I got this beauty in for some TLC. I have worked with older Fender Showman amplifiers but this one was different: different tubes in a Fender amp?
The Fender Showman was introduced as the first Fender piggyback amp on which the top could be screwed on a loose cabinet. One person that was the instigator for this line of amps was one Dick Dale that needed volume from his amplifiers; he is known to blow up most of the amps he used in that search for volume. Fender took the opportunity to create a louder amp by enlarging the output transformer, switching to a solid state rectifier but also by changing the cabinet design to withstand the power of four 6L6 powertubes. The 15" speaker was introduced in the guitar world by this. The first Showman's had a design in which they combined the output section of a tweed twin with a new preamp section combined with a new sort of tremolo: the harmonic tremolo. These models are known as the 6G14 models.
After these models the AA763 model was introduced (some with the known white tolex, some already with black tolex) and a special run was only done in 1963 in which the output tubes were replaced by 7355 tubes. These models lost the 'harmonic tremolo' design and the newly designed optocoupler tremolo was introduced; this tremolo circuit became a standard on later Fender amps. The standard Showman amp had an output impedance of 8 ohm driving a single 15" 8 ohm speaker. Later the Dual Showman was introduced that changed the output impedance to 4 ohm (an impedance more common to Fender amps) so more speakercabs could be used.
The 7355 tube was known as a different tube compared to the 6L6 tube as it was used in HiFi models and delivers 18W per tube instead of the 25 / 30W of the 6L6. Ampeg used these 7355 tubes in some of their models and the size of these tubes mimic the size of a 6V6 (quite smallish tubes). Nowadays the 7355 tube is quite rare BUT the 7591 tube, a direct replacement of the 7355 tube's pinout, is more common and available. The 7591 is not the same tube though as the 7591 tube has some different values (higher Mu at the grid), different size (size is equivalent of a 6L6 tube) and will sound different compared to the 7355 tube. The 7591 tube will be more fitting into the sound of a Fender amp compared to the 7355 tubes due to better handeling of power distortion. Both the 7355 as the 7591 tubes needs a octal socket but the pinout is different compared to a 6L6 tube.
Soundwise the tubes venture to the HiFi world with a more spacious sound and more clarity. The mid emphasis on these tubes are different compared to the standard 6L6 tubes and the 7355 / 7591 tubes provide more warmth.
The circuit
With the amp opened up you can spot the beautiful blue mold caps, the dual cathode electrolytes and the push back cloth wires. The inside of the amp looks pretty standard for 196X Fender amps. Underneath the doghouse five 22u / 500V electrolyte caps are available. On the sockets of the powertubes turrents were located to give extra strength to the different pinout of the original 7355 tubes; I don't know if these turrents were common as I can hardly find any gutshots from 7355 tube equipped Fender amps.
A schematic can be found easily here and here. Yet a known schematic of a 7355 tube Showman is not available. For the pinout you can use the datasheet here.
As you can spot this amp uses a slightly different schematic as the first set of electrolyte caps are both 22u / 500V and are put in parallel providing a single 40u, 500V cap. Later version of the amp had two 70u / 350V electrolyte caps set in series with bleeder resistors (220k) parallel to the caps providing a single 35u, 700V electrolyte.
The sound
When the amp came in the amp sounded dull, lifeless and really compressed. Opening up the unit revealed old electrolyte caps with signs of leakage. Measuring them with a dedicated tool brought that these caps were not suitable anymore for the purpose they had and needed to be replaced. Also the electrolyte caps in the audio path were leaky.
After the recap the amp came back alive. The sound became clear, punchy and less compressed. From the former owner the BIAS was set low (approx 30%) and I can't grasp why this was done. I adjusted the BIAS to a suitable 72% and with that the dynamics, output and tone increased.
The 1963 Showman amp is a beauty! It is a shiny example of Fender sounds from the 60's with a warm and welcome tone that suits most music styles, yet it can also be used for more piercing mid emphasis by adjusting the EQ. And although the EQ is limited to two controls (only bass and treble), the knobs give enough options to create a variable amount of sounds and feelings. The onboard tremolo is fitting the style of the amp and gives the amp a vintage character it suits.
With guitar this amp is a perfect marriage but with bass the unit really shines in tone and warmth fitting pre 70's recordings. It lacks the output and mid emphasis of modern amp but you'll gain so much more from this one. And those weird output tubes? You get used to them!
The links
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