Princeton Reverb "random" fuse popping help

[SelmaSedlak]

Hi,

i am still having some problems with "random" fuse popping on my blackface princeton reverb build.

I have narrowed it down to this:

1) The fuse burns only occasionally - i had one week of normal operation, then it burned. It was alright for another week, then it burned again.

2) In never burns the fuse while operating (playing). Measured voltages also seem to be on spec.

3) When the fuse burns, it is burned immediately when the amp is being switched on.
The power light does not even remotely light up.
This lead me to a hypothesis that it might be already burned by the previous powering down the amp. But this is probably not the case, as I checked the continuity across the fuse-holder after every switch off.

4) It never burns with no tubes in the amp.

5) It burns with just the rectifier 5U4GB in.

I am using the standard 0.5A slow-blow at 230 VAC.
The rectifier does make some rattling noises at startup, which fade a little after a couple of seconds, but never completely go away.
I have checked the filter caps for shorts and did not find anything suspicious.

Before i try the obvious - ordering a new rectifier and trying it in - i would appreciate any advice on what other things might cause this problem and I should check.
Thanks!

schematic:https://www.tubeampdoctor.com/media/pdf/80/ee/16/full_documentation_bf-14-rev_3.pdf

[SILVERGUN]

I am using the standard 0.5A slow-blow at 230 VAC.

Is your input voltage 230V?

[SelmaSedlak]

Is your input voltage 230V?

The mains voltage in my country is 230V.
From what i know for Princeton Reverb 0.5A fuse is used for this mains voltage, and is doubled in countries which use 120V mains.

[SILVERGUN]

That is correct. I just wanted to make sure.

You'll have to monitor current with a meter to see whats happening.

Also, double check that your fuse really is a slo-blo.

[dwinstonwood]

Hi SelmaSedlak, I'm trying to follow your rectifier wiring.

What is the brown and yellow wire on pin 3 of the 5U4GB? There is also a red wire connected to pin 5. I'm probably wrong, but I think pins 3 and 5 are "No Connection" pins on the 5U4GB.

I'm pretty sure 8 & 2 are the yellow 5VAC heater connections, and 4 & 6 are the red plate connections.

Sorry if I'm overlooking something obvious.

[SILVERGUN]

Hi SelmaSedlak, I'm trying to follow your rectifier wiring.

I checked over that too and found that the wiring follows the layout.
It looks like The Tube Amp Doctor is just using pins 3+5 as dead tie off points for optional taps.

[SILVERGUN]

After looking at the pic of the build I only have one other question:
Is there a physical connection between pins 4+5 on this tube socket?
I can't tell from the pic.

[SelmaSedlak]

Thanks, SILVERGUN and dwinstonwood!

You'll have to monitor current with a meter to see whats happening.

Until now i have avoided taking current measurements on the mains side. I would need to get some more sutitable DMM probes for that. My current hypothesis is that it also happens very quickly, so I'm unsure if I will be able to read it on a digital amp-meter.

Also, double check that your fuse really is a slo-blo.

The fuse is labeled T500 ma and that should be right. I also used fuses from two different purchases, so that should rule out a mislabeling of the fuse. This puzzles me a lot, because the fuse seem to burn out in an instant, in case the failure happens.

What is the brown and yellow wire on pin 3 of the 5U4GB? There is also a red wire connected to pin 5.

SILVERGUN is right, the layout uses the unconnected pins 3 and 5 as dead tie points for the unused 220V and 240V taps of the power transformer primary winding.

Is there a physical connection between pins 4+5 on this tube socket?

Yes i have double-proved that pins 4 and 5 on the noval tube sockets are connected.

[sluckey]

Go ahead and measure the mains current just to see how closely it is approaching the .5A fuse rating. It's possible that there is really no fault in the amp, just normal current inrush/spike that occurs in every amp at turn on. A thermistor in series with the PT primary can slow down the inrush current and probably solve this problem. Using a 3/4A or 1A fuse could probably stop those nuisance fuse blowings and still give the amp some normal protection too.

[jordan86]

I hate to sound lazy but if it only happens with a particular rectifier why not just start by replacing that? There could be other issues I guess but the few times I’ve blown fuses on fire up, it has always been a bad rectifier. Swapping it solved the problem. It’s good to be thorough but you could also be chasing a ghost and possibly cause more problems pulling things apart. 

As an aside, I don’t love the 5U4. i run a 5AR4 in my PR. I’d probably go 5Y3 or 5V4 if I were in your shoes. A 5Y3GT would probably give you about 390v on the 6V6 plates. A 5v4 would be around 425v. And 435v for a 5AR4. 5AR4 is a slow warmup rectifier too. If you like headroom, more Deluxe ish. A 5AR4 would be great.

Sorry to be unhelpful :)

[SelmaSedlak]

I have followed sluckeys and SILVERGUNs  advice, found some old hook probes and attached them in place of the wire between the fuse holder and the switch and made current measurement

I have done about 20 power ons before the failure occurred. Just the rectifier tube was installed.
All of them followed this pattern: initial peak dropped within a second to 0.15A where it stabilised.

The highest measured peaks were 0.83A, 0.72A, 0.55A the rest was bellow 0.5A.
I would assume a 0.5A slow-blow fuse would withstand these values. Of course they were measured with a DMM so the actual peak current might be probably larger.

On the last attempt when the failure occurred the values displayed were only 0.22 A and then 0.

Using a 3/4A or 1A fuse could probably stop those nuisance fuse blowings and still give the amp some normal protection too.

I have found a 0.8A fuse in my 5E3 build, so i might give it a try and check.

A thermistor in series with the PT primary can slow down the inrush current and probably solve this problem.

I have never worked with thermistors. From what i found, i would need a NTC thermistor. If i decide to go this way, would mounting it between the fuse holder and the switch a good way to go?

I hate to sound lazy but if it only happens with a particular rectifier why not just start by replacing that?

If i had a spare rectifier at hand, that would be the first thing to do. But because I would need to order it, I try to rule out other possible causes of failure. From my pedal building history i have learned that in 99 % the problem lies not in faulty components, but in my mistakes. Of course with tubes the ratio is probably smaller.
Anyway, I will probably try another rectifier tube, if I don't find another cause of the failure.

Thanks for all the helpful messages!

[jordan86]

On second thought...your TAD kit did not include a bias adjustment pot. I guess they went true to vintage spec. Seems quite impractical though for 2021. Prob best to stick with a new 5U4 unless the bias is just way cold already and you want to run it more at its full potential.  In that case, you could possible try the 5AR4. Get more plate voltage and volume and a slow warm up.

[PRR]

.......... the actual peak current might be probably larger.....

The idiot helper says the first half-cycle may be 1.2 Amps, falling very quickly to under 0.4A RMS (which may be a program bug; <0.2A seems more likely).

[acheld]

I dunno, but if the fuse blows with only the rectifier tube in place, and that tube makes rattling noises at startup, well then . . . seems to me the rectifier tube is a reasonable place to go at this point. 

[sluckey]

I dunno, but if the fuse blows with only the rectifier tube in place, and that tube makes rattling noises at startup, well then . . . seems to me the rectifier tube is a reasonable place to go at this point.

I agree. And I suggest a 5AR4/GZ34 rather than a 5U4 because of the slow warm up and because it only requires 2 amps rather than 3 amps for the 5U4. This means your mains current surge would be slightly less at turn on.

[ac427v]

Replacing the rectifier makes sense to me. But I was surprised to see that this circuit calls for a 1 amp slow blow fuse. Many other similar sized amps use a 2 amp fuse. In fact, I somehow used a 2 amp fuse in my PR build. Don't know why. Now I am curious to know if others have had this problem?

[sluckey]

Replacing the rectifier makes sense to me. But I was surprised to see that this circuit calls for a 1 amp slow blow fuse. Many other similar sized amps use a 2 amp fuse. In fact, I somehow used a 2 amp fuse in my PR build. Don't know why. Now I am curious to know if others have had this problem?

1A is the size if operating on 120V. .5A is the size when operating on 230V.

[glass54]

Hi Selma
I have had similar experiences with many Fender amps. (Occasionally other brands  )
The nominal mains voltage in Australia is 240VAC 50Hz. Quite frequently in my lab, I measure 245V to 247V. This is more than the 200% of original USA mains voltage. I find using the next value fuse in the range fixes my problem of "intermittent blowing fuses" during turn-on. In this case I would choose a T630mA fuse.
You are still getting the protection you need in case of major circuit failure in power supply/power amp but you are also "kind" to your power transformer primary winding. I would prefer not to go to T750mA and probably avoid T1A. (I think PRR's simulation points to this direction)
Australia was supposed to go to 230V to meet international standards but I'm not sure what happened there.
Kind regards
Mirek

[PRR]

> supposed to go to 230V

247V(124V) is only 8% hot of 230V(115V), and 99% of people do not know or care.

The last time it kinda really mattered was incandescent lamp lifetime. Inkys are out of fashion and 120V or even 130V are far more common than 115V lamps. CFLs semi-regulate and LEDs do better because they suck less.

[Medley]

I dunno, but if the fuse blows with only the rectifier tube in place, and that tube makes rattling noises at startup, well then . . . seems to me the rectifier tube is a reasonable place to go at this point.

I bet dollars to doughnuts it's a JJ 5U4 as well. I replaced one of those that came with my 5F4 kit I built last year because it too rattled on first power up. Didn't inspire confidence; I replaced it with a NOS JAN Phillips. And the 63 Vibroverb I just finished building blew it's JJ GZ34 on it's 3rd power up. Not a fan.

[eggbert]

Maybe blowing smoke in da wind but i had a bassman 100 amp doing the same thing, i found the heater wire fragment was just touching pin3 on power tube, didnt see it until i used a magnifying glass,  bent pins back away snipped the wire and the issue never happened again. Coincidence?

[tubeswell]

Is your input voltage 230V?

The mains voltage in my country is 230V.
From what i know for Princeton Reverb 0.5A fuse is used for this mains voltage, and is doubled in countries which use 120V mains.

Use a 1A slo-blo. I’m in a 230v mains country and use a 1A T fuse in a PR clone with no problems. (The silicon grain oriented steel laminations in modern PTs can cause underrated or barely rated mains fuses to blow)