Fuse on the Cathodes?

[leadfootdriver]

I'm almost done with my layout. I wanted to put a fuse after the rectifier on the HT. So, I was checking out a Mesa Boogie drawing, and seen that they had a 1 amp fuse on the 4-powertube cathodes. What gives? Wouldn't it be better the fuse the HT for the whole amp instead?

What's you thoughts on what should be fused?

[pdf64]

After a major power tube short damaged an OT primary winding (the B+ was fused at T500mA after the rectifier but didn't blow quickly enough), I bought a load of internal chassis fuseholders and am on a program of fusing the ground returns of each power tube cathode on all my amps.
It has the significant advantage that the fuse can be quick blow and sized appropriately to the most likely failure mode; 250 to 330mA for 6L6GC / EL34, 100mA for EL84 / 6V6.
These are in addition to a (slow blow) fuse on the PT B+ winding.
I can't see any real benefit in the arrangement you describe in the Mesa amp, compared to a regular B+ fuse, though it would eliminate the switch on current surge from charging the reservoir cap; is it a quick blow type?

[trobbins]

If you add say a 10k resistor in parallel with each cathode fuse, then it may save the tube from damage - if the tube wasn't the problem that caused the cathode fuse to blow.

[leadfootdriver]

IT's a 1 amp fast blo.
http://www.webphix.com/schematic%20h...boogie_290.pdf

I was reading in my Wizard book that said it's good to fuse the center tap of the OT.

[pdf64]

Agree, the 10k resistor is good mitigation from the cathode getting pulled up to excessive levels and compromising the heater-cathode insulation. It can be combined with an LED for fuse failure indication.
In reality though, the cathode voltage doesn't seem to rise much when the ground return is opened, about 30V is the number at the back of my mind. However, that's not an exhastive representation of every power tube ever made.

[R.G.]

I have fused cathodes. In a different way, though. I used a high voltage MOSFET for the fuse, and sensed the current electronically, turning the MOSFET off for overcurrent which was more than X and lasted longer than Y. That was about a decade ago.

Worked. Had a little tinkering to do to get the time constants right. I tested it by clipping the wire to the bias supply, among other ways. Never could kill a tube or the power supply with this in place.

[Steve Conner]

I never liked cathode fuses. I can think of internal shorts in the tube that would bypass them, for instance cathode to grid, or cathode to heater.

A cathode to grid short would likely lead to a cathode to heater short right after the fuse blew.

When I played with regulators for B+, I just gave the regulator a current limit that would allow it to survive a short. This actually worked: I once plugged in a pair of metal 6L6s that turned out to be seriously faulty in some way, and the limiter gave me plenty of time to turn the thing off again.

[Gtr_tech]

A fast acting fuse in line with the B+ is always a good idea. The HV winding doesn't really like a short on it, and I've seen xfmrs damaged by shorted output tubes and some dummy with a 20a fuse for the mains. In fact, I have to source a pwr xfmr for an old Supro that this very thing happened to...

In another example, like PV does it, the HV fuse is on the inside. Most dummies won't pull the amp apart after it "quits working" due to a tube short, so they end up taking it in to be serviced because even with the new tubes, it still won't work. Mains fuse is ok, lights on...nobody's home. And if the HV fuse was on the outside, they replace the output tubes, fuse blows again, there could be spike diodes that have shorted. So if the HT fuse was on the outside where some dummy could severely overfuse it, they could "fix" the amp into submission requiring a new pwr xfmr.

IMO, every guitar amp should be idiot proofed. Musicians are not known for be'n real savvy about gear....

[J M Fahey]

IMO, every guitar amp should be idiot proofed. Musicians are not known for be'n real savvy about gear....

Amen, Brother

[leadfootdriver]

What if I decide to fuse each leg of the PT secondaries? How do I calculate the fuse rating? If the PT is rated at 300ma's, does that mean each leg puts out 150ma's?

[pdf64]

'What if I decide to fuse each leg of the PT secondaries? How do I calculate the fuse rating? If the PT is rated at 300ma's, does that mean each leg puts out 150ma's?'
Yes, I've not tried the Merlin approved arrangement yet but I think that's right, as each leg / phase will conduct the full current but for only half of each cycle (due to the rectifier). During the non-conducting half cycle the fuse would have chance to cool down. So therefore the average current will be half what the centre tap 'sees'.
If these are internal type fuses, try to get fuseholders with insulated covers, to mitigate for dufus moments when working on a live amp.
Steve must be right about shorts to the heater somehow being common; otherwise, how do all those 100ohm heater balancing resistors end up being burnt out? However, for a new build at least, cathode fuses seem worthwhile, as an addition to a regular B+ winding fuse.
Gtr_tech, re quick blow fuses, my feeling is that they may be unreliable in circuits with high switch on surge current, ie charging up the reservoir cap. Is that not really a concern, or do you mitigate by using PTC thermistors in series etc?
Pete.

[leadfootdriver]

My amp will be a 3-12ax7, 2-EL34 amp. In 50W Marshalls, they put a T500ma fuse after the rectifier. Would it then be a safe bet to go with 250ma's on the PT secondaries?

[pdf64]

It should be, as for this type of full wave rectifier, Idc = Iac.

[markr14850]

What if I decide to fuse each leg of the PT secondaries? How do I calculate the fuse rating? If the PT is rated at 300ma's, does that mean each leg puts out 150ma's?

You should be sure to understand all the implications that arose in this thread: http://music-electronics-forum.com/t27323/

[leadfootdriver]

It should be, as for this type of full wave rectifier, Idc = Iac.

Actually, it's a half wave.