Amp keeps blowing fuses at startup

[Baguette]

So I've built this 5F6ish amp and it sounds great with no issues.

Yesterday the customer (a pro player) calls me in and tells the amp has blown 3 mains fuses at startup (or a few seconds after start up). He brings the amp back to me, and the amp works dandy in my shop. I do the following tests :

- PT test : I run it with a pair of EL34 biased really hot at 80mA each. After 3 hours, the PT is cooking hot, but no issues and nothing blows.
- just in case, I remove the power tubes / rectifier (I leave only the 3 preamp tubes) and "cook" the PT for an hour. It gets slightly warm.
- I swap the GZ34 with an equivalent Weber Copper Cap to save a couple of watts from the PT, just to make it run cooler.
- I test the original 6L6GC tubes (Groove Tubes 6L6GE). Make them run for 3 hours biased really hot at 60mA or so each. Lightly tap on them in the end and the thing don't bulge.
- The amp sounds great. No hum no nothing, just great sound.
- Today I plug it in and and everything's ok.

The customer take it back, switch it on and FUSE BLOWS. He replaces it and now it works.

The amp has 230V primary voltage. Weber Copper Cap GZ34 equivalent at the input. No standby switch. 2 6L6GC biased @ 40mA or so each with 470V B+. Mains fuse is a 1.5A Slo Blo.
Now when I think of it, while doing measurements right after the amp was done 3 weeks ago, the tip of my meter inadvertently went touching both pin 2 and pin 3 on a 6L6 socket (plate to cathode). Big electric arc for 2 or 3 second and then the fuse blew. I'm still using the same tubes as they seem to work perfectly, despite this misadventure.

How comes this sucker blows fuses, and how comes they seem to blow at the customer's room?
Could the plate to cathode short kinda ruined one tube, or worse the OT and be the cause of this fuse blowing problem?
I'm stumped. The player's got a big show on friday and I'm extremely worried. I lended him a Blackface Twin Reverb as a backup but the band prefers my amp by far.

Thanks for any insight.

[Raybob]

1.5A seems a bit small for a 5F6A.  Without calculating, I would think you'd at least want a 2.0A Slo-Blo.

[stratele52]

1.5  A too small. I'll use 2 A normal blow like Fender amp use.

[Baguette]

I'm in Europe with 230V on the primaries. I took the original US value (2 or 3 amp slo blo for the big dual 6L6 Fender amps) and halved it since line voltage is doubled.
Is that wrong?

[Willabe]

Could it be that even thou the fuse is a slo-blo type but because the fuse is a little underrated that the in rush current it taking out the fuse at start up?

I just noticed that your using a GZ34 and real ones have a controlled warm up time, so there should be no in rush at start up.



                        Brad      

[Colas LeGrippa]

Baguette, augmente la valeur du fusible de 100mA jusqu'à ce qu'il supporte le courant.  Par exemple, débute avec 2A, puis 2.1 A ainsi de suite , sans dépasser trop la valeur suggérée.
Utilise des fusibles de type 'slo blo'  , c'est-à-dire supportant un courant plus élevé, lors de la mise sous tension.


Colas

[RicharD]

>I'm in Europe with 230V on the primaries. I took the original US value (2 or 3 amp slo blo for the big dual 6L6 Fender amps) and halved it since line voltage is doubled.
Is that wrong?

It is correct.  I=E/P  I will be 1/2 where E doubles and P is a constant.  A 2A sloblo would prolly still be safe.  I'm thinking chopstick the heck out of the power supply.  Something may be loose causing the intermittent issue.

[Baguette]

I did chopstick all solder joints and it is all nice, shiny and tight down there.
1.5A Slo Blo seems like a good value (remember, it 230VAC line voltage here). Fender 59' RI Bassman and Super Reverb come in Europe with 1.5A mains fuse.
I'm totally perplexed on this one.

Could it be a bad PT? The laminations get cooking hot after a couple hours of use with a hot biased set of power tubes.

Thanks for your replies.

[Willabe]

If the fuse is _ only _ blowing up at start up, I don't see how the PT getting hot _ after _ a couple of hours would be why the fuse blows.

You say you have the OP tubes biased hot, seems like the PT is not rated high enough (B+ mA's on secondary) to handle the load.
So it's working too hard and it's heating up.


                              Brad     

[sluckey]

There may not be anything wrong with the amp. Can you check the AC line voltage at your customers place?

This '59 Bassman Service Manual shows the fuse at 1.6A for 225/240VAC models. It also has a thermistor TH1 in the PT primary. This thermistor has a negative temp coefficient and is used to prevent high inrush of current at turn on by presenting a higher resistance when it's cold. As it heats up, the resistance drops allowing normal current to flow to the PT. It's a slow turn on feature.

This thermistor would help your situation. A 1.6A slow blow fuse would help. And going back to the real GZ34 (slow warmup) would help also. These three things may stop the fuse from popping at turn on.

[Baguette]

Thanks Mr Luckey. Hopefully you're right and I'm worrying for no reason.
But still, I don't like the idea of an amp intermitently blowing fuses at startup being on a big stage.

[chocopower]

"amp intermitently blowing fuses at startup being on a big stage."

that could be a clue.
I don´t know in your country, but here, in spain, voltaje suply for outdoor events cames from engines gasoil generators who use to be.... "inestable"

And i agree, a Bassman could work perfect safe with a 2A fuse.
In the past i have some similar problem with 2 blackface bassman when i changed the USA power trafo for a 240v one.
Both use 2A in 110v sittuation. I follow the rule and put 1A. Fuse blow always.
Them try 1,5A, and still blow often.
2A work since them without problem in both amps.

[Baguette]

Thanks for sharing Chocopower.
Are you using 2A Slo Blo or Fast Blo fuses on those amps?
Right now the amp has 1.5A Slo Blo and it has blown 2 of them. The 1A Fast the owner had on hand all blew immediately, no surprise here.

For some reason, I'm a bit scared of going up to 2A Slo Blo.

[Geezer]

I would suspect the Weber copper cap. There have been problems reported with those.

If you've ever been inside one (I have) there are resistors & a thermistor stuffed in there, and it all is bunched up & "balanced" in the center of the metal tube. If the assembly moves at all, it can short out on the metal wall & cause problems.

Going back to an actual glass tube GZ34 will have multiple good results, IMO.

$0.02

G

[Baguette]

The Copper cap is not the problem. It blew a couple fuses with a JJ GZ34 as well.

[Geezer]

The Copper cap is not the problem. It blew a couple fuses with a JJ GZ34 as well.

Very good!

Carry on............. (my wayward son) 

[chocopower]

Thanks for sharing Chocopower.
Are you using 2A Slo Blo or Fast Blo fuses on those amps?
Right now the amp has 1.5A Slo Blo and it has blown 2 of them. The 1A Fast the owner had on hand all blew immediately, no surprise here.

For some reason, I'm a bit scared of going up to 2A Slo Blo.

Slow blow.

[Baguette]

Guess I'm forced to go the 2A way. Seems pretty wild to me, although IIRC 50W Marshall had a 2A fuse in the mains slot.
What you guys think, is a 2A fuse too much for a regular Bassman running on 230VAC?

[plexi50]

Could be that the line voltage is higher at his place than at yours. I notice last week that my domestic line voltage is now 125VAC instead of the 120VAC i saw a few months ago. Go figure /