What is the HT fuse for on a tube amp?

[talustalus]

Also... mine keeps blowing at power up of the amp.

Tried with 3x new HT fuses. Each one blows immediately upon power up.

Any shared knowledge on a possible cause will be much appreciated.

thank you

[JamesPeters]

When the HT fuse fails, lots of people think "it's just a bad fuse..." and replace it. That's not how fuses work though. The fact it failed indicates something needs to be addressed.

That fuse is in line with the high voltage part of the circuit, and its failure is most often associated with a failing output tube. You need to change the output tubes, put a new fuse in, and bias the amp. Even if "the tubes look good"...because tubes don't fail quite like light bulbs. Also "but it sounded fine right up until the first fuse blew" doesn't count either. Things always work right until they don't. (I've heard this many times so I'm not poking fun at you.) Once you do this and the amp works correctly, dispose of the old tubes; don't keep them around "as spares". That might sound obvious but I've caught people doing it, reusing the tubes, and having the same problems as before (then blaming me for it somehow). Don't put yourself in that spot. It sucks that "perfectly fine looking tubes" fail and you need to throw them away, but that's life.

There's nothing else even worth mentioning to check, that's how likely it is that it's the output tubes. Preamp tubes usually won't draw enough current (even when they fail) to blow the HT fuse. And if it were something in the power supply besides that, it'd probably have blown the mains fuse (and/or a transformer).

Of course if an output tube is causing the fuses to fail, there's the possibility of more damage, but it would most likely be something common such as a failed screen grid resistor on an output tube socket.

Depending on the amp, it might not require biasing. Which amp is it?

[talustalus]

Originally Posted by JamesPeters

When the HT fuse fails, lots of people think "it's just a bad fuse..." and replace it. That's not how fuses work though. The fact it failed indicates something needs to be addressed.

That fuse is in line with the high voltage part of the circuit, and its failure is most often associated with a failing output tube. You need to change the output tubes, put a new fuse in, and bias the amp. Even if "the tubes look good"...because tubes don't fail quite like light bulbs. Also "but it sounded fine right up until the first fuse blew" doesn't count either. Things always work right until they don't. (I've heard this many times so I'm not poking fun at you.) Once you do this and the amp works correctly, dispose of the old tubes; don't keep them around "as spares". That might sound obvious but I've caught people doing it, reusing the tubes, and having the same problems as before (then blaming me for it somehow). Don't put yourself in that spot. It sucks that "perfectly fine looking tubes" fail and you need to throw them away, but that's life.

There's nothing else even worth mentioning to check, that's how likely it is that it's the output tubes. Preamp tubes usually won't draw enough current (even when they fail) to blow the HT fuse. And if it were something in the power supply besides that, it'd probably have blown the mains fuse (and/or a transformer).

Of course if an output tube is causing the fuses to fail, there's the possibility of more damage, but it would most likely be something common such as a failed screen grid resistor on an output tube socket.

Depending on the amp, it might not require biasing. Which amp is it?

Thank you for what sounds like qualified information.

The amp is a Bogner Uberschall Twin Jet.

I decided to remove two tubes and run the amp at half power. The new HT Fuse survived!

This means one of the two power tubes I removed are the bad tube. For now I'll just keep running the amp at half power (it is a full 150W with all four tubes - which is excessive).

*NOTE: for anyone wanting who might need to try this in the next century with this particular amp, only tubes in positions 7&9, or 8&10 can run together as a pair.*

[JamesPeters]

There are 2 things you should now do, if you plan to keep running it like that:

1) Choose current ratings for HT and mains fuses that are half what they normally are. Using half the output tubes cuts the current draw of the amp roughly in half (preamp tubes do draw some current but the most of it is for the output tubes). If you use the normal values of fuses while only running half the output tubes, you're risking more potential damage to the amp if those tubes fail since the fuses won't fail until they're handling twice the current they should (under the "half power" condition). It's not a huge risk, but it's worth doing especially since fuses are cheap.

2) Double the impedance. That is: hook a 16 ohm cabinet up to the amp's 8 ohm output (for example). This will provide a closer impedance match when running half the output tubes. The amp was designed with a transformer matching 4 output tubes to a load. Running it with 2 tubes changes things. Doubling the impedance like that compensates for this. It'll sound more normal and be safer in general.

Also make note of which output tube sockets you aren't currently using (sitting vacant). If a problem occurs later when you use a matched quad of output tubes again, it's likely one of those 2 sockets has a failed screen grid resistor. Making a note of this now is a good idea. It's easy enough to test for, but it can't hurt to make a note of it.

[talustalus]

Thank you very much for the info.

[Geoff Waddington]

Originally Posted by JamesPeters

There are 2 things you should now do, if you plan to keep running it like that:

1) Choose current ratings for HT and mains fuses that are half what they normally are. Using half the output tubes cuts the current draw of the amp roughly in half (preamp tubes do draw some current but the most of it is for the output tubes). If you use the normal values of fuses while only running half the output tubes, you're risking more potential damage to the amp if those tubes fail since the fuses won't fail until they're handling twice the current they should (under the "half power" condition). It's not a huge risk, but it's worth doing especially since fuses are cheap.

Yup.

Originally Posted by JamesPeters

2) Double the impedance. That is: hook a 16 ohm cabinet up to the amp's 8 ohm output (for example). This will provide a closer impedance match when running half the output tubes. The amp was designed with a transformer matching 4 output tubes to a load. Running it with 2 tubes changes things. Doubling the impedance like that compensates for this. It'll sound more normal and be safer in general.

Had a quick look at the schematic, there is a 3 positon output impedance selector -- 3 taps on output secondary, so you are likely good to go there.

Has true bias supply with trim, might want to touch that up.

Originally Posted by JamesPeters

Also make note of which output tube sockets you aren't currently using (sitting vacant). If a problem occurs later when you use a matched quad of output tubes again, it's likely one of those 2 sockets has a failed screen grid resistor. Making a note of this now is a good idea. It's easy enough to test for, but it can't hurt to make a note of it.

When it comes to how many screen resistors I've replaced over the years -- well... wayyy toooo early in the morning to count that high

[talustalus]

Originally Posted by Geoff Waddington

-- 3 taps on output secondary, so you are likely good to go there.

Has true bias supply with trim, might want to touch that up.

Indeed you can change the output impedance (4, 8, or 16 ohms). What do you mean by "3 taps on output secondary"?

And touch up trim / true bias?

Thanks v much!

[talustalus]

Originally Posted by JamesPeters

There are 2 things you should now do, if you plan to keep running it like that:

1) Choose current ratings for HT and mains fuses that are half what they normally are. Using half the output tubes cuts the current draw of the amp roughly in half (preamp tubes do draw some current but the most of it is for the output tubes). If you use the normal values of fuses while only running half the output tubes, you're risking more potential damage to the amp if those tubes fail since the fuses won't fail until they're handling twice the current they should (under the "half power" condition). It's not a huge risk, but it's worth doing especially since fuses are cheap.

Need some clarifying advice here.. If the main fuse is rated 5A, would you go with 3A or 2A? Or must it be exactly 2.5A?

No problem as such for the 1A HT fuse, as they have a 500mA rated fuse.

[JamesPeters]

I'd use 2.5A, but 3A is ok (better than 5A when used like this). Be sure to use the same type, which I assume is a slow-acting variety. If you're having trouble finding fuses locally you can order them from a place like mouser.com.

If the bias was set fine before, it won't need any adjusting now. The decrease in current draw due to using fewer tubes will slightly increase the plate voltage (since there's less load on the power transformer), but adjusting the bias to compensate isn't necessary. That's because the difference in plate voltage will be less than 10%, so adjusting the bias by 10% is splitting hairs. It can't hurt to check the bias, but if you are not capable of checking the bias or biasing the amplifier without taking it to a tech, I wouldn't say this is an important thing to do. As long as the amp works fine the way you have it now and it sounds fine. Besides, the way most people think about adjusting bias is a bit flawed. People will tend to pop a number into a calculation to figure out what they think the optimal bias point is. That's okay as a starting point but the truth is there's a range which makes sense to use and it is fairly subjective at that.

When Geoff was talking about the output taps, he was referring to the wires that come off the secondary of the output transformer. It is something that is relatively invisible to you because you're not inside the amp, so don't worry about it. He was just confirming that the amp is capable of being set for different impedances.

[talustalus]

Originally Posted by JamesPeters

I'd use 2.5A, but 3A is ok (better than 5A when used like this). Be sure to use the same type, which I assume is a slow-acting variety. If you're having trouble finding fuses locally you can order them from a place like mouser.com.

If the bias was set fine before, it won't need any adjusting now. The decrease in current draw due to using fewer tubes will slightly increase the plate voltage (since there's less load on the power transformer), but adjusting the bias to compensate isn't necessary. That's because the difference in plate voltage will be less than 10%, so adjusting the bias by 10% is splitting hairs. It can't hurt to check the bias, but if you are not capable of checking the bias or biasing the amplifier without taking it to a tech, I wouldn't say this is an important thing to do. As long as the amp works fine the way you have it now and it sounds fine. Besides, the way most people think about adjusting bias is a bit flawed. People will tend to pop a number into a calculation to figure out what they think the optimal bias point is. That's okay as a starting point but the truth is there's a range which makes sense to use and it is fairly subjective at that.

When Geoff was talking about the output taps, he was referring to the wires that come off the secondary of the output transformer. It is something that is relatively invisible to you because you're not inside the amp, so don't worry about it. He was just confirming that the amp is capable of being set for different impedances.

Much obliged!

[JamesPeters]

Glad to help.