Blown fuse, current draw?

[johnsonad]

Hello,

Today I rolled in a pair of Mullard 12at7's and when I fired the amps up (from a cold start) one blew the fuse.  The tubes  test fine on my B&K 747.  I replaced the fuse with a 1.5 amp quick blow (only one I had around) and put in the 12at7's I had in it before.  No issues.  Later I remember I had a 1A fuse in a different project and switched them out.  It blew the second 1A fuse.  Tomorrow once it cools off I'll run some voltage checks.  These amps belonged to Doc and are factory built.  Any suggestions?

Do certain tubes have more of a current draw and risk blowing the fuse on start up?  The instructions do no specify but should the amps use quick or slow blow type fuses?  Also am I risking smoking a board somewhere by running this 1.5 amp fuse?



Regards,

Aaron

[JC]

When tungsten filaments are cold, they have very low resistance.  It rises rapidly as they warm up, but for that first instant they draw a lot more current than the rating for operation.  Tubes or light bulbs, same deal.

Tube manufacturers used various approaches to what was sometimes referred to as "controlled warm-up".  Some worked better than others.

I would suggest trying slow-blow fuses of the correct rating with the Mullards.  I would be surprised if this did not solve the problem.

As far as what risks, if any, you are running by using fuses of a somewhat higher rating, that would be a question best left to Doc and/or Paul I expect.

[johnsonad]

I put the usual tubes back in that have worked fine with a 1A fuse and it blew again.  I've just started the cold resistance checks.  Any suspect ones I should be looking for?  I'll post the out of speck values once completed. As of now, the zeners are off in a couple positions.

[JC]

My first inclination would be to look for something that might have changed around the 12AT7 socket.

But, I am unclear: Did the amp operate properly with the 1.5A fuse?

[johnsonad]

The amp did run fine on the 1.5 amp fuse.  I then changed it back to a 1 amp fuse and it blew.

All cold resistances listed in the manual were measured.  These are the ones which are outside 15%  All tests were done with a BK Precision 2890A DMM:

T9: 1.4m
T11: 5.3m
T14: 15.3m
T15: 24.8m
T19: 1.4m
B1: 1.4m
B3: 11.6m
B6: 1.3m

Thanks in advance.

[JC]

You will want someone more familiar with the circuit than I am to discuss those readings with you.  In the meantime, I think I would still try the 1A slow-blow fuse if I were you.  This is nothing more than a wild guess, you understand, but I'm guessing that in swapping the 12AT7s, you may have actually cleaned up the sockets a bit, so the cold-inrush current might just be sufficiently higher than it was to take out the regular-blow fuse before the circuit stabilizes.

BTW, have you measured the line Voltage at the outlet you are using?  It may be useful to know.

[Doc B.]

Spec fuse is 1A slo-blo. When you say zeners are off, do you mean LEDs are not lighting up when the amp is operational? If so, some components on the green PC board may have blown for some reason when the tube was changed.

[johnsonad]

Hi Doc.  When I wrote the zeners were off I meant it only in values.  I wanted to run the resistance values by you guys before firing it back up again and checking voltages.   Thanks again.

[johnsonad]

All right, did the voltage checks with a 1.5 amp fuse in and a regulated 119.3v on the variac.  All four LEDs light up on the shunt reg board and all of the voltages are within 15%.  I left the 1.5 amp fuse in and it is playing sweet music right now.  Thoughts or suggestions?  Is it safe to just leave that 1.5 amp fuse in?

Thanks in advance,

Aaron

[Doc B.]

If the 1.5A is a fast blo it is probably OK, but I would suggest putting in the stock spec 1A slo blo when you have the opportunity.

[johnsonad]

I ordered some 1 amp slow blo fuses from the Rat Shack today.  These Japanese fuses aren't marked fast or slow but it's working.