Hello everyone,
I sent this question to RG but his spam/junk filter most likely ate my emails again so I'll kick it to the hive mind and see what all of you have to think about it, plus I'm sure RG will chime in. Now, I am building a Vox UL 715 amp using RG's pre-amp pcb board but the power section is hand wired. My problem stems from the fuse rating that I will need to use. Included in my post are three schematics for everyone's perusal. The original Vox power section (os 113) schematic which shows a 1 amp slow blow fuse. Another one that was redrawn by Vintagekiki who has a 2 amp slow blow fuse. Since I am using the same power transformer specs that the original ac-15 has I will be using an EZ81 rectifier tube here is also the original ac-15 schematic that uses a 2 amp slow blow fuse.
So my question is which one do I use? It might be just as simple of an answer for me just to use the 1 amp without any worry but I'd like to have a second opinion on this. Is their any math equations that I should know about here or am I just being paranoid in my decision?
Also, my apologies if this is in the wrong section of Amp Garage. Not sure if I should have posted this in the Vox/Highwatt section or not.
Dennis
Foolish question possibly, but a fuse size on a UL715 power section
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Dennis Perusse
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Foolish question possibly, but a fuse size on a UL715 power section
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Stevem
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Re: Foolish question possibly, but a fuse size on a UL715 power section
I can’t see for the life of me why the whole amp, as in the AC-15 schematic would need anything more then a 1 amp slow blow fuse.
What I would do is add a 100 ma fuse into the OT center tap.
When the main AC fuse blows in a amp 99% of the time it’s due to a shorted output tube and EL84s it seems love to short.
This fuse will act much faster then waiting for the main fuse to blow if a output shorts out and this will save the PT from a lot of unnecessary stress and cost all of 4 bucks to add to a build.
A 100ma fuse is more then enough to hold up to 17 watts out of a pair of 84s.
PS.
Actually without the heater draw of a rectifier tube you might get away with a fast blow 1 amp fuse.
Due your first fire up with a 1 amp slow blow and the try a fast blow and see if it holds up to 4 or 5 start ups and full cool down sessions.
What I would do is add a 100 ma fuse into the OT center tap.
When the main AC fuse blows in a amp 99% of the time it’s due to a shorted output tube and EL84s it seems love to short.
This fuse will act much faster then waiting for the main fuse to blow if a output shorts out and this will save the PT from a lot of unnecessary stress and cost all of 4 bucks to add to a build.
A 100ma fuse is more then enough to hold up to 17 watts out of a pair of 84s.
PS.
Actually without the heater draw of a rectifier tube you might get away with a fast blow 1 amp fuse.
Due your first fire up with a 1 amp slow blow and the try a fast blow and see if it holds up to 4 or 5 start ups and full cool down sessions.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Foolish question possibly, but a fuse size on a UL715 power section
Maybe 2A was for 120v and 1A was 230v. Totally agree with Steve that you won't need more than 1A, but it's not uncommon to see commercial amps with unnecessarily large ratings. Fuses aren't exactly precision devices and vary a lot between manufacturers anyway. You might hit 3/4A with all of the knobs up while hitting the input hard.
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Stevem
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Re: Foolish question possibly, but a fuse size on a UL715 power section
Are you talking about the total ma load, or just the output section?
I was going by the RCA tube manual that called out 92 ma , 300 B+, 17 watts.
I was going by the RCA tube manual that called out 92 ma , 300 B+, 17 watts.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Foolish question possibly, but a fuse size on a UL715 power section
I simply opened the STBY switch and connected my Fluke 87V (set to measure DC current) across the open switch terminals to measure the total B+ current. Here's the schematic for reference...
https://sluckeyamps.com/VAC15/Vox_AC15.pdf
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Stevem
- Posts: 4576
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Foolish question possibly, but a fuse size on a UL715 power section
In terms of the whole amp then look at a Fender deluxe reverb.
That amp has more tubes and more wattage then the Vox and is powered on only a 1 amp SB fuse.
That amp has more tubes and more wattage then the Vox and is powered on only a 1 amp SB fuse.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Dennis Perusse
- Posts: 25
- Joined: Sat May 27, 2023 1:37 am
- Location: Amesbury Mass
Re: Foolish question possibly, but a fuse size on a UL715 power section
Hello,
Okay, I think I might put in a 1.5 amp slow blow fuse. I figure if it pops then I'll put in a 2 amp and I'll consider it good. If it doesn't then I will at least have a margin of error and split the difference. I appreciate everyone's thoughts as I get closer to finishing this project.
Dennis
Okay, I think I might put in a 1.5 amp slow blow fuse. I figure if it pops then I'll put in a 2 amp and I'll consider it good. If it doesn't then I will at least have a margin of error and split the difference. I appreciate everyone's thoughts as I get closer to finishing this project.
Dennis
Re: Foolish question possibly, but a fuse size on a UL715 power section
If a 1.5A pops, don't put in a 2, fix whatever caused the 1.5 to pop. Unless you have a defective fuse, you shouldn't come close to getting a nuisance blow with a 1.5A. 1A is a safer bet, especially while you are testing the amp.
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Dennis Perusse
- Posts: 25
- Joined: Sat May 27, 2023 1:37 am
- Location: Amesbury Mass
Re: Foolish question possibly, but a fuse size on a UL715 power section
Ok, I'll do that then. I'm almost done wiring up things as my parts came in.
Dennis
Dennis
Re: Foolish question possibly, but a fuse size on a UL715 power section
I find it strange that some happily measure and set output stage tube currents etc to better than 5-10% accuracy, and spend many $$ on tubes, but don't take the time to measure mains ac current at idle, and when cranked, for their nominal mains AC voltage, and then buy and use an appropriately valued and blow-rated fuse (if deemed worthwhile).
As indicated, it is not uncommon to find default fuse values used by manufacturers that can cope with a wide range of mains AC voltage and operation conditions (eg. bias settings), but that doesn't mean a user can't determine and install a lower fuse value that imparts some better hope of saving internal parts should an accident or fault occur. Sure, peace of mind is important that the mains fuse won't blow when it shouldn't, but peace of mind also comes from knowing what your amp is doing and how you are protecting it.
Imho if you were thinking of making the secondary side more bullet-proof, given you are hand-wiring yourself, then at least add series 1N4007 to each EZ81 anode. And if entertaining the inclusion of a fuse then I highly recommend placing it in the power transformer secondary CT link - and if you can't find someone who has gone through that process and chosen a suitable fuse, then the link below may be a useful (albeit technical) guide.
https://dalmura.com.au/static/Valve%20amp%20fusing.pdf
As indicated, it is not uncommon to find default fuse values used by manufacturers that can cope with a wide range of mains AC voltage and operation conditions (eg. bias settings), but that doesn't mean a user can't determine and install a lower fuse value that imparts some better hope of saving internal parts should an accident or fault occur. Sure, peace of mind is important that the mains fuse won't blow when it shouldn't, but peace of mind also comes from knowing what your amp is doing and how you are protecting it.
Imho if you were thinking of making the secondary side more bullet-proof, given you are hand-wiring yourself, then at least add series 1N4007 to each EZ81 anode. And if entertaining the inclusion of a fuse then I highly recommend placing it in the power transformer secondary CT link - and if you can't find someone who has gone through that process and chosen a suitable fuse, then the link below may be a useful (albeit technical) guide.
https://dalmura.com.au/static/Valve%20amp%20fusing.pdf