HT fuse blows in SVT 3 Pro

[shughey]

I have an SLM made in USA SVT 3 Pro that has been blowing the 1/8 amp 250V HT fuse. I'm using the correct slow blo fuses and it I don't see any arcing or a big catastrophic event (flash, smoke, burning smell, etc.) when it happens. It typically happens during power on. The fuse won't blow everytime, it "seems" random. I can run the amp for an hour and it won't blow, but if I power it on and off a few times over the course of a day, it may blow. The mains fuse is ok. When I inspect the couple of blown fuses I've used during troubleshooting, I've noticed they don't appear visibly damaged, but do measure open.

None of the MOSFETs are shorted and the amp otherwise appears to operate normally (I haven't noticed a loss or increase in power or loss of functionality of a particular knob/feature on the amp). I had a JJ ECC803S in the 12AX7 driver spot for a while. I recently installed a higher-gain JJ ECC83S in that spot. I'm wondering if this is playing into whatever the issue is. Also, while troubleshooting another issue with the mute circuit, the line and neutral wires of the convenience plug arced since I didn't have them properly isolated. Really bad, but it did happen and that initially blew the HT the first time. I've done some troubleshooting, but all the schematics I've found seem to slightly differ from my amp's layout, so it adds to the complexity of narrowing down the root cause. I'm not an EE, but an eng tech and most of my daily work is not with audio amps.

 

[agedhorse]

That fuse has nothing g to do with the MOSFETs. It’s part of the HV tube plate supply.

Just note that there are errors in the schematics on line.

Use proper safety procedures.

 

[shughey]

That fuse has nothing g to do with the MOSFETs. It’s part of the HV tube plate supply.

Just note that there are errors in the schematics on line.

Use proper safety procedures.

Good to know about the errors noted in the 3 Pro schematics floating around.

I'll be practicing proper safety precautions while working on the amp. Just a bit of a tough one for me since it's not a fault that is easy to replicate like the issue I had with the mute circuit for instance.

 

[shughey]

That fuse has nothing g to do with the MOSFETs. It’s part of the HV tube plate supply.

Just note that there are errors in the schematics on line.

Use proper safety procedures.

So I took a look at the B+ voltage going to each tube and the DC voltage looks fine per the table in the online schematic. I measured the ESR of the electrolytics that are in parallel with the HV supply, and they measured low per ESR table on my meter (not sure if this absolves them from being at fault however). Any suggestions on where else to look or even something I may be missing with the measurements I already took? Is it possible the issue is on the other side of the transformer or on the bridge rectifier part of the secondary, or not probable?

 

[agedhorse]

If you took ESR measurements in circuit, the caps will look low and leaky.

It's possible that one of the caps is intermittently faulty, not common since they caps are well spec'd though. If you remove for testing, you need to be very careful not to damage the PCB.

 

[shughey]

If you took ESR measurements in circuit, the caps will look low and leaky.

It's possible that one of the caps is intermittently faulty, not common since they caps are well spec'd though. If you remove for testing, you need to be very careful not to damage the PCB.

yes, all ESR measurements were in-circuit. Taking them out does look painful. All I have is a solder removal braid and I'd have to peel all that silicone (?) off. Was holding off on removing those at least until I rule out whatever lower hanging fruit there may be.

 

[agedhorse]

Yes, braid can be very problematic. You want to remove the cap in a way to minimize damage to the PCB.

 

[crazyBassClown]

I recently installed a higher-gain JJ ECC83S in that spot. I'm wondering if this is playing into whatever the issue is.

If the fuse started blowing just after installing this tube the first thing I would do is change it back and see if it stops. Long before attacking the board and measuring caps.
It is possible it has an intermittent short internally or some other failure, and it is also possible it is not the problem at all. If the fuse still blows with a different tube in there then you can safely assume it's something else.
edit: preferably the previously known good tube...

 

[shughey]

If the fuse started blowing just after installing this tube the first thing I would do is change it back and see if it stops. Long before attacking the board and measuring caps.
It is possible it has an intermittent short internally or some other failure, and it is also possible it is not the problem at all. If the fuse still blows with a different tube in there then you can safely assume it's something else.
edit: preferably the previously known good tube...

I'm in the process of seeing whether it blows with the original tube back in place. Since this issue is intermittent, I may have to give it some time before I can rule it out. Since it seems to happen during power on, I'm powering it on and off every now and again throughout the day.

 

[crazyBassClown]

Power on may also indicate a problem when cold. If you are feeling adventurous/bored you could try spraying freeze spray onto suspect components and see if it triggers a fuse blow although this is probably more helpful for things that fail when hot and getting them to work again by cooling.
Tubes will probably not be very happy if sprayed though(I've never tried it personally)

 

[mike buzzetti]

Power on may also indicate a problem when cold. If you are feeling adventurous/bored you could try spraying freeze spray onto suspect components and see if it triggers a fuse blow although this is probably more helpful for things that fail when hot and getting them to work again by cooling.
Tubes will probably not be very happy if sprayed though(I've never tried it personally)

NEVER freeze spray at tube !!! if it HOT it will most likely break.
WHats the current rating on the fuse ?? if you can find one with a slightly higher rating you might get away with it. say like a 1.25 A vs 1.0 amp

 

[crazyBassClown]

if you can find one with a slightly higher rating you might get away with it. say like a 1.25 A vs 1.0 amp

Using a higher rated fuse than the manufacturer recommends is a good way to go from blowing fuses to burning up stuff...
May make it easier to diagnose though

 

[Old Garage-Bander]

NEVER freeze spray at tube !!! if it HOT it will most likely break.
WHats the current rating on the fuse ?? if you can find one with a slightly higher rating you might get away with it. say like a 1.25 A vs 1.0 amp

Good advice on the freeze spray.
Cautionary though on increasing the fuse. I know you are suggesting a very minor change, but this is a slippery slope. What if the next guy, or the same guy at some point in the future tries to bump it up again?
If the properly sized fuse keeps blowing, then there is a reason for it. It means something is wrong and could get wronger (yeah I know) in the future. When you increase fuse size you also place more strain on the circuit it was intended to protect. You don't want a transformer or a tube protecting the fuse.

 

[mike buzzetti]

Using a higher rated fuse than the manufacturer recommends is a good way to go from blowing fuses to burning up stuff...
May make it easier to diagnose though

Hence the 20 % increase , the circuits are designed for 50-100 % over spec normally ( Aged can give you better #'s for new stuff ) dont use FOIL ! from a cig pack like I used to do as a kid LOL!

 

[Old Garage-Bander]

Using a higher rated fuse than the manufacturer recommends is a good way to go from blowing fuses to burning up stuff...
May make it easier to diagnose though

Yup. Better ways to do this than bigger fuses.

 

[mike buzzetti]

Another option is to pull the output tubes and see if it persists , If it does ,its not tube related.
do you have an old set of tubes ?? if so try them and see what happens , it sounds like it is a surge current issue .,

 

[mike buzzetti]

Yup. Better ways to do this than bigger fuses.

Hence the 20% increase , well within design spec 99% of the time.
or spend $500 and send it to a tech ?? up to the owner.

 

[wcriley]

Damn! You guys have me wanting to spray tubes and other hot things with Insta-freeze.

 

[Old Garage-Bander]

Hence the 20% increase , well within design spec 99% of the time.
or spend $500 and send it to a tech ?? up to the owner.

Yes. I acknowledged that it was a small change. But the OP has to decide to fix the amp or put a bandaid on it.
If it needs $500 in repairs, it needs $500 dollars in repairs. A qualified tech should be able to give a fairly accurate estimate for needed repairs. The OP then has to decide if it's worth it. He'll only be out the diagnostic fee, which is usually waived if he decides to have it repaired.

 

[agedhorse]

N
WHats the current rating on the fuse ?? if you can find one with a slightly higher rating you might get away with it. say like a 1.25 A vs 1.0 amp

Never.

Another option is to pull the output tubes and see if it persists , If it does ,its not tube related.
do you have an old set of tubes ?? if so try them and see what happens , it sounds like it is a surge current issue .,

There are no output tubes.

Hence the 20% increase , well within design spec 99% of the time.
or spend $500 and send it to a tech ?? up to the owner.

No, it's not "within the design spec". The fuse is selected by the designer based upon the needs of a circuit, if the needs are 20% higher then the designer would have spec's a 20% higher fuse.

As it is, as long as the caps haven't been "upgraded", the sizing is adequate.