Hello,
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.
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.
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?
Where was the probe when it slipped? Do you have clip leads?
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
John Revilla
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!
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.
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.
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.
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.
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.
Oh, OK...somehow I thought you already checked R18. If you see B+ on the other side of R18, the OPT is fine.
Russ,
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.
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.
Russ,
I can't measure current on the right channel. Voltage is at B+ using R18. What can be the problem now? Thanks. Godspeed.
I can't measure current on the right channel. Voltage is at B+ using R18. What can be the problem now? Thanks. Godspeed.
Help. Amp still not working. Here are the voltage measurements: with just rectifier in place
B+ =420
B- =-281
R18=420
R29=420
R34= -176
R33= -149
Caps =420
R14= -178
R25 =-283
Anyone? Thanks. Godspeed.
B+ =420
B- =-281
R18=420
R29=420
R34= -176
R33= -149
Caps =420
R14= -178
R25 =-283
Anyone? Thanks. Godspeed.
R33 and 34 should change when you turn the bias adjustments. What happens to the two channels when you do this? You see current floating through the bias resistors on one channel but not the other?
Hi Russ,
Thanks. R34 voltage change (more positive) when I adjust the pot but not with with R33. Yes, I can't measure current on R18. I already change the CCS chip. I will also replace the follower chip later today if necessary. Godspeed
Thanks. R34 voltage change (more positive) when I adjust the pot but not with with R33. Yes, I can't measure current on R18. I already change the CCS chip. I will also replace the follower chip later today if necessary. Godspeed
Hi Russ. Thanks for your help. I think I got it. Preliminary measurements are all good. I replace Q1, Q2, U1, U2 and both the 100k pots. I was able to measure 175 volts on the plates of 5842s and 65ma on R18 and R34. After 5 minutes of being on, the amp retained all the measurements mentioned. I'm happy but the problem is that by replacing all those parts, I cannot pinpoint what was the culprit originally. Thanks again, Sir. Godspeed.
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