Did I just fry my tubes?

[leib10]

Stupid me decided to try to swap out a quad of EL34s while I’m sick. I accidentally turned the bias pot the wrong way and the current shot up to about 170 mA for about 15 seconds before I realized what I had done and switched the amp off. Are these tubes toast, or is that too short of a time period to damage them?

EDIT: yep, one of them just popped. Luckily, it didn’t take the amp with it.

 

[Chubbs]

I have a hard time believing they actually got up to 170ma and didn’t blow given the limited range of most bias pots, but I don’t know bogner’s very well.
I bet they’re fine. I once forgot to connect some stuff properly and ran a new set of el34’s as hot as possible for about the same amount of time, fixed it and they’re still going strong. And I wasn’t sick just retarted.

 

[baron55]

I have a hard time believing they actually got up to 170ma and didn’t blow given the limited range of most bias pots, but I don’t know bogner’s very well.
I bet they’re fine. I once forgot to connect some stuff properly and ran a new set of el34’s as hot as possible for about the same amount of time, fixed it and they’re still going strong. And I wasn’t sick just retarted.

The bias points measure the pair, so 170 was for the pair and not a single. So probably 85ma per tube.

 

[baron55]

Stupid me decided to try to swap out a quad of EL34s while I’m sick. I accidentally turned the bias pot the wrong way and the current shot up to about 170 mA for about 15 seconds before I realized what I had done and switched the amp off. Are these tubes toast, or is that too short of a time period to damage them?

EDIT: yep, one of them just popped. Luckily, it didn’t take the amp with it.

A good tip is when biasing a new amp. With the tubes removed, measure pin 5 of the power tube socket and see which way the - voltage reads when turning the bias pot. A higher negative voltage means less current, a lower negative voltage = more current. The UU uses PCB mounted trim pots (like most amps) the issue is you are going at them from the chassis side of the amp which means they will turn opposite of what you would expect.

 

[RedB4Black]

The bias points measure the pair, so 170 was for the pair and not a single. So probably 85ma per tube.

Are you sure about that? Each pair of tubes has a bias knob to adjust them together yes, but the test points have 3 spots. Middle is ground. Right point is one tube of the pair and left point is the other… so when you measure, you get the measurement for that particular tube… the lines on the chassis pointing the tube being measured

 

[glip22]

Are you sure about that? Each pair of tubes has a bias knob to adjust them together yes, but the test points have 3 spots. Middle is ground. Right point is one tube of the pair and left point is the other… so when you measure, you get the measurement for that particular tube… the lines on the chassis pointing the tube being measured

Yes,
The OT sees four tubes as two pairs. Two primary leads come from your OT, one to each inner power tube socket. The outer tubes are connected to the inner tubes. Make sure to take a reading on your screen resistor for that tube that popped. You may have an open or dead resistor in that spot.

 

[baron55]

Are you sure about that? Each pair of tubes has a bias knob to adjust them together yes, but the test points have 3 spots. Middle is ground. Right point is one tube of the pair and left point is the other… so when you measure, you get the measurement for that particular tube… the lines on the chassis pointing the tube being measured

Yes, most likely, I use a current probe (measures plate current) not screen and plate like most probes and the bias test points on the amp.

 

[hopkinWFG]

I had a similar occurance too with my Wizard MC2.... the biasing pot seems to be in opposite direction... clockwise is decrease and anticlock wise is increase.... nearly took my new tubes away... it went up to 50plus mA and tubes started to redplate luckily turn the pots the other way round in clockwise direction and mA backed down.... tubes cooled back to normal.... close encounter here...

But if ya tubes arent redplating i think ya tubes are just fine... personally felt the bogner mains at 5A !!! Its way too high ! Should wind down the mains rating to 2.5A ? Or 2FA for a better protection?

Just power back up and inspect closely the tubes on standby for any case of unsual appearance and also when on standby off... check for arcing inside tubes and etc.....

 

[baron55]

I had a similar occurance too with my Wizard MC2.... the biasing pot seems to be in opposite direction... clockwise is decrease and anticlock wise is increase.... nearly took my new tubes away... it went up to 50plus mA and tubes started to redplate luckily turn the pots the other way round in clockwise direction and mA backed down.... tubes cooled back to normal.... close encounter here...

But if ya tubes arent redplating i think ya tubes are just fine... personally felt the bogner mains at 5A !!! Its way too high ! Should wind down the mains rating to 2.5A ? Or 2FA for a better protection?

Just power back up and inspect closely the tubes on standby for any case of unsual appearance and also when on standby off... check for arcing inside tubes and etc.....

The 5A is also for the entire power supply, power tubes typically in 100-watt amp pull only 500ma at full tilt. The heater circuit pulls a min of 6 amps on the power tubes. Plus, the current draw on the DC supply to the switching DC filaments for preamp tubes, etc. The HT fuse which is the smaller one is specifically for the tubes. But the fuses are designed to protect the amp and not the tubes. 99% of the time the screens are the ones that go, and they have a much lower power rating and are not directly fused.

 

[Fusionbear]

In a superlead plexi, I can see the current shoot up to 200ma for a moment as you are playing. Maybe it's just the fuse that blew?