Princeton Reverb - arcing inside rectifier tube

[Bob M.]

Hi all,

I've got an original BF Princeton Reverb on my bench. It arrived with one dead 6V6 power tube and a blown fuse. I examined the power amp and power supply, put in a new fuse and new pair of 6V6 power tubes and started testing. The bias supply needed some attention. One resistor had drifted too high (replaced) and I also replaced the bias set resistor as the bias was set a little too high, now lowered to the mid-20 mAs per tube. This amp uses the 5AR4A/GZ34 rectifier tubes, as do the early BF Princeton Reverb amps and I noticed the B+ is pretty high in this particular amp, about 425~430 Vdc, depending on the various 5AR4A rectifier tubes I subbed in. I subbed in different rectifiers for two reasons, 1) the 5AR4A rectifier that was in the amp when it arrived (a Mesa Boogie branded tube) looked a little cloudy/foggy inside the bottle (but very minor) and that had me take notice. 2) I thought I saw some arc-ing inside the rectifier bottle which instantly blew a fuse. The power transformer seems fine, drawing no current when unloaded (all tubes removed) and the filter caps are new (the cap can is not wired up, although present). The first filter cap is rated at 450 Vdc and I wondered if I I need a 500 volt rated cap in its place and if that could be the cause of the arc-ing inside the rectifier tube, as there is not much headroom between the B+ voltage and the first filter cap rating. It also did the same thing again with another known-good 5AR4A rectifier tube, blowing a fuse again. The bias circuit is stable and delivering expected voltages, the screen grid resistors are new and measure spot-on. The screen voltage on the power tubes is about 12 volts below the grid voltage. This arc-ing, fuse blowing is not happening all the time but occasionally, infrequently, but it's got to be solved, nonetheless.


What are some of the conditions that can cause arc-ing in a rectifier tube?

Thanks for your advice and expertise,

Bob M.

[Helmholtz]

What are some of the conditions that can cause arc-ing in a rectifier tube?

Reasons for arcing can be a gassy (poor vacuum) tube or intermittent contacts causing high (kV) flyback voltages. Increased current is not a probable reason.

[nevetslab]

Hi all,

I've got an original BF Princeton Reverb on my bench. It arrived with one dead 6V6 power tube and a blown fuse. I examined the power amp and power supply, put in a new fuse and new pair of 6V6 power tubes and started testing. The bias supply needed some attention. One resistor had drifted too high (replaced) and I also replaced the bias set resistor as the bias was set a little too high, now lowered to the mid-20 mAs per tube. This amp uses the 5AR4A/GZ34 rectifier tubes, as do the early BF Princeton Reverb amps and I noticed the B+ is pretty high in this particular amp, about 425~430 Vdc, depending on the various 5AR4A rectifier tubes I subbed in. I subbed in different rectifiers for two reasons, 1) the 5AR4A rectifier that was in the amp when it arrived (a Mesa Boogie branded tube) looked a little cloudy/foggy inside the bottle (but very minor) and that had me take notice. 2) I thought I saw some arc-ing inside the rectifier bottle which instantly blew a fuse. The power transformer seems fine, drawing no current when unloaded (all tubes removed) and the filter caps are new (the cap can is not wired up, although present). The first filter cap is rated at 450 Vdc and I wondered if I I need a 500 volt rated cap in its place and if that could be the cause of the arc-ing inside the rectifier tube, as there is not much headroom between the B+ voltage and the first filter cap rating. It also did the same thing again with another known-good 5AR4A rectifier tube, blowing a fuse again. The bias circuit is stable and delivering expected voltages, the screen grid resistors are new and measure spot-on. The screen voltage on the power tubes is about 12 volts below the grid voltage. This arc-ing, fuse blowing is not happening all the time but occasionally, infrequently, but it's got to be solved, nonetheless.


What are some of the conditions that can cause arc-ing in a rectifier tube?

Thanks for your advice and expertise,

Bob M.

Am I understanding you correctly.....the screen voltage is 12V below the grid voltage? I could believe that if you meant the plate voltage, but certainly NOT the grid voltage.

[Leo_Gnardo]

Could be that old filter cap is getting leaky with age, and that's causing your rectifier to arc. Yes it would be a good idea to go to 500V caps, for the first two sections. Nothing like having a little margin, especially with high voltage. Personally I replace the can cap with individual caps, after pushing the old cap terminals down as much as possible to make room. Then I place the new individual caps bridging over the site of the old one, + ends on the board and - ends to solder lugs placed under the power transformer's mounting nuts. Good to clean the chassis where those go too, it's often greasy/ waxy / corroded there. Some folks like to use the CE multisection cap instead, about a $45 part PLUS shipping. See why I use the individuals, I'm a cheapskate.

If you want to prove whether you need to replace that multisection cap or not, you could remove wires from the lugs for the first 2 filter sections, use clip leads to substitute known working caps & see what happens with that rectifier.

There are some amp owners who take pride in having "all original parts", they'd better expect to have breakdowns though. Electrolytic caps approaching 50 years old are nothing but time bombs. Preventive maintenance is where it's at if owner's going to really use the amp. If he wants to put it in his museum, and point it out to his friends "oooh! all original parts" well he better not plug it in.

[Helmholtz]

(Real) arcing is always initiated by over-voltage - not over-current. Over-voltage is often produced by interruption of current in a circuit containing inductors (PT secondary).
As soon as the rectifier arcs, mains current increases and eventually the mains fuse blows.

I recommend to measure HT and mains currents - but much more importantly clean and re-tighten rectifier socket contacts.

[g1]

(Real) arcing is always initiated by over-voltage - not over-current. Over-voltage is often produced by interruption of current in a circuit containing inductors (PT secondary).

I'm not sure real arcing is what we are talking about here. The flashing light seen inside the rectifier tube, often the result of excess current. Not arcing like we see at a power tube socket.

[Bob M.]

nevetslab,

My error, hurrying too much and not editing myself properly.

I meant to that screen voltage is about 12 volts below the plate voltage on the power tubes.

Thanks for pointing this out.

Bob M.

[Bob M.]

Many good suggestions here which I will follow up on.

Just to further detail what I'm observing inside the rectifier : a blue spark - mini lightening - if you will, followed immediately by the blowing of the amp's 1 amp slo-blow fuse on the rear panel. I've run this amp for hours on my current limiter with no problems and no unexpected results that I would experience from any other healthy BF PR.

The only other situation that I can think of where I've experienced rectifier tube arc-ing in the past is when I've added too much filtering (too high UFs) to the 1st filter cap but this is, to say, with only certain rectifier tubes but this is a stock amp with replaced (new) filter caps of the stock values. Besides this problem, the amp has been gone thru, drifted parts replaced, new electrolytics, in otherwise great shape.

Leo Gnardo, the filter caps have been replaced with discreet individual caps recently. They look new and measure new. I'm not a fan of the can caps and as a matter of course, I always replace older ones I find in circuits. I agree with you that replacing electrolytics is like a normal maintenance item, like tires and belts on a car. I think I'll up that 1st filter cap to a 500 volt one. Thanks for your comments.

Bob M.