Guitar Amp Conversion
1958 Bouyer 241x
The little French-Guy.
Originally designed as a single-ended 6L6 PA amplifier. With a few modifications, the 241x cuts a fine figure as a bedroom amp for guitar.
This amp came to the Tube WorkShop in early 2020. An old PA amplifier from the Paul Bouyer company in Montauban, France. Equipped with a transformer-balanced line input ("Ligne"), the low power that is generated from just a single 6L6 power amp tube is particularly striking. Even in the preamp, very little gain is produced with just a single 6J5 triode. So everything is designed to send already pre-amplified, low-impedance signal sources to loudspeakers.
The idea was to turn this little amp into a bedroom guitar amp, since it actually has all the requirements you need for it.
Determination of age
Well, determining the age was very easy in this case, since we continental Europeans are very meticulous about documenting everything well. A simple look at the nameplate attached says that this amp was built on 03/10/1958.
After researching on the internet I believe that this amplifier was designed and built by Bouyer for the French national railway company SNCF for announcements, as the nameplate has the heading "Amplificateur SNCF" and a corresponding logo. SNCF stands for "Société Nationale des Chemins de fer Français"
But it is interesting that the logo on the type plate corresponds to the first SNCF logo, which apparently was only used until 1947.
Ultimately, however, it seems that an old amp that used to be used to make announcements in French train stations will in future make guitarists happy as a "bedroom amp".
functional but immaculate.
The amp was in pristine, untouched and working condition. Great!
If you had the right connection cables, you probably could have just plugged it in and music would have come out.
Of course, there is no circuit diagram for the amplifier. Ok, that's no problem... the circuit is simple enough that we can paint this ourselves from the wiring.
The case made of sheet steel with a hammered look will not win design-prizes and has more of an industrial flair, but it is extremely practical. With only 2 screws the top flap can be removed to access the tubes, with 2 more screws a flap on the bottom can be removed and you have access to the complete circuit. A very clever and service-friendly solution, and perfectly suited to the original application as an announcement amplifier in train stations.
The circuit is wired point-to point, but at the highest level. While with other amps you often find a tangle of components and cables, there is a quasi "military order" here and it is simply eye-candy to see the precision and attention to detail of the work here. This is craftsmanship at the highest level that I have only encountered in a similar form in HiWatt amplifiers. Great respect for Paul Bouyer and his staff!
At this point it hurt a bit to use the soldering iron for the first time and modify this work of art.
It is striking that the 241x is very American in its mix of components with RCA and JAN tubes and 2 Mallory capacitors as cathode bias. The period after the Second World War, which was strongly influenced by the USA, is evident here.
The first thing on the agenda was to analyse the circuit of the amp and to draw a circuit diagram. This takes some time, but is quite easy to do with a simple amplifier. Let's go...
Although the high voltage after rectification by the 5Y3 tube is "only" approx. 285V under load, a very complex filter chain with series-connected capacitors and corresponding strapping resistors were designed into the amp (to increase the dielectric strength) and an additional filter choke with a high inductance of 10H was also integrated. This is more than bullet proof!
I can only guess the reason for this, but I assume that the amp was trimmed for maximum operational reliability due to its intended use. In the original wiring of the amp, the power supply capacitors are only "weakly challenged" and one can sometimes go bad without the amp immediately quitting its job.
By the way, the mains transformer is one of the most beautiful I've ever seen.
When modifying the guitar amplifier, all old electrolytic capacitors were removed from the circuit for safety reasons, but left in the chassis for aesthetic reasons. The renewed power supply contains electrolytic capacitors from TAD with a dielectric strength of 500V, which are more than able to cope with the voltages of the 241x even without series connection and "strapping".
Also with the 5Y3 rectifier tube which ramps up quickly and there may be some overshoot until all other tubes are heated and conducting.
The filtering was slightly increased overall, but generally left close to the original values. You will find out later why I introduced a 3rd voltage node for an additional amplifier stage.