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Doc Jr 8156 22nd March 2013 03:49 AM

Foolish Mistake
I had my Tubelab SE (300B) for more than 4 years without any problem. Tonight, I was setting the bias on the right channel via R18 (about 65ma) when my voltage meter lead slipped and a spark occurred. Right after, I cannot measure any current on R18 and no music is flowing on the right channel. I turned the amp off and switch tubes, I thought I killed my right channel 300B but after switching, still the right channel is dead with music on the left.

All tubes are glowing. R18 and R29 measured at 10 ohms. No music on right channel. Did I kill my right channel mosfet (Q1)? Thanks for helping.

rknize 22nd March 2013 05:25 AM

So you measure B+ on both sides of R18? What about the tube plate? Best to check just to verify that the OPT is OK. Aside from that, yeah it sounds like you may have killed R34 or the FET. You might also want to look carefully at the PCB traces. Take some measurements on both sides of R34 to see if the bias voltage is there and that it changes when you turn the pot.

Where was the probe when it slipped? Do you have clip leads?

Doc Jr 8156 22nd March 2013 04:57 PM

Thanks Russ. No, I don't have clipped leads. I think I touch the jumper. How do I check if the OPT is okay? Is there a way to check the FET? I'll measure voltages and post later. I am away from my home. Godspeed.

John Revilla

rknize 22nd March 2013 06:12 PM

The DRV_B+ jumper? Maybe it got shorted to R34. We'll know if the primary is open when you check the plate voltage at the tube socket. Be careful when probing around such high voltages without clip leads!

Doc Jr 8156 22nd March 2013 06:22 PM

Hi Russ,
Yes, the DRV jumper. I'll check R34. You mean the plate voltage for the 300B? I do have clipped leads but was just doing a quick adjust when it happened. Thanks. Godspeed.

rknize 22nd March 2013 07:48 PM

Yeah, if you have B+ on the plate of the 300B, you know your OPT is OK. it probably is, but you never know. Since you saw a big spark, I'm assuming the DRV jumper (with B+ on it) shorted to R34. If you shorted B+ to the FET side of R34, I wouldn't expect anything bad to happen unless the back-EMF killed the FET. So I'm left to wonder if the shock simply popped R34 itself.

Doc Jr 8156 22nd March 2013 07:53 PM

Thanks Russ,
I'll measure and report. Godspeed.

John Revilla

Doc Jr 8156 23rd March 2013 07:08 AM

Hi Russ,
Is the plate for the 300B tubes pin#3? I measured 390v on the left channel (working) but 0v on the right. R18 is dead. R34 is 100 ohms. How can I be sure that my opt (Electraprint) is not damage? Is there another way to test the opt? Thanks.

rknize 23rd March 2013 05:40 PM

Oh, OK...somehow I thought you already checked R18. If you see B+ on the other side of R18, the OPT is fine.

Doc Jr 8156 23rd March 2013 08:16 PM

I change R18 and got B+ on the right side now. All voltage measurements plate, R18, R34) are the same as the left (good channel). I'll try with output tubes and see is music will flow. Godspeed.

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