Bad hum on Epiphone BC30 low power mode.

snowblind

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

snowblind

Finally got around to rebuilding the switch for the 15/30W mode select. These switches are the worst and to further complicate matters they use a 25mm square bezel and apparently nobody on the planet now makes such a thing. The hum only occurs in 15W mode which if you look at the schematic just grounds out part of the output section turning it from a class A/B push-pull into a class A. Kills half the tube in effect. Can anyone work out where the hum might be coming from? Bad output tube maybe? I will try swapping the tubes out but if the thing works ok with the whole output section running I'm thinking that may not be the issue.

ICBM

snowblind said:

Finally got around to rebuilding the switch for the 15/30W mode select. These switches are the worst and to further complicate matters they use a 25mm square bezel and apparently nobody on the planet now makes such a thing. The hum only occurs in 15W mode which if you look at the schematic just grounds out part of the output section turning it from a class A/B push-pull into a class A. Kills half the tube in effect. Can anyone work out where the hum might be coming from? Bad output tube maybe? I will try swapping the tubes out but if the thing works ok with the whole output section running I'm thinking that may not be the issue.

No, it switches between ‘pentode’ and ‘triode’ mode, both Class AB.

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.

snowblind

Thanks for that. If you look at the back of one of these things it does say 30W Class A/B and 15W Class A and I'm an idiot so I believe it. 

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.  

ICBM

snowblind said:

Thanks for that. If you look at the back of one of these things it does say 30W Class A/B and 15W Class A and I'm an idiot so I believe it.

Not your fault. There is unfortunately so much bullshit about “Class A” that it’s much easier to list the amps that are actually Class A than the ones which claim to be but aren’t… a good working assumption is that none of them are!

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.)

snowblind

Makes sense to a degree. The Socal 50 50W version does use EL34s and it is largely the same layout. It has a 25/50W switchable option too. Maybe the design works better there. Or would if they hadn't used the same crappy switches. Honestly the construction of the switches relies so much on luck and goodwill it is amazing anyone could consider it a good manufacturing choice. Another nice design let down by poor execution. The standby is a sword of Damocles hanging over these things from birth.

snowblind

Pair of russian 5881s turned up today. Let's see if it is the power tubes......

snowblind

Aaaanddd no. Still hums on 15W mode. New tubes do sound nice and sparkly though. Better than the original Sovteks I would go so far to say. 

pt22

Dumb question. Are you standing too close while testing with a single-coil guitar? I had a 633 Engineering Butterfly that hummed like the devil if I was anywhere near it. As soon as I moved 10 or so feet away it quieted down. 

snowblind

No. If it was pickup from the transformers it would do it in 30W mode too. Good shout all the same. Things can get a bit noisy if I get too close to the HiWatt with the strat for instance but that's not the problem here. Hum kicks in as soon as it switches modes. I have two of these BC30s. The first one did hum a bit on both settings when I first got it but a new rectifier tube cured that. The second has also had a new 5AR4 but no difference. 

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. 

ICBM

I would remove the switch, make the amp permanently 30W, and think about whether it could be usefully re-purposed elsewhere in the signal path where it’s not as critical, eg as a cathode bypass cap switch, or something in the preamp.

snowblind

I probably will go down that route. The switches themselves are garbage so getting them out of the picture entirely would be no bad thing. I tend to run amps clean and do all the work with pedals so don't need a great deal of flexibility from the amp itself. My good BC30 is in a twin amp setup with the HiWatt where it adds some fill to the sound and helps with movement from the wobby stuff like the vent. Generally set and forget apart from matching volumes.

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.

snowblind

@ICBM Question for you if I may. If that mode select switch were to operate on only one out the two power tubes would that cause the hum?  Effectively this would have one tube in triode mode and the other as tetrode. Although I reckon I have rebuilt the switch and it seemed to test ok if there is a break elsewhere and only half of it is having any effect I'm guessing that would not be a happy outcome? 

ICBM

If the switch wasn’t working properly on one side it would shut that valve down - the screen grid must be connected either to the HT or the plate for the valve to work. That would certainly cause the amp to hum, but it would also sound oddly distorted.

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.

snowblind

Ok. I'll do some digging with that as the guiding theory.

snowblind

Had a chat with Lyle (aka psionic audio) who suggested adding in some screen grid resistors. The problem he suggests is similar to one that afflicts Tone King Sky Kings which have an "Ultralinear mode" switch on them which does effectively the same thing the Epi is doing.

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.

ICBM

snowblind said:

Had a chat with Lyle (aka psionic audio) who suggested adding in some screen grid resistors.

If the schematic above is correct then it already has screen resistors - R45 and R46.

You could possibly increase their value, although they're the standard for 6L6s.