Marshall Mercury 2060 – Repaired!!
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 your 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 unplug 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.
Finally! This thing is working and sounding loud and clear! Ear deafening treble! All the reports are right, for sure, this thing doesn’t need a tone pot to turn up, it needs a different filter to allow more bass through.
I just can’t believe what a simple problem it was all along, but one that I would not have suspected, but could have found with a bit of common sense in hindsight, but then I would not have learned so much along the way with all the research I’ve done, wanting to know as much about anything I do, generally.
The transistors arrived today, and I replaced T1 and T3 then measured the voltages from the circuit.
Disappointed to find T1 at 0V again, I knew that was still not going to solve the crap sound problem without a working gain stage 1.
T3 however, had something like correct values – 28V – a bit high but ok, and seemed to be working, as I could hear the mains hum being modulated when the controls were turned up (it takes a bit of time for resonance to happen). At least something was fixed properly. It sort of worked before, but seemed temperamental.
After a cup of tea and some thinking, I wondered what could be “blowing” the T1 tranny – that I assumed was happening on turn on, and assumed that the high voltages I was reading like 33V at T2 collector (lucky that tranny survived), and 295-330V ripple at point X (later found out its the Trem that causes that variation on the DC meter) was that maybe someone had changed the mains transformer for a higher output one in an attempt to make it more powerful or something, putting more than 30V across T1 CE which is the max it can take from the BC184 spec sheet.
If T1 was short, I thought, it would also give about 0V at the collector (the same, I thought, as having R5 open circuit) and also account for higher than normal voltages across the rest of the circuit now that the T1 gain circuit had no voltage dropped across it – can you see how I got sent down the wrong diagnostic path there?
Only then, thinking about the 0.3V at T1 collector, and not exact 0V, did I start to think that resistors could fail also, (last thing on my mind in terms of more likely components such as transistors, then electrolytics).
I had already checked the capacitors for shorts, but not checked the resistors.
The problem was R5 – the 220k resistor at the collector of T1, which was open circuit!
Luckily, I had bought a 220k to make the cap drain lead at the start of this project, so used that, and that got it all going.
I don’t know how I’m going to record a demo of this yet, so you can hear it, but I’ll be taking it to college to show some of the guys, as my lecturer is a Marshall fan and has a couple himself, so will try and record something there using the studios.
I will add all the voltages that I read for this amp on a copy of the circuit diagram to help anyone else that has one of these amps, to refer to.
It’s been a long old journey, but I have learned a ton of stuff, and definitely feel more confident about tackling the Aston Electronics 5W kit build – which also arrived today.
I’m really pleased I didn’t give up on this, like a almost did 2 weeks back, just to be able to hear what this thing actually sounds like after all the work (which is a LOT better than I thought it would be), and know that I can sell it on knowing it has a few years (decades?) left in it yet. The more I play it the more I like it, (you have to play with the guitar volume and tone controls to get a nice balance) but you won’t get much distortion starting until it’s near full volume (LOUD in a bedroom), and then it’s slight, like a typical old blues amp sound. I just hope someone out there likes toppy amps and wants to give it a good home.
I’ve got other things to get on with now and want to move on. I hope this has been bebficial for those who may have followed the saga, and you now have a better idea of what is inside your guitar and you have a better insight into how it and your guitar/bass work.
My Readings (all T bases are 0.6V):
EL84 pin voltages:
1 0V NC
2 0V grid 1
3 10.9V (cathode) k, grid 3
4 3.4V ac (heater) this is from the 6.3V transformer secondary winding that powered the original mains bulb)
5 3.4V ac (heater)
“Heater filament. In the earliest valves the heater filament and Cathode K was the same element, described as a directly heated Cathode. Approx 1940 onwards (except rectifier valves) the majority of valves, the heater filament and Cathode are separate elements. The Cathode is in-directly heated by the filament. The majority of valve filaments is 6.3V. Early vehicle batteries were 6.3V.”
6 1.4V ac NC (noise?)
7 311V (anode)
8 1.2V ac NC (noise?)
9 311V (anode) grid 2
Final KEY calculation for Valve Bias Current:
The voltage across the 200R pot (that I added as a modification) is 7V. Thus, the bias current (total current passing through the valve from plate to cathode) is the same as that passing to earth via R15 and my 200R biasing pot. This means I can take a voltage reading across the pot to earth (7V), and as the pot is at its maximum resistance value from when I did it by eye as the EL84 was running too yellow (in the last post), I can divide the pot voltage by its resistance:
7V/200 Ohms = 0.035A or 35 milliAmps.
The power dissipated by the Anode plate (the electrons boiling off the heated cathode and being attracted to it) can now be calculated also:
The Anode voltage is 311V.
The Cathode voltage (R15) is 11V
The voltage across the valve is 311-11 = 300V
Power dissipated by the valve = P = 300 x 0.035A = 10.5 Watts
This is a nice amount, just below its design max of 12W, that shouldn’t over tax the valve and shorten its life, and of course it isn’t distorting the sound until near max volume, which is fine.
Checking this against safe settings values for the EL84, I’m happy that I’m not over-driving this valve too much from these pages:
“Assuming that you have a cathode biased amp using EL84s, you do not want to dissipate more than 12 watts, which is the quoted “design center” value on every EL84 data sheet that I have seen. Exceeding this value can dramtically shorten the life of the power tubes and possibly cause major problems if one should fail while in operation. It’s not a bad idea to at least check the plate dissipation when replacing the power tubes in a cathode biases amp – no two power tubes are exactly the same. Some may tend to move more current than others. If you find that the plate dissipation is over 12 watts, increase the size of the cathode resistor until the plate dissipation is at 12 watts or under – 10.5 to 11.5 watts is a pretty good operating range and will give longer tube life. Cathode bias is generally very forgiving due to its very nature. However, don’t assume that just because your amp is cathode biased that it is bulletproof – you just never know what you are going to get with new manufacture tubes.”
Marshall Mercury 2060 – Repaired!!