Orange OR15 blowing HT fuse

[pontiacpete]

Hi, This amp is blowing the HT fuse, a 250mA slo blo. Immediately I think the tubes might be going, so I put in a new fuse and new el84s and it still blows the fuse. This amp is pretty new but it's over the warranty.
I did a quick search on the web and sure enough there are others who have had this problem with the HT fuse but under warranty.
When sending it back they were told that the fuse holder was replaced and in one case the 12AT7 was also replaced.
In my feeble mind it doesn't seem very likely that a fuse holder would cause this problem. But is it possible and worth the trouble of finding the proper holder and swapping it out?
The old tubes are pulling about 34mA/tube and 38mA with the new tubes.
I checked the ESR on the first cap a 220uf 450v cap, and that measures about 1.7 ohms. Does this warrant taking this apart and getting a cap that fits in place?
I just wondering how you would deal with a repair like this?
I'm almost inclined to just up the HT fuse to a 500mA. Tried this and no problem seems to arise on the limiter and without limiter. Flipping from standby to the 15 watts or the 7 watts does not blow the fuse nor is there any sign of anything burning, nothing.
How bad of idea it to up the rating of this cap?

Thanks for any suggestions!
pete

[Leo_Gnardo]

No schematic, we don't know for sure the location of this fuse in circuit. But if it is between rectifier & main filter cap, holey smokes 220 uF is a way big cap and would cause a bit of a current surge as you go from standby to operate mode, that could easily burn even a slo blo 250 mA cap. 500 mA may hold for that cap-charging surge but affords you less protection should an output tube short. It's sort of a hobbes choice. Might try some in-between fuse values if you can easily get 'em, say 300 or 315 mA, 400 mA, use the smallest one that will hold thru the surge period. I'd hate to see an output transformer get fried because an EL84 shorted & then the fuse held instead of opened.

New output tubes, that's good but new doesn't always mean working properly. Hear any noises from them, rattles if you tap 'em while the amp's on? If so, replace them, check your bias current & make sure it's in the ball park, do install the spec'd 250 mA fuse then see if it holds. Also, what's the high voltage, B+ in your Orange?

[pontiacpete]

It has 338vdc for B+.

Thanks for the suggestions!

[pontiacpete]

I have a couple of 315mA fuses, not sure if they are slo blo because there's no marking.
However it blew the first go around.

[Leo_Gnardo]

I have a couple of 315mA fuses, not sure if they are slo blo because there's no marking. However it blew the first go around.

I can't find an OR15 schematic, at least not fast. If it is in fact rectifier > fuse > stby switch > main filter cap it may make sense to change the order of appearance so the filter cap charges on power up, then the fuse isn't stressed by having to charge it up when switching to go mode. Also I'm sure you checked to make sure bias voltage at the EL84 grids is what we expect it to be, somewhere around -11.5 to -12.5 V, right?

Also, the hi voltage is similar to Fender's Blues Jr, maybe a dab higher. I set bias in the high 20's in those amps say 26 to 29 milliamps. I'm sure your EL84's would be a bit happier in your OR15 if set similarly.

[Jazz P Bass]

OR 15 schematic:

Orange OR15 SCH 115V-230V.zip

[pontiacpete]

Oh,,,, here it is.

[pontiacpete]

There's 11v on the cathodes. This is cathode biased. Isn't 34mA about right for cathode biased pair of el84s?

Sorry B+ is 324vdc.

That might be tricking moving the cap? Would have to think about how to do that.

[Leo_Gnardo]

Oh,,,, here it is.

Similar to what I suspected. If you were to wire together the OT center tap and the hot side of the first dropping resistor, put fuse between them and the filter cap, I suspect you'd put a stop to the fuse popping & still have good circuit protection. Might not be a bad idea to also have a larger fuse say 500 mA in the location of your present one in case of a "gone to hell" situation, say one of the rectifiers goes bad or main filter cap shorts. That amp uses some whopping big filters too, 220 uF then 100 uF. Keeps hum down all right but it apparently likes to eat fuses on charging them up.

You'd be moving the wiring not the cap itself. Might mean cutting circuit traces on the PCB.

Now I see it's self-biased. 34 mA not awful high but you might be able to dial it down a smidge with a larger value cathode resistor. Now that I see what's going on in the power surprise, I'm a lot less worried about bias current at this level, that's not apparently the cause of your fusen-poppen.

[52 Bill]

Doesn't seem to have a standby switch. Does the fuse hold at turn on with the output tubes removed?

[pontiacpete]

It's not shown but actually it does have a 3 position switch with the standby between the 7 and 15 watt modes.

[Leo_Gnardo]

Doesn't seem to have a standby switch. Does the fuse hold at turn on with the output tubes removed?

SW1A acts as a standby, also selects hi voltage secondary tap for low or hi power settings. Still a good test to see if fuse holds with output tubes removed.

Now that I've had a peek at the schematic, and that SW1A feature, it wouldn't be possible to move standby to another position, without adding another switch. Drat.

Might it be a way out, to partially charge C25 at low power setting then switch quickly to full power?

[R.G.]

If I might make a general debugging suggestion - one huge problem with problems involving the words "fuse" and "blow" is that before the fuse blows you're very lucky if you were looking in the right place, and after the fuse blows, not only does nothing work you're out another $0.50 to $2.00 for another fuse. It gets expensive and frustrating.

One huge advantage is if you can replace the fuse with something that holds current to some limiting value without blowing. This is the whole reason for existence of the light bulb limiter. It holds things at a pre-disaster level so you can poke around and find what's wrong.

Another way to do this is with a current limiter. Here's my take on one version of this: Tube Amp Current Clamp. This little circuit can be set to limit at any current I = 0.6V/R2. So to limit to 200ma (before your fuse would pop), you need R2 = 0.6v/0.2A = 3 ohms. 3.3 ohms limits to I = 0.6V/3.3 = 182ma.

That being said, it's almost certainly
- rectifier diodes
- B+ filter caps
- C24 cathode bypass cap
- R41/R42 not grounding the output tube grids
- C22/C23 leaking.

[jeffects]

I'm having the exact same problem with an Orange OR-15. If I use my light bulb current limiter, the HT fuse does not blow and I have output. If I don't use the current limiter it will blow consistently when going from standby to 15watt. Was there ever a difinitive solution to this? Thanks.

[pontiacpete]

Sorry I can't remember what I did to resolve the issue. Maybe you should start a new thread.