Tube Rectifier Simulator Resistor EXPLODE!!

[MarshallPlexi]

Okay here are my measurements on the amp.

The B+ Winding has a 330R 50W resistor on the center tap to ground.
The cathode resistor is: 200R/25W
B+ Supply is: 351
Plate Voltage (reference Cathode) is: 323
Screen Voltage (reference Cathode) with 1K/5W resistor is: 316 (7.7V drop across the resistor)

I later changed the screen resistor to a 1.5K resistor.

Screen Voltage (reference Cathode) with 1.5K/5W resistor is: 310 (11.42V drop across the resistor)

The plate current on the EL34's is between 57 and 58mA which given the plate voltage puts it at about 76%. The amp sounds great as is.

Given the data here, can anyone see anything that might cause any failures?

Thanks so much guys!

[Merlinb]

As a general rule, what is the safe margin to use when selecting a fuse? Is a 250mA fuse too small in that it might blow during transient surges?

I usually use the smallest value that doesn't blow at switch on (maybe a touch larger so it doesn't blow at a really awkward moment). This is normally more than large enough that it won't blow on musical transients. Usually ends up being about 3x the DC current, depending.

[MarshallPlexi]

I usually use the smallest value that doesn't blow at switch on (maybe a touch larger so it doesn't blow at a really awkward moment). This is normally more than large enough that it won't blow on musical transients. Usually ends up being about 3x the DC current, depending.

In that case I'm leaning towards a 500mA fuse.

[MarshallPlexi]

Absolutely.
Power dissipated in the resistor is= I(squared)xR
For 1A fuse: 1x1x200= 200W
For 0.25A fuse: 0.25x0.25x200=12.5W

Given this formula, if I use a 500mA fuse with a 330R resistor, I get:

0.5 x 0.5 x 330 = 82.5W

So that means that in order for the B+ fuse to blow BEFORE the resistor, I have to use a resistor that is an 82.5W or better (100W) or at least a lower value fuse which I can't really see doing given that my constant B+ current is at least 150mA. Figuring a safety margin of 3x the constant current, I arrive at the value of 500mA.

Before I order some 100W resistors tomorrow, would someone make sure that my logic is correct?

Thanks!

[Merlinb]

So that means that in order for the B+ fuse to blow BEFORE the resistor, I have to use a resistor that is an 82.5W or better

That's probably overkill, as the resistor only has to withstand that power for a second or two until the fuse blows.

[J M Fahey]

Agree, it looks like overkill to me too.
I'd start with a 0.25A fuse, for the simple reason that *in your case* you won't be having "big turn-on transients" since you have an important series resistor (those 330 r ) plus you increased the cathode resistor which further lowers the current.
In case those 0.25A fuses blow, well, post that for us to know and if they don't blow after 10 to 20 shows or rehearsals, post too.
Theory is great, but can permanently be corrected by experiment.
0.25x.025x330=21W , good for a 25W resistor.
Besides that, your *cathode* resistor is in series with the other, and you're not putting a 100W resistor there !!
Test and post.
I guess your amp must be a sweating, swampy, breathing Blues machine.

[MarshallPlexi]

I measured the current on the B+ Winding and when in low power mode the amp idles at 120mA and maxes out at 160mA when played wide open.

When in high power mode the amp idles at 90mA and maxes out at 250mA when played wide open.

With this information in hand I went with the 400mA slo-blo fuse as the 250mA would have been too small and the 500mA would have been too big.

So far no problems.