Hiwatt DR103 Fuse Blowing

[Pryde]

Hello,

I have a DR103, schematic attached, that is only popping the 6.3A main fuse when the amp is cranked past ~2 o'clock per the owner. As you can imagine that level would be ridiculously loud so I can't test that here.

The amp works as should until REALLY cranking it. What might I be looking for with this issue? Thanks for some insight.

[g1]

These are slow-blow fuses?
Possibly a power tube that is breaking down under high current conditions.

[Enzo]

That is what dummy loads are for.

My first question is this: is he using a real speaker cord, or is he using a regular guitar cord to connect the head to the speakers? Is whatever cord he is using in good shape? Is his cabinet good. In other words, he blows fuses with his whole setup, and I bet all you have is his head.

Could be a failing tube, do any of the power tubes rattle if you shake them? Could be an arc in the OT that only strikes above certain signal voltages.

Bang on the top of the amp while it is running. Use your fist. Does that make the amp crackle or anything? Louder volumes means more vibration from the speakers getting into the amp.

[Pryde]

That is what dummy loads are for.

My first question is this: is he using a real speaker cord, or is he using a regular guitar cord to connect the head to the speakers? Is whatever cord he is using in good shape? Is his cabinet good. In other words, he blows fuses with his whole setup, and I bet all you have is his head.

Could be a failing tube, do any of the power tubes rattle if you shake them? Could be an arc in the OT that only strikes above certain signal voltages.

Bang on the top of the amp while it is running. Use your fist. Does that make the amp crackle or anything? Louder volumes means more vibration from the speakers getting into the amp.

@g1: yes these are slo-blo fuses 6.3A rated

Thanks Enzo. I will complete the checks you recommended and report back. I should be able to drive it into a dummy load and hook up my current bulb limiter and test that way.

I will follow up with him regarding his setup. I did ask him if he was certain of cab impedence but not about the cab cord.
Thanks

[Enzo]

I am not as concerned with the cab impedance as I am with the cab having solid 100% jacks and wiring. The speakers themselves could even have intermittent tinsel wires that only shows up above certain levels of excursion. If you run a speaker "one notch off" on impedance, it won't make an amp blow fuses.

Your amp will not drive to high power levels well with the bulb limiter in place.

[Chuck H]

+++ to what Enzo said. I'll add that a fault found when bonking the amp could also be dirty tube sockets and oxidized tube pins. Or a shoert or fault in a tube or the amp as noted.

I'm posting because I noticed that you intend to use your current limiter. That's probably a good idea for initial testing to see that nothing is going to catch on fire, but unless you find an obvious failure in the physical testing or initial electronic checking you'll probably need to remove the current limiter to induce the problem that blows the fuse.

[Pryde]

Gotcha. I thought the bulb would brighten as the amp was cranked up assuming it is drawing excess current. He did inform me the impedence correct and he uses an actual speaker cable for the hookup.

[Enzo]

So we still need to see if the cord and speaker have good solid connections. Make him bring them to you, do not accept, "the cord HAS to be good, it is new" as an excuse. new things can be bad too.

yes the bulb will brighten the more current the amp draws, and the brighter the bulb gets, the lower the voltage the amp sees. So a bulb at half brightness might leave 60-70vAC for the poor amp to run on. remember the bulb is in series with the amp.

[Chuck H]

So we still need to see if the cord and speaker have good solid connections. Make him bring them to you, do not accept, "the cord HAS to be good, it is new" as an excuse. new things can be bad too.

And... If the problem IS vibration induced it could definitely have something to do with the cabinet rather than the amp. It's the whole rig that failed somehow. Maybe it's not the cord, speaker or jack. But maybe it is.

[Pryde]

OK got it apart and on the bench. Immediately blew a fuse at low volume. Removed all power tubes and put in new fuse to check for bad tubes...

Flipped the amp on and the fuse held BUT the power transformer started to crackle and smoke, uh-oh.

Big problems apparently. It did it with the standby off still. Would it be a primary side short perhaps? How can I confirm. Thanks for the help

[Chuck H]

That's odd. That the fuse popped with tubes in but the transformer tries to fry, but doesn't pop the fuse with the tubes out. Sorry if this is insulting, but did you install the correctly rated fuse?

The obvious next move would be to confirm the fuse and then lift the PT secondaries and try again.

[Jazz P Bass]

It appears that your power transformer is toast.

If it is crackling & smoking with the standby off.

[Chuck H]

It appears that your power transformer is toast.

If it is crackling & smoking with the standby off.

There's still the rectifier and first filter stage behind the standby switch. If either or both are bad that could compromise a new PT as well. But yeah, once the PT is crackling and smoking there isn't much to think about.

[Pryde]

So I disconnected the red secondaries from the diode rectifier network, removed the H.T. fuse, and disconnected the first filter cap and still lighting up my bulb bright. Without the limiter the mains fuse goes immediately (no magic smoke now). I gather the primary is shorted then and likely nothing in the secondary circuits themselves caused this? Want to make sure before getting a new PT in there.

[Pryde]

Would this be the PT I need? Doesn't appear the original Partrige replacements are available. Seems to spec out correctly except there is no multi-tap primary, only 120v: (amp is in USA so not a big deal).

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