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 Рthe 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.

19 thoughts on “The Lamington Reverb – a better valve reverb unit”

  1. Hi Grant. My first amp build was a Lamington. No experience, no skills and just a circuit diagram to work from. I’m now very much hoooked on tweaking/fixing/building amps. Thanks (I think).

    Where in the signal chain of the lamington amp would you put the lamington reverb? As an effect prior to the amp? After the first gain stage? Where would you suggest? Cheers.
    Jeff

    1. Hi Jeff, really great to hear you have successfully built a Lamington amp.
      With regards to my Lamington reverb, it is designed to be connected to the front of an amp, as a Fender 6G15 or an effect pedal.
      Regards, Grant

  2. Hi Grant. Have no much experiences with tube amps building (just a few mods) but this looks like very nice project for the beginners.
    Is there any easy way to add footswitch to this circuit?
    Thanks for sharing.
    Regards, Radim

    1. Hi Radim,
      Thanks for your interest in the Lamington Reverb. Yes, it is quite easy to add a footswitch to the circuit. Just add the switch between the reverb return output (ie pin 7 of V1b) and ground. Have fun!

  3. I observe that there is mains isolation through the psu and toroidal step up transformer.
    1. The circuit diagram references ‘chassis earth’ but there is no earth connection on your diagram. Please elaborate.
    2. Would you please explain how safety is maintained (a failed solder joint could expose 300+VDC to the chassis and hence the guitar and player in the absence of any other earth).
    3. Would you also provide details of the source of your 12VAC PSU with an earth pin?
    I am 90% through this build and need to be sure it is safe. Thanks.

    1. Hi, yes my design provides mains isolation via the 12V plugpack. This means that the chassis of the reverb unit is isolated from the mains, but as you say is not earthed. Typically, when used with an amplifier, the signal earth of the reverb is connected to the amplifier earth. Either way, the reverb unit is safely removed from the mains.

  4. Hi, I’ve been looking at your stand alone Reverb. I’ve already built your valve tester.
    Just a query or two.
    1. Why the power pack and then the step up back to 240v, why not just go with 240v supply and a small step down to 12v for the heaters?
    2. I know that the pin outs are different, but the EL84 is more readily available than the 6BX6, any reasons for not using this?

    1. Hi Syd,
      Great to hear you have built my valve tester.
      Re the Lamington Reverb, I assume you mean connecting the unit directly to the mains 240V? If so, that is not a proposition for safety reasons. There would be no isolation between the reverb and the 240V mains meaning that there is a real risk of the mains lead being reversed making the chassis live. The plugpack used in the Lamington Reverb provides full isolation for the circuitry.
      Re the choice of 6BX6 for the tank drive valve, I used it as it has a much reduced heater current requirement than an EL84. There are a heap of valves that can be used in place of the 6BX6 and they are listed in the description. If you get stuck, let me know and I can send you one for the cost of postage. Cheers!

    1. Hi Andrew, an EF86/EF36 is not really suitable for the drive pentode in the Lamington Reverb. They are lower current preamplifier valves, rather than a higher current RF pentode like the EF80/6BX6. If you get stuck finding a 6BX6, email me and I can supply you with one. Cheers, Grant

  5. Hi there,

    Great reverb design. I have built this but I used a tube town toroidal that is providing both the HT voltage as well as heater voltage (6.3V) eliminating the plug pack/step up combo.

    My problem is the reverb has quite a pronounced hum when I blend in the reverb, full dry the hum is gone. I’ve implemented a hum loop block circuit for the signal grounds but that hasn’t helped, as well as elevated the heater tap onto the cathode of the the 6BX6. Also played around with reverb tank position but that hasn’t helped either. It’s a low frequency hum.

    Also chopsticked various signal and power wires to no avail so I’m stumped, any ideas?

    I’m guessing maybe something to do with heater wires, I noticed in your schematic that they are running off of a 12V arrangement where as mine are coming from 6.3V off the toroidal.

    Any ideas/help would be appreciated, this reverb has a lovely tone besides the hum problems.

    Regards,

    Dale.

    1. Hi Dale, it can help if you can confirm if the hum is 50Hz or 100Hz. If it is 50Hz, you have a magnetic hum induction issue. If it is 100Hz, it is power supply related. Cheers, Grant

      1. Using a graphic EQ to target frequencies, a lot of the hum is reduced when I cut at 100Hz but that is the lowest the EQ goes down. I will say that I am using a tank with an output of around 600ohm versus the 2500ohm of the suggested tank so not sure if that is causing issues?

        Losing a lot of general volume when I use the reverb unit versus plugged straight into the amp too but not sure if that is normal behaviour.

        Cheers,

        Dale.

        1. Hi Dale – Hmmmm….hard to say if that is mains hum or power supply hum. The tank is fine with a 600ohm input impedance. There should not be a significant reduction in volume if the reverb is bypassed – I suggest you recheck your wiring – perhaps the hum issue is related to the volume drop? Cheers.

  6. Hello Grant,

    Thank you so much for the schematics this looks awesome. I’m building one right now, and if i didn’t just get a nice little tube amp (without a reverb tank) i’d be building a Lamington amp to go with it.

    I’m wondering what you thought about putting an SPDT switch just where the input comes in creating the option to “true bypass” to just before the output, in the form of a stomp toggle. Obviously I won’t be smashing my biscuit tin version and i never stomp my stomp boxes anyway but yeah, can you think of any reason why this wouldn’t work or would be a bad idea?

    Thank you for your time,
    Jonus

    1. Hi Jonus,
      Thanks for your kind comments. Yes, you can add a true bypass to the reverb, and another option is to add a SPST switch across the reverb tank output – this is how Fender amps switch the reverb in and out. Have fun!

      1. Hello Grant,

        Thank you for your prompt reply. Sorry but I’ve got some noob questions. This is the first audio-related circuit I’ve ever made, having made coil/Gaussian guns in high school from disposable camera flash circuits and other than that only high school physics pracs.

        I assume the ground from capacitor 8 is also going to the chassis, or is it going to signal ground?

        Is the signal ground essentially a line from the input ground and the output ground with all those signal grounds (like from capacitor 3, and resistor 3) connected to that line?

        Thanks again for your time,
        Jonus

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