This duo of 18-Watt, twin-channel, all-valve combos with valve-driven Tremolo were originally produced from 1966 to 1968 and were named the 1973 (2x12”) and 1974 (1x12”). The 1973X and 1974X are painstakingly accurate reissues of these highly respected and much sought-after originals. Aside from their differing speaker combinations and cabinet sizes (the 1974X shares the same cabinet size, the 1973X is larger), the amps are identical…
The 1973X and 1974X are all-valve, 18 Watt, two-channel combos with valve-driven tremolo and no negative feedback in its cathode-biased output stage. The amplifier section is extremely straightforward: Channel 1 (the non-tremolo channel) only boasts two, self-explanatory controls: Volume and Tone; while Channel 2, the Tremolo Channel, boasts four – namely Volume and Tone, plus Speed and Intensity for the valve driven tremolo circuit.
Three ECC83s (12AX7s) in the pre-amp, a EZ81 rectifier and a pair of EL84 power valves working in push-pull. All valves are of the highest quality available and go through a meticulous grading and testing process. The way the three ECC83s (V1, V2 & V3) in the pre-amp are utilised is as follows:
- V1 is dedicated to Channel 1, the non-tremolo channel. As Channel 1’s pre-amp has a single gain stage, each half of the valve (the ECC83 being a dual-triode) acts as a dedicated gain stage for the channel’s two inputs (which are identical).
- V2 acts as the amplifier’s phase-splitter.
- V3 is dedicated to Channel 2, the Tremolo channel. To be precise, one half of the ECC83 acts as the single pre-amp gain stage for the channel’s two inputs (High and Low) while the other half acts as the oscillator for the amp’s tremolo circuit.
Due to the fact that the EZ81 rectifier valve and also the pair of EL84 output valves attain extremely high temperatures when the amplifier is in use, their bases are made from the highest grade ceramic available.
Important Note: In order to comply with the strict safety requirements of modern legislation, the amplifier’s valves have been housed in a removable, vented aluminium cage. This is obviously a deviation from the original spec. but has no tonal or performance impact on the amplifier what-so-ever.
The protective shroud covering the power valves (shown here in a 1974X)
In typical Marshall fashion, the tone network is post gain and passive. The tone circuit involves a blend of high frequency pre-emphasis and passive high frequency cut – the mix of which is dependent upon the setting of the Tone control on the channel being used.
With the obvious exception of the valves, whenever possible components used are sourced from European and American manufacturers – including some custom built, ‘double can’ (a.k.a. ‘dual gang can’ or ‘dual electrolytic’ – meaning two capacitors in a common case) power supply capacitors due to the odd value used in the original – one of many steps taken to ensure maximum authenticity.
This is exactly the same as the original in terms of dimensions, thickness and matrix pitch. The material used is EM42. The reason we didn’t use a board with the exact same chemical composition as in the original units is because that material doesn’t pass current safety legislation regarding flammability.
Because these combos were produced relatively early in Marshall’s history, Jim had yet to strike up his now well-known relationship with transformer manufacturers, Dagnall and Drake. As a result, both the power and mains transformers used in the originals were ‘off-the-shelf’ devices purchased from a highly regarded, general electronic component supplier. This company, whilst still in existence, does not have records of exactly who manufactured the original transformers.
Output and mains transformers are vital components in an amplifier as they influence performance, sound and feel. Consequently, we worked extremely closely with our associates in Dagnall’s R&D department in order to duplicate the original transformers in all areas. To do this we spent a great deal of time and attention studying and analysing the constructional methods and materials (e.g.: lamination grade, insulation, coil spacing, etc.), used in both transformers so we could match everything as closely as possible and also ensure that the all-important electrical characteristics and performance were identical. With Dagnall’s expert help and dedication, this goal was achieved.
In the originals the speakers were hardwired directly to the transformer. To make the re-issue more user-friendly, the internal speakers are not hardwired to the transformer – instead the speaker cable is fitted with a jack and then connected to the amp via one of the two speaker outputs on the rear panel of the combo. The rear panel also boasts an impedance selector with three options – 4, 8 or 16 Ohms.* These additions (the original didn’t have speaker outputs or an impedance selector) give the user maximum flexibility, enabling the owner to use an external cabinet or two (by disconnecting the internal speaker(s)), if they wish.
*Note 1: Adding speaker outputs and the impedance selector has no effect on the tonal authenticity of the 1973X, they merely add to its potential flexibility.
*Note 2: On the original output transformer you could choose from one of three taps: 8 Ohms, 16 Ohms and something called a ‘100 Volt line load’ – a quaint offering that’s completely irrelevant to musical applications. So, we replaced it with a 4 Ohm tap instead – a far more useful option, the addition of which has no effect what-so-ever on the performance of the transformer.
Mains (Power) Transformer
Just like the ‘off-the-shelf’ device used in the original, the re-issue’s mains transformer is a ‘drop through, half-shroud’ type. In order to comply with strict, current-day safety legislation the custom-manufactured Dagnall transformer we’re using is physically larger than the one in the original but, as with the output transformer, we went to great lengths to ensure that its performance mirrors that of the original. We paid particular attention to exactly replicating an effect called ‘regulation’ – which is the way that the voltage from the transformer that feeds the valve circuitry varies according to the power delivered in the speaker.
As in the original, the chassis we’re using is made from aluminium. While the original chassis was open-ended though, the re-issue’s is closed-ended. This has been done purely for strength/constructional integrity reasons (all the open-ended originals we’ve seen have been quite badly deformed) and has no sonic effect (positive or negative).
A definite contributing factor in the sound of the vintage 1973 and 1974 amplifiers is the way the Celestion T1221 Greenback loudspeakers’ sound softened with age. In order to establish exactly the sound we were trying to recreate we listened to many vintage correct Greenbacks from our Museum here in Bletchley. We then met up with Celestion to decide a strategy to recreate the gorgeously smooth tones of our vintage Greenbacks which are over 35 years old. Celestion revisited the 1967 recipe for the original 20 Watt, ceramic magnet, 15 Ohm, Greenback T1221 speakers used in the 1973 and supply it to us exclusively. They meticulously duplicated every critical vintage parameter from winding lengths, coil former dimensions and edge treatment to using the original dustcap material and adhesive. In sound tests these speakers were close, but still lacked something of the sonic signature of the originals.
The Celestion 15 Ohm, Greenback T1221/67 G12M Special speaker used in the 1973X
Further investigation uncovered that the magnetic properties and cone make-up of the vintage speakers varied slightly from the newly manufactured speakers. This slight sonic discrepancy led us to investigate the possibilities of somehow ‘ageing’ the speakers. Celestion’s proprietary ‘ageing’ is largely achieved by the following two things:
- Matching the magnetic flux to that of our vintage references in order to duplicate the output of the older speakers. Doing this not only affected the ‘loudness’ of the speakers but also altered the tonal balance, warming up the low end and also making the top-end less aggressive – exactly like the originals.
- Specially manufacturing the cone material to be more ‘pulpy’, recreating the ageing effect and thus further softening the sound of the combo.
Rear Panel Improvements
The five features listed below are all ‘deviations’ from the original that have been added for user-friendliness, practicality, added flexibility and/or improved serviceability. Please note that none of these have any sonic impact (positive or negative) on the combo in any way.
Speaker Output Jacks
As mentioned earlier, no speaker output jacks were present on the original – the internal speaker was hardwired directly to the output transformer and there was no provision for external cabinet usage. Adding these allows you to use an extension speaker cabinet with the internal speaker or, if you wish, disconnect the internal speaker altogether and drive one or two extension cabinets.
Important Note: These amps should never be used without a speaker load attached, and the minimum rated impedance should always be met.
Another feature that wasn’t present on the original but, in conjunction with the Speaker Output Jacks, adds to the unit’s flexibility.
H.T. and Mains Fuses
In the original these two fuses were located within the chassis itself, making it extremely time consuming and cumbersome to access them, should one blow. Making both fuses easily accessible via the rear panel is another improvement.
Tremolo Footswitch Jack
The original footswitch was hardwired to the combo.
Note: The detachable Tremolo on/off footswitch (supplied) is an exact, sand-cast copy of the original.
The original was hardwired to the unit, for serviceability and sheer practicality the mains cord on the re-issue is detachable.