[Rob Woodwind]

Hi.....I just got a bias probe( from TT) & 1st tested on the Ruby tubes in my Laney Cub 10..mvolts were 31.5   fine. The I put TAD 6V6GT-STR power tubes,put the bias probe on ..No reading at all,tried both tubes. After I put the Ruby tubes back in & NO sound at all from the amp??  fuse is OK, cannot see any burned out parts on the PCB. Any idea what happened?
Bias probe ohm was 1.0 no load
Speaker was connected
I can head a buzz from the power transformer, not sure about the preamp one though.
 
 I also tried the probe again on the Ruby tubes..nothing
Is it possible the bias probe can short circuit the amp?

Thanks for any help

[Rob Woodwind]

Hi.....I just got a bias probe( from TT) & 1st tested on the Ruby tubes in my Laney Cub 10..mvolts were 31.5   fine. The I put --- 6V6GT-STR power tubes,put the bias probe on ..No reading at all,tried both tubes. After I put the Ruby tubes back in & NO sound at all from the amp??  fuse is OK, cannot see any burned out parts on the PCB. Any idea what happened?
Bias probe ohm was 1.0 no load
Speaker was connected
I can head a buzz from the power transformer, not sure about the preamp one though. There is power to all 4 tubes still.
 
 I also tried the probe again on the Ruby tubes..nothing
Is it possible the bias probe can short circuit the amp?

Thanks for any help

[Lebowski]

Do you have a Voltmeter for some basic tests?
Then again, just did a quick search on the schematic. The one I found said 6V6 and not 6L6.
Although they seem to have the same pin layout, I wouldn't expect them to work just like that.
The adapter by itself should definitely not be able to short circuit anything.
So please check your tubes first and then we can go on from there...

Cheers!
Max

[Rob Woodwind]

Hi have a voltmeter of course
tubes are 6V6  & the original Ruby tubes
before it died there was a fairly loud buzz from the transformers, now only a slight buzz from the largest (power? ) one
when I tried to read the mv with the adapter on the 6V6, no glow in tube & smelled something burning
Thanks for the help
Rob

[Lebowski]

Oh my... I totally misread that, my bad  ! Of course you wrote 6V6 !
From that far it's hard to judge what's going on.
My next step would be to pull the 6V6 tubes and measure plate voltage and bias voltage.
I've had some fairly loud buzz from the output transformer. One tube of a pp-pair broke down, it had 100mA bias current (instead of 20) and was redplating instantly, that massive imbalance caused the ot to buzz like crazy...
General rule of thumb, try to minimize the time you power the amp on as long as it's not working as it should.

Hope this could be of some help.
Cheers, Max

[Rob Woodwind]

Hi OK will measure the voltage & see..
Cheers Rob

[Rob Woodwind]

Voltage at pin 3 was 340 v, on both power tubes, on pin #5 it was -19?? nothing on any other pins
maybe this says why its dead??

[geowicht]

What about the heater voltages? Did you check them? what is their reading?

[cca88]

What about the ScreenGrid voltages?

To check the heaters - please remember: it is AC there


AND
MIND THE SAFETY RULES

[Rob Woodwind]

How do I check the heater /Screen grid voltages?
Thanks

What about the ScreenGrid voltages?

To check the heaters - please remember: it is AC there


AND
MIND THE SAFETY RULES

[Lebowski]

Screen grid: V between pin 4 and pin 8 (ground) - should be a little less than your plate voltage, and that's DC.
Heater: between pin 2 and 7.
Something like 6-7 VAC.
Cheers!

[Rene2097]

Voltage at pin 3 was 340 v, on both power tubes, on pin #5 it was -19?? nothing on any other pins
maybe this says why its dead??

Really nothing at the other pins? You should see 330ish Volts at the screens (pin 4).

René

[Rob Woodwind]

Hi  found out R2 1K fusable resistor is burned out, so have ordered a new one..hope that solves the problem, as no voltage on pins 4. I think what happened is when I used the bias probe, the tube pins were turned 1 notch, as fried R2
Will post when I replace it ..

[Rob Woodwind]

OK replaced the R2 with a 1K  fusable .225W (all I could find) & now it works fine again