Ampeg B-15 Trouble

[CaptainKirk]

I'm working on a B-15-Nf. It came in with a complaint about distorted sound when playing the Low E. The owner claims the distortion occurs only when the amp is mounted on the cabinet. When I played through it, the distortion seems to occur when the amp is not on the cabinet too. Hmm . . .

I found it needed a cap job which is complete. The sound tightened up and became more defined. The owner retubed the amp before I go it with Groove Tube 6L6GC's and 5AR4 and Amperex 6SL7's. I also swapped the rectifier tube on the idea of possible microphonics but the distortion remains.

I also noticed the bias level in each 6L6 is 50-53ma (cathode current). This seems a bit high but I can't imagine that would cause distortion. I also checked the speaker by connecting it to another amp - no problem. I connected the B-15 head to another cabinet - no distortion.

Any Ideas?

[tim]

Remember that this is only a 40 watt amp (at best)...when you are playing a low E, you are pulling a fair amount of current through the tube pair. Try backing off the bias current a bit and see if it cleans up at all. You mentioned that there was no distortion through another cabinet. Was the speaker compliment the same? Also, this will sound obvious but several years ago, I had a customer bring an amp in that he said was "distorting" on low notes...what was happening was that it wasn't distorting but one of the casters on the bottom of his cabinet was up in the air a bit and was vibrating, making noise when a loud, low note was played...probably not your issue but it's something to tuck away in the "chalk it up to experience" locker.
Also, the B-15 is not really a loud amp by todays standards.

[txstrat]

I agree. I built a B-15 copy for the bass player of my then band. He complained about a rattle/distorting sound on the low E too. It really sounded distorted but what had happened was, the magnet of the speaker had come loose. Since the speaker had been fixed the amp's played great.
Not that it has to be the speaker in any case, but the B-15, although it's only 40w, produces very deep/low notes that can bring speakers to their limit.

[Rob Mercure]

If this amp has the factory 15" speaker then wattage is irrelevant as the speaker is rated for, I believe, 100 W. Has the customer checked his bass through another amp/speaker combination? Bass rattles are often an artifact of playing style and low action interfering with a fret. And it's extremely unlikely that a rectifier would cause a rattle at a specific frequency as the storage time of the filter caps would "ride out" the rattle.

Rob

[Ted]

Those cathode currents are very high.

I'd expect to see high a maximum of high 30s for combined anode and screen current.

Check the bias voltage at the smoothing capacitor - should be around -51V.
If its lower than that (i.e. closer to 0V) suspect the capacitor. The dropper resistors in the bias circuit also drift with age - check their values.

[robyo]

I have a '65 b15n which may have had the same issue. Turned out the cab has a double baffle which became slightly loose over the years. There are 4 nuts inside the speaker cab you could need to sung down.
maybe worth checking out.
Rob

[Ted]

I have a '65 b15n which may have had the same issue. Turned out the cab has a double baffle which became slightly loose over the years. There are 4 nuts inside the speaker cab you could need to sung down.
maybe worth checking out.
Rob

Good point. B15s tend to sound awful if any of the baffle screws come loose.

Another thing worth checking is the seal round the top of the cab where the amp sits - it should be perfectly airtight, If not you can put self-adhesive rubber draught excluding tape round it. (Fliptops do the proper seal).