Additional fuses advice

[22uf]

I've built a 5F2A and it sounds wonderful. Really happy with it. I used vintage transformers which I couldn't easily replace. I wanted to add some additional fuses for peace of mind, as this amp is getting gigged a lot going forward. Seeking advice..

I'm in Europe and I run the amp on 242v exactly AC off a variac - the sweet spot for my ears.

Right now all I have is the main fuse on the AC "FS1" which is 1 Amp 250V antiSurge/slow blow.

I have also added "backup" 1N4007 diodes in series before the 5Y3 rectifier.

Now I am running these NOS Brimar 6V6's at 100% PD so I think I need some extra protection for when one inevitably fails?

Google and Mr Keen suggests:

R.G. Keen: 300 to 500 milliamp slow blow fuse for the 5F6A to the power transformer high voltage center tap (B+ Fuse). 200 to 300 milliamp for a 5E3. 4 amp slow blow fuses added to each 6.3v heater line (2 parallel fuses = 8 amp limit). 4 amp slow blow fuses added to the rectifier's 5v heater lines. I don't recommend a fuse on the power transformer's bias tap because if it blows the power tubes will run away with max current and burn up. I highly recommend you at least install the B+ fuse to protect the power transformer.

So I reckon, 100mA should be plenty for the B+ in a 5 watter and the rest as above?

[kagliostro]

So I reckon, 100mA should be plenty for the B+ in a 5 watter and the rest as above?

I agree with you for a 100mA B+

Franco

[22uf]

I did some more googling and found a response from tubeswell from last year..

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

Seems like it maybe isn't necessary to go this far in a 5F2A.

[HotBluePlates]

I've built a 5F2A ... I run the amp on 242v ... the main fuse on the AC "FS1" which is 1 Amp 250V antiSurge/slow blow. ...

The original 5F2-A used 0.75A with 120v AC.  Since your line voltage is double, the current through the fuse will be halved.

I don't know whether 0.375 Amp fuses are available, but I would suggest looking for something between 350-500mA.  That will ensure the fuse offers similar protection as the original circuit received.


Different Topic:
I know we assume the last-version was the "best, most-evolved version" but I'd recommend looking at adding a choke to your amp, as found in the earlier 5F2 Princeton amp.  The output transformer should source its voltage from the filter cap after the choke.

If you do this, hum will be greatly reduced (even if you had the 5E2's 8µF filter caps, and not a single 16µF).  I can only imagine Fender sought to eliminate the cost of the choke, and somehow also felt they needed to get an extra part-watt with the change found in the 5F2-A.  But it doesn't really make the amp better, and all the earlier Princeton amps had this other power supply arrangement that delivered less-hum.   

[22uf]

Many thanks. 

Noted re: the choke. I think I got lucky with mine as I don't have any appreciable hum in this one at all. My pedalboard produces more noise than the amp. 

I did bump up the filter for the 6V6 screen to 22uf though so it runs: 32-22-8.

The only other modification, apart from the diodes on the 5Y3, was to drop the V1A cathode bypass down to 10uf.

I "accidentally" discovered that if I put a 10k in place of the 22k dropping resistor to the pre-amp, I can get pre-amp voltage up to 285 which basically turns the amp in to a totally clean pedal platform. I wish I could make that switchable but I don't know how I could get 300v dc on a switch without dying..

[HotBluePlates]

I did bump up the filter for the 6V6 screen to 22uf though so it runs: 32-22-8.

It's good you're having no issues!  I've encountered lots of folks who weren't so lucky.

I used to own a 1954 5B2 Princeton that was hum-free with 8µF-8µF-8µF.  That was all due to 2 stages of filtering before the feed to the OT.