Gibson Mercury II ......... bit of a challenge !
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This page was last modified on Sunday, January 31, 2010 00:45
1) A thing in this state would normally get broken up for spares, but this one's a bit special. I got it from Graeme (Buzz) Elliott up in Cumbria. He sold it on behalf of his 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.....
Have a look at Alastair Duncan and John Fisher's web site about Cumbrian bands (huge amount of stuff and well worth a visit).
Click on the section about Jimmy Henshaw and The VIPs.
This describes 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......
Update. 22nd June 09
Having looked into it a bit more, this amp's history is a bit more complicated. I tried to contact previous owners but got nowhere, so we'll have to stick with what we've got for the time being.
2) So....What to do with it ????
Well, for a start, I'd like to get it working and actually use it. This isn't a strip down/rebuild job, it's a one off and deserves some sympathy. We'll see how it goes. What's it worth? Moneywise, £205 plus postage, so not a lot. As a piece of rock history, priceless.
3) This is going to get complicated now because it isn't actually a standard Mercury inside the box. The power section has been rebuilt at some time as a typical 1970s 4xEL34, 100watt job. Output valve bases, electrolytics, mains and output transformers are new. The choke is original Gibson.
Who did it and when (and why)? I don't know yet, 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 probably came out of another amp.
These dates tie in nicely with what Buzz said about it being still as Eight Hertz bought it in 1975.
Did Jim Henshaw mod it? Possibly. He was an engineer as well as a musician and would have been more than capable. Sadly Jim died in 2007 so we may never know. If anybody remembers any more please get in touch.
4) One thing I'm pretty sure he didn't do was spray paint the whole thing grey. What an interesting time I've had trying to scrape the bugger off.
5) 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.
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.
6) 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 supplied 110v power plug was only two pin and could usually be inserted either way round. This feature isn't required with UK three pin, grounded plugs.
The existing switch is rated at 3A at 125V or 1A at 250V. If you take 180 to 200watts mains power as a ball park figure for a 100watt valve amp then it would draw roughly 800ma at full power so (if it can stand the switch-on surge) this switch should survive. However, 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.
7) 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.
8) 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.
If anyone's got any info about the amp or any of the people mentioned I'd love to hear from you. Updates uploaded when I get time to work on it.
9) 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
Mu: 9.6mA/V
Valve b.
Manufacturer: Unknown
Date code: none
Insulation (hot) cathode to heater: 15M
Ia: 69mA
Mu: 9.2mA/V
Valve c.
Manufacturer: Mullard
Date code: Xf3 B5F4
Insulation (hot) cathode to heater: 2M
Ia: 79mA
Mu: 11mA/V
Valve d. (Heat damage + locating lug missing)
Manufacturer: Unknown
Date code: none
Insulation (hot) cathode to heater: 25M
Ia: 76mA
Mu: 8.5mA/V
The cathode to heater leakage on V c doesn't really matter in this circuit as there's very little potential between them.
10) 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.
11) 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 looks suspiciously like something out of an old Selmer (good news if it is).
12) 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 it, I'm happy that it's safe to use again.
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 could do with a 125V feed rather using extra series resistors from the incoming 240 volts. This arrangement would use the mains TX primary as an auto-transformer.
13) 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 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.
This is the other cap. It 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.
14) Power Switch Wiring.
I finally made a start on rewiring the power switch. This is it loomed up on the front panel and ready to refit. The three unused contacts are where the polarity selector would have been. This is version two, the first try had the two HT (red) leads tied in with the mains wire.
15) More soon.