5F2A B+fuse question - how to...

Greegooryy

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Hello!

Building my first tube amp, a 5F2A, and i wondered where and how to fuse the B+?

It seems there are several options?

1/ fuse the center tap of the HT = between PT HT CT (0v) and the negative side of the first filter cap?

As in the Fender 57 Champ 5F1 RI schematic, here: https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-57-Champ-Schematic-Rev-B.pdf

It's the F4 fuse, T250mA250V, on the red/yellow wire.

or,
2/ fuse between 5Y3 pin 8, and the positive side of the first filter cap.

As in the RG Keen article here:




And why would i choose one way over the other?



Would a 250mA fuse (as suggested in the Fender schematic) be a good value? I'm not sure how to calculate this value...


There is a handy 5F2a layout by King Fan here,
it's post 1 of the thread:


Thanks a lot!

Gregory
 

dan40

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Welcome to the forum! I have always installed them inline between the rectifier and first filter cap. I think a T250ma fuse would be a good starting point as both amps draw roughly the same current. The letter "T" in the fuse rating indicates it is a "timed" or "slow blow" fuse so be sure that you use the same.
 

tubeswell

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Fusing the B+ in a 5F2A is overkill IMHO. The 6V6 is cathode biased, so you've got next-to-no-chance of loss of bias as a failure mode. If the rectifier tube fails short, the best protection for the reservoir cap (and PT) is 2 x 1000V 1A SS diodes (1 each in series with each rectifier tube plate and each respective end of the HT winding). If the reservoir cap fails short (or if you get a heater-to-cathode or heater-to-other-electrode short), the Mains fuse will blow (if you have rated it properly). If the screen filter cap fails, the most you're like to see (assuming 360V at the reservoir cap) is 40mA or so across the 10k supply resistor to ground (or assuming 290V at the screen supply node, then it'll be 14mA or so across the 22k supply resistor if the pre-amp filter cap fails), so even a hard-to-find 125mA rated T-fuse won't blow. YMMV
 
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Greegooryy

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Welcome to the forum! I have always installed them inline between the rectifier and first filter cap. I think a T250ma fuse would be a good starting point as both amps draw roughly the same current. The letter "T" in the fuse rating indicates it is a "timed" or "slow blow" fuse so be sure that you use the same.
Thank you, Dan40!!

Yes, they are basically the same amp - 5f2a has a tone pot and a little bit of extra filtering. The slow blow's are a bit harder to find, but i have them already...

Much appreciated!
Gregory
 

Greegooryy

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Fusing the B+ in a 5F2A is overkill IMHO. The 6V6 is cathode biased, so you've got next-to-no-chance of loss of bias as a failure mode. If the rectifier tube fails short, the best protection for the reservoir cap (and PT) is 2 x 1000V 1A SS diodes (1 each in series with each rectifier tube plate and each respective end of the HT winding). If the reservoir cap fails short (or if you get a heater-to-cathode or heater-to-other-electrode short), the Mains fuse will blow (if you have rated it properly). If the screen filter cap fails, the most you're like to see (assuming 360V at the reservoir cap) is 40mA or so across the 10k supply resistor to ground (or assuming 290V at the screen supply node, then it'll be 14mA or so across the 22k supply resistor if the pre-amp filter cap fails), so even a hard-to-find 125mA rated T-fuse won't blow. YMMV
Thank you very much, Tubeswell!
The 2 1N4007 diodes are installed already, as you advised!




Do you think it is wise to fuse other secondary windings of the PT, as RG Keen advises in his article?
The 5v or 6V windings?
Or would that be unnecessary, in your opinion?





For the fusing of the primary winding - mains fuse, i would use a 500 mA fuse, although it seems a 1A fuse is standard on these amps -

1A seems a bit too high, according to my calculations (i'm in Europe - 23Ov mains voltage).

My math:

The PT is a Hammond 291AEX, i'll be using the 275v as advised many times on this forum.

The following values are taken from the PT's spec sheet:

275v winding x 100 mA capacity = 27,5 VA
6,3v winding x 2,25 A capacity = 14,175 VA
5v winding x 3A capacity = 15 VA

This adds up to 56,675 VA

Divide this by 230 volts main voltage over here = 0,246A

nearest value up is 250 mA,

double that for margin = 500 mA

Of course, a 5F2A would not even be using the full capacity of this Hammond 291AEX PT,
so there seems to be even more margin?

Would you agree with my calculations?



Thanks a lot!!
 

tubeswell

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The gist of RG's article's on fusing PT secondaries is you need to select fuses that are rated sufficiently to deal with start-up surge current, but will blow if there is a short that makes the PT conduct heavily over the normal expected peak current. Usually this is 2 x the peak. For heaters, it depends on the number and type of tubes and its difficult to get 'right'.

An Amp SVT-CL (with 6 x 6550s and 1/2 a dozen pre-amp tubes) will have a much bigger startup surge on the filament winding when the filaments are cold, than a champ (with 1 x 6V6 and 1 x 12AX7). In a 12AX7 alone, peak heater filament power spikes to almost 8W at startup before it settles down to about 1.8W.

But a 12AX7 filament is designed for this. So is any other type of tube filament.

If you want to protect against startup surge, a surge protector will provide better in-rush current limiting than a fuse (a fuse is just a simple blow/no-blow situation).

As far as the mains fuse goes, rule of thumb is if you're in a 'euro' mains voltage place, the size of the Mains fuse is about 1/2 of what is needed for US-mains. (The V.A is a function of 'V' as well as 'A', and 2x the 'V' calls for 0.5 x the 'A' - all other things being equal - to get the same amount of total power on the secondaries)
 
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Greegooryy

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Thank you, Dan40 and Tubeswell!!

As the usual USA value for the mains fuse in a 5f1/5f2a is 2A,
i'll use 1A for 230 volts european mains voltage.
slow blow.

The explanations were interesting to read. Much appreciated!

Tubeswell, how and where would one put a surge protector inside an amp to deal with startup surge? Where could i read more about this?

Wishing you a nice day!!
Gregory
 

Kev-wilson

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You'd find a plug in 'surge protector' in your local hardware store or Screwfix kind of place, common on a pc's extension lead so to send any surges to ground.
 

tubeswell

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... how and where would one put a surge protector inside an amp to deal with startup surge? Where could i read more about this?
Google is your friend e.g. 'startup surge current on cold electron tube heater filaments' got me this: https://audioxpress.com/article/con...end-the-life-of-your-amplifier-s-vacuum-tubes

(Note: The suggested regulation is for 150mA 12VDC, so you'll need a beefier circuit for output tubes)

Seriously - I wouldn't bother with this in a champ. Much easier to do what Kev ^said^, or just forget about the heater surge altogether - the tubes are designed for it. I reiterate that an appropriately rated mains fuse is the best protection in a champ
 

Greegooryy

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Sorry for the lateness of my reply (got side-tracked),

and thank you very much for your good advice, Kev-wilson
and
Tubeswell!

Wishing you the best,
Gregory
 




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