Hi Folks! I'm new here and hope to plan to stay for a while.
Here is the story, a friend of mine, who once played in a DIY punk band, just gave me this guitar amp because he no longer needs it and it has some troubles and I was the fellow he thought could get this bad boy back to life.
It has four Electro Harmonix EL34 and four various 12AX7 (not super sure, two are ducktaped to the insides)
It works, but has loads of problems and I would like to know where to start.
1. It hums quite loudly - like 30% of the volume is hum.
2. The third EL34 runs too hot - got orange once and started sparky, sparky stuff, immediately I shut it off. Still runs.
3. third and fourths of EL34 are darkened.
4.Volume pot is broken, apparently it runs at max at all times (cause for problem 2?)
5.Duct tape
6. Soldering
I would like to know what would be the right order to revive this Punk. Troubleshoot, maybe get some approximate schematics this could be based upon?
It is not a pretty sight! You have been warned!
If you wonder how this thing used to perform, look up a band called Bikes Against the Machine. They should be in last.fm.
Here is the story, a friend of mine, who once played in a DIY punk band, just gave me this guitar amp because he no longer needs it and it has some troubles and I was the fellow he thought could get this bad boy back to life.
It has four Electro Harmonix EL34 and four various 12AX7 (not super sure, two are ducktaped to the insides)
It works, but has loads of problems and I would like to know where to start.
1. It hums quite loudly - like 30% of the volume is hum.
2. The third EL34 runs too hot - got orange once and started sparky, sparky stuff, immediately I shut it off. Still runs.
3. third and fourths of EL34 are darkened.
4.Volume pot is broken, apparently it runs at max at all times (cause for problem 2?)
5.Duct tape
6. Soldering
I would like to know what would be the right order to revive this Punk. Troubleshoot, maybe get some approximate schematics this could be based upon?
It is not a pretty sight! You have been warned!
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
If you wonder how this thing used to perform, look up a band called Bikes Against the Machine. They should be in last.fm.
Looks like a death trap
Someone has already been in there replacing parts.
I love the "just tape this tube and socket on there, its probably not gonna catch fire"🙁
First question: Are you comfortable working with these voltages?
Someone has already been in there replacing parts.
I love the "just tape this tube and socket on there, its probably not gonna catch fire"🙁
First question: Are you comfortable working with these voltages?
I think couple of guys have been in there, she's a real mess..
I have done some electronics repairs previously, dishwashers, transistor amps, pcs and even projectors, but not a tube amp. I'm a mechanical engineer, so it is not really my thing so mush, I do it for the DIY spirit. I do take some safety precautions, but I assume that the caps don't hold charge well due to their age, but I think I will be able to find a high wattage resistor to discharge them.
I have done some electronics repairs previously, dishwashers, transistor amps, pcs and even projectors, but not a tube amp. I'm a mechanical engineer, so it is not really my thing so mush, I do it for the DIY spirit. I do take some safety precautions, but I assume that the caps don't hold charge well due to their age, but I think I will be able to find a high wattage resistor to discharge them.
I know it's bad, but I really wish to have a functioning tube amp for my Strat. I think I have some options sourcing parts from this old beast. I guess it would be quite a lot of work rebuilding the entire thing and getting new tubes when it is in a reasonable condition, but why not. The case, metal construction, transformers, most of the caps seem in reasonable condition. What is the plausibility I need new ones? How to find out?
Does anybody know a circuit diagram I could follow? 8 Ohm speaker cabinet intended.
Does anybody know a circuit diagram I could follow? 8 Ohm speaker cabinet intended.
If a valve is glowing red then it faulty or the grid bias to it is wrong.
Could be a dodgy coupling capacitor.
Or a fault in the bias setting or circuit.
Could be a dodgy coupling capacitor.
Or a fault in the bias setting or circuit.
To me it looks like your best bet is going to try and basically make a schematic for the way it sits and find out if it's based on anything and work from there replacing electrolytic caps and burnt up components, or you could simply keep the transformers, sockets, and maybe the tubes if they're good and you could rebuild the amp into any kind of amp you would want as long as the preamp uses the same number of triodes.
1. the humming is possibly caused by AC heaters, try putting the heaters to ground reference via a resistor. the hum should be reduced.
2. Try finding the bias pot and adjust it all down, then slowly turn it up until a proper amount of current is flowing through the valves. (you can find instructions on google on how to do this )
3. Possibly because the are aged, and the bombrament of electrons caused the glass to darken. also this can be the cause for hum.
4.replace it.
2. Try finding the bias pot and adjust it all down, then slowly turn it up until a proper amount of current is flowing through the valves. (you can find instructions on google on how to do this )
3. Possibly because the are aged, and the bombrament of electrons caused the glass to darken. also this can be the cause for hum.
4.replace it.
I got most of the hum away by finding three dead caps. Replaced them, but something still seems to be wrong. One EL34 still overheats, measured the voltage, it said 5.4v. Guess it is too low. But why to they overheat then? Amps? The preamp is supposedly a Mesa Boogie dual rectifier clone. I found the previous two guys who got around this piece of kit. They both said it is a mess.
My next step is to do the biasing, can it be the cause for the overheating?
You should be able to check the bias voltage with the amp on standby - that is, if it has a standby switch. You want to see a minimum of -42vDC but for the time being, adjust it for the most negative it will go. Check and confirm this voltage on pin 3 of each power tube socket.
The amount of hum and non-functioning volume control points to a bad ground connection at least on the volume pot. Without a schematic, it will be hard to find bad grounding. If you take the time to draw out the schematic, you'll probably find bad connections, solder joints etc. Plus, we'll be able to help you with any design problems.
Good luck!
..Joe L
The amount of hum and non-functioning volume control points to a bad ground connection at least on the volume pot. Without a schematic, it will be hard to find bad grounding. If you take the time to draw out the schematic, you'll probably find bad connections, solder joints etc. Plus, we'll be able to help you with any design problems.
Good luck!
..Joe L
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