• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Ampeg B-15nf blowing fuses

Jimmyupstairs

Jimmyupstairs

Nov 23, 2017
15
9
#1
so after replacing speaker in my 1966 Ampeg B-15nf and getting excited about it after 2 days of continuous playing 2.5A fuse blew. Replace 6L6s and again blew fuse. Replaced rectifier tube and again blew fuse. Removed pre amp tubes again blew fuse. Ordered new 40/40/40 MFD filter cap in can from fliptops figuring this original cap was shorting. Anything I need to know before re-soldering it in? I ordered new clip for this but not sure if this comes with what I need to isolate cap casing from chassis. Read something about being able to reuse original paper for isolation ?
 
#2
Jimmyupstairs said:
so after replacing speaker in my 1966 Ampeg B-15nf and getting excited about it after 2 days of continuous playing 2.5A fuse blew. Replace 6L6s and again blew fuse. Replaced rectifier tube and again blew fuse. Removed pre amp tubes again blew fuse. Ordered new 40/40/40 MFD filter cap in can from fliptops figuring this original cap was shorting. Anything I need to know before re-soldering it in? I ordered new clip for this but not sure if this comes with what I need to isolate cap casing from chassis. Read something about being able to reuse original paper for isolation ?
Click to expand...
Jimmyupstairs said:
so after replacing speaker in my 1966 Ampeg B-15nf and getting excited about it after 2 days of continuous playing 2.5A fuse blew. Replace 6L6s and again blew fuse. Replaced rectifier tube and again blew fuse. Removed pre amp tubes again blew fuse. Ordered new 40/40/40 MFD filter cap in can from fliptops figuring this original cap was shorting. Anything I need to know before re-soldering it in? I ordered new clip for this but not sure if this comes with what I need to isolate cap casing from chassis. Read something about being able to reuse original paper for isolation ?
Click to expand...
DDB93492-C873-494C-9479-6A62771BE51D

DDB93492-C873-494C-9479-6A62771BE51D

B15ng
 
#4
Does the amp work for a while and then the fuse blows or does it blow as soon as you toggle the power switch?

I’m wondering if there’s a nick or an opening at a hot spot in the power line wire sheathing that is causing a short. Lift the wires and carefully inspect all the wiring up to the primary of the power transformer.

Next step is to isolate the power transformer. Unplug all the tubes. The polarity cap is often removed when a three conductor power cord is installed. If it is still be used in the amp, it’s possible that it’s shorted. Unsolder the leg on the polarity switch.

I can’t see how the power cord is wired. The cord black wire should go to the end of the fuse holder. The side fuse holder terminal should go to the power switch. The other end of the power switch goes to one transformer primary. The white wire connects to the other transformer primary. This is shown in the TB Portaflex Wiki:

Technical - Amplifier | TalkBass.com.

If you don’t see any obvious shorts in the transformer wiring and the fuse continues to blow, remove the transfomer. Reinspect all the wires. With the transformer wires disconnected and isolated, if the fuse still blows, chances are there is a short inside the transfomer. It would need to be replaced.
 
#5
It blows within 2 seconds with some 2A fuses I had around. When I used a 1A fuse for the pre amp tube removal test it was instant with no sound at all. I will check the wiring you described. Hopefully the fuse protected the $250 transformer !
 
#6
May I humbly suggest that you take the amp to a tech. Throwing new parts at a problem is not the way to repair your amplifier. Everytime you blow a fuse you are causing more damage. You have to go step by step to diagnose what is causing the fuse to blow. Having a device to limit inrush current is a requirement to aid in the diagnosis.
 
Last edited:
  • Like
Reactions: SAlcott, Balog, SamanthaCay and 4 others
#8
I will try the 3A fuse good point. I should replace the 50 year old electrolytic cap anyway if I want to keep the amp. Do you think I need to worry about discharging it before starting work given that it has been off for 5 days? I would think the current would find a way to leak out after 5 days :-)
 
#9
Jimmyupstairs said:
I will try the 3A fuse good point. I should replace the 50 year old electrolytic cap anyway if I want to keep the amp. Do you think I need to worry about discharging it before starting work given that it has been off for 5 days? I would think the current would find a way to leak out after 5 days :)
Click to expand...

Knowing the B-15 as I do, if they are original, yes change the electrolytic caps. The one inside your amp is not original though, it might be fine. There should be a date code on the cap can that is in top of the chassis. If the one inside was changed, maybe the one on top has been changed as well.

I wouldn’t presume that any circuit is safe unless I tested it with a volt meter. Chances are it is after five days. It takes nothing to discharge the caps, it is always a good idea to do so.

Technical - Amplifier | TalkBass.com

You don’t need anything fancy, stick a resistor (around 2W) between two wires, cover it with heat shrink tubing or an insulator such as electrical tape, an alligator clip on ench end. Clip one end to the chassis, the other to a small screwdriver. Use the handle of the screwdriver to insulate yourself. Or you can buy one from Weber as shown in the link.
 
  • Like
Reactions: spaz21387
#10
May I suggest that you actually identify the actual fault before assuming a solution. I think you will find the repair attempt more successful. Constantly replacing fuses and trying is a good way to increase the amount of damage.

If you can't troubleshoot and identify the cause, I would suggest a qualified service tech.
 
  • Like
Reactions: SAlcott, Balog, SamanthaCay and 8 others
#11
beans-on-toast said:
Knowing the B-15 as I do, if they are original, yes change the electrolytic caps. The one inside your amp is not original though, it might be fine. There should be a date code on the cap can that is in top of the chassis. If the one inside was changed, maybe the one on top has been changed as well.

I wouldn’t presume that any circuit is safe unless I tested it with a volt meter. Chances are it is after five days. It takes nothing to discharge the caps, it is always a good idea to do so.

Technical - Amplifier | TalkBass.com

You don’t need anything fancy, stick a resistor (around 2W) between two wires, cover it with heat shrink tubing or an insulator such as electrical tape, an alligator clip on ench end. Clip one end to the chassis, the other to a small screwdriver. Use the handle of the screwdriver to insulate yourself. Or you can buy one from Weber as shown in the link.
Click to expand...
 
#12
0850B27F-B2DF-4C1C-B1E2-F429AC467A05.jpeg
Here is 40/40/40 MFD 500V cap. Does it look original Beans on Toast?
 
#14
beans-on-toast said:
That is the type that was originally in the amp. There may be a year month date code, last line of the text printed on the cap.

There’s a B-15NF service manual and revision schematics here: Schematics if you don’t have what you need.
Click to expand...
Thanks. I got this amp 15 years ago for $200 from a Japanese musician who was selling everything before moving back and I know who he bought it from too. I brought it too my tech Joe at the time who has since moved to San Diego and he replaced the big blue electrytic cap and the plug with a 3 prong. I have been letting the amp sit in my garage since the speaker blew 10 years ago so I don’t think that was good for that old electrolytic cap which if it’s not original is at least 15 years old probably 20 since Max the owner before the guy I bought it from was a mess....not the kind of guy replacing caps in his b15 so i figure it’s at least 20 years old and I should replace it while parts are still available.
 
  • Like
Reactions: beans-on-toast
#15
I wouldn't change the cap unless you KNOW it's the problem. Forget everything you have read on the internet about how you have to replace caps like some people replace underwear. I see more amps screwed up by indiscriminate parts swappers than just about everything else.
 
  • Like
Reactions: SAlcott, Balog, petrus61 and 2 others
#16
agedhorse said:
I wouldn't change the cap unless you KNOW it's the problem. Forget everything you have read on the internet about how you have to replace caps like some people replace underwear. I see more amps screwed up by indiscriminate parts swappers than just about everything else.
Click to expand...
It’s hard to have that definite knowledge at this point because the amp won’t power up so I can’t check voltages. If I want to measure resistance /capacitance with a meter I have to remove the element from the circuit and in the case of this > 20 year old electrolytic capacitor which is a pain to remove I might as well replace it after I do all the work to remove it. The electrolytics dry up over time and turn the cap into a short circuit. Also Joe my tech who moved to California is telling me this is the logical next step before we get ultimately to the $250 power transformer.
 
#17
Good grief, what happened to systematic troubleshooting. 5 minutes with a real tech could get to the bottom of this without guessing, without removing parts, and without charging the customer for unneeded parts.

This makes me sick.
 
  • Like
Reactions: SAlcott, Balog, Kelly robinson and 1 other person
#18
The good news is that the capacitors will be available in the future. They are currently being manufactured by CE Manufacturing (aka tubesandmore), many suppliers sell them. Invalid Link Removed. It’s important to use the isolation washer to prevent ground loop hum. The old one can be reused if it isn’t damaged, it’s riveted to the chassis.

I can't comment on the condition of your cap can but based on the ones that Ampeg used that I've seen, there is a good chance that it needs to be replaced. The can sits next to a very hot running power transformer, it sees stress over time because of this. The FP cap cans can have a long service life if operated conservatively, that isn't so much the case in these amps.

Most of the vintage B-15's that I've seen have had their power transformers replaced, they have a high failure rate. As I said, they run hot. Although expensive, the good news is that the Heyboer replacement units that fliptops sells are excellent performers. I hope that a transformer failure isn't the problem in your case.

Since your amp has sat for 10 years, at the very least the electrolytic caps should be reformed if they can be. When an amp is in storage, it should be run for a half hour or so in playing mode (standby off) at least once a year to reform these capacitors.

Andy is correct in saying that too many people rush to recap amps. Techs make bread-and-butter money doing this sort of work when it isn't necessary. They do this to pad out their billing. Choosing to do it because you want reliability is a different story, you should do what you feel is best.
 
Last edited:

Latest posts

Top Bottom
Back