Blown 1 OHM current-sense resistors

[Gaz]

Could running an amp set to 8 OHMs into a slighty-less-than 4 OHM load cause the the 1W 1 OHM current sense resistors to blow? If so, why? It seems as though that's what happened. The amp in question is a non-master 100W with one bias resistor per pair of tubes. The tubes 'seem' fine as well, which is a bit puzzling.

Thanks for any thoughts.

[imaradiostar]

Probably not, but it could cause the screens to get hot, current to spike and all manner of crap to happen. Maybe. Or Maybe not.

jamie

[Leo_Gnardo]

I wouldn't think anything short of a serious internal arc would crap out your current-sense R's. I've been installing Dale WW 3W 1% one-ohms for years now & never popped one yet. Ampeg puts similar 10-ohm R's in their SVT-CL's & similar, but I have seen those open up. Usually there's a tube with lightning bolts inside involved and its 220-ohm SG resistor is also open, so chalk those up to faulty tubes

Oh well, think of it as a fuse at the - end of the tube.

[Gaz]

Thanks, Jamie. It does seem related since it only happened when the impedance was accidentally mismatched. So the theory would screens getting hot and causing a current spike. Could you expound on that a little

[Gaz]

Yeah, I'm totally with you, but tubes seem just fine. So weird.

[Leo_Gnardo]

In order to dissipate 1W, your 1 ohm resistors would have to have an amp of current passing thru them. AND resistors don't just give up at the point of their power rating, they'll generally take a beating and keep going, maybe a bit off spec but they keep going anyway. What type resistor are you using? Might consider going to wirewound. I get mine from Mouser.

Carbon comp might be OK in this situation due to their toughness in current-pulse applications BUT generally available in 10% or 5%, not what I'd call fine-tuned enough for a precision bias current measurement.

[Gaz]

These are the ones I was using. Not wirewound, but not cheapies either. CPF11R0000FKEE6 Vishay / Dale | Mouser

Yeah, I know, that's why I'm really puzzled because the idea of them passing 1A I can understand if the bias supply goes bad, but I would think they tubes would be deader than dead at the point too. The amp was fine for a whole year, then the change of impedance, a little rock and roll, then poof.

[Leo_Gnardo]

Here's my favorite 3W 1% WW, also Vishay/Dale:
RS02B1R000FE70 Vishay / Dale | Mouser
71-RS02B1R000FE70

83 cents each, 75 for ten @ a time on down the line.

[Gaz]

Here's my favorite 3W 1% WW, also Vishay/Dale:
RS02B1R000FE70 Vishay / Dale | Mouser
71-RS02B1R000FE70

83 cents each, 75 for ten @ a time on down the line.

Thanks for the link. What was your original rationale behind choosing 3Ws?

[Enzo]

Post hoc ergo propter hoc is a logical fallacy. DOn;t lock your mind around a cause and effect relation just because two things happen at the same time. The other day I sneezed, and at that exact moment, a police car siren went off on the street right out front. I am still trying to figure out how my sneeze caused that.

[Gaz]

Fair enough, but I'm just looking for a cause somewhere. If anyone's seen this before it's you Any other ideas?

[Gaz]

I found this old thread looking for related problems. External bias jack - AMPAGE Forum

How does the diode across the bias resistor work exactly?

[Enzo]

Has anyone ever seen a burnt resistor in the cathode lead of a power tube? Sure, probably most of us. What caused it? Excess current through it, and the only circuit path for that current is through the tube. generally it means tube failure, or perhaps something shorted on a socket. It takes a whole 1amp just to get 1 watt into a 1 ohm resistor. Not easy to get that from the B+ supply for any length of time, but possible. A plate to cathode short is darn rare, but a broken screen grid could short anything to anything. A screen to cathode short could do it, but then the screen resistor enters in series.

Then perhaps the cathode gets shorted to the heater. With a grounded center tap, then 3vAC with tons of current available is across that resistor. That would draw 3A. Now we are talking 9 watts, that will burn her up for sure.

Your bias is not going to be off so far that the tube conducts a couple amps, even if the B+ could supply it.

[Enzo]

Diodes don;t conduct until sufficient voltage is across it - something like a half a volt. SO parallel the 1 ohm resistor, when the voltage is under half a volt, the diode does nothing. When the voltage across the resistor gets up to that half a volt, the diode starts to conduct. The diode maintains roughly that voltage across itself. So the voltage across the resistor never gets over that half a volt. SO the resistor never burns out.

[Gaz]

Has anyone ever seen a burnt resistor in the cathode lead of a power tube? Sure, probably most of us. What caused it? Excess current through it, and the only circuit path for that current is through the tube. generally it means tube failure, or perhaps something shorted on a socket. It takes a whole 1amp just to get 1 watt into a 1 ohm resistor. Not easy to get that from the B+ supply for any length of time, but possible. A plate to cathode short is darn rare, but a broken screen grid could short anything to anything. A screen to cathode short could do it, but then the screen resistor enters in series.

Then perhaps the cathode gets shorted to the heater. With a grounded center tap, then 3vAC with tons of current available is across that resistor. That would draw 3A. Now we are talking 9 watts, that will burn her up for sure.

Your bias is not going to be off so far that the tube conducts a couple amps, even if the B+ could supply it.

Thanks, Enzo, you have a way with words! I can suppose there is an intermittent short that hasn't surfaced again (like I said, the amp is working fine again), but would it make sense that both current sense resistors would blow from a short in a single tube?

I found a few old interesting threads with more about the diode:

http://music-electronics-forum.com/t29274/
http://music-electronics-forum.com/t14015/

Do you think it's a good idea to add them, or go with larger watt resistors instead? Or both? I'd love to make this part of the circuit more reliable and safe (so the cathode cannot potentially float up to B+!)

Thanks!!