Help!! I think I broke it ....

PaulO · Post by **PaulO** » Mon Feb 27, 2006 10:47 pm

Well, I have never biased an amp before. But I have a book on it!

I got me an ocilloscope, a VOM, a 100 Watt resister set to 16 ohms, a sine wave signal gernerator for input and a voltage variac.

Hooked everything up per the book. Stepped up the in voltage to 117 volts per spec, and tried to adj bias pot.

Oh no, nadda . ... can't be that easy.

Next day, I decide to go back at it. Flipped on the variac at the previous 117 volt level from the day before, hit the power on the JTM and vershizzel ..... the rectifier tube starts to pop and fizz. Lightening strikes in the glass.

Is that because I had left the variac at 117 volts? Should I have stepped up the voltage to the power vs. hit it at 117 volts?

Am I in it for just a new rectifier tube or a lot more than just that?

Anybody got any ideas?

Paul

Guest · Post by **Guest** » Tue Feb 28, 2006 6:27 pm

Anyone?

neikeel · Post by **neikeel** » Wed Mar 01, 2006 6:17 am

I am sure you need a new rectifier.

I doubt the power going on suddenly was an issue, more like a short somewhere.

I do not bias my amps that way so cannot comment, I use the 1ohm resistor between pins 8 and 1 and to earth (1mV across this resistor = 1mA)

calculate for 70% of plate dissipation at idle (=35-42mV on your meter = 35-42mA of current).

Anyone use a variac/oscilloscope/bias probe routinely, I can imagine the trace you get when setting up but you will always compromise with one of these amps and valves unless perfectly matched?

PaulO · Post by **PaulO** » Wed Mar 01, 2006 7:32 am

I purchased 2 of the 1 ohm resisters, 1% . I am going to do what you suggest .... sounds a lot easier to do than to set up all the other gear.

If I have the GZ34 pulled from the amp, apply power, what voltages should I read from the various pins to ground to determine if a short esists someplace?

Or maybe it was just dumb luck the tube shorted out internally while I was cocking around with hit ... I hope that is the case ...

neikeel · Post by **neikeel** » Wed Mar 01, 2006 3:03 pm

If it was working ok before I suspect the latter.

Remember crocodile clip to earth insulated probe and one hand in pocket (sorry if Grandmothers and holy eggs springs to mind

)

Good luck

(see the difficult ones Flames, Larry and Billy help older rookies help new rookies

)

p.s. What tubes did you go for?

PaulO · Post by **PaulO** » Sat Mar 04, 2006 10:30 pm

Well, I got a new rectifier tube ... China .. only one I could find in Syr ... and all is well. In fact, amp sounds a lot better ... I gotta believe the old tube was on its way out to begin with. I took some voltage readings w/out any tubes in it and they all seemed within range so I took a chance and put all tubes back in and fired it up.

The tube cost me $18 so I didn't want to fry it on power up.

Whew!