blowing fuses

[Slartibartfast]

I am using the JE Labs schematic diagram on my 2A3 and if I have been listening to the amp, power down, and come back an hour or so later and power up the amp again, the fuse blows. It is a 3A slo-blo. What is good value bleeder resistor to use for the PS caps?

Thanks,

Robert

[dcriner]

Hi, Robert. I'm not familiar with your schematic. But, the discharge time constant = T = 1/RC. Pick your T (in seconds) and solve for R. Maybe 100K would be about right?

You must select a resistor with adequate power rating for normal operation. P = V^2/R. Double power rating for safety margin.

But the thing I'm wondering about is -- If the filter caps are staying charged, then when you turn on the power, I'd expect the current surge to be less than if the filter caps were fully discharged.

I think of a bleeder resistor for improving safety against shock for people working on an amp.

[Slartibartfast]

I'm not familiar with your schematic. But, discharge time constant = T = 1/RC. Pick your T and solve for R. Maybe 100K would be about right?

You must select a resistor with adequate power rating for normal operation. P = V^2/R. Double power rating for safety margin.

But the thing I'm wondering about is -- if the filter caps are staying charged, then when you turn on the power, I'd expect the current surge to be less than if the filter caps were fully discharged.

I think of a bleeder resistor for improving safety against shock for people working on an amp.

Well I am not pleased with these Rat Shack slo-blow fuese. These are the typical glass fuse, but the short version for fitting in an IEC connector. They do not seem very substantial. There is a thin wire with a ball in the middle of it. IT is nothing like a traditional sized slo-blo fuse.

Here is the wiring diagram.

[TomMcNally]

Robert ...

Your amp already has bleeder resistors, the
220K and 68K in series ...

I'd look for another problem, assuming you
built it per Joe Esmilla's schematic.

... tom

[dcriner]

Robert ...

Your amp already has bleeder resistors, the
220K and 68K in series ...

I'd look for another problem, assuming you
built it per Joe Esmilla's schematic.

... tom

Agree with Tom.

Something else is puzzling to me here. The schematic calls for a 2-A slo-blo fuse. You're using a 3-A slo-blo, and it's still blowing.

[Slartibartfast]

Agree with Tom.

Something else is puzzling to me here. The schematic calls for a 2-A slo-blo fuse. You're using a 3-A slo-blo, and it's still blowing.

I meant to say (or type) 2A fuse. Yellow_Light_Colorz_PDT_02

Tom,

You are right. I was not paying attention.

Robert

[TomMcNally]

Did you build the amp pretty much by the schematic ?
Any changes ? Same transformers, including the two
small 2.5 volt filament transformers for the 2A3's ?

[Slartibartfast]

Did you build the amp pretty much by the schematic ?
Any changes ? Same transformers, including the two
small 2.5 volt filament transformers for the 2A3's ?

The only difference is I had to use a 33µf (630v rating) cap in place of the 100µf cap on the 68kΩ resistor. I was short one cap. I also used 2.5v 3A filament transformers. But those transformers will not pull more than the 2.5A that the 2A3 tubes require, right?[/code]

[TomMcNally]

You could go up a fuse size ... say to 2.5 slow blow or
even 3 amps fast maybe and see if the nuisance fuse
blowing stops. The bigger transformers may have
taken you over the edge in start up current.

[Shannon Parks]

A Keystone IRCL may help, too.

[Slartibartfast]

A Keystone IRCL may help, too.

keystone? You got some splainin to do, Lucy!

What is a keystone?


Robert

[Shannon Parks]

It's an inrush current limiter. They look like a black disc capacitor. Put one in series on the primary side and you are good-to-go.

[Slartibartfast]

You could go up a fuse size ... say to 2.5 slow blow or
even 3 amps fast maybe and see if the nuisance fuse
blowing stops. The bigger transformers may have
taken you over the edge in start up current.

Is this because the 3A filament transformers are allowing the 2A3 tubes to draw 3A on startup?

[EWBrown]

The cold 2A3 filaments have a very low resistance, as compared to the "hot" resistance - so do light bulbs, for that matter...

So there is a brief instant when the cold filaments place a very high load on the filament winding, a matter of less than a second for these DHTs.

Also, using SS diodes in place of the tube rectifier, in conjunction with high-value (100 uF or greater) electrolytic caps, can cause the SB fuse to blow from the sudden inrush, just step up to the next higher fuse rating, or elaew use an ICL as Shannon suggests.

I'd suspect the Rat Shack fuses, they sometimes seem to have a "hair trigger" and can blow prematurely. There are also several variations of "slow blow" fuses, some are more rugged than others. Look for fuses at teh local car parts place, they usually have the standard size fuses in addition to those weird "blade" type automotice fuses.

On a related, but different topic, I've found that the DIY-35 with the CL-90 inrush current limiter, if I power off the amp, then power it back up within a few seconds, it sometimes blows its fuse, because teh ICL is still hot and its resistance is still very low. Not a fault of the design, just the result of a bad practice on my part -I should give it a few minutes to cool off and let teh ICL get back up to its higher resistance (90-120 ohms cold) state first.

HTH

/ed B in NH

[Slartibartfast]

Well I put in a Rat Shack 3A fast-blo and it blew real fast. Yellow_Light_Colorz_PDT_04


So I put in a 3.1A slo-blo from rat shack and it seems to work fine, until I can order a CL-90. Maybe then I can go back to the 3A fast blo fuse.