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Saurav 2nd January 2005 01:38 AM

Amp intermittently blowing fuse at power up
 
What should I be looking at? I don't see any shorts anywhere. I haven't noticed the rectifier tube flash. The amp is a 2A3 SET, with Sovtek 2A3s being run a little hot, and ECC99 driver tubes at 20mA. What size fuse should I use? I've been using a 2A slo-blo, can I up that to 3A? It did it for the first time about a week ago today was the 2nd time. As usual, I haven't changed anything in over a month.

Thanks,
Saurav

rcavictim 2nd January 2005 05:38 AM

You are probably suffering the occasional peak current surge when the moment your switch closes happens to coincide with a sinewave peak in the mains supply. Slo-blo fuses are helpful but if you go too big on the fuse rating you lose protection for the small problems that are often easier to fix. How about wiring a two ohm 5 watt resistor in series with the power cord. Better yet, go to www.rfparts.com and buy one of those little NTC (negative temp coefficient) resistors that are specifically designed to go in series with the mains supply and limit turn on surges. They start off looking like about a 10 or 20 ohm resistor and then as they heat their resitance quickly drops taking them effectively out of the circuit. They are cheap at just a couple or three bucks each and look like a disc capacitor IIRC.

fdegrove 2nd January 2005 05:50 AM

Hi,

Quote:

They start off looking like about a 10 or 20 ohm resistor and then as they heat their resitance quickly drops taking them effectively out of the circuit. They are cheap at just a couple or three bucks each and look like a disc capacitor IIRC.
No tube gear drawing more than an ampere (whatever) should be without.
I use them everywhere...

Other than that, as per rcavictim's advice and I'd also recommend a slo-blo fuse on the primary side of the powerxfomer, fast blow type at the secondary side (B+).
I leave the heaters as they are, never caused trouble in the past.

One remark, an NTC is NOT a fuse but when it dies it goes open circuit so you're safe.
It's just that when you're troubleshooting that it's not always obvious to think about them when they're dead.

Cheers, ;)

Saurav 2nd January 2005 07:31 AM

I have only one fuse, and it's the first thing the power sees, it's even before the switch. And it's a slo-blo. The NTC is a good idea. I could maybe get a handful, in case those die too.

Thanks.

SY 2nd January 2005 01:59 PM

The other way to handle this is with a zero-crossing relay, one that fires when (as you would expect) the line voltage crosses zero.

rcavictim 2nd January 2005 04:36 PM

Quote:

Originally posted by SY
The other way to handle this is with a zero-crossing relay, one that fires when (as you would expect) the line voltage crosses zero.

I used this solution in my 300B driven 805 SET integrated amp project. Use two SSR's, one for mains and one for Hi-B+ (have separate plate suply xfmers). I didn't mention it in this case because it is ovekill and expensive for the posted problem but Sy is correct. In my case I wanted to be able to use the LVDC control feature (no radiated hum) on toggle mains control switches located inside the copper shielded box containing the low level audio volume and source selection circuits. SSR's worked out great! Using DC activated SSR's required the addition of a well filtered ~9 volt DC power supply on the chasis that was alive whenever the plug was in the wall.

This low current, low voltage control loop also returns to ground through the removeable tube cage mounting pins(banana plugs and jacks). No cage, no power. With up to 1600 VDC in there and some 300 joules of stored energy I felt this was a must. Someone else (likely less aware) will own this fine amplifier someday after me and this was mainly done to protect them.

Saurav 9th January 2005 09:46 PM

Well, it looks like my rectifier tube has died. It's making a rattling sound from inside the tube. So maybe that's why I've been having these sporadic problems. At least, it sounds like it's coming from the rectifier, and it doesn't happen if I power up the amp with the rectifier out. Time to buy a new 5AR4, and I'll probably take my 2A3 operating point back to the classic 15W plate dissipation. I have Sovtek 2A3s and I'm running them quite a bit hotter, and they seem to be fine, but I guess the rectifier didn't like it.

I could replace it with an SS rectifier too. But I like the slow B+ ramp-up, and this rectifier seemed to be the simplest way of achieving that. I looked around Triode Electronics, and there's the standard Sovtek 5AR4, which is what I have now, and it looks like they're selling a new JJ/Tesla 5AR4. Does anyone have any experience with that? Is there any other tube that is pin compatible with the 5AR4, is indirectly heated, and is beefier? My input cap is about 10uF, which should be well within 5AR4 limits, right. I'm probably running about 75mA through each 2A3, and 20mA through the ECC99s, so that's about 200mA total. Hmm.. maybe that's my problem right there. I found one page that said 250mA, but TDSL says 160mA. Which is it? If it's 160, then that's the answer to my question about why it died. OK, how much current I can send through it depends on what voltage I run it at, right. I don't remember that off the top of my head.

Anyway, is there a beefier substitiute for the 5AR4? If not, I'll just get a new one and back off my 2A3 operating point. That'll also let me try out fancier 2A3 tubes :)

Thanks,
Saurav

Saurav 9th January 2005 10:45 PM

More info... the PT secondary shows a DCR of 46 ohms. I also measured 23 ohms from each end to the CT, which has a 10 ohm resistor to earth ground, and I measure 33 ohms from the secondary taps to chassis. There's no reading between the secondary and primary windings, or primary to chassis. So I don't think I've killed my power transformer, right? I think I bought the Hammond 272JX, which should be 300-0-300 @ 250mA.

el capitan83 9th January 2005 11:25 PM

about that NTC resistor...
where does that go in the amp?
after or before the mains fuse?
i am guessing...before?

Saurav 9th January 2005 11:33 PM

I would put it after. I don't like having anything in my amp live if the fuse is blown, so the fuse is the first thing the incoming power sees. But that's just me.


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