Last night one of my MkIIIs blew a fuse. I heard it happen ... loud pop. (the speaker is fine) Pulled the tubes and they look good. Tube sockets are clean. I can't see any physical damage to anything on the unit. I don't have a tube tester but I have some spare tubes. FWIW, the caps on this amp were replaced by a previous owner. The amp sounded good until this event.

What is the best way for me to proceed?

Thanks, Bob, this is exactly what I will do tonight.

One other question, is it necessary or desirable to have a load across the output terminals? Would a high wattage 8 ohm resistor be satisfactory?

OK, so I replaced the fuse, with a resistor hooked up to the output terminals. Everything OK. Then put the amp back into the system. Have been listening for a couple of days. No problem so far.

I know you say the capacitors were replaced by a prior owner, but take another look at the amplifier and make sure that the power supply capacitors (the four-section can capacitor) were replaced, and that it was done with capacitors of sufficient voltage rating.

Also, a tube...either an output tube or the rectifier...could arc over intermittently and blow the fuse.

The replacement cap is a 30/20/20/20 525v, which is what the schematic calls for.

I seen recommendations that the new cap should be rated at a higher voltage. Also one vendor is selling a cap that has higher capacitance, I think 80/40/30/30 ... something like that.

Is this a worthwhile improvement?

If you were going to replace quad capacitor then all by means order the 80/40/30/20, wire the capacitor with the 40uf connected to the 5AR4 and the choke between the 40uf and 80uf section with the audio transformers red wires (center tap) connected after the choke to the 80uf section.

The improvement you might/will hear is tighter bass if you are playing the amp loud that is bass heavy.

Sal

hup_d_dup wrote:The replacement cap is a 30/20/20/20 525v, which is what the schematic calls for.

I seen recommendations that the new cap should be rated at a higher voltage. Also one vendor is selling a cap that has higher capacitance, I think 80/40/30/30 ... something like that.

Is this a worthwhile improvement?

525 volts is marginal at best. Quite often the AC line can be at 130VAC which puts a 525V capacitor in the danger zone, especially during turn-on. The delayed-on characteristic of the 5AR4 helps, though. I have gone to series-connected 350V electrolytics on my Mark II, making a 700V rating, safe with its 5U4 rectifier which has no such delayed-on characteristic.

The previous poster said that if you were to use the 80mfd section of that 80/40/30/30 cap, the feed to the output transformer should be placed in the second position, after the choke...and I quite agree.

As to your original fuse-blow, it's possible that a short occurred across a section of the quad electrolytic, which was then blown out by the current before the fuse blew. This happens quite a bit on old, marginal electrolytics, leaving no evidence that anything ever happened.

You didn't say...did you replace the tubes? When one of my Mark II blew a fuse lately, the cause was one of the Mullard EL34 output tubes that arced over intermittently. No evidence of runaway or of any other problems...just a bright arc, a POP and the blown fuse.

Wow, this is great info. Thanks for such quick replies!

What was the outcome with your amp, did you find out what was causing the fuse to blow?

Sal

No, the amp is running. However, I have discovered that my bias is high even at minimum setting. I don't know if this was a previous condition or if it's related to the fuse problem. I'm going to get a bias repair kit.

Thanks for your concern.

Bob Latino wrote:Hup,

Some people, when they do this, trace the wire from lug # 1 and find that it goes to ground? and say "This can't be right ? Positive to GROUND" but it is. The bias system runs a negative DC voltage to control the flow of current through the output tubes and everything is in REVERSE - positive to chassis ground.

Bob

Is that why my bias capacitors keep blowing up...

Only kidding....

Sal