So, I bought a non-working Ceriatone Matchless DC30 clone for a song (original owner thought the PT was shot, so he bought a set of DC30 iron from West Labs, but sold it to me as is with the original iron and the WL iron for less than the built chassis from Ceriatone would be). I tested the original Ceriatone PT yesterday, and it's fine - it may be a bit under-built compared to the proper iron, but all voltages are on spec coming from the PT into the 5AR4 recto. I'm hoping that I can salvage the amp without having to put in the West Labs iron, so I can repurpose that for an AC30 clone.
Here's where it gets weird - with no tubes, the power supply dropping string isn't dropping any voltages with no tubes apart from the 5AR4 in there. The B+ voltage off of the rectifier is ~365V, which actually seems a bit high from what I'd expect (285 * 1.2 = 340 volts or so, but the PT is currently putting out closer to 300v unloaded). All of the B+ points are also showing the B+ of 365 or so. I also checked the plate of V1a, and that also is showing 365v. I checked all of the power resistors, and they are all on spec. I'm wondering if this is due to the weird Matchless PSU design where all of the filter caps tie off from the screen supply (meaning that there is no current draw across the dropping resistors until the amp has tubes in it). I'm hesitant to just throw tubes in the amp and see, since it seems like a waste of a 12AX7 to send 350v to the plate and nuke it.
That's completely normal in any amp. The tubes are what's drawing the current, so without them there is no voltage drop across the resistors in the PS.
Gaz wrote:That's completely normal in any amp. The tubes are what's drawing the current, so without them there is no voltage drop across the resistors in the PS.
See, and now I feel like a Dumass, since I've only ever checked the B+ off of the rectifier when firing up an amp without tubes.
Sounds like it might be situation normal for this amp then....
put the tubes in and bring the amp up on a variac while measuring amperage if the amperage spikes as you bring up the voltage from 0-117vAC you will be able to see that the circuit has a problem and power down before any harm is done to the tubes or parts.
Trace continuity threw the circuit based on the schematic make notes of anything that is weird and fix it as you go. if nothing pops up as out of place meter components for prpoer values. if that doesn't give you a result of solution.
I hate to say it. but in my experience when taking on a partly built or non working clone it's often faster, easier, and less frustrating to gut the entire amp and rebuild it from the scratch then trying to troubleshoot something that may have never worked.
I agree with Sellout only if your troubleshooting skills aren't up to snuff and you aren't properly equipped to debug tube equipment. You need more than a multimeter to do anything other than basic checking. At minimum you should have a function generator (so you don't have to pluck the guitar with your foot, I think we've all been there ) and an oscilloscope to trace out the signal path. Without these you will end up shooting in the dark and I wouldn't recommend any sort of audio probe for working with HV circuits. On a guitar pedal you can get away with a small amplifier to poke around in a circuit to see where the sound stops but again, there isn't really a safe way to do this with HV tube circuits.
Post some detailed pics if you can, maybe we can spot something that you are missing. It's probably just a single bad part, wrong value, bad solder joint or a missing connection.
selloutrr wrote:I hate to say it. but in my experience when taking on a partly built or non working clone it's often faster, easier, and less frustrating to gut the entire amp and rebuild it from the scratch then trying to troubleshoot something that may have never worked.
Well, this was a Ceriatone built clone, and I've double checked the circuit, so it should be OK as long as the wiring is up to par, which it appears to be.
selloutrr wrote:I hate to say it. but in my experience when taking on a partly built or non working clone it's often faster, easier, and less frustrating to gut the entire amp and rebuild it from the scratch then trying to troubleshoot something that may have never worked.
Well, this was a Ceriatone built clone, and I've double checked the circuit, so it should be OK as long as the wiring is up to par, which it appears to be.
Edit: I put in tubes and brought it to life - it sounds like a DC30 for sure. There is some mild 60 cycle that I think that I can get rid of with a quick chopstick of the amp, but in terms of health, it seems to be OK. Still debating putting in the West Labs PT since the Ceriatone one seems underbuilt and has a slight mechanical buzz to it.
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) and an oscilloscope to trace out the signal path. Without these you will end up shooting in the dark and I wouldn't recommend any sort of audio probe for working with HV circuits. On a guitar pedal you can get away with a small amplifier to poke around in a circuit to see where the sound stops but again, there isn't really a safe way to do this with HV tube circuits.
