here is a question
For a number of reasons most manufacturers fuse before capacitor banks
I come across one amplifier 6 output transistors per ch 40.000 uf per ch supplied with 50+50 volt Hybrid, trs output, trs drivers, tube first stage, and fused between output trs and capacitor bank ...
All fine but for no reason it seems that this amplifier may blow fuse in one rail
pop the hatch , replace the fuse and ready to go amplifier plays like hell biased according to the manual and seem to operate flawless
then again after few days blows a fuse and here we go again
Checked the basics almost everything appears to be fine, contacts,soldering , insulation , bridges and so on while the weird thing is that the amp is a monoblock so owner has another one that behaves exactly the same
Fused after the capacitors has anything to do with it ? Most of the times fuse issue takes place at powering up ....
Amplifier is Vincent SP-T100
Any ideas ?
For a number of reasons most manufacturers fuse before capacitor banks
I come across one amplifier 6 output transistors per ch 40.000 uf per ch supplied with 50+50 volt Hybrid, trs output, trs drivers, tube first stage, and fused between output trs and capacitor bank ...
All fine but for no reason it seems that this amplifier may blow fuse in one rail
pop the hatch , replace the fuse and ready to go amplifier plays like hell biased according to the manual and seem to operate flawless
then again after few days blows a fuse and here we go again
Checked the basics almost everything appears to be fine, contacts,soldering , insulation , bridges and so on while the weird thing is that the amp is a monoblock so owner has another one that behaves exactly the same
Fused after the capacitors has anything to do with it ? Most of the times fuse issue takes place at powering up ....
Amplifier is Vincent SP-T100
Any ideas ?
If the design were flawed, then all the amps of that model would be blowing fuses. If there is an intermittent problem, chances are static measurements won't reveal it.
Is this your amp for your own use? or is it a customer amp returned to customer but returned with more blown fuses? Amps do not work isolated, they work in an environment. SO consider the speaker wires could be frayed, or bitten through by a house pet or mice. The speakers themselves could have some internal issue. Are the speaker loads proper for the amp, or are extra speakers wired in?
Do you witness the blowing of the fuse? Or do you turn it on to find the fuse has failed? If you are there when it blows, is there any sonic evidence? Look at the blown fuse. is it just open? Or is ther black evidence of a large spark inside the glass?
Fuses come in slow blow and fast, are yours supposed to be slows and we are using fasts?
Monitor mains current draw. The amp may be biased correctly, but the system still might be drawing too much current. Of it may draw a good amount at idle, but ramp up to excess levels at high volumes. You could also put a current meter in the fuse line to see if the fuse is handling current close to its value or has enough headroom.
And a connection problem could exist. Monitor current draw while it is operating, then use your fist or a rubber mallet and whack the top or ends of the amp to see if jarring it physically causes any current spikes.
Is this your amp for your own use? or is it a customer amp returned to customer but returned with more blown fuses? Amps do not work isolated, they work in an environment. SO consider the speaker wires could be frayed, or bitten through by a house pet or mice. The speakers themselves could have some internal issue. Are the speaker loads proper for the amp, or are extra speakers wired in?
Do you witness the blowing of the fuse? Or do you turn it on to find the fuse has failed? If you are there when it blows, is there any sonic evidence? Look at the blown fuse. is it just open? Or is ther black evidence of a large spark inside the glass?
Fuses come in slow blow and fast, are yours supposed to be slows and we are using fasts?
Monitor mains current draw. The amp may be biased correctly, but the system still might be drawing too much current. Of it may draw a good amount at idle, but ramp up to excess levels at high volumes. You could also put a current meter in the fuse line to see if the fuse is handling current close to its value or has enough headroom.
And a connection problem could exist. Monitor current draw while it is operating, then use your fist or a rubber mallet and whack the top or ends of the amp to see if jarring it physically causes any current spikes.
Hi Guys
The convention for fusing is that all windings of the PT must have a fuse. A centretapped winding should have fuses at each end - a CT fuse does not provide full protection.
All of the above fuses are slow-blow to accommodate cap charging and transformer magnetisation currents.
Fuses after cap can be either slow or fast blow depending on the circuit and the type of protection required. For example, in a tube amp, a fast-blow HT fuse that is ONLY for the OT is fine.
The fact both monoblocks of the OP behave the same way suggests that there is a design flaw consistent to the model. "the amp is a monoblock so owner has another one that behaves exactly the same"
Since it is a hybrid amp with a tube front-end, it is likely that there needs to be a turn-on mute between the circuits that shunts the connection point to ground. This will allow coupling caps to charge without using the semiconductor junctions as the charge path. In a typical hybrid output stage, the semis will stabilise a lot faster than the tube does, so some means of keeping it stable during this period has to be implemented by the designer.
Fuses placed as described by Sajti are not preferred. They are effectively in the audio path and can introduce distortions of their own.
Have fun
The convention for fusing is that all windings of the PT must have a fuse. A centretapped winding should have fuses at each end - a CT fuse does not provide full protection.
All of the above fuses are slow-blow to accommodate cap charging and transformer magnetisation currents.
Fuses after cap can be either slow or fast blow depending on the circuit and the type of protection required. For example, in a tube amp, a fast-blow HT fuse that is ONLY for the OT is fine.
The fact both monoblocks of the OP behave the same way suggests that there is a design flaw consistent to the model. "the amp is a monoblock so owner has another one that behaves exactly the same"
Since it is a hybrid amp with a tube front-end, it is likely that there needs to be a turn-on mute between the circuits that shunts the connection point to ground. This will allow coupling caps to charge without using the semiconductor junctions as the charge path. In a typical hybrid output stage, the semis will stabilise a lot faster than the tube does, so some means of keeping it stable during this period has to be implemented by the designer.
Fuses placed as described by Sajti are not preferred. They are effectively in the audio path and can introduce distortions of their own.
Have fun
Could be something as simple as fast blow fuse instead of slow blow.
Perhaps someone has designed in a fuse that is borderline and sometimes blows when caps are powering up.
40,000 is quite a high capacitance and will put quite a load on the fuse on power up.
Many amps with that size of capacitor will use a soft start circuit.
Perhaps someone has designed in a fuse that is borderline and sometimes blows when caps are powering up.
40,000 is quite a high capacitance and will put quite a load on the fuse on power up.
Many amps with that size of capacitor will use a soft start circuit.
sorry guys but none of the speculations match the case. so one by one :
--Speaker load ? nothing to do with it done it also without load ( fuse blew in front of my eyes)
--Scope says there is no oscillation ( looked very carefully )
--Fuses are typical fast 15A fuses ( originally ) ( to my understanding those are already quite high )
--Yet again the fuses are after the caps so i expect that they don't see inrush current from the charging of caps
--Struth might have a point here there is a sequence on powering up on the various stages and one more relay beyond protection relays that probably controls first power to the tubes and then activates the rest of protection circuit in the output. There is a chance that the time line of this circuit has some type of failure if the capacitors that create the time constant failed through the years or values are drifted ( need to verify that but at this point i don't have a schematic )
--did witnessed one fuse blown in front of my eyes but at this point it was a no load condition Then i replaced the fuse ( while before i made all known tests ) and after that amp is working like a charm
--fuses blew at no load conditions and no volume conditions so i dont think that it has anything to do with excessive power or too low load ....
So i expect after this analysis that the only think that makes sense is to make sure that the time line of powering up is working correctly ....
--Speaker load ? nothing to do with it done it also without load ( fuse blew in front of my eyes)
--Scope says there is no oscillation ( looked very carefully )
--Fuses are typical fast 15A fuses ( originally ) ( to my understanding those are already quite high )
--Yet again the fuses are after the caps so i expect that they don't see inrush current from the charging of caps
--Struth might have a point here there is a sequence on powering up on the various stages and one more relay beyond protection relays that probably controls first power to the tubes and then activates the rest of protection circuit in the output. There is a chance that the time line of this circuit has some type of failure if the capacitors that create the time constant failed through the years or values are drifted ( need to verify that but at this point i don't have a schematic )
--did witnessed one fuse blown in front of my eyes but at this point it was a no load condition Then i replaced the fuse ( while before i made all known tests ) and after that amp is working like a charm
--fuses blew at no load conditions and no volume conditions so i dont think that it has anything to do with excessive power or too low load ....
So i expect after this analysis that the only think that makes sense is to make sure that the time line of powering up is working correctly ....
Two things cross my mind.
One is a thermal/mechanical problem - short to heatsink, bad insulation washers/silicone/mica pads. I had this sort of thing and it was a PITA to figure out. It turned out one of the mica insulators was contaminated and produced intermittent shorts.
Two, oscillation. The power stage can oscillate only on one half producing voltage peaks and excess current through decoupling caps.
Two and a half
- it's a Vincent, check the caps. One I had used imitation WIMA caps that eventually all went bad.
One is a thermal/mechanical problem - short to heatsink, bad insulation washers/silicone/mica pads. I had this sort of thing and it was a PITA to figure out. It turned out one of the mica insulators was contaminated and produced intermittent shorts.
Two, oscillation. The power stage can oscillate only on one half producing voltage peaks and excess current through decoupling caps.
Two and a half
- it's a Vincent, check the caps. One I had used imitation WIMA caps that eventually all went bad.try slo blow fuses.
or monitor temperature on individual output transistors if one is getting hotter than the others in a no load idle condition it may the culprit (after cooling while your replacing the fuse it operates fine until the cycle repeats.which is frustrating as the next incident can be spaced by months sometimes before it does it again.)
or monitor temperature on individual output transistors if one is getting hotter than the others in a no load idle condition it may the culprit (after cooling while your replacing the fuse it operates fine until the cycle repeats.which is frustrating as the next incident can be spaced by months sometimes before it does it again.)
Here's a thought.
If its just one of the rail fuses that seem to blow every now and then and its not oscillation the explanation could lie in how the amp powers up. Either one of the rails is coming up before the other and you have a state during power up wherein the problem channel is drawing a lot of current. Can you put a small (0.1 Ohm) resistor in series with the supply rails and measure it? What's your line voltage doing?
Seems to me that since it does not blow on every power up cycle, if I understand it correctly, then it might also be related to which half of the mains cycle the power is applied.
Happy hunting . . .
If its just one of the rail fuses that seem to blow every now and then and its not oscillation the explanation could lie in how the amp powers up. Either one of the rails is coming up before the other and you have a state during power up wherein the problem channel is drawing a lot of current. Can you put a small (0.1 Ohm) resistor in series with the supply rails and measure it? What's your line voltage doing?
Seems to me that since it does not blow on every power up cycle, if I understand it correctly, then it might also be related to which half of the mains cycle the power is applied.
Happy hunting . . .
if you connect the fuses after the big capacitors, you preserve the fuses by the switch-on inrush current (at t=0 the capacitors are discharged and present themselves as short circuits). BUT when you switch off the amplifier, the capacitors are charged and must discharge: the discharge current pass through the fuses and if the switch-off transient opens a low impedance way, via final power transistors as example, the discharge current can open the fuse: with a memory scope measure the discharge current of the capacitors where the fuse blows.
Perhaps a gentler turn on would solve your problem.
I installed post-cap rail fuses in my 1.3K amp, to cut the number of blown output transistors to maybe only one instead of 9 at a time. 1/4" phone plugs will be pulled out sometimes stumbling around on a dark stage. To cut the lamp dimming at turn on, and the audible whomp, I also installed a CL-101 NTC thermistor in the AC between the circuit breaker and the transformer. The turn on is nice and gentle, and the rail fuses are not blowing. If you have a smaller amp a higher resistance GE thermistor would be in order. heat shrink around the thermistor seems to be the usual installation.
I installed post-cap rail fuses in my 1.3K amp, to cut the number of blown output transistors to maybe only one instead of 9 at a time. 1/4" phone plugs will be pulled out sometimes stumbling around on a dark stage. To cut the lamp dimming at turn on, and the audible whomp, I also installed a CL-101 NTC thermistor in the AC between the circuit breaker and the transformer. The turn on is nice and gentle, and the rail fuses are not blowing. If you have a smaller amp a higher resistance GE thermistor would be in order. heat shrink around the thermistor seems to be the usual installation.
I also found that a fuse before capacitors wasn't good when something failed.
The fuse would blow but there would still be a lot of power in the capacitors to discharge into a faulty circuit.
I used to run a mobile disco and tripped over a speaker lead and ripped it out of the back of the speaker. Luckily my amp also had a fuse after the output transistors and this blew before the transistors. I replaced the fuse and carried on.
On SMPS I always add a thermistor on the mains input live leg.
The fuse would blow but there would still be a lot of power in the capacitors to discharge into a faulty circuit.
I used to run a mobile disco and tripped over a speaker lead and ripped it out of the back of the speaker. Luckily my amp also had a fuse after the output transistors and this blew before the transistors. I replaced the fuse and carried on.
On SMPS I always add a thermistor on the mains input live leg.
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