Dumb Bias Probe Mistake. Used Dcv Instead Of Dca. Something Popped.

[MPRawll]

I grabbed my bias probe, hooked it up to my 6l6 Ibanez TSA30, flipped it on. POP. Tiny amount of smoke. No sound. Turned it off. Took the probe out. Put 6l6 back in. Turned it on and all tubes warmed up. No sound...

Guessing it is because I was hasty and had it on DCV instead of DCA? (which EuroTubes has NOT VOLTS in all caps... moron...)

All fuzes appear intact.

Anyone know what was most likely to blow in this scenario before i just start taking caps and resistors out and checking them? Visual inspections shows nothing...

Schematic uploaded if you are interested.

Bought this as a project since it was cheap due to noise problems. Replaced all preamp grid and plate resistors as well as the coupling caps, which helped. But realized the guy had replaced tubes without biasing. Grabbed the bias probe and was hasty...

 

[mickeydg5]

I would also be worried about the meter.

Test all fusses with meter resistance.

If capable give voltages around the power tube sockets/pins.

 

[MPRawll]

Meter appears to be functioning normally. Good idea. I'll test the fuses.

As for voltages, should I be turning on the amp? Fearing I will pop something else with more smoke...

If so, how should i measure? i am a hobbyist and get concerned about measuring high voltage when i am not 100% sure of procedure.

 

[mickeydg5]

Well if you aint sure and all fuzes test good then you may want to think about a bringing to a technician.

Depending on state of meter operation you could have blown a tube or supporting component, if it is not a fuse.

I have to ask. Why are you doing projects if you do not know electricity and electronics?

 

[MPRawll]

Trying to avoid the tech and use this to learn since I got it cheap and I am not sure its worth more money... not my main amp or anything.

I might take out some resistors and measure them. Just didn't know if anyone else had made this bone headed move and knew where to start... Not sure I understand the implications of using volts instead of amps here on the multimeter, but it seems to have had an impact...

 

[MPRawll]

I know measuring plate voltage from pin 3 to ground give plate reading. Just wondering if that is the same for each pin?

 

[mickeydg5]

Different circuits in the meter. Introducing high impedance/resistance can make differences to specific operations of the tube.

Well if you can measure plate then you can measure the rest. Main thing is be careful. And yes the amplifier has to be ON and operating, all gain and volume controls at zero. Take your time. Write things down.
Just measure all 8 pins on each power tube with reference to ground. Pins #2 and #7 will be AC for heater/filament.

Pull the power tubes and measure just in case.

 

[Dogs of Doom]

the setting shouldn't do anything, but give you a bad reading...

 

[mickeydg5]

I have never tried putting a ten meg or so circuit inline with a plate before or whatever happened. I do not know how the tube will react.

 

[mickeydg5]

Pull the power tubes and measure just in case.

 

[MPRawll]

Thats what I was wondering. DC for all except AC for 2 and 7?

 

[MPRawll]

I did test most suspect resistors and capacitors in the power tube circuit while on board. I know that isn't very accurate so I might pull some to test, but the resistors all seem in expected range and no cap seems bad... I need to test power filtering caps but those were not as easy to get at. i was expecting to find one that was gone after the pop and smoke.

I also tested D5 which seems to be some polarity protection or flyback diode but it seems ok on board as well with the diode test.

All fuses also test fine.

One last test - tried a set of older tubes that were known good. No sound... So it wasn't the set of 6l6's that blew during the ill fated bias mistake...

 

[MPRawll]

Pull the power tubes and measure just in case.

When you say pull the power tubes... I thought removing preamp and running the amp was ok but removing power amps and turning on the amp was not good? Worried for my transformers and I can’t find any other obvious problems...

 

[ampmadscientist]

I grabbed my bias probe, hooked it up to my 6l6 Ibanez TSA30, flipped it on. POP. Tiny amount of smoke. No sound. Turned it off. Took the probe out. Put 6l6 back in. Turned it on and all tubes warmed up. No sound...

Guessing it is because I was hasty and had it on DCV instead of DCA? (which EuroTubes has NOT VOLTS in all caps... moron...)

All fuzes appear intact.

Anyone know what was most likely to blow in this scenario before i just start taking caps and resistors out and checking them? Visual inspections shows nothing...

Schematic uploaded if you are interested.

Bought this as a project since it was cheap due to noise problems. Replaced all preamp grid and plate resistors as well as the coupling caps, which helped. But realized the guy had replaced tubes without biasing. Grabbed the bias probe and was hasty...

Set meter on DC volts, highest scale 600V or 1000 Volts.
Test between pin 3 of each socket, and ground. Red probe on pin 3, black probe to chassis.
What is the reading?
Test between pin 4 and ground what is the reading?

If no voltage on pin 3, but voltage IS on pin 4, could be blown transformer.

Also, open meter and check for blown fuse...

 

[mickeydg5]

When you say pull the power tubes... I thought removing preamp and running the amp was ok but removing power amps and turning on the amp was not good? Worried for my transformers and I can’t find any other obvious problems...

You are testing voltages in the power amplifier stage. It is ok to pull the power tubes and turn the amplifier ON. The amplifier cannot operate without the power tubes but the voltages will still be present, although a bit higher because of no loading.

At this point we do not know what is wrong. You can pull the power tubes in case one or both have an issue. It is to be on the safe side. The choice is yours.

I see Ampmadscientist is trying to get some of the same information.

 

[ampmadscientist]

When you say pull the power tubes... I thought removing preamp and running the amp was ok but removing power amps and turning on the amp was not good? Worried for my transformers and I can’t find any other obvious problems...

Do the voltage test on pins 3 and 4 of the output tubes sockets.
Tubes in or out, it will still tell you,,,

Then do this:
write down the voltages on all the tube sockets pins:
post it here.
Output tubes sockets
pin 1:
Pin 2 AC volts:
Pin 3 DC volts:
Pin 4 DC volts:
Pin 5 DC volts:
Pin 6 DC volts:
Pin 7 AC volts:
Pin 8:

 

[MPRawll]

Cool

Do the voltage test on pins 3 and 4 of the output tubes sockets.
Tubes in or out, it will still tell you,,,

Then do this:
write down the voltages on all the tube sockets pins:
post it here.
Output tubes sockets
pin 1:
Pin 2 AC volts:
Pin 3 DC volts:
Pin 4 DC volts:
Pin 5 DC volts:
Pin 6 DC volts:
Pin 7 AC volts:
Pin 8:

Cool thanks for the advice everyone. Here are the reading. Voltage on 3 not 4...

So does that mean... not transformer?


Pin 2 : 4

Pin 3 : 519

Pin 4 : 1

Pin 5 : -28

Pin 6 : 1

Pin 7 : 4

 

[ampmadscientist]

Cool


Cool thanks for the advice everyone. Here are the reading. Voltage on 3 not 4...

So does that mean... not transformer?


Pin 2 : 4

Pin 3 : 519

Pin 4 : 1

Pin 5 : -28

Pin 6 : 1

Pin 7 : 4

If there is voltage on pin 3 the output transformer is still working.

If there is no voltage on pin 4 the choke (or resistor) might have blown.
In this amp it's either a choke or a resistor not sure.

Have to find a schematic...

 

[mickeydg5]

Not transformer.

Was it the same for both power tubes/sockets? We really needed both.

That is with the power tubes out. I kind of expected that.

Give voltages DC on the other side of the screen resistors R9 and R16, both being 470 ohms at 1 watt.

 

[MPRawll]

It was the same for both tube sockets


Other side of both screen resistors read 0.31 volts.