DISCLAIMER: The following is NOT to be taken as a definitive procedure for repairing ANY electronic device and the author takes NO responsibility for any damage or injury that results from anyone using this guide. It is intended for educational purposes ONLY.
If you have ANY doubt about making modifications or repairs to your own equipment then seek advice from relevant qualified persons.
Valve amplifiers use and can store high AC and/or DC voltages that can KILL. You have been warned!
Ebay guide quote to bear in mind:
“I don’t usually get on my high horse, but I keep on seeing various eBay items that explain how to service your own valve amplifier, etc.
Note: For anyone who pooh-whos this and chooses to ignore what I am about to say, at least do one thing; ALWAYS ensure you have a RCB (sometimes called RCD) into which you connect your amps power supply – These units are similar to what is used in the garden with hedge trimmers – If a full short occurs then the mains 240VAC (or 115VAC) is automatically disconnected. All modern houses and public venues should be equipped with these in place of the old fusebox, but hundreds of thousands still use the fusebox as the only form of protection, which are not instant by a long margin. The bottom line here is better safe than sorry.
Unfortunately most people I come across have not even considered this fundamental safeguard, which costs around a fiver.
The thing I find so amazing is that they will happily remove the chassis and dabble about inside [innocently] thinking that only 240VAC is present – well it jolly well isn’t. Simply put, and with particular respect to the output tubes/valves (EL34, 6L6, EL84, etal) they require a high Voltage Direct Current (VDC) to drive them and this can easily be in the 400 – 500 VDC range. What does this mean to you if you complete the circuit – one hell of a shock or even possible death. Also, with DC voltage you do not “shake” the same as if shocked with AC (Alternating Current), which runs at 50 – 60 cycles per second or Hz (hertz). With AC voltage you at least stand a chance of letting go, with VDC your muscles will “clamp-on”.
Whenever I get an amp in for repair or a service, I always do 3 things before even switching on the power:
1. Check that if it has a mains EARTH connection that is in place and has not been disconnected. Amazingly some musicians will disconnect their amps earth wire to stop hum and then they wonder why they get shocks when they touch the microphone stand, which is earthed!!!.
2. Check that the mains fuses (and any internal ones for that matter) have not been fuse wire wrapped or tinfoil wrapped. Again, I have seen this cardinal sin being committed and why? “Well, the amp keeps on blowing fuses and this has stopped it” is a stock answer. Keeps on blowing fuses – doesn’t that tell you something???
3. Make a visual inspection to see if there are any other peculiarities present e.g. discolouration due to high temperatures or a blown component.
If you are still intent on dabbling (and everyone has to learn), then find a friendly professional repair guy/gal who will allow you to watch the way they treat [with great respect] a valve amp when it comes in for repair/service.
This only touches on the subject, but at least it might help someone understand and think twice before removing that chassis”
There is a big effect difference between voltage levels that break through the skin conductivity – which happens to be in the 400 to 500V DC range of our amp circuits! BE CAREFUL! Drain the secondary capacitors when the circuit is off and always check the circuit with a multimeter on both AC and DC sides before touching components or working on it – with the mains UNPLUGGED. If the Mains is plugged in, the plug socket and switch contacts are LIVE still, with the switch in the OFF position.
If you are testing the circuit after a change, make sure all you test leads, probes and croc clips are well insulated from your fingers and chassis contact points (chassis edges can be sharp too). I always connect the croc clips and meter and double check. I then plug in the fused mains side. I use the mains toggle switch to power the secondary with the indicator light connected so I can SEE power also, as well as checking the on/off position of the switch BEFORE I plug in the mains. I switch OFF and unplugged the mains, checking the meter voltage or current level has dropped to 0V, (or use a drain lead on the capacitors ASWELL) before I move any croc clips to a new circuit point for another measurement.
For the sake of a few extra seconds, why risk safety by not switching off and draining the circuit?
A 240V AC shock will certainly help you not make that mistake again if you ever forget any of this – if you are lucky enough to not get away with it! My last one left my right arm aching for an hour!
A 500V DC shock may mean you don’t get to worry about any of this ever again…but now you know why.
Ok, that’s the scary bit over, so let’s continue the potential fun/frustration/satisfaction of fixing something that was broken – if possible.
History: This all came about due to me thinking about building one of the online guitar amp valve kits from sites such as in the US here:
or the UK here:
Once I had decided I would buy one and build it over Xmas, I saw this Marshall on Ebay, for spares or repair, won it, and collected it the same night.
Initial research on valves and valve amp kits meant watching lots of YouTube vids on valve amp repair, valve operation and manufacture, (some great old stuff too!) listed below.
VERY IMPORTANT – Draining the capacitors before working on any valve amp first:
There are loads more of course, so do your own research and I advise buying a book before you even power up any amp that you don’t know the condition of, so you:
A: minimise any further damage
B: get familiar with the circuit and components visually, and spot obvious component faults like burn outs, capacitor leakage, blown fuses (why did it blow in the first place?) etc.
C: You can test short circuits that should not be there with a multimeter, like mains wires shorted to earth, and transformer wires shorted to earth etc.
Building Valve Amplifiers (Paperback)
Valve Amplifiers (Paperback)
by Morgan Jones (Author)
So, the amp arrived home and I took some initial shots of its general condition, model number, serial number (00331), power info etc:
I researched some forums about the history of the amp and found a circuit diagram for the 5W version, though mine is badged as a 25W version. I will compare component layout and values as best as I can when I get to that point.
I made an initial reminder list of important things to do once I have removed the chassis from the cab e.g:
- Safety first – check capacitor charge, particularly on C17-18, as these charge to 275V on the circuit (point X), where the mains transformer gets rectified after the diode D1
- Check for shorts and correct continuity on mains input lines and transformer lines; check fuse
- Use an RCD in the mains chain when first powering up at the end of repair
- Check valve model IS an EL84 (if possible from valve print number)
- Order new EL84 or equivalent asap if valve looks duff
- Print circuit diagram and check off components present on PCB
I removed the speaker board from the front – the wood screws were missing anyway, so it fell out when tipped forward.
I removed the amp chassis from the cab by undoing by a few turns, the two wood screws underneath the chassis, behind the speaker mount board.
DANGER! Check for residual charge on the capacitors with a multimeter set on the highest DC range, and move down the range. I found 0V across the 2 largest blue electrolytics – as expected as I have had the amp 24 hrs, and it has probably been days or weeks since last powered up.
I hoovered out the spiders webs etc. First thing I noticed was the EL84 power output valve was blackened at the top, which would follow from what I know so far – that there was very faint audio coming from the speaker according to the previous owner when he powered the amp and tested it with a microphone. This would follow if the power valve was dead. If you follow the circuit above, you can see a path for pre amp audio that can make its way to the speaker output transformer even with the power valve removed – looks like it may be only a replacement valve required at this stage?
The seller is a 2nd hand store dealer with little tech knowledge, and said that this amp had come from an old man’s basement.
I briefly checked the PCB tracks on the outside of the PCB for signs of non-original soldering and to see if any tracks or components were corroded enough to make me doubt if there was continuity to any, or definite visible breaks which would be a big problem. Only one area looks a little corroded here:
I next removed the valve retainer spring and undid the 5mm nuts that hold the PCB to the chassis and tilted it back to get a better look at the components in the other PCB side – all components look like quite new condition. This amp must have been stored in a dry environment.
I removed the EL84 valve using the “circular wiggle” technique from the Net vids I have watched.
Pretty burned out!
Here’s some info on the Philips EL84 valve originally made in 1953:
The PCB board also names the capacitor and resistor numbers, so the next step will be to check off the component values against the schematic I have, and try to log and re-draw any different component positions I find if different from the 5W circuit. I could not find a 25W circuit from this site or any other – it appears they are one and the same amp – maybe Marshall did some dodgy number crunching – Peak to Peak power instead of RMS etc. to get the 25W figure? Most web info rates it as 5-10W.
so may have to do more research to see if one exists – all I have read on the Mercury so far states they were all 5W versions.
The only electrical work I did today was check for short circuits and continuity on the mains lead and mains fuse – all ok.
I did find some useful tone control related info for the Mercury on a forum:
The tone control has to be set at 0 else the highs are just ear biting pick axe in y face horrors. Any value changes or small rewires I could do to get it a bit more useful? c7 does not exist on my board, it’s just c8
The pre-amp section’s I’m sure are not push the output valve enough, the amp can take any pedal well and they can all be set at 100 level and as much gain as I want and the amp starts to sing, problem being when I switch off there is a massive volume drop so i’d like to get the pre amp pushing it more. the amp is mine to keep so I’m not bothered about breaking into the board or adding a sub board etc.
I also don’t mind disconnecting the vib circuit and using the traces for any improvements.
C7 is a standard “bright” capacitor, looks like C8 is an extra bright capacitor.
Try removing these (desolder one leg temporarily of each cap to see the effect removing each one has). Remove C8 first.
You can also try increasing the values of C6, C10, C13, to get more bass through the amp. start off with (say) 47n for C6 & C10, then (say) 100n for all three. This may give too much bass, but its what your own ears tell you is good, as to where you end up.
Before playing with any valve amp, unplug the power cord from the wall, and discharge the power supply capacitor C18. Use a 220k 1W resistor with insulated aligator clip leads. Dont electrocute yourself. Be careful
Hope this helps…..
I don’t want to modify any components in this amp – certainly until I have heard the default sound anyway – as it is rare, so I want to leave it as original as possible. I will probably sell it once it is working anyway, and move on to a kit build project after this, but this info is useful for understanding what the various capacitors do to input impedance and tone.
Ordered a replacement EL84 from Ebay (Voc Roc) for 13.50 pounds inc postage.
Cut the speaker wire near the speaker solder point, leaving a bit of the red and black sleeve to know which wire to solder back, to free the chassis up and get a better look at the speaker frame also.
The code T1692 led me to this site:
where they have one for sale of the same type – a “green back”:
Celestion (Rola) G12 “Small” Green Back 4Ohm. Model: G12 | Date Code: BJ27X Dates Feb 1976, | Frame Code: T1692 | Cone Code: 1777″
Which looks the same as mine, except mine is the 10″.
My date code is GJ15X
Here’s date code sites:
I know this amp was made from 1972-3, so the Celestion speaker seems to have been replaced at some point, and made on 15th July 1976. What the X means, I don’t know.
It measures 4.4 Ohms DC resistance, so would be a 4 Ohm AC impedance speaker.
I found the red speaker cable solder joint shorts to the chassis, which puzzled me for a mo, then I checked the circuit diagram which shows one side of the speaker coil is actually grounded:
Once the chassis was free, I could get a good look under fluorescent light to see the state of the tracks on the PCB – which aren’t so good in places, and there is evidence of prior track repair attempts, from a sharp solder spike on one track. I will have to research conductive track paint and possibly redo all the tracks.
I wrote a capacitor check list on paper from C1 to C25 and CT, which are the highest numbers printed on the PCB and compared all the values I could read against the values on the circuit diagram.
This was a good thing to do, as I could check how close my actual circuit is to the diagram, and apart from a few different values, it all adds up suitably. I have a CT that I realised related to C17 and C18 on the diagram, as the PCB has shapes for two capacitors, whose roles are filled by one large 32nF capacitor that has two anodes, separated by a 1k resistor, and one cathode at earth.
C4 and C23 are absent from my circuit.
I’ll continue this Post on day 4 later.