Amp intermittently blowing fuse at power up - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd January 2005, 01:38 AM   #1
diyAudio Member
 
Join Date: Sep 2002
Location: Oregon, USA
Default 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
  Reply With Quote
Old 2nd January 2005, 05:38 AM   #2
diyAudio Member
 
rcavictim's Avatar
 
Join Date: Jun 2004
Location: Next to an open wormhole NW of Toronto
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.
__________________
On April 01,2012, my radioastronomy Project TARGET SETI search succeeded. The signal came via extradimensional subspace, not radio waves. I now get a lot of 'visitors'. I took my avatar pic of one friend watching me in my work yard from just 20 ft. away.
  Reply With Quote
Old 2nd January 2005, 05:50 AM   #3
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
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,
__________________
Frank
  Reply With Quote
Old 2nd January 2005, 07:31 AM   #4
diyAudio Member
 
Join Date: Sep 2002
Location: Oregon, USA
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.
  Reply With Quote
Old 2nd January 2005, 01:59 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
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.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 2nd January 2005, 04:36 PM   #6
diyAudio Member
 
rcavictim's Avatar
 
Join Date: Jun 2004
Location: Next to an open wormhole NW of Toronto
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.
__________________
On April 01,2012, my radioastronomy Project TARGET SETI search succeeded. The signal came via extradimensional subspace, not radio waves. I now get a lot of 'visitors'. I took my avatar pic of one friend watching me in my work yard from just 20 ft. away.
  Reply With Quote
Old 9th January 2005, 09:46 PM   #7
diyAudio Member
 
Join Date: Sep 2002
Location: Oregon, USA
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
  Reply With Quote
Old 9th January 2005, 10:45 PM   #8
diyAudio Member
 
Join Date: Sep 2002
Location: Oregon, USA
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.
  Reply With Quote
Old 9th January 2005, 11:25 PM   #9
diyAudio Member
 
Join Date: Jul 2002
Location: pennsylvania
about that NTC resistor...
where does that go in the amp?
after or before the mains fuse?
i am guessing...before?
  Reply With Quote
Old 9th January 2005, 11:33 PM   #10
diyAudio Member
 
Join Date: Sep 2002
Location: Oregon, USA
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
AB Amp keep blowing fuse karlranks Solid State 3 29th May 2009 01:19 AM
Peavey 400 blowing fuse on power up Barnacle Solid State 11 2nd February 2009 10:49 AM
fosgate 1998 series power 500.2 amp blowing fuse jbrooksgametech Car Audio 13 26th November 2008 06:21 PM
Fuse housing melting but fuse not blowing, ppi amp rawadia Car Audio 21 2nd August 2008 10:39 AM
Fuse keeps blowing Vivek Everything Else 4 1st August 2003 06:31 AM


New To Site? Need Help?

All times are GMT. The time now is 09:41 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2