So finally got the BC30 I bought recently and which was battered by UPS in transit out of its box to look at. Amp powers on ok and everything seems to work, mostly. Problems are a bad hum when in 15W mode, a lot of noise on the channel switching and what sounds like some microphonics on the tone stack switch. The hum practically disappears in 30W mode.
I have swapped out the chinese 5AR4 rectifier for a Sovtek option. This cured a hum on my other BC30 but sadly not here.
I have shuffled the valves around a bit internally to see if any of problems move. They do not.
From what I can see this amp has probably never been out of its box since it was built back in 2006. Looks pretty much unused.
On that basis then I am thinking that likely suspects are the electrolytics.
Might be simple corrosion issues but the usual "operate the switch a few times" doesn't change anything.
Knowing that the switches in these things are garbage that will be one point of inspection. Might get lucky and it is just a bad switch.
Visually there are no obvious signs of decay. None of the caps appear blown and there is nothing untoward leaking out to be seen with the naked eye.
If anyone has any other suggestions as to where the issues might lie I'm all ears. Below are a shot of the insides and a pdf of the schematic.
https://drive.google.com/file/d/1AaK4mTm2GJqeBbHv7bb058XqqmDKx3CZ/view?usp=sharing https://drive.google.com/file/d/1cTej5iRfWWVsNT93Ftfw_N8vGt3C0CSk/view?usp=sharing
Old, overweight and badly maintained. Unlike my amps which are just old and overweight.
Comments
Technically, since 5881s are beam tetrodes and not pentodes, operating them as triodes defeats the point of the beam tetrode design and often causes more hum. It’s not a good idea even with true pentodes - it’s just a bad design for a bad reason.
I would just run the valves correctly as pentodes (tetrodes) and use the switch to disconnect C29 instead, which will limit the power when overdriven.
"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
Since I never use 15W mode normally there is the temptation just to bypass the switch altogether. Doesn't do anything for the sound overall other than thin things out. Easy enough just to turn it down although it would be nice if whatever the problem is causing the hum wasn't there. I've seen a master volume mod for one of these so if it stays around I might try that for a laugh.
My Mesa Blue Angel says “Pure Class A Power” on the front panel too, and it absolutely isn’t.
The schematic clearly shows it’s a screen grid switch which runs the valves in triode mode, although it also incorrectly labels the other mode as pentode. (It would be if they were EL34s, but 6L6/5881s are beam tetrodes.)
"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
From what I can see this thing has been in its original box unused for nearly 20 years so there's a good chance there is a bad electrolytic in there somewhere. I've also noticed that compared to the other one the drive channel seems a bit weak but that may just be down to oxidation in the pots and switches where it has sat in storage.
I'm going to stare at the schematic for another couple of hours to see if I get any inspiration then I will probably cave and take it to someone competent. Luckily I know someone who was an old mate of Jim Marshall so I'm covered on that front.
There is a bit of a worry that it could be one of the transformers. Guitologist has a video where he worked on one of these (not his finest work I will say) but part of the fix involved replacing the power transformer. I'm not convinced that the fault all along wasn't just the standby switch but whatever. It all worked in the end but the p/t cost nearly as much as the amp would have done new.
Since it is only 15W mode which I never use the temptation is just to take the switch out of the circuit altogether and fix it in 30W. That has the advantage of getting the crappy switch out of the equation thereby removing the risk of it disintegrating in the future and causing more damage.
I don't really want the thing to end up as just spares for the other one but there does come a point at which the investment stops being sensible. I always favour fixing stuff instead of throwing it away but given the plethora of decent 2x12 combos around which can be had for the cost of a tank of gas or a couple of month's worth of wine if I were still drinking (expensive tastes in plonk) , coupled with the fact that I don't need another amp, limits the resources I can put into it. At the very least it would make a decent 2x12 expansion cab.
"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 links into the switches are just standard quick-connects so I'm thinking just bridge the wires with something like a blade fuse and wrap it all up out of the way.
I would be inclined to either follow the wires back to the PCB and connect those points permanently with wires, of if that's too tricky then cut the push-connectors off, solder and heatshrink the wires together. Don't bodge it with something that could fail or come loose later.
"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
Some of the issues he has dealt with in the Tone Kings sounds remarkably similar to the Epiphone although the Tone Kings retail for $3K compared to the $5-600 the BC30s went for when they were new. Don't think the Epi has some of the design issues the Tone King boards seem to have either.
You could possibly increase their value, although they're the standard for 6L6s.
"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