supply used an old Philips valve power transformer which was up to the fairly high heater current load (4x 1.25A + 3x .3A = 5.9A) and had a ht winding of 110v which applied to a voltage doubler provided 300v HT. I decided to use fixed bias as it allows higher plate to cathode voltage for the output valves than cathode bias. The technique of getting a negative bias voltage from a voltage doubler is an old trick from guitar amplifier designs. I added a 10 ohm resistor in each 6CM5 cathode earth return to monitor cathode current and act as a fuse under overload conditions. Note that the circuit doesn’t have any provision for individual adjustment of output valve standing currents. This was because I used 4 Radiotron 6CM5’s from the same batch with very similar characteristics. A couple of other 6CM5’s that I tried varied a bit in standing current. It may be helpful to modify the bias supply with 4 x 50K trimpots in parallel and then in series with an 82kOhm resistor to ground across the bias supply in place of the one 100kOhm trimpot. With each trimpot wiper independently feeding each output valve this arrangement could then provide individual bias control over each valve. If the bias voltage needs to be increased, the value of the 220nF capacitor feeding the bias rectifier can be increased.

Construction was fairly easy as I took a leaf out of many current hi-fi amp designs. I simply made up a square plinth of timber which has a 12 by 12 inch aluminium sheet (from Dick Smith or Jaycar) fixed to the top. Easy to make and it makes a great furniture piece. I checked the frequency response figures – the amp has a wide frequency response with sustained bass and sounded very nice indeed – even my wife immediately noticed a dramatic difference in sound to our existing solid-state hi-fi amplifier. The S/N ratio for this amp was outstanding – maybe the C-core power transformer, or the star earthing, or the provision of a centre-tap on the heater winding, or my 100µF overkill filter capacitors on the small signal HT? The noise and hum are just about inaudible listening directly to the drivers of my quite sensitive speakers – certainly better than other amplifiers I have used over the years, and pretty amazing for a valve amp! Overall, a lot of fun to build, and a most satisfactory result.

Here is an underside image of the original design:Construction2

Since I developed this amplifier, I have received many requests for an alternative power supply for the amp. The original used an old TV power transformer which is not available any more and so I looked at an alternative power supply using similar voltage multipliers to my Lamington designs. Here is a schematic for an updated version of the power supply adapted to use currently available transformers here in Australia. Click on it for full size.

6CM5 Hi-fi PS

 

A new 2W low power amp design

I have just finished prototyping a new 2W Valve Heaven amp design called the “Lamington Junior”. I’m pretty pleased with it, as the design has been a long time in development. The design brief was for a low power amp that delivered a significant amount of clean, “chimey” headroom and also a great overdrive tone which is what it delivers! What is especially cool is that it runs from a 12V AC plugpack, so a newcomer to amp building can do so with no contact at all with the 240V mains making it a safer amp to build.

I am currently building a second prototype which will be easier to construct. I’ll soon have the design details including schematics available here at valveheaven.com. Soon to come will be construction manuals and kits for prospective builders.

Watch this space 😉

***UPDATE*** 13/1/2015  

Well, as promised, I have now posted the full design details and schematic diagram for the Lamington Junior here. I am very pleased with this amp, and it has now become a favourite here to play through at home, and I feel sure it will become as successful as its big brother, the Lamington 🙂

I will be making full kits and a construction manual for this amplifier available early Feb 2015 for $195 plus post for the DIY kit and $49 plus post for the DIY manual. Kits and the construction manual are available for pre-order by emailing me at grant@valveheaven.com

***UPDATE*** 16/2/15

Several Lamington Junior kits have now been purchased with the builders very happy with their new amp. I just received this email from a customer who had never constructed an electronic project before:

Hi Grant,  I finally got it working (1k8 was in the wrong spot), all good. Thanks for your help, I will play with this one for a while and then i will try the Lamington 15w. Since I have no previous electronic experience I found the whole kit very easy to follow as a beginner. Thanks again!