Gibson Mercury II ......... bit of a challenge !
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This page was last modified on Wednesday, July 28, 2010 20:00
1) You'ld 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 in 1995 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.....
(I've since been told by Buzz that the list of previous owners is actually a bit more complicated, so apologies to anyone who got missed out)
Have a look at Alastair Duncan and John Fisher's web site www.fenderstrat.co.uk . 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.
...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 VIP's music is still very much available. Amazon have a compilation CD, with 28 tracks of studio and live recordings, on their preview system.
Audio CD (November 27, 2006) The Complete V.I.P.s [Import][Original recording remastered] Label: Repertoire ASIN: B000K2RD3K
Update - 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
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. Presumably the speakers were changed at the same time although I haven't found a date code on them yet.
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. These dates tie in nicely with what Buzz said about it being still as Eight Hertz bought it in 1975.
3) The whole chassis has been sprayed with grey paint at some point, probably to stop the rust (what an interesting time I've had trying to scrape the #### off.)
4) The speaker cabinet when new would have had one 15" and one 10" CTS/Jensen speaker 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 cabinet back to normal, but what would be the point? The original speakers won't take the power anyway and it would destroy forty five years worth of history. Apart from cleaning and minor repairs, I don't intend to do much to it.
5) So why did it stop working? Well I've got more sense than to turn it on so I'll take a guess. There is evidence of a short between a screen grid resistor leg and one of the heater wires. The resistor has been very hot and burnt off the printing. The broken valve you can see in the other pictures was probably just accidental damage. Unfortunately, history or not, the damaged wiring around the output valves has got to be replaced if the thing is ever going to be gigged.
A lot more worrying is the corrosion around the mains on/off switch. Gibson used a four way power switch designed to work with the USA mains system. The Mercury handbook explains it very well.
...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 essence 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 required with UK three pin, grounded plugs.
The existing switch is rated at 3A at 125V or 1A at 250V, although 240V operation wasn't an option on the original amp. Given the state of it, it might be safer to fit a new one. The original fuse holder has been changed to a 20mm version floating in a grommet (presumably to clear the 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 other components are the modifications.
This is what the original would have looked like.
7) Progress:
Work on cleaning up the chassis is well under way (lot more to do). The front panel was a lot worse than it looks in the 'before' pictures. Virtually all of the plating was corroded and floating on very active rust and was just falling off. I'm quite surprised how much of the screen printing survived. Shame it got into this state, but hopefully it won't get any worse now.
I'm having trouble finding replacements for the missing control knobs. They were made by Rogan (who also made knobs for cookers) and measure 28.5mm across the circular part. I need seven altogether.
8) Output valves.
Got out the AVO valve tester today.
I took out the four EL34s 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 position with the screen resistor shorted to the heater wire.
AVO test results should be: Ia = 76mA, mutual conductance = 11mA/V
Tests settings VA = 250V, VS = 250V, VG= -13.5V
Valve a.
Manufacturer: Mullard
Date code: Xf3 B5G1
Insulation (hot) cathode to heater: 24M
Ia: 76mA
Gm: 9.6mA/V
Valve b.
Manufacturer: Unknown
Date code: none
Insulation (hot) cathode to heater: 15M
Ia: 69mA
Gm: 9.2mA/V
Valve c.
Manufacturer: Mullard
Date code: Xf3 B5F4
Insulation (hot) cathode to heater: 2M
Ia: 79mA
Gm: 11mA/V
Valve d. (Heat damage + locating lug missing)
Manufacturer: Unknown
Date code: none
Insulation (hot) cathode to heater: 25M
Ia: 76mA
Gm: 8.5mA/V
The cathode to heater leakage on 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 incoming mains power is controlled by a single pole, single throw unit at the back of the assembly (the third contact isn't connected inside the switch).
Underneath the SPST section is a steel disc and the pressed on actuator.
Below the 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.
The next drawing is a representation of what goes on inside the wafer.
The source code stamped into the metalwork indicates a manufacturing date of September 1963. This ties in nicely with the early 1964 pot codes.
10) Mains wiring
I know I was going to leave as much as possible alone, but, since the mains wiring is so badly corroded, I've got to re-do it. The paxolin panel on the transformer is broken, it's missing the selector link and it's been hard wired to the 245V position. The link's a bit vulnerable sticking out of the back of the cabinet so the mods are probably not a bad idea. I've put it back to normal for the time being just to test the other voltage taps. The actual transformer is an old Selmer. (See update in section 1)
11) Indicator lamps
The neon indicators are the original Leecraft "Snaplite"s rated at 125volts, 1/3watt. Both were suffering from brittle insulation on the connecting leads and one lead actually fell off when I moved it. The orange lamp also had an open circuit series resistor.
The only way in is to cut around the white part of the body at the wire end.
One new 47k resistor later, plus some new wires, and they're Araldited back together again. The black bands are glue-lined heat-shrink sleeving. Now the power switch is clean and I've had a look inside I'll have a go at re-using it.
All I've got to do now is figure out how to wire it. If the mains input selector gets put back together then the neons will be fed from the 125V tap, using the mains TX primary as an auto-transformer.
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 (plus I'm trying to keep things original). They both read OK for value on a bridge and I've had a quick go at re-forming one of them. So far it's going well although I need to rig up a higher supply voltage.
The first photograph shows one of the cans up at 80volts drawing 6microamps through a 1K resistor. The lid off a Nitromors can is just to save the ceiling if it goes bang. I've got to put it into something a bit safer before taking the voltage up any more.
The other cap was reading just over 34uF on a Marconi bridge before I knocked it setting up the photograph!. The internal oscillator is running at 1kHz rather than 100Hz but it'll do for the time being.
I didn't put any of the dents in the can (the other one's just the same) so these things must have been around the block a bit in the past. (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 down to the bare chassis and powder coat it, but that would have meant disturbing all of the vintage wiring and that wasn't the idea.
14) Switch On.
After checking the earth continuity and high voltage insulation, the amp was powered up slowly with a variac to start reforming the capacitors. It's worth remembering that the front end power supply 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 to come 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 re-secure the detent mechanism. 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 over 100watts rms into a dummy load. There's a lot of ripple with a continuous sine wave and the bias current needs setting up but we're nearly there.
15) It Works!!
I didn't have a quick way of measuring the bias currents directly (no cathode resistors) so I set the bias voltage to -34 volts 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 noises or smoke and all the pots were fairly quiet. Plugged in the workshop Dano U2 (I wanted something cheap to drop if I had to run) and set everything half way.
This thing is LOUD and the sounds are definitely vintage. There's none of the extreme brightness that you'ld get from modern designs and even the U2, which would normally shatter glass, sounded usable. I've only played through it for ten minutes or so but it seems to be cleaning up slightly so I'm guessing that it's burning off five or ten years of damp and decay.
The next step is to get the tremolo working. The oscillator is OK but the LDR assembly and footswitch are missing and the wiring has been altered. Should be fun.
16) Back soon.