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:

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.


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.


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.


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


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.

43 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.

    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!

      1. Hi Grant,
        Thank you for the great project. Reverb was sucessfully build and I still really like it.
        Do you think it is possible to use two reverb tanks and to switch between them when play? My first reverb tank was MOD 8EB2C1B. Sound was perfect but wanted longer decay, so I bought MOD 9EB3C1B (both have 3 springs). Now Im satisfied (at least for more surf stuff) but sometime miss the sound of shorter one. There was something in the sound which cannot be simulated just by turning the knobs.
        What would you suggest?

        1. Hi Radim, great to hear you are enjoying your Lamington Reverb unit. Re switching between tanks, it could be achieved by using a double pole double throw (DPDT) switch. The middle (wiper) contacts would be connected to C4 for one half of the switch and pin 7 of V1b for the other half. You would then connect the “hot” leads of each reverb tank to the outside contacts of the switch. The switch would then select between the tanks. Hope that helps!

  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.



    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.



        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,

    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,

  7. Hi Grant
    Nice build, I like the idea of using both signals from plate and cathode of the first stage.
    I am doing something similar but with a bigger input impedance reverb tank and a 6DX8 as a driver and recovery.
    I wonder if you have noticed any “phaser effect” as it seems signals are out of phase when they are mixed in the output pot.

    1. Hi Niquel, yes there is a real advantage to using a pentode as a current source to drive the tank. There were no phase issues with the prototype as reverb by nature involves random phase additions and cancellations meaning that it does not matter how the reverb signal is mixed with the dry signal.

  8. Hello, Some one is helping me build it as i have no idea.. but i have a gibbs tank that is 4AA1C1C which input is 8 ohm and output is 500ohm.. are we able to use this tank?

    many thanks

    1. Hi Wayne, the 4AA1C1C tank is a low impedance tank with those specs. My design requires a 600-800 ohm input impedance and approximately a 2275 ohm output impedance.

      1. Thanks so much for clarification. Any suggestions what we can build with this tank any links to plans that would suit please? Thanks so much for helping.

  9. Hi Grant, Fender reverb circuits generally couple the drive tube to the tank with a transformer. I’m wondering why you chose a capacitor instead?


    1. Hi Larry, the rationale to use a directly driven high impedance tank with a pentode is discussed in the project description.

  10. Hello Grant. I built this and it works great. However there is a buzz which increases when I turn up the volume with the reverb engaged. There is no buzz when just the dry signal passes through. What would cause this? Thank you.

    1. Hi Issac, a couple of things you might look at. If the noise decreases when the tank is moved, you will need to re-orient it. Also, make sure that the junction of the two valve heaters is grounded.

  11. Hi Grant,

    In the photo there appears to be a resistor attached to the heater lead. Should I be adding this to my circuit and at which rating?


    1. Hi Lorin,
      You have a good eye! The prototype did use a resistor in the heater circuit to drop a couple of volts as I used a 14V plugpack. If you use a 12V plugpack, no resistor is required.

  12. Hi Grant,
    Have recently built the Lamington III amp. It sounds awesome.
    I’m wondering if it’s possible to build the Lamington reverb with the power supply coming from the Lamington III instead of using a plugback.

    1. Hi Adrian,
      Great to hear your Lamington III build went well.
      It is not possible to power a Lamington Reverb circuit from the Lamington III power supply as there is not extra power available from the Lamington III supply. Regards, Grant

  13. Hi Grant. I’ve had a crazy idea, but not sure if it would work so wanted to run it past you to see if I’m nuts or not.
    I’d really love to build this reverb circuit into the 15W Lamington, and been trying to figure a way to do it. Assuming the power supply was beefed up to handle the extra load, could you slip this in at the beginning of the Lamington preamp after V1a; and drop the preamp valve (V1a) from the reverb circuit? Basically replace V1a in the Lamington with the entire reverb circuit? The only difference I can see between the two circuits is the 68K grid stopper resistors(?).

    Link to a schematic with my idea:

    Note: there’s a tremolo on there too, still working on that guy…

    Thanks Grant.

    1. Hi Graeme,
      My thoughts with any integration of reverb in an amp are that care needs to be taken with not only blending dry and wet signals but taking into consideration the different signal levels and impedances required by the tank drive and return circuits as they integrate with the preamp stages. The Lamington Reverb was designed as a stand alone reverb unit and that application requires quite different design to a built in reverb. A quick look at your circuit shows that the use of V1a as the reverb preamp means that when reverb is switched in, the dry signal only has two rather than the three gain stages required for the Lamington.
      If you want to see examples of commercial amps that integrate tanks into a preamp, look at the Fender Deluxe Reverb circuit.

  14. Thanks Grant. I had a feeling that may be the case, so I have another drawing that switches the whole reverb circuit after V1a:

    However, sounds like there’s more to it than just squishing the circuit in verbatim, looking at this new drawing am I correct in saying the signal will be amplified from V1a (which the reverb is not designed for) and adding the circuit in will change signal path impedances in those stages too? Sounds dangerous way down there in the first stages.
    Thanks for your suggestion of looking at the Princeton Reverb. That design was the basis for a potential “option 3” I was cooking up; injecting the circuit between the last preamp stage and the phase inverter. My concern with this approach was that the Lamington reverb as designed to amplify tiny line-level signals, at this end of the amp it might not be strong enough to make much of a difference.

    I have a couple of 6BX6 valves and a 800 ohm tank which is why I wanted to try your design in the 15W Lamington, would be sweet! Do you think with a bit of perseverance (trial and error) it could work, or do you think it’s too hard (full disclosure, I’m a beginner – but you probably guessed that already 😀 )?

    1. Hi Graeme,
      Yes, integrating a reverb circuit into an amp does involve a range of design issues. My reverb unit was designed to be a stand alone unit and can’t be simply added to an existing preamp. A better option is to rebuild your preamp along the lines of a typical Fender reverb amp. HTH

      1. Thanks Grant, appreciate your help. Taking this all on-board I think what I’ll do is make an amp using some “known-good” plans, but build it in a way to let me add a reverb later once it’s “bedded in” and I know a little bit more about what I’m doing. Really keen on getting a high impedance capacitor coupled reverb working, even have the right tank waiting to go :-).

  15. Hi Grant, thank you for this really cool design!
    I was very curious, so built one and did a direct A/B comparison to a Fender ’63 Reissue Reverb Unit.
    With some small tweaks there is almost no difference, the Lamington sounds even a bit fresher to my ears.
    I made a short video, I hope you like it. If you don’t mind I’ll leave it online:

    1. Hi Andreas, thanks so much for the update on your Lamington Reverb build – it looks good!
      And thanks also for the Youtube clip – it confirms that current drive is preferable to voltage drive with a more extended high end. Cheers!

  16. Hi ,
    I really want to try it 🙂 Thanks for share
    Maybe a stupid question 🙂
    A heater at 12V AC work really ?
    I was thinking the maximum is 6.3V ( tolerance 10% up or Down )
    or DC heater can used but some other part

    1. Hi Seb, in the reverb unit the heater supply is 12V but the two valve heaters are wired in series – 2 X 6V heaters in series to make for a 12V supply. HTH!

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