A '75 Marshall Artiste has just arrived from
@Exorcist dad. Beautiful condition. The only worry is the power tubes are running red hot, like the whole tube seems on the point of exploding red hot. I quickly turned off the amp. Sounded great though. This is only when the standby is off.
Is this simply a Biasing issue?
Here's the link to the photos that clearly show the circuit board.:
http://www.thefretboard.co.uk/discussion/152857/
Is the bias control the black disc-like part on the lower RHS of the circuit board above the standby switch?
Does anyone know where I should set up the multi-meter on the artiste?
And lastly regarding draining the caps, is turning the amp off with the standby off exactly the same as a dedicated current drain? Other than touching the bias, I have no plans on fiddling around, having been electrocuted once already (holding my mate's bass guitar while he tried to fix his bass amp where the wires in the mains cable on the bass amp had managed to join, sending the mains through the bass through me and out of my guitar into my amp... once bitten, twice shy).
Comments
You can often spot ones that are on the way out if the bias trimmer is right up to the end of its travel, and especially if a previous tech has bodged the feed resistor by changing it to a smaller value or piggybacking another in parallel - this is the *wrong* solution and will actually increase the leakage in the caps even if it temporarily gives a bit more voltage. Given that the circuit works correctly with the original values, it's simply faulty logic to modify the circuit rather than fixing the cause of the problem.
If either of the caps has failed, always replace both. If it's a '75 it will be a PCB one, and in my opinion it's acceptable to snip the old caps out (as close as possible to the cap body) and solder the new ones to the wires, rather than lift the board which is a big job on these - undo all the pots and jacks, if you really have to.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
A lot of the modern models (JTM60/JCM600, DSLs etc) do use a bridge rectifier and it's yet another of the weak points in the design, although the jumper connectors to the adjuster board on the DSL/TSLs are the most common cause of trouble...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
What was less good is that on a lot of 70s Marshalls, the standby switch works by disconnecting the HT secondary of the power transformer, and the bias voltage is derived from this too via a larger than normal resistor. The problem is that with the large resistor, it takes a long time for the bias voltage to rise and so for the first few seconds after turning the standby to 'play', the power valves run wide-open.
The solution is luckily simple - move the bias feed wire (the white one) to the PT side of the standby switch so it's always on. The Artiste in the pic in the thread linked above has either had it done, or left the factory like that - the bias supply is the white wire (3rd from the right-hand end of the PCB), the resistor above it and the diode above that.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I've just seen this and feel terrible! It seemed fine when we tried it at my parents house (and fine 20 years ago when I borrowed it for band practice) but it has never been serviced in its life... I certainly don't remember the valves glowing.. but I guess I may have never looked while it was powered on. I sincerely hope its a quick fix :-( Please let me know how you get on.
Simon
Thanks for that - I know what you mean about the weight, when he decided to sell it I had to carry it down the stairs for him, I'm not sure my back has fully recovered! :-)
Cap failures when they're subjected to voltages they haven't seen for years are very common, which is why I always advise that an old amp that hasn't been used for a long time should not be just 'turned on to see if it still works', but powered up carefully under controlled conditions by a tech. It can be spectacular if it's one of the main filters that goes.
All electrolytic caps will need replacing sooner or later anyway.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The use of a single diode to produce valve bias is not really poor design since the current required is microscopic. There might even be an advantage since the ripple produced is at 50Hz, well below the capabilities of most guitar speakers! A bridge produces 100Hz, about "A" on the bottom string? Mind you, the filter caps needed are low in value and cost in either case.
Could of course be a hangover from pre-semiconductor days when a valve diode was needed to provide bias for big Mother PA amps? These went to Copper oxide then Selenium rectifiers. Both of which were less reliable than the valve! But then, big M PA amps usually had a relay bias interlock to save the amp's ass if the bias volts failed. I think the big Ampeg has that IC?
Half or full wave design I still think a dedicated winding is the most elegant solution.
Dave.
Well I wouldn't go THAT far Fends! The HT-40 uses a bridge bias rect and I have never known one fail. It is a wee DIL thing but rated at 1A and so massively over rated for the current required.
That is the case with bridge rectifiers in general, they are so well above the current demand of even a big valve amp that they hardly ever fail, that is of course assuming the mnfctrs don't pare the spec to the bone! No need because even a 1200V 10A bridge is remarkably cheap. TVs of old sailed closer to the wind with 1.5A devices but again, rare to have one fail.
Dave.
That's not quite how it works, the mean time between failure would be the same.