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Gibson Mercury II .........  bit of a challenge !

 
 
  
 
 

 1) You'd have to be ever so slightly mad to take on a project in this state, but this one's a bit special. I got it from Buzz Elliott up in Cumbria. He sold it on behalf of a friend who had owned it since 1975.
 
You can see Buzz on stage with The Bullfrogs in 1995 at
 
and in 2010 with Slagbank at
 
This is the gist of what he said about the Mercury.
 
...In the mid 1970s the other guitarist in my school band 'Eight Hertz' aquired the amp from another local band called 'Rue & The Rockets'. They had bought it from Jimmy Henshaw who played with Carlisle 1960s band 'The VIPs'. We used it for about five years, after which my friend stopped playing regularly, and the amp was put in storage. Unfortunately, over the past 25 years it has deteriorated into quite poor condition due to being kept in various unsuitable places such as attics, outbuildings and garden sheds etc.....
 
... The speakers in the cab are the same ones that were in when it was bought by my friend, but we do not know if they are the originals. They are still working and sound loud and clear. The speakers are Celestion G15C 50W 55Hz bass resonance.....
 
 
               
 
 
Buzz has very kindly looked into the amp's history a bit more.
 
Hello again Chris,  I have been trying to do a little research regarding the Gibson Mercury and I can confirm the following:-

Steve Wilson (Eight Hertz)  owned it from 1975/6 until you purchased it.

I originally thought that Steve got it directly from 'Rue & the Rockets', but he tells me that he actually got it from a family band called 'The Macauleys', who appeared on the UK Television show 'New Faces' round about 1974/5, I've tried to find out info' on the net about the 'Macauleys' on this programme, but to no avail.

The rest of the story is pretty much as I told you:- that they purchased it from 'Rue & the Rockets' who used to use one either side of the stage, (we used to call them the Gibson pyramids), and they got it from Jimmy Henshaw, I'm not quite sure of the dates that the amp changed hands............
 
Now I've tried to track down The Macauleys too and, although there are New Faces contestant lists, they're incomplete. 
Have a look at Alastair Duncan and John Fisher's web site
 
This site has been growing for ten years now and it has a huge amount of detail about 60s and 70s bands based in and around Cumbria, charting their influence on the British music scene.
 
If you go to the section about Jimmy Henshaw of The VIPs, there's a description of a VIPs' gig at the Scotch Of St James club in London on 26th September 1966. (Thanks to Alastair and John for permission to use it).
 
...Once the interval arrived, Henshaw was approached by another good friend, Chas Chandler (of the Animals). Chas grabbed Henshaw’s arm, keeping his voice low. “I’ve got a guy with me who wishes to jam with your band”. Jimmy quickly replied “That’s OK, he can plug into my lead amp”. Henshaw knew how to build amplifiers AND how to produce a bigger output than the manufacturer’s recommendations – however, he was concerned when this strange but polite and friendly guitarist began to wind up the volume and tone controls. “My name is Jimi Hendrix. Could we start with ‘Summertime Blues’ then see how things go?”......
 
Forty years later in 2006 the Cumbrian based 'News & Star' published an article with a photograph of Eight Hertz on stage and a follow up.
 
 ...Only an observant musician like Jimmy Henshaw would notice in last week's picture of group Eight Hertz that one of the guys was using his old amp. "It's the one I used when I played with The VIPs during the sixties and the one that Jimi Hendrix plugged into on his UK debut," remembers Jimmy...
 
Sadly Jim died in 2007 after a long career in the music business.  All the more reason then to get his old amp up and running and to keep as much of the existing wiring and components as possible. Not that easy unfortunately given it's present condition. If anybody has more information or memories please get in touch.
 
The VIPs' music is still very much available. Amazon have a compilation CD, with 28 tracks of studio and live recordings.
 
Audio CD (November 27, 2006) The Complete V.I.P.s [Import][Original recording remastered] Label: Repertoire ASIN: B000K2RD3K
 
Thanks also to 'juckreizdelmenhorst' for the YouTube post at  http://youtu.be/yoTAdGFqoh8. Walter Johnstone would probably have been the drummer on this recording, not Mike Kellie.
 
23rd March 2010
 
Thanks to everyone who got in touch and a special thankyou to Brian Atkinson who sent the following email. The references to the electronics won't make much sense until you read the rest of the page, written some time ago.
 
Hi my name is Brian Atkinson From Carlisle. during the late sixties / early seventies i was employed by JP Dias Ltd music dealer in carlisle and remember the gibson mercury amp you are repairing, i repaired it many times after Jim Henshaw had parted with it.I can confirm that Jim did the mods to it. The transformers are indeed selmer although at first the gibson mains trans powered it till a big burnout in the output stage took the transformer as well due to someone trying the old silver paper round the fuses trick.The amp probably had been run without a speaker cab connected as the octal bases had "tracked" between heater and anode pins 2and3.The re build included new mains trans mc murdo octal bases screen feed and grid stopper resistors but can't remember if i fitted the caps they sound like the 32 mF 500 volt 500 mA caps that were used in the selmer tv100 type amps.I haven't seen this amp or it's duplicate which belonged to frank kenyon for years although i am still in touch with one of the "sister" amps previous owners. As for the speaker cab i would be shocked to discover Jim had fitted the 15" celestion spider frame speakers which probably came from a vox T100 cab(2x15") or from 2x vox T60 cabs which had a 15 and a 12 in them.As i recall the knobs were all black apart from the power switch that was red.Don't recall any problems with that type of power switch but i liked the idea of it.The 10" driver from the cab was a sealed driver for high frequency use and there was a crossover in the cab which i may still have !! For some reason i think the cab had 4 x 10" celestion drivers that marshall used but maybe that was the "sister" amp.Hope i have been some use and feel i should mention Jim was very much a local music and Electronics Hero looked up to by all the local musical beginers like myself and by established players Sadly missed !! R.I.P jimmy!  Regards Brian Atkinson
 
As Brian points out, Frank Kenyon also used a Mercury setup. I wonder where that is now.
 
 

14th October 2010

 

Another thank-you to Brian and Keith Jefferson for coming up with this photograph of The VIPs on stage at the Star Club in Hamburg. Jim is seated at the keyboards on the right with his Mercury in the backline. Frank Kenyon is standing in front of his matching set-up. 

 

Greg Ridley is on bass on the left, Mike Harrison is on vocals and Walter Johnstone on drums.

 

 

 

 

 


2) So....What to do with it ????
 
This is going to get complicated now because it isn't actually a standard Mercury inside the box. Over the years the power section has been rebuilt as a typical 1970s 4xEL34, 100watt job. Output valve bases (including two extra ones), electrolytics, mains and output transformers are new. The choke is original Gibson.
  
When was it first modified ? I don't know but it was probably after the Hendrix session.
  
The pot source/date codes indicate early 1964 so the original amp must have been made after this and I can't see anybody, especially a working musician, stripping a very expensive, one/two year old import.
 
One of the replacement caps is marked with the rectangular RS logo (instead of the Radiospares flying man) and I don't think this appeared until 1971.
 
The speakers are a bit of a puzzle. If the Celestion web site is to be believed, their date codes (26LG and 29LG) could indicate the 26th and 29th of December 1962 or the 26th and 29th of November 1997. Since December 26th was a Bank Holiday (Boxing Day) and the factory at Ipswich apparently didn't open until 1968, that would make them 1997.  
 
 
 
 
 
Celestion state that the other LG possibility (November 1974) would have the day code after the letters, e.g. LG26. Informed opinion though suggests that the date code list is over-simplified and that some speakers from this period were actually stamped day-month-year. That and the fading of the cone material (plus the amp's history) make November 1974 the most likely date.
 
I asked Celestion (very fast and helpfull reply by the way) who agreed that although mixed up codes from that era were rare they weren't impossible.
 
The electrolytic supplying the preamp is marked P 74 02 (Feb 1974?) and the two larger Erie caps are AYN KB 706 (June 1967?). The Eries look fairly worn and, with clamp marks at both ends, they were probably second hand when they were fitted.
 
 
           
 

 
3) The whole chassis has been sprayed with grey primer at some time, probably to stop the rust. Nice to know somebody cared. Bugger to get off though.
 
 
 
 
 
 
 

 
4) When it was new the speaker cabinet would have had one 15" and one 10" CTS/Jensen driver with a crossover. The normal output power of a Mercury was 35W so the originals obviously had to go when the amp was modded.
 
Replacements are Celestion G15C 15" rated at 50W each (see update in section 1)
 
Part of the tuning port has been cut away next to the old 10" position and the hole in the baffle has been blanked off.
 
 
 
 
 
It wouldn't be too difficult to put the speakers back to normal but there wouldn't be any point. The originals wouldn't handle the power anyway and it would destroy forty five years worth of history. (Have a look at section 16 to see what happened next)
 
 
 
The back of the cabinet is in two parts with a tilt-back leg recessed into a centre strut.
 

 
 
5) I don't know when the rig was last used, but the whole thing is too far gone to just switch it on. Apart from dire state of the mains wiring one of the screen grid resistors has been running hot and burnt off the printing. Right next to it is a hole melted through the heater wiring. It's possibly an old short that's been pulled apart.
 
 
 
Unfortunately, history or not, a fair amount of wiring will have to be replaced before it's safe to use.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The most worrying things is the mains on/off switch. See section 9 below.
 
The existing switch is rated at 3A at 110V A.C. or 1A at 250V, although 250V operation wasn't an option on the original amp. A new switch would be favourite given the state of it.  The original fuse holder has been changed to a 20mm version mounted in a grommet (presumably to clear the new mains transformer) but the original Leecraft 125v neons are still there.
  
 
 
 
 

 
6) Gibson Mercury Schematic
 
This is what survives of the original circuitry. The shaded area is Gibson. The rest of the components are modifications.
 
 
 
 
This is what the original would have looked like.
 
 
 
 

 
7) Clean Up
 
The front panel was a lot worse than it looked in the 'before' pictures at the top of the page. Most of the plating was sitting on top of rust and it was just falling off. It's surprising how much of the screen printing survived. Shame it got into this state but, hopefully, it won't get any worse.
 
 
 
 
 
A big problem is trying to find some replacements for the missing control knobs. They were made by Rogan (who also made knobs for cooker). They measure 28.5mm across the circular part and fit split, 1/4", 24 spline shafts. There are seven missing altogether including the red one that fits the power switch.
 
 
 
 
 

 
8) Output valves
 
Got the AVO valve tester out today.
 
The four EL34s came out some time ago so I'm not sure which one went where but, judging by the damage to the base, valve d was in the socket where the screen grid resistor shorted out.
 
The AVO test results should be: Ia = 75mA, mutual conductance = 11mA/V
 
Test settings were: VA = 250V, VS = 250V, VG = -13.5V
 
Valve Manufacturer Date Code Isolation (hot)
Cathode to Heater 
IA Gm 

 Mullard

xf3 B5G1 

24M 

76mA 

9.6mA/V 

b

 Unknown

None 

15M

69mA

9.2mA/V 

c

 Mullard

 xf3 B5F4

2M 

79mA

11mA/V 

d

 Unknown

 None

25M 

76mA

8.5mA/V 

 
The cathode to heater leakage in valve c doesn't really matter in this circuit as there's very little potential between them.
 

 
9) Power Switch
 
The worlds most complicated on/off switch, made by CTS. The source code stamped into the metalwork indicates a manufacturing date of Septmber 1963. This ties in nicely with the early 1964 pot codes.
 
Gibson used a four way switch designed to work with the USA mains system. The Mercury handbook explains it's operation.
 
"A convenient switch has been provided for performing a total of three functions, turning the amplifier on and off, putting the amplifier in 'Standby' and selecting the proper polarity of the power source which reduces the A.C. hum and other extraneous noises to a minimum. In the 'Standby' position the amplifier is kept 'warmed up' and ready to play at the instant the switch is advanced to the 'On' positions. Two 'On' positions have been provided. The hum and hiss levels on one of the 'On' positions will be noticeably lower - this is the correct 'Polarity' for using the amplifier.....
.....In locations of extremely high noise levels, clipping the 'Ground Clip' to a grounded object will greatly reduce objectionable noise and hum....."
 
 
What the switch does in practice is couple one or other of the incoming power wires to the chassis via a capacitor. The user selects the switch position which gives the least hum and noise. This feature isn't needed with UK the wire mains systems.
 
The incoming mains power is controlled by a single pole, single throw unit at the back of the assembly (the third contact seen in the photo isn't connected to anything inside the switch).
 
 
          Removing the rear section reveals a steel plate and a pressed-on actuator.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Below the steel plate is a three pole, four way, ceramic wafer switch which takes care of the HT switching and mains polarity selection. The moving contacts are held between two rotating plastic mouldings which are welded together. Apart from pulling the wafer off the spindle, this is as far as dismantling can go.
 
Now that the power switch is clean and I've had a look inside, I'll have a go at re-using it.
 
 
 
Below is a representation of what goes on inside the switch.
 
 
 

 
10) Mains Wiring
 
I know I was going to leave as much as possible alone but, since the mains wiring is so badly corroded, it's got to be replaced. The paxolin sheet holding the transformer voltage selector is broken, it's missing the selector link and it's been hard-wired to 245V. Since the transformer is bigger than normal, the selector link would be vulnerable sticking out of the cabinet so the mod isn't a bad idea. I've put it back to normal for the time being just to test the other voltage taps. The transformer itself comes from an old Selmer amp. (see update in section 1)
 
 
 
 

 
11) Indicator Lamps
 
The neon indicators are the original Leecraft 'Snaplite's rated at 125V, 1/3watt. Both were suffering from brittle insulation on the connecting leads and one lead actually fell off when it was moved. The orange lamp also had an open circuit resistor.
 
The only way in is to cut around the body at the lead end.
 
 
 
One new 47k resistor later, plus some new wires, and they'e all Araldited together again.
 
The existing neons can be run from the 125V mains tap using the TX primary as an autotransformer. Just got to figure out the best way to rewire it now.
 
 
 
 
 

 
12) Electrolytics
 
I've been having a look at the 32uF cans to see if they can be rescued. They're lovely old Eries rated at 500V and 500mA ripple and I just can't bring myself to change them for some skinny modern things. They both read OK for value on a bridge and I've had a quick go at re-forming them. So far it's going well although I need to rig up a higher supply voltage.
 
Both cans have dents that don't match the clamps so say must have been round the block a bit before. (see update in section 1) 
 
 

 
13) More rust 
 
The top of the chassis was so badly corroded that I've given up and painted it. The rust has been ground off and the bare metal primed with zinc based, cold galvanising paint. On top of that there are two coats of grey primer and a gloss coat. The transformers were removed and painted separately but the top mounted valve bases were just masked off to avoid disturbing the wiring. Boyd Coddington might not have been impressed but it hasn't turned out too badly and now everything can go back together. It would have been a lot easier to strip the whole thing back to bare metal and powder coat it but that would have meant stripping out all the vintage wiring and that wasn't the idea.
 
 
 
 
 

 
14) Switch On
 
After an earth continuity and high voltage insulation test, the amp (minus valves) was powered up gradually with a Variac over several hours to give the capacitors a chance to reform. It's worth remembering that the DC voltages will be higher than normal if the valves aren't in and drawing current.
 
The good news:- The power switch worked and the HT started coming up.
 
The bad news:- Turning off the power to go and make the tea produced a loud clunk and instant darkness as the mains RCCB tripped out. 
 
The culprit was that d****d power switch again. It was fine moving clockwise but wear in the detent mechanism was allowing the internal contacts to short out when moving back to the off position. The result was a short between the HT section and mains neutral (sections C and D in the switch diagram above).
 
(Long pause for jumping up and down and swearing a lot)
 
I was going to scrap the switch but, after a day to think about it, I stripped it down and managed to repair it. It now copes with a 750V DC isolation test with no problems.
 
The separate 125V feed to the indicator lamps works well and the switch operation looks original, Off - Standby - On - On. Finally the amp is producing 100W rms into a dummy load.
 
 
 
 

 
15) It Works!
 
I didn't have any way of measuring the bias current directly (no cathode resistors) so I set the bias voltage to -34v as shown for the Selmer PA100. Turning out the lights and just getting rid of the red-plating gave a final setting of -36v.
 
 
Plugged it into a Marshall 2x12, stood well back and switched on. Success. No loud bangs or smoke and all the pots were fairly quiet. Plugged in the workshop Dano U2 (wanted something cheap to drop if I had to run) and turned everything up to halfway.
 
 
This thing is loud and definitely vintage. No extreme top end, but a good thick sound (not easy with a U2). I've only played through it for ten minutes or so but it seems to be cleaning up slightly so I guess it's burning off ten years of damp and decay.
 
 
 
 
 
 
 
There were two main problems. The first was a very loud background hum, eventually completely cured by re-fitting the two screening plates around the pots. The second was mechanical noise from a couple of microphonic EL34s. This can only be cured by replacement and that will have to wait.
 
The chassis is held in the cabinet by four 'spire' or 'U' nuts. The originals are long gone, so these are M5 replacements obtained from ADI Components in Derbyshire.
 
 
 
 
One potentially dangerous area is the metal mesh covering the bottom cut-out of the cabinet.This can get bent backwards when the chassis is pulled out and stick up into the electronics.
 
 
 
 
Be sure to cut off any wire that can't be flattened out and secured.
 

 
16) Cabinet Tragedy
 
Thought it was time to have a closer look at the cabinet. Big mistake!
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
     
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
This must be the most unlucky cabinet in the world. What looked like part of Hunstanton beach falling out of the bottom was actually the remains of the plywood after the woodworm left. They probably left when the floods arrived to saturate the bottom four inches of their home. Oh well. Put the kettle on.
 
 
 
 
 
 

17) OK. So it's next year and the Sun's out again. Time to get back to it.
 
First step was to cut away all the rotten ply in the side panels.
 
 

    DSCN0299small.JPG DSCN0303small.JPG

Next, the reinforcing batten along the lower front edge had to be split up with a wood chisel. The screws which hold the baffle board to it had disintegrated and wouldn't shift. 

  DSCN0301small.JPGDSCN0295small.JPGDSCN0297small.JPG

 

 

 Meanwhile, the bottom of the speaker baffle has been soaked in wood hardener and fitted with a new support batten. The scrap wood is to hold the plywood layers flat while the hardener dries.

DSCN0304small.JPG 

 

Had a look for replacement plywood but came across the old problem. Sixties cabinets were made from 3/4" ply which isn't available anymore.

The modern version is 18mm in Europe and (so I'm told) 23/32" in North America. That's about 18.3mm.

Genuine, old fashion 3/4" (especially if it's got wet) is actually a nat's over 19mm and that causes problems when you butt two sections together. The base panel won't matter, but the new side patches might need packing out.

We'll find out soon.