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Advice on safe troubleshooting method please

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Posted on February 7, 2016 at 12:04:05
jorisdallaire
Audiophile

Posts: 356
Location: Quebec city
Joined: July 26, 2012
 Hello,

One vacuum diode on my DIY amp red-plated a few moments ago. Didn't want to leave it on longer to check if the other vacuum diode was red-plating too, I unplugged the thing right when I saw the problem. Two 6AX4 vacuum diodes are connected as full-wave bridge configuration, with two UF4007s on the other side. The mains fuse didn't blow, though. It is a 2A slow-blow.

Didn't have time or will to check if other tubes were overloaded either. Is it safe if I plug another pair of diodes in there and power on just long enough to check the situation further?

If something goes wrong I just quickly disconnect the damn widow-maker.
That would tell me if it is the diode that suddenly died or the problem is elsewhere. If the situation is OK long enough I'd also measure the plate temps with my IR thermometer gadget.

Is that method looks OK to you, or should I haul the amp to the bench and open to check and measure the internals just to be on the safe side?

I'm better at designing than repairing it seems ;-)
Thanks for any help,

Joris in Quebec city

 

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RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 13:54:12
REL54
Audiophile

Posts: 1944
Location: Virginia Coast
Joined: January 3, 2003
 I would certainly suggest you go with you last option of hauling the unit to the bench and going inside to check the internals. Look for burned or discolored resistors, bulging caps, or any other component that looks as if it has been over heated or burned. Also, be on the look out for lose wires or component leads that may have come loose at the solder joint.

You might not detect the problem visually but you don't have anything to lose by looking around. You just might find the problem before you energize the thing again and blow up something else.

roN

 

RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 14:17:22
Stuben
Audiophile

Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
 Joris,

Were you listening at the time?

Stuben

 

RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 14:31:35
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 I'm NO designer, but WHY the Slo-Blo fuse?
Perhaps a 3 amp REGULAR fuse?
Too much is never enough

 

RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 15:41:21
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 It's bench time. Don't risk serious damage to the power supply or transformer. It does sound as if something is drawing too much current through the tube. If the tube was shorted, you'd probably have a nice little light show.

 

RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 15:55:29
jorisdallaire
Audiophile

Posts: 356
Location: Quebec city
Joined: July 26, 2012
 Hello,

The amp is more demanding on startup than when running; hence the slow-blow fuse. It doesn't blow at the initial load, which is 2X nominal.

 

Thanks guys! To the bat-cave bench! {nt}, posted on February 7, 2016 at 16:00:48
jorisdallaire
Audiophile

Posts: 356
Location: Quebec city
Joined: July 26, 2012

 

RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 16:09:04
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 WAY past what I know, but here is the Elliot Sound Products article on 'soft start'.

Too much is never enough

 

RE: Advice on safe troubleshooting method please, posted on February 7, 2016 at 16:14:12
jorisdallaire
Audiophile

Posts: 356
Location: Quebec city
Joined: July 26, 2012
 Hello Stuben,

I wish I was, but I just laid down my Stax earphones on my desk, I was out the office door when I heard a strange "electrical" noise. I turned and saw the red-plated diode, and immediately removed mains power cord.

So I have no idea of the source of the problem or damage done :-(((

 

Vacuum diodes already provide soft-start. (nt), posted on February 7, 2016 at 16:51:10
jorisdallaire
Audiophile

Posts: 356
Location: Quebec city
Joined: July 26, 2012

 

RE: Vacuum diodes already provide soft-start. (nt), posted on February 8, 2016 at 07:06:05
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Vacuum rectifiers will provide a "soft" start, IF they are constructed with cathode sleeves. Directly heated vacuum rectifiers turn on almost as quickly as SS diodes do.


Eli D.

 

RE: Advice on safe troubleshooting method please, posted on February 11, 2016 at 06:03:51
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 With this sort of problem, one method I like to employ is to apply power slowly through a variac, closely monitoring AC current draw with a kill a watt. And checking voltages. If something is drawing too much current you should be able to isolate it and figure it out. This is an easy way to find bad components without destroying good ones.

Don't overlook that it could also just be a bad tube.

 

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