Trainwreck Rocket blowing mains fuse...

[freakshow__]

Hey all

I have a Rocket clone I made that has been fine for about a month of playing. It started blowing the mains (2A slo) fuse all of a sudden. Here are the things I have done so far to remedy:

Check voltages, it looks fine everywhere as far I can see

visually look at components for problems...nothing is burned etc...

changed tubes....it was fine for a week but is now back to blowing fuses when you go to "ON' from standby. I don't think tubes are the issue here (read below) but who knows.....

The thing that concerns me is the fuse holder....the spring in it was very weak to begin with and I noticed it was actually stuck so I freed it back up after blowing 3-4 fuses. It is one of the cheaper thread-in type ones for reference.
I'm wondering if it being loose has caused some arcing when you go to "ON' from standby, causing a little bit of melting to happen inside it each time...which is making a bad connection over time? It worked for a week after I freed it up....Does this make sense?

I read on another forum of a guy that had the same problem, once he changed the fuse holder to a better one his fuse issues went away.

Does anyone have any other ideas where to look for a problem?

[freakshow__]

[xtian]

Your Rocket has a tube rectifier. I would NOT use the Standby switch at all, ever. Leave it in "run" position. See if this solves your issue.

[Colossal]

Nice build.

Not related to your issue, but I would ensure that there is some space between the power tubes' cathode resistor and the 220uF bypass cap.

[freakshow__]

xtian

I’ve been reading this in many forums….any amp using a gz34 rec tube should not need to use a standby switch…..wish I knew this before because i would just build all rockets with no standby switch anymore! funny….this is the only rocket I’ve built with this issue.

[Mark]

Very nice build quality.

What value filter caps are on the first stage?

Typically, there is 80uF there, the GZ-34 won’t like that. I use my standby switch as I have a 40uF cap on one side of the switch and another 40uF on the other side of the switch. I charge up the first 40uF, then the GZ-34 and the first filter caps charges the second cap. This may ease the burden of switch on.

Is there a Zener diode behind the filter caps?
What rail voltage does your amp have?
Why did you use 1K screen grid resistors on the output valves?
I experimented with this and found the 1K resistors reduces bass response.

[Littlewyan]

It could the fuse holder. Did you swap valves at the same time as freeing up the spring though? And how is your standby switch wired? Is it between the rectifier and filter caps?

[freakshow__]

i usually go by the ceriatone layout with the exception that I don’t do the 1/2 power / full power thing that is shown.

for filter caps i usually use 3x 30uf and 3x 22uf in the cap ‘block’ i use the F&T brand that I get from amplified parts.

i swapped tubes first….still blew fuses so i ‘fixed’ the fuse holder after that. it worked for a week afterwards. this is why i think there’s an issue with it.

i buy most of my parts from RJ and he supplies the 1k resistors…that’s why I use them.

[Littlewyan]

Try swapping the fuse holder then. It's possible that it's causing the issue. I know I've blown fuses on one of my amps when I've flicked the power switch too slowly. I've never fully understood why but I think it's something to do with the connection being made on and off really quickly by an arc and doing something with the transformer. I think old car ignitions worked in a similar way, inductors/coils and something flicking power on/off really fast to generate a high voltage.

[ViperDoc]

GZ34 max filter cap value is 60 uF.

[freakshow__]

littlewyan

I agree. i think there is a small lightning show inside the fuse holder at power up. i think it damaged the contacts inside and now it just arcs. after a bunch of power ups it causes a bad connection and blows the fuse.

i’ll let everyone here know if this truly IS the case.

weird though huh? i was only able to find 1 other individual on the forums who had this same issue which was the fuse holder.

we’ll see.

[wpaulvogel]

It creates heat because of the loose connection and that melts the solder in the cup of the fuse. It’s most likely not an over current situation. What happens if you leave the shrink tubing off the sprung end of the fuse holder. That’s probably what’s interfering with the spring. The holder is probably shot but I’d leave the heat shrink off the new holder.

[freakshow__]

i did remove the shrink tubing. that’s how i discovered the messed up spring…. i should get the amp thursday. i’ll be sure to post if the fuse holder is the culprit. fingers crossed.

[freakshow__]

changed the fuse holder to the ‘high quality’ style one that amplified parts sells. So far so good. i have powered up the amp 8x and no issues yet. i’m going to power it on/off several times this week before giving it back to the owner to be safe.

weird that i haven’t seen much out there for this issue….guess there’s a first for everything?

remember kids…..only use quality fuse holders!

[Mark]

That’s definitely a strange fault.