I have a power amp I ALWAYS run from a Variac as it is designed for 115v and my wall voltage ranges from 120-125v. I have a Kil-a-watt on the Variac so I can ensure I am delivering 115v. I also know, via kil-a-watt, it draws just under 2amps.
The Amp has a 5amp slow blow fuse.
The Variac has a 5amp fast blow fuse.
In the past couple months I have blown a several fuses in the variac. This has never happened before and now it seems to be a chronic issue.
After first fuse replacement the amp ran fine for 10 or more hours.
After the second fuse replacement the amp ran fine for an hour.
I opened up the amp and checked values and voltages - everything seems fine.
Powered it on and it ran for a few hours and blew the fuse again
Tonight I made a Dim Bulb tester using a 300w incandescent bulb.
I powered everything on and the bulb had a slight glow and amp ran fine.
I then put the Kil-a-watt after the bulb and powered everything on; the Kil-a-watt showed under 100 watt. Turn the Variac to max and still only 100 watts.
I'm assuming I should have gone with a larger bulb but also not entirely sure.
My goal was to run it connected to Dim bulb and when the amp acts up I would see a change in the glow and could investigate.
So ultimately, what size build should I run?
The Amp has a 5amp slow blow fuse.
The Variac has a 5amp fast blow fuse.
In the past couple months I have blown a several fuses in the variac. This has never happened before and now it seems to be a chronic issue.
After first fuse replacement the amp ran fine for 10 or more hours.
After the second fuse replacement the amp ran fine for an hour.
I opened up the amp and checked values and voltages - everything seems fine.
Powered it on and it ran for a few hours and blew the fuse again
Tonight I made a Dim Bulb tester using a 300w incandescent bulb.
I powered everything on and the bulb had a slight glow and amp ran fine.
I then put the Kil-a-watt after the bulb and powered everything on; the Kil-a-watt showed under 100 watt. Turn the Variac to max and still only 100 watts.
I'm assuming I should have gone with a larger bulb but also not entirely sure.
My goal was to run it connected to Dim bulb and when the amp acts up I would see a change in the glow and could investigate.
So ultimately, what size build should I run?
It almost sounds like you could entirely remove the Variac and find the right wattage bulb to use continuously for the 'small' voltage drop.
This would introduce a degree of current limiting to your amp, but with large power supply capacitors, you would still have 'dynamic output power'.
More information about your power amplifier would probably really help others to make a more comprehensive comment / post 🙂
This would introduce a degree of current limiting to your amp, but with large power supply capacitors, you would still have 'dynamic output power'.
More information about your power amplifier would probably really help others to make a more comprehensive comment / post 🙂
Last edited:
It is a Dynaco based model from https://www.tubes4hifi.com/amps.htm#ST120 <-- ST120, specifically.
I built it from a kit about six years ago.
In the six years the only issues I've had are:
Apparently the transformer is designed for 115-120v. Monitoring my line voltage I found it can swing between 120-125v on any given day. The 127v must have been an anomaly. I never had the issue again since on the variac.
Sometime in the past few years they have changed the transformer for one that can deal with higher line voltages.
When running normally it draws just under 2amps according to the Kil-a-watt that sits between the variac and amp.
My end goal is to run the amp with the circuitry exposed while on the dim bulb so if/when I see the bulb light up I immediately investigate and locate the suspect component. Also, I could be on the dim bulb while probing with a chopstick (so I do not kill myself).
I built it from a kit about six years ago.
In the six years the only issues I've had are:
- Red plate on one output tube. I replaced the tube and the BIAS resistors on all the output tubes.
- In the first week I blew the 5amp slow blow fuse a couple times - thats when I discovered my line voltage was 127v.
Apparently the transformer is designed for 115-120v. Monitoring my line voltage I found it can swing between 120-125v on any given day. The 127v must have been an anomaly. I never had the issue again since on the variac.
Sometime in the past few years they have changed the transformer for one that can deal with higher line voltages.
When running normally it draws just under 2amps according to the Kil-a-watt that sits between the variac and amp.
My end goal is to run the amp with the circuitry exposed while on the dim bulb so if/when I see the bulb light up I immediately investigate and locate the suspect component. Also, I could be on the dim bulb while probing with a chopstick (so I do not kill myself).
I would have thought the variac fuse (assuming an input fuse) should be a time delay type and not fast. Variac's have high inrush current and any voltage fluctuations or brownouts I would think could easily pop a fast type.
Does the amp 'act up' or are you assuming the fuse blowing means the amp 'has acted up' which are two totally different scenarios?
Does the amp 'act up' or are you assuming the fuse blowing means the amp 'has acted up' which are two totally different scenarios?
In all cases where the variac blows its fuse there is ZERO audible indication of an issue.
Music is playing and then fades out as the caps drain.
The first time the fuse blew I jumped to the amp immediately and I "believe" I saw a puff of smoke but it also could have been imagined or a reflection off my glasses from an overhead light. When I opened up the amp and checked everything I could not find any indication of a burned out component.
I referred back to my build document and checked all voltages and followed the initial power up sequence and everything checked out.
Initial sequence as I recall:
At this time I must assume there is something askew with the amplifier.
The variac has been in place for years.
The first fuse that blew in the variac was in place for several years.
Since this is a "intermittent" issue and does not appear to be heat related - at least not obviously - my hope is the dim bulb would be a canary in the coal mine that I can monitor and investigate with brightness changes.
What I do not want to do is just start replacing components in hopes of resolving the issue.
Thank you for taking the time to reply to my post.
Music is playing and then fades out as the caps drain.
The first time the fuse blew I jumped to the amp immediately and I "believe" I saw a puff of smoke but it also could have been imagined or a reflection off my glasses from an overhead light. When I opened up the amp and checked everything I could not find any indication of a burned out component.
I referred back to my build document and checked all voltages and followed the initial power up sequence and everything checked out.
Initial sequence as I recall:
- Power up with zero tubes in place and check voltages at the rectifier and maybe a couple other places.
- Power off, install rectifier. Power up and check voltages again in a few places.
- Power off, install output tubes on left channel. Power up and set bias, check voltages.
- Power off, install output tubes on the right channel. Power up set bias, check voltages.
At this time I must assume there is something askew with the amplifier.
The variac has been in place for years.
The first fuse that blew in the variac was in place for several years.
Since this is a "intermittent" issue and does not appear to be heat related - at least not obviously - my hope is the dim bulb would be a canary in the coal mine that I can monitor and investigate with brightness changes.
What I do not want to do is just start replacing components in hopes of resolving the issue.
Thank you for taking the time to reply to my post.
It sounds like its just random fuse blowing tbh. I really would try an anti surge fuse. Maybe your mains supply is less constant lately and suffers transients that would pop a fast blow fuse. Also check any plugs/sockets/wall sockets/fuse holders etc in the mains supply to the variac.
If the amp fades out when the fuse pops then its not drawing excess current in the scheme of things at that time. One random thought is whether the amp or the variac has any 'self healing' mains filter caps across L and N. Its all a bit unlikely but worth considering.
I would think if one of the output valves for example had an intermittent issue you would here a definite pop as something shorts internally.
Put a time delay fuse in 🙂
If the amp fades out when the fuse pops then its not drawing excess current in the scheme of things at that time. One random thought is whether the amp or the variac has any 'self healing' mains filter caps across L and N. Its all a bit unlikely but worth considering.
I would think if one of the output valves for example had an intermittent issue you would here a definite pop as something shorts internally.
Put a time delay fuse in 🙂