Pulling Current

[kdmay]

I am 90% the way through building a Rocket and thought, before fitting the preamp board, I'd measure some voltages.

I am using Vox AC30 iron.

I fitted a Sovtek 5AR4, that has been used successfully in other amps. No other tubes fitted.

I am using a 1A SLO-BLO Mains Fuse and a 500ma Fast BLO HT fuse.

I am based in Australia, so running 240v. When I check the AC in, it's 237v, so all good.

Anyway, when I flick the mains switch I keep blowing the mains fuse (twice). Now, further reading suggests I should be using a 2A fuse, but my maths suggested 1A should be fine.

Question is: can a tube rectifier (tested as good) pull enough current to blow a fuse? Or is there some dodgy wiring (I checked and traced the wiring to the schematic and it looks fine).

Cheers

[Diablo1]

You certainly have a wiring problem, or a bad filter capacitor, or a power transformer with an internal short. Make yourself a lightbulb limiter....google it. Plug your amp into your limiter to help you find your short. Perhaps your rectifier tube is shorted, pull that 1st and check it out on your lightbulb limiter.

[kdmay]

Thanks - that was the next plan of attack, remove rectifier, then power up. Will report back.

[Cliff Schecht]

Are you using fast blow or slow blow fuses? The inrush current from tube amp heaters when you flip on the switch is usually enough to pop a fast blow fuse rated at what seems to be the right operating current. Ever notice when the lights dim sometimes when you flick on a big tube amp? Those heaters draw a LOT more current instantaneously when cold! I've been told the number is up to 4-5x what the nominal current is (very dependent on your tubes of course) but either way, make sure you are using slow blow fuses for the mains fuse!

[kdmay]

OK - so I removed the rectifier and by this point only had 2 x Fast Blo 1A fuses left (the SLO BLOs had already BLO'd!).

Pop!

Then I removed the Heater wiring and tried again. Pop.

In both cases, it was instant.

Next I pinched the last remaining fuse from an existing amp. It was a 2A SLO BLO, which I now think is most suitable.

Before I powered on, I went through everything I could think of.

I measured the resistance across all the PT windings. The primary was quite low (around 9ohms IIRC) and the HT secondaries was around 39ohms. None of the windings were open circuit to ground, which seems to suggest that nothing inside the PT is shorting. But I am absolutely no expert at this.

I measured everywhere on the amp I could think to check if there was an incorrect ground connection. Nothing. Double checked wiring and it looked fine.

So, I powered up with the 2A SLO BLO. This time it made a weird humming noise I've not heard before (this is my 4th build). It was different from the normal transformer hum, louder and more "mechanical".

After around 5 seconds, before I had a chance to read the secondaries, pop!

Hopefully I haven't damaged the PT by allowing too much current in.

The smoking gun is pointing towards the PT, but all the windings seemed ok.

Any ideas?

[passfan]

If...you have completely disconnected ALL secondary windings and blown a 2 amp fuse 5 seconds after powering up the primary; how much more damage do you think you could do ? I'd say it's time for a new transformer , which was probably your problem from the start.

That mechanical noise you heard was a winding inside having what we call a hissy fit. It sounds like it is content to having it's hissy fit on a permanent basis as well, sorry for your luck. It happens to all of us.

For the record, I believe when your secondary voltage goes up to 240 the current should go down by the same ratio. Basic ohms law states that 120 volts times 3 amps is 360 watts or 240 volts times 1.5 amps is 360 watts.

In the future, best to rig up a bulb limiter. You could have a winding blowing across itself and not to ground which is going to draw more current simply by affecting the windings resistance. Hitting it time and time again is only going to make it worse not to mention waste your time buying more fuses which I believe at this point...you're out of

[kdmay]

You could have a winding blowing across itself and not to ground which is going to draw more current simply by affecting the windings resistance.

Thanks for your response.

Is there any way of testing this?

Yep - I am outta luck and should have listened all that time ago when wiser people told me to build that lightbulb limited for my first build!

I guess I've been lucky up until now......

[passfan]

Yep, and you just did it, the poor mans way. You could meg it out but you would need a megger. A hipot test would also tell you but even more expensive equipment would be needed. It sounds like an existing condition which you could have done nothing about. Popping everything else time and time again is going to cost you money down the road and, if you're like me having to get everything mail order,the added time waiting for it to arrive.

[kdmay]

Problem sorted!

It's with a slightly bruised ego that I advise the issue.

In the Rocket schematic, where the HT secondaries connect to the rectifier, my brain has interpreted that to be the heaters. For some reason, in my head I had assumed the top of the winding in the schematic was the HT and the middle was the heaters. (No when I look at it, it's very clear to me!)

So, I had wired the HT to the heaters. Given one side of the HT was also going to the first filter cap, due to the centre tap to ground I had created a ground reference via the cap. Pop!!



The good news is that the PT seems to have survived. I am measuring 420vdc no load, which seems quite high but I'm sure once the glass is in pulling current that will come down. Don't reckon I'm going to get the ~320vdc I was shooting for though.

The bad-ish news is that my VVR isn't working (not adjustment going on). Need to do some reading, as I'm not sure whether this will correct itself under load. I did have a slight issue, as I hadn't used a mica spacing and the drain was directly connected to ground (via the chassis). A few more pops! Not sure whether this damaged the Mosfet.

Anyway, thanks for everyone's help. It goes to show that my troubleshooting skills need some fine tuning (I had measured the cap resistance to ground and measured the exact secondary winding resistance, thinking this was correct!!!).