Selmer Constellation 20

CAUTION

LETHAL VOLTAGES PRESENT THROUGHOUT THE CHASSIS

1) Dating from about 1965, this is the second generation of Constellations, uprated to 20 watts and fitted with 2 x 12" speakers, in this case R&As. It has two channels with two inputs each, volume controls for each channel, shared treble and bass plus speed and depth controls for the tremolo. It's had a major burn up at some time and been stuck in someone's shed for years. That's a real shame, because it's not in bad condition externally and it'll make a really nice rehearsal / recording amp when it's up and running again.

This one is beyond economic restoration to original condition, but it'll make a very good renovation project.

2) First sign of trouble is the large hole in the paxolin circuit board. It's beyond repair so I've made a new one out of fibreglass. The track layout and heavy solder tinning is exactly the same as the original apart from the area around the electrolytics. Resistors and electrolytics are brand new while the bias diode and Wima caps are original.

3) R&A Loudspeakers:

Selmer seem to have used several speaker suppliers.

John Chambers at Champ Electronics has some pictures of a Constellation fitted with RTCs at

http://www.chambonino.com/work/selmer/selm5.html

and Steve Russel and Tim Fletcher have a Celestion G12 version at

http://www.vintagehofner.co.uk

These two were made by R&A, 'Reproducers And Amplifiers Ltd' of Frederick Street, Wolverhampton. They were known for supplying good quality speakers to major radio and radiogram manufacturers, hence 'Higher Fidelity'. I believe the Company was dissolved in 1973. They had no connection with Richard Allan despite the initials.

I haven't tried these yet, but the coils are intact and don't rub and they appear to be original to the cabinet. At a guess, I'd say that they're a version of the model 7120 without the fancy chrome plating and with a bell cover to make them look more impressive. If that's the case, they're 15ohm units rated at 10watts with an LF resonance of 50Hz. In 1965 the standard versions would have cost £4 18s 6d each.

The bell cover bolt has sheared off in the magnet on the left and the rust bug has got to both frames. It's not terminal though and I've test sprayed the loose cover with red oxide and CarPlan 'Smoke Blue' touch up paint. The match is almost perfect but, sod's law, I don't think they make it any more. Oh well, try again. (See section 10 below for "after" pictures).

4) Schematic

This is a CAD version of the old Selmer drawing No.2372.....

.........and the corresponding main board layout.

If you compare the track layout with the original circuit board you can see that V5 has had a bit of a bad day and taken out the screen grid resistor, the track and the board. Probably couldn't face playing Yellow River any more.

I should think the amount of damage was down to the home made fuse. The original 1amp Belling had already vapourised.

There's another problem on the actual front panel circuit board. The track that joins C8 and C9 to the wiper of the bass pot is missing. There's no sign of it ever having been there so I guess they're all the same. (Just tried making the connection and the bass pot stops working. Strange)

5) This is the front panel pcb as found.

6) On the way to recovery.

7) More fireworks.

How bad can this thing get? This is where the mains switch has arced across to the chassis and blown a lump out of it. The switch, although it doesn't fit, is original and gives some idea of how dangerous old amps can be.

It gets worse. Someone has disconnected the mains earth wire and carefully taped it back. Thinking about it, the earth must have been connected when the switch shorted for the first time or there wouldn't have been any current draw. Now though the whole thing is a death trap.

Have a look at these pictures and then check out the Watkins Copicat section for a discussion about Portable Appliance Testing (PAT).

8) Pot Troubles

Bit of a problem finding replacement pots. The front panel is spaced away from the pcb by an extra pot nut so the mounting bush has to be fairly long. Haven't found anyone who does all the values at a decent price yet.

Update Aug 2010.  Maplin now have a range of pots made by Omeg which have a suitable thread length.

The bodies are smaller (20mm) and the connecting lugs are for a pcb but they fit the front panel. That's about the nicest thing you could say about them because the construction and feel are disappointing. The 1M pots (the two with black inserts) have a ridiculous amount of side play in the spindles although the others are just about OK. They also need extra nuts (M10 x 0.75) which aren't generally available. Mouser or Conrad stock them or, for an easy life, you could just buy extra pots.

The track side of the control panel board faces the metalwork and, to avoid shorts, two strips of felt are glued to the aluminium. These are quite thick and it's important to space the board away from the panel to prevent mechanical stress.

In this case there's an extra shakeproof washer between the first pot nut and the panel to make up the gap.

9) Close Relatives. The Selmer Stadium.

I was trying to find a schematic for the older Constellation 14 on the internet and only came across odd separate sheets for the Stadium. The Stadium is basically a Constellation 14 without the magic eye tremolo indicator and so I've put these sheets onto one drawing as a start. If anyone's got any Constellation 14 schematics I'd appreciate a copy. This is only a first attempt so there might be a few mistakes on it.

10) Just got round to the speakers

Above left is the frame and cone masked up and grey primered after rubbing back to bare metal.

This is the one with the broken bell cover stud. The dust cover has to come of to fix that. This chassis was so badly rusted that the whole magnet assembly was taken off for painting (It's a bugger to put back on).

The 2BA brass studding actually goes right through the magnet and is held on by a nut at each end.

Above: Before and after.

Above left is the speaker baffle showing the damage to the scrim that supports the grill cloth.

 

The cloth itself was OK so (about 300 staples later) it has new scrim.

11) It's Alive !!!! (You won't know what the hell I'm on about unless you watch Frankenstein movies)

Mostly back together now. Tidying up to do on the wiring and a new upper back panel to make. The only problem that caused a bit of head scratching was motor boating (the putt-putt-putt noise an outboard makes when it's ticking over). This was down to connecting the anode leads to the output transformer the wrong way round. That gives you positive feedback through R31 and results in low frequency oscillation.

 

The second problem was an annoying hum when the tone pots (either of them) were turned clockwise. I thought it might be normal because of the exposed signal wiring but, after a lot of swearing and head scratching, it turned out to be a brand new ECC82 with what must have been heater to cathode leakage. Nothing showed up on an AVO valve tester though. Put in a old Mullard and everything was quiet as a mouse.

 

With a collection of old valves just to try it out, the sound is outstanding. Being sonically illiterate, I won't try to describe it but it's a pleasure to play through.

 

The schematic in section 4 above now has test voltages shown. They were taken with a 10Mohm digital meter with old valves and will vary slightly on other units. Near enough though.

12) Back Pain

Am I missing something or are there a whole bunch of weird people out there who collect back panels? Every time an old amp turns up the back's gone.

Left is an inside view of the new one. It's 6mm plywood covered in Fender Tolex. The glue is Evostick 528 impact adhesive. It doesn't show up too well in the photograph, but there's a square section stiffening rib running along the bottom edge.

There were a few different versions of the ventilation hole, some with mesh, some without.

The Fender Tolex pattern is slightly courser than the original Selmer covering. It's about as close as you'll get though and it'll tone in much better when the rest of the cabinet is polished up.