Blowing Fuses....

[Bob M.]

I have a Princeton amp (no reverb) that was totally rebuilt about 2~3 months ago with all new parts and new tubes with the exception of the 4 section multi can cap which seemed okay at the time I did the work. The amp worked like a dream for these few months but now blows its fuse (1 amp slo-blow) after about 2 to 3 hours of being on. The amp didn't seem different in performance at all and didn't seem especially hot or overheated when the fuse blew. I've been involved in some intense 5 to 6 hour a day rehearsals everyday, so the amp has been on for a long time at a stretch but the band plays very quietly so the amp has not been pushed very hard at all. All voltages read normally from my original spec sheet created when the work was done when I disassembled, benched and tested this amp. I should mention that I tied together two sections of each 20uf@450 Vdc cap, creating two 40uf@ 450Vdc caps. These were used to filter the PI and the preamp. I used new caps for the B+ and screen when I did the rebuild.

That's the background. The first thing I suspected was the 40 year old multi can cap and I replaced this with new, dedicated caps for the PI and preamp filtering. I reduced the preamp filter cap to 22uf@350Vdc as the voltage at this point reads about 325Vdc.

The amp still blows its fuse in the same fashion, after 2~3 hours of use. Next, I replaced the two 6V6 power tubes and the 5AR4A/GZ34 rectifier tube with the exact same result. I'm beginning to suspect the power transformer, which is original. Obviously, there is something causing the current draw to rise over time, with maybe a leaking voltage somewhere. I've gone over the amp and its wiring and it looks good to me. I've had another very good tech, who I trust, do the same and he says it looks fine. This amp did work very well in about a dozen gig situations over a two month or so period before developing this problem.

What should I try next before replacing the power transformer? Or is that the next thing to try?

Thanks for your time and expertise,

Bob M.

[Twist]

Did you replace the electrolytic cap in the bias supply circuit? It could be allowing the bias voltage to drift over time.

[Amp Kat]

It is possible that the bias is drifting or the coupling cap giving it up which also causes the bias to fluctuate. I would monitor that bias voltage after a few hours or when it starts to do it and the plate voltage also to find out where the current draw is coming from or what is giving it up when it gets hot. I don't think it's the tranny but if you don't find what is causing it then it could damage it. Are the tubes red plating when this happens and does it start humming right before it blows ?

[Enzo]

I don't think it is bias. Even when tubes red plate, it doesn't usually blow fuses, the tubes just sit there and bake.

Pull that fuse, and in its place, connect your DMM as an amp meter. Monitor the current the amplifier is drawing. What do you read? The fuse might be blowing two different ways. It could be running about normal, and at some point, a momentary spike takes the fuse. OORRR... it could be drawing much heavier current than normal and slowly climbing until it takes the fuse. OORRR... it could be coincidentally drawing just a little over the fuse rating, and following the fuse performance chart, it takes 3 hours at 110% - or whatever - before the fuse blows.

Will it pop the fuse if you don't play it, but leave it running for several hours? Or does it have to be played.

In any case, the nature of the overload leads us to different things depending.

You suspect a transformer? Are either of them getting hot?

[lowell]

maybe i didn't read it right am misunderstood... but in tying the 2 20uf 450v caps together you're actually getting 40uf at 225volts, not 450volts. Maybe these aren't taking the voltage? just a thought

[Enzo]

Uh no, wiring caps in parallel does not reduce their voltage rating. It is done all the time. Lots of Marshalls for example.

[lowell]

Enzo, you are exactly right. I just double checked that. Thanks for correcting me on that. Actually I read that when capacitors are in parallel the voltage rating is equal the smallest voltage rated cap. So 3 caps in parallel - 50uf 100v, 50uf 300v, 50uf 500v = 100 volt rating for the group.

[Bob M.]

Thanks for all the thoughtful replies.

I should have said it before but I converted this amp to cathode bias at the time of the rebuild. I'm using a big 10 watt resistor@ 470 ohms and I'm bypassing it with a 33uf cap @ 68Vdc (both brand-new parts). The cathode bias is about 34.5 Vdc at pin 8 of either 6V6 and has been very stable (In fact, all voltages have been stable, when the amp is cool or first turned on). At no time have I observed the power tubes redplating. I tested and observed alot at first to make sure all was stable and fine (and it was) for about the first 4~5 gigs, just after rebuilding it. It seems that I have to be playing the amp to blow the fuse but that's not definitive because I'm involved in rehearsals with my head stuck in my charts, reading. I'm also playing in stereo with a 2nd amp so when the Princeton blows its fuse, I still have sound and I'm situated much nearer to the 'other' amp. Of course, I've started paying more attention to the situation now. The other problem is I'm away from my home (and my bench/shop) on the other coast with only minimal tech stuff (no ammeter) but I do like the idea of monitoring my current draw. I might re-size the normal stock 1 amp Slo-Blo fuse to 1-1/2 amps. I do happen to have one of those oversized Allen Princeton replacement power transformers with me. I might put that in and see what happens or what doesn't happen. I'll report back after that.

Thanks,
Bob

[Enzo]

You don't have a basic hand DMM with you? Most have current scales. I have meterd outlets and stuff, but on my bench, most of the time it is faster to pull the fuse and stick my meter across the fuse holder for a quick read.

No one on the road with you has a meter? Go buy one of those cheap meters at Harbor Freight or something. For trend spotting like this, it only has to come close, it need not be lab quality measurements.

[Bruce / Mission Amps]

Of the many amps over the years I have modified, refurbished, repaired or rebuilt... the Princeton series and the Vibro-Champ amps are the ones that have had a history of of bad power transformers.

Also, I don't know what your plate voltage is on the power tubes there so maybe the power tubes, at close to 75ma and a hot PT, are idling in too high of current for that little 1a slo-blo fuse.

[Bob M.]

The experts (dia)log in...

Well, Enzo and Bruce,

Thanks so much for your insights. I guess I'll pick up a meter (what a good idea - why did I think of that!) I'm in that purgatory between the tour (and hiring of road crew - it's called rehearsals) and just being the guitar player (I'm supposed to know about notes, not (milli)amps). Once I find an amp that sounds flat-out great, I get hesitant to change things on it so I'd like to fine-tune it rather than make wholesale changes to it. I will check that bias situation. Even though I have a cathode biased amp (more being revealed!), I have borrowed John McIntyre's very cool ideas about PI, and bias from his Prince 'O Wails amp. The bias is adjustable (thru a 15 turn pot) and is tied to a PI measurement as I recall, but I reviewed my notes re: the McIntyre mod and he says to set bias between 32 and 33 Vdc across the 470 ohm cathode resistor. I'm sorry to follow this like a lemming but it did sound very, very good and it wasn't redplating. I was pulling about 33mA per 6V6 tube - high but it was great sounding (I know, just before they explode!!). Maybe I'll try a lower, more realistic current, in view of my plate 400Vdc plate voltage. Anyway, a great sounding amp - guys - thanks for the great ideas to keep it that way.

Bob M.