Single Ended Amp Problem - Fuse Blowing
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Re: Single Ended Amp Problem - Fuse Blowing
One capacitor getting extremely hot means it is drawing a lot more current than it's supposed to ergo it is faulty. It's not PTs fault.
Aleksander Niemand
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Life's a party but you get invited only once...
affiliation:TUBEWONDER AMPS
Zagray!-review
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Life's a party but you get invited only once...
affiliation:TUBEWONDER AMPS
Zagray!-review
Re: Single Ended Amp Problem - Fuse Blowing
Circuit breakers are a good idea. Ones like this can be used:
https://taweber.powweb.com/store/fuseord.htm
https://taweber.powweb.com/store/fuseord.htm
Re: Single Ended Amp Problem - Fuse Blowing
The mains fuse is deliberately intended to be the weakest link in the chain (so to speak) and will blow when too much current is drawn through it, in order to prevent a fire.
W.r.t. a tube amp, this invariably means that either there is an internal short in the PT, causing too much current to be drawn through the fuse, or more load on the secondary windings than intended (which in turn causes the higher-than-anticipated load to be reflected through the primary and thence through the mains fuse) which is caused by a short on something connected to one or more of the secondaries.
So you have to look for the thing that is causing the short, and you do this by a process of deduction.
If its not an internal short within the PT itself (which can be determined by disconnecting all the secondaries and seeing whether the fuse still blows), then your next suspect is a short in the rectifier, or a short on the B+ rail directly to ground.
A DC short across one of the diodes can cause high VAC to be dumped into the filter caps, causing them to fail (electrolytic filter caps cannot withstand high VAC) and short.
A badly wired component (where the bare B+ wire is somehow touching the chassis directly) can cause the short.
However, if your first (reservoir) filter cap after the rectifier is okay, then its unlikely to be the rectifier or (even for that matter) the first filter cap (or a badly wired B+ wire in between all of those) which is causing the short, so we move on to the next filter cap, and so on.
You will get a clue as to whether the filter caps are shorting by measuring the DC voltage at the high-voltage side of each of the filter caps. If the DC voltage at one of these DC supply nodes is a lot lower than expected, then that is the point where too much current is going through the ground return (determined in accordance with Ohms law: E = I x R).
Now it could be that lower DC voltage is either caused by a short in the filter cap, or a short somewhere in the circuit (like say for example, a DC voltage divider for a bleeder network or a fixed bias supply) that sources its power supply from that particular filter cap node, so you look for other clues to determine which is more likely.
Another sign of too much current is, excessive heat, which is determined by power dissipated through a component. Watts = Volts x Amps ( or P = V x I or E x E x R). In the case of a shorted filter cap, the internal resistance (R) will be low, allowing more direct current (I) to pass through it. The more current that passes through it, the more it will heat up.
In this regard, the higher temperature of your second filter cap indicates that this is where the short is (and I bet that this would be where the DC voltage is really low). The next thing to do would be to replace that filter cap and see if the fuse still blows.
W.r.t. a tube amp, this invariably means that either there is an internal short in the PT, causing too much current to be drawn through the fuse, or more load on the secondary windings than intended (which in turn causes the higher-than-anticipated load to be reflected through the primary and thence through the mains fuse) which is caused by a short on something connected to one or more of the secondaries.
So you have to look for the thing that is causing the short, and you do this by a process of deduction.
If its not an internal short within the PT itself (which can be determined by disconnecting all the secondaries and seeing whether the fuse still blows), then your next suspect is a short in the rectifier, or a short on the B+ rail directly to ground.
A DC short across one of the diodes can cause high VAC to be dumped into the filter caps, causing them to fail (electrolytic filter caps cannot withstand high VAC) and short.
A badly wired component (where the bare B+ wire is somehow touching the chassis directly) can cause the short.
However, if your first (reservoir) filter cap after the rectifier is okay, then its unlikely to be the rectifier or (even for that matter) the first filter cap (or a badly wired B+ wire in between all of those) which is causing the short, so we move on to the next filter cap, and so on.
You will get a clue as to whether the filter caps are shorting by measuring the DC voltage at the high-voltage side of each of the filter caps. If the DC voltage at one of these DC supply nodes is a lot lower than expected, then that is the point where too much current is going through the ground return (determined in accordance with Ohms law: E = I x R).
Now it could be that lower DC voltage is either caused by a short in the filter cap, or a short somewhere in the circuit (like say for example, a DC voltage divider for a bleeder network or a fixed bias supply) that sources its power supply from that particular filter cap node, so you look for other clues to determine which is more likely.
Another sign of too much current is, excessive heat, which is determined by power dissipated through a component. Watts = Volts x Amps ( or P = V x I or E x E x R). In the case of a shorted filter cap, the internal resistance (R) will be low, allowing more direct current (I) to pass through it. The more current that passes through it, the more it will heat up.
In this regard, the higher temperature of your second filter cap indicates that this is where the short is (and I bet that this would be where the DC voltage is really low). The next thing to do would be to replace that filter cap and see if the fuse still blows.
He who dies with the most tubes... wins
Re: Single Ended Amp Problem - Fuse Blowing
Hi Phil,Phil_S wrote:Circuit breakers are a good idea. Ones like this can be used:
https://taweber.powweb.com/store/fuseord.htm
Have you used any of these?
I was wondering how these work compared to a Slo Blo fuse?
Can they handle the in rush current and not open?
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Single Ended Amp Problem - Fuse Blowing
Tom,
I did try them once. It is only good on the AC mains and not at all -- due to it's 125v rating -- appropriate as a DC or B+ fuse, as in the CT of the PT. They're OK, but nothing to write home about. I mean, if you build it right, the fuse doesn't blow. My general attitude is to stick with the tried and true.
In this instance, I think the OP is problem solving, so it is a) cheaper, and b) less time consuming. Given it's mitigation of the hassle factor, I think it is a useful tool to keep in the box and bring it out when you need it.
Phil
I did try them once. It is only good on the AC mains and not at all -- due to it's 125v rating -- appropriate as a DC or B+ fuse, as in the CT of the PT. They're OK, but nothing to write home about. I mean, if you build it right, the fuse doesn't blow. My general attitude is to stick with the tried and true.
In this instance, I think the OP is problem solving, so it is a) cheaper, and b) less time consuming. Given it's mitigation of the hassle factor, I think it is a useful tool to keep in the box and bring it out when you need it.
Phil