Selmer Truvoice Zodiac Twin Thirty "Croc-Skin"

As purchased with non-original grillcloth
Non-original grillcloth replaced with Speaker Grills

amp side view
amp top view
amp top view

Last Update 11-08-2009

I used to own a Boogie MKIIa – it was great at what it does but pretty much a one trick pony. I sold the boogie when the blue grilled Vox Valvetronix AD60VTH modelling head was more to my liking, especially with the 2x12 celestion alnico loaded Vox cabinet that I bought from Northcoast Music.  Years later I found this slightly battered Selmer Truvoice Zodiac Twin Thirty "croc-skin" combo amp in a local store.  They had no idea what it was.
To be fair I had no idea what it was either, and it didn't help that it was missing the original front panel.  This is what it was SUPPOSED to look like:

stock amp front
stock amp side
Damn, I'm missing that way-cool german gothic badge letters.  Note the difference in the letters on the image on the right - I'd LOVE to find those.

Only the rear panel badge betrayed its origin:

stock amp mains

I wasn't familiar with vintage amps beyond fenders, marshalls, and voxes. When I got home and checked my books and the net, I decided to go back and check it out.  Knowing it was similar to Vox amps, I brought my strat to audition it.  When I returned to the store, closer inspection revealed it was indeed the “croc-skin” amp from 1963-1965 and it was the most desireable Selmer model on the vintage market - the 30w Twin Thirty "croc-skin" with celestion alnico speakers.  I already had the Valvetronix which offered a lot of amp, so this Selmer had to be unique enough to justify taking it home.  You gotta be careful of old tube amps though – they can be a maintenance headache and the older the amp the more likely it will need an overhaul.  This thing is over forty years old and way past its expiration date.  There is a risk of replacing failing components like leaking capacitors, worn tubes, or burned out transformers - potentially expensive repairs.

So in the best beach blonde delivery: what does the Selmer have that a modeller doesn’t?  I’ve had flight time with vintage british valve amps and I know what they sound like.  Vox AC30, AC15, plexi marshalls, and sound city.  I’ve even heard some tweed fenders. The Valvetronix can deliver all of those. But the Valvetronix cannot deliver the crispy juicy jangle of this Selmer amp, especially on single coil guitars.  This 30w combo has a nice high end that doesn’t pierce the ears or is overbearing.  It's sort of like hearing a Fender tweed with the pillows removed from your ears.  I really like the variations in tone between the “Selectortone” and the various pickups on my strat.  My attempts to emulate the Selmer with the Valvetronix were futile, could not even get close - even though both amps had the same celestion alnico speakers.  Even though it is a 30w amp, it is deceptively LOUD (aided by the excellent SPL of the alnico speakers).

You can get some really sweet blues or funk stuff dripping out of this thing although it is reluctant to overdriving.  It's the only amp I've heard that can make my Les Paul approximate the twang of a strat (very few amps can do that).  This is not an amp to reach for creamy sustained distortion but if you slam a Gibson hard enough it starts to sound like early AC/DC (listen close to AC/DC and you will hear minimal overdrive – the unique crunch of the Young brothers comes from their hard strumming technique).  It’s got the spank of a fender tweed or blackface, the jangle of a Vox, with the raw sting and bite of a Marshall power tube amp.  Interesting combination of tones and it responds quite well to dynamics of playing whether you feather the strings, flatpick, or slam them.

These Selmer amps are underrated and relatively unknown, especially in the US where there was no distribution of any kind.  Back in the day, you had to travel to England to buy a Selmer then bring it back to the US.  And back in the "croc-skin" days there was no AC mains selector for US110VAC, you had to retrofit a step-up transformer ($$$)!  My amp already had the step-up transformer intact, there's a hole in the back panel to accomodate it.  Selmer was making guitar amplifiers starting in the late 40s, well before the first Vox amps in the late 50s.  They were the most popular guitar amp in the UK and Europe until Vox out-manuevered them with their savvy marketing and group endorsements.  Selmer did defend their second place throughout the sixties and their most famous endorsers were Gerry And The Pacemakers with their "wall of Selmers", croc-skins no less!  But Marshall and Hiwatt squeezed them out of the market by the 70s.

The Twin Thirty has two channels – channel one possesses a volume and tone control (no separate bass/treble) and is slightly hotter and fuller than channel two.  Channel two has identical controls in addition to the “Selectortone” and the tremolo.  The “Selectortone” is a basically a tank RLC circuit with varying RC components switched in and out via a pushbutton piston switch panel not unlike the AM radios on old cars of the same era.  An RLC circuit is a notch filter and can cut only, no boost - the opposite of the bandpass action of a wah-wah pedal.  The result is tones unlike anything you’ll get out of standard “tone stacks” in typical guitar amps, and they’re especially effective with a single coil guitar like a strat.  Buttons are marked low bass, bass, medium, treble, high treble, and rotary control which makes the tone control effective.  The tone control attenuates the high frequencies.

The tremolo was well designed and sounded very effective.  Selmer used a unique visual indicator in the form of a tube that emitted a blue-green glow.  This tube was fitted to the front panel between the speakers and was visible through a bezel in the grillcloth.  This was called the “blinking eye” in which the tube flashed in synchrony to the tremolo rate so you had a visual indicator of tremolo!  But be sure the RCA plug on your footswitch has a secure connection or the tremolo won't work - that's what was wrong with my amp.

The “croc-skin” is a highly striking visual feature – a two tone tolex style of black and facsimile crocodile scales.  “Croc-skin” was the british fashion rage during the early 60s.  Although there are some slight cigarette burns on the top of the amp, the tolex has no tears and is in fine condition.  When Vox and Marshall popularized the all black tolex, these “croc-skin” patterns quickly became old hat, which is why they were made only for two years.  Today they are desirable in vintage amp circles although not well known and highly underrated.  Selmer never had US distribution for these amps so examples in the US are rare, especially the old “croc-skin” line (I don’t recall “croc-skin” being a fashion style in the US).

This amp has the original Celestion alnico speakers which sound excellent – they are identical to the Celestion “alnico blues” installed in Vox amps with the exception of the magnet cover, which has zero bearing on the sound.  The speaker is supposed to have a datecode that can be used to date the amp but there IS no datecode on them (my amp tech couldn't fine them either).  My serial number is 13xxx which is lower than a 1964 specimen at serial #16723 so it is likely I have a 1963 model, the first year they made them.  However a forumite did note that my specimen has grey handles instead of black.  The tolex style that preceded the "croc-skin" was a two tone blue-grey - Selmer made the Twin Thirty in this style and as you can see from this early blue-grey specimen it has the same grey handles as my amp.  So I probably have a transitional specimen - "new" croc-skin tolex with "old" grey handles and a serial number landing it in its first year of production.  Whoa!

Unfortunately it doesn’t have the original grillcloth and it is missing the “blinking eye” bezel and large german gothic “Selmer” letters on the front.  As far as I could ascertain there were no sonic indications of failing leaking caps and the chassis and metal components were free of rust.  It just came back from a vintage amp tech and he did very little to it - all the tubes, sockets, caps, everything was in good condition.  He was amazed it was this good for the miles it had on it.

The grillcloth not being original and absolutely no luck at locating replacement grillcloth that is an exact replica of the original, I decided that ugly grillcloth had to go.  So upon unceremoniously tearing off the grillcloth, I was greeted with a surprise.  There are no holes in the speaker panel for the SELMER badges!  The holes that are in a typical Twin-Thirty speaker panel are NOT in my specimen.  After some research I found a catalog entry for this amp and this is what the amp may had looked like.  There are some holes on the bottom of the panel from wood screws so this may have had the separate panel with the badge letters on it.

The speaker panel had been secured with four woodscrews, one on each corner.  The factory arrangement was three per side.  The original holes were still there so I replaced the wood screws with T-nuts and long screws.  Believe it or not, this does impact the tone.  I also replaced the bolts/nuts securing the speakers as some were missing and the threads are non-standard - neither standard or metric would fit them.  They were rusty so they were replaced with T-nuts and bolts.

I spent six months prowling for an empty cabinet to scavenge grillcloth and badge letters from.  Two came up, but the buyers did not want to ship to the states.  I gave up looking for one.  Since no original grillcloth was available, I opted for speaker grills.  I decided that a set of gold-finished speaker grills would look good on this.  Gibson has them on their amps but I cannot find the grills separately anywhere.  So I simply bought black speaker grills and a can of gold paint from the hardware store.  Some quick painting and they were done.  As you can see from the pics at the top of this page, I think it looks better than as purchased (I sure would like it restored to original).

Asking price?  Very reasonable compared to market prices.  The vintage Celestion alnico speakers alone were worth what I paid for it.

These amps are so rare in the US that this one has a step-up transformer to UK220VAC since the mains selector has no position for US110VAC.  I thought I would re-sell it for a profit, but the more I play it the more I'm leaning towards keeping it. I would like to restore it to stock, but the grillcloth, large german gothic badge letters, and “blinking eye” bezel are serious unobtanium.  The best source is ebay UK, since Selmer amps were more prevalent in the UK.  It’s a good thing the “croc-skin” tolex is in good condition because no one makes anything like it today.

An inspection of the schematic reveals a remarkably simple amp.  Tube arrangement is 3xECC83 (12AX7), 2xEF86, 2xEL34, and tube rectifier (UK 50hz=GZ34 or 5AR4, US 60hz=GZ32 or 5U4).  What was surprising was how quiet this amp is.  Little if any noise or hum can be heard, even with the single coil pickups under fluorescent lights.  There’s a reason for this, which I’ll explain below.

The Selmer Truvoice Zodiac Twin Thirty is an interesting amplifier design.  The power amp is a class A/B with negative feedback, similar to the Marshall JTM45 design.  In fact the output transformer is the same as the one used in the JTM45.  The preamp is totally unique though, and this is where it gets interesting.  The ECC83 is the input gain stage while the EF86 is the gain recovery stage after the tone filters (the Twin Thirty has only a single "tone" control so no "tone stack" with separate bass/treble controls).  Each channel has independent ECC83/EF86 sets.  Gain recovery stages after the tone filters were usually reserved for the high end amplifiers in a product line.  Instead of diming the ECC83 to full gain at the input stage like most guitar amps, the gain in the Selmer Twin Thirty is divided between the ECC83 and the EF86.  This exploits the optimum features of both tubes.
All active components – tubes, transistors, opamps - have a parameter called gain-bandwidth product.  The gain is inversely proportional to the bandwidth meaning as gain increases the bandwidth decreases (high frequency response is sacrificed for gain).  Gain-bandwidth product is also dependent on the device’s parasitic capacitance, which exists in any active component.  The lower the capacitance, the better the bandwidth.  Parasitic capacitance is a product of the internal components of the device during the fabrication process and it cannot be reduced or eliminated.

The ECC83 is optimal for the color it imparts on guitar signals but its inherent parasitic capacitance limits its high frequency performance.   That is why an ECC83 tube has a brighter clean tone than an overdriven tone – with the ECC83 pushed into heavy metal overdrive, you’re compromising high frequencies.  You’re adding ear-pleasing even ordered harmonics at the expense of high frequencies.

EF86s are optimal in that they have higher gain, lower noise, and lower parasitic capacitance than the ECC83 and were popular as input stages for the microscopic signals from vinyl record needles in hi-fi stereo systems.  They are very clean pristine audio tubes at high gain.  But they impart little harmonic color and tend to sound “sterile” with guitars (precisely why they are ideal for phono records).  EF86s also are notorious for being microphonic and when operating in high gain they tend to “squeal” in a hostile vibrant environment like a guitar combo amp.

So the Selmer design exploits the best features of the ECC83 and EF86 – the ECC83 is the input stage where it imparts its color on the guitar and the EF86 is post the tone filters where its superior gain-bandwidth and low noise is exploited.  The Twin Thirty divides the gain between the ECC83 and EF86 – by operating the ECC83 at lower gain and the EF86 at higher gain, the gain product of both stages is equivalent to a proper guitar preamp.  With the ECC83 operating at less gain it is capable of better bandwidth (read more high frequencies).  The EF86 breezily passes the same high frequencies.  This is why the Selmer Twin Thirty sounds so crisp and juicy – the high frequencies of single coils shines through on this amp like nothing else.  This is also why the Selmer is so quiet – the low noise EF86 carries the bulk of the amplifying chores.  An unfortunate compromise of this design is lack of sufficient overdrive at the input stage because the ECC83 is not dimed out like a typical guitar amp. 

The Zodiac Twin Fifty - 50w instead of 30w - shares the same preamp but the power amp is a different circuit and the alnico speakers were "upgraded" to Goodmans whose heavier wattage can better withstand the fifty watt muscle.  If you put two 15 watt celestion alnicos - total 30 watts - in a fifty watt amplifier, the amp will make quick toast of those valuable alnicos.  The Thunderbird models (adds reverb and separate bass/treble controls) do not have the same valve arrangement (no EF86 tubes) so both the preamp and power amp are different.  So the Twin Thirty is unique in the Selmer line.

This is an amp that is sensitive to technique - baby the strings and you get that sweet juicy tone of a Vox, then if you slam 'em it will break up like a Fender Tweed.  Nice dynamic tone changes you can control with your fingers.  This is also an amp that will highlight sloppy picking technique, so I look forward to developing my playing on this amp.  You're not going to get a Selmer Twin Thirty to distort into creamy overdrive - and these things do not react well to pedals, a trait shared with most pre-Marshall british amps.  As you approach 10 on the volume knob you can hear the EL34 power tubes distorting, which tends to be transient distortion and you won’t get much sustain in this design.  Coupled with the rectifier tube, this transient distortion “sags” which softens the impact of power tube breakup and imparts a dynamic change in tone on the transients.  This is a very pleasant trait for blues music.

As per most 1960s british amps, the Selmers do not like most pedals.  My VestaFire RV-2 dual channel spring reverb sounded great as did my brother's Mesa Boogie V-Twin.  But my Vox Tonelab SE... meh.  The clean amp models were good but the dirty amp models were not pleasant.  Yes I did put the line out in AMP mode.  This amp just does not like its preamp signal to be over-processed.

However I *did* find one very useful pedal - the Morley A-B footswitch.  You can route to either A or B, or to both at the same time.  Since the Selmer was a two channel amp, my experiments driving BOTH channels were interesting.  The selectortone can get some really tinny sounds which make you think "where would I use that?!?"  Well using the A/B switch to drive BOTH channels, the tinny channel combined with the full-ranged channel will yield a full-range sound with a nice crispy attack.  Using combinations of selectortone this really expands the variety of tones out of this amp - and it seems quite content to do this, contrasted to pedals!

Selmer was very wise to have separate chassis for preamp and power amp.  The hum-inducing transformers of the power amp are in a separate chassis in the bottom of the cabinet, a safe distance away from the high gain preamp that is sensitive to interference whose chassis is in the top of the cabinet (back of amp with panels removed).  Very few guitar amps use this design, unless you are using a modern modular rack system with separate preamp and power amp units.  This is the primary reason why the Selmer Zodiac Twin Thirty is a very low noise/low hum amp.

Another advantage to the separate power chassis is improved heat dissipation.  Accumulated heat gradually destroys electronics over time – caps, tubes, transformers, everything.  The hotter the heat, the faster they break down.  With the common knowledge that heat rises, Selmer put the heat generating components – the power tubes and transformers - in the bottom of the cabinet where heat has plenty of room and few obstacles to rise away from the chassis.  Today fans are used to force cool air on heat generating components – that works great UNTIL the fan breaks down (fans have a tough life and they don’t last forever).  Selmer chose a passive system that is not vulnerable to a malfunctioning fan.  That also kept the price reasonable by eliminating the cost of a fan.

So why is that important?  A cooler amp is a more reliable amp.  Amplifier heads such as Marshalls have the disadvantage in that the heat generating components are in a (almost) closed space – all that heat has to escape through a small air vent in the top of the head (don’t block those vents!).  Newer head designs have a fully vented front panel allowing maximum ventilation.  Fender and Vox amps have the heat generating components mounted underneath the chassis and since heat rises it can’t effectively escape around the chassis, it has to escape out the back of the amp (don’t place the amp flush against a wall!).  Today you have to have a spare amp ready if your main amp goes down.  That isn’t a problem with Selmers (unless you neglected replacing the capacitors past their expiration date).

Whoever designed these amps was one clever and creative engineer.  A shame his name was lost to history...

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