Intermittant HT fuse blowing

[adamsapple]

I gotta '65 Fender Bassman that I converted to JTM 45 specs, using the orginal PT with separate filament trns. for rectifier and tubes and new OT for EL34's from Anitque Elect. Supplies. Everything runs fine, sounds good etc. but playing on max volume the HT fuse intermittantly blows. So I placed an ammeter across the HT fuse holder sans fuse and watch the current while playing guitar thru on max. I get an average current reading of about 225ma with peaks of about 425ma. Never has the reading gone near 500ma. So first are these current readings normal for a '45 ckt on 10 and 2nd if I never go quite near 500ma why does the fuse blow. Used both sloblo and fast fuse and both blew, also used different sets of power tubes and still blew. I'll add that at one point I used the amp with a talk box (before the conversion) and forgot to provide a load and blew the mains fuse after about an hour of playing but the amp still runs an provides correct voltages. Please help

[Twist]

What is the idle current for each tube with no signal ? Don't rule out an intermittent short in a new tube. It may be that the sudden current draw is too quick to observe on a current meter and thus the fuse blows.

[pdf64]

The currents look reasonable (what's the B+ at idle and max power?), but what meter are you using? Reason it matters is that there's a lot of ac on top of the dc, which you are going to miss without the right meter set correctly. This ac will heat the fuse, which is what is causing it to blow. My fluke 187 will measure ac+dc simultaneously, and the ac is true rms (ie actual heating power), rather than an average approximation. But if you just try measuring the ac at max output, and add it to the dc, it will give you an idea of what the total current is.
In addition to a HT fuse, I advise that you fit a fuse in each power tube cathode return to ground, because you can use regular quick blow fuses at that point (try F330mA) and so avoid stressing the transformer windings if a tube shorts, as a slow blow takes a long time to blow even at twice it's rating.
Hope that helps - Peter

[adamsapple]

Ive got the tubes idling at 34ma with 445V on the plates. These tubes work fine in my other amp so I've ruled out the tubes. As far as the meter its a BK and i had it set to ac current if i can remember right. The idle current thru the meter reads about 77ma. I like running a non-master volume amp on 10 to get power amp distortion so would i be better off getting a 100W power transformer instead of a 50W. Is the only difference btwn the 2 current handling?

[Twist]

I don't think a higher power OT would solve the problem unless you have developed an intermittent short in your speaker transformer. Tube amps are forgiving for some abuse but playing at "10" continuously will require a lot of trips to the bench. But hey, if it sounds good to you and you got the bucks for parts and tubes more power to ya !!!

[Steve Conner]

Well, it sounds like either an arced tube socket, a burnt screen grid in one of the power tubes or an intermittent short in the OT. Playing the amp without a load until the mains fuse blew could have caused any of these three faults.

I'd try new power tubes, as well as checking the power tube sockets for signs of burning, and replacing them if they are burnt. (Some folks prefer grinding the burnt bits out with a Dremel.) Replace the OT as a last resort because it's so expensive. I wouldn't recommend the 100W version as it has a different ratio and is a mismatch for only 2 tubes.