Bad TSE Input Tube?

[jeffx]

I have finally completed my 300B TSE board and and am now checking the voltages. Twice now when putting the 5842s in to adjust to 175v I believe I have fried the 2SK2863 and possibly the 10M45S on the left channel(both replaced after first instance). Also, R25 now appears to be shot.

After turing the amp on there was a "spark" in the left channel 5842, then I started to smell the resistor. The left channel at the coupling cap reads 5v while the right channel adjusts to 175v. I tried the "left/bad?" 5842 in the right channel, and it seemed that the same thing was about to happen because I could smell the resistor(R14?) heating up, so I turned it off immediately.

Could this be the result of a bad 5842 or something else on the board? The 5842s I have are Amperex gold pins purchased off Ebay and supposedly NOS.
Anyone have a source for tested inexpensive 5842s?

[jeffx]

I ordered a replacement pair of 5842s and have replaced both Q2 and U3. Doing the initial setting of max negative voltage on the output grids checks I now get positive voltages(approx 160v) and another smoking resistor(R6?). Any thoughts on what is bad now? Any help is appreciated!

[Evenharmonics]

How were the voltages for 2 steps prior to putting input tubes during "checkout" process? Checkout

Write down voltages for each channel.

Step 1:
B+ =
B- =
OPT filament =

Step 2 (w/ rectifier tube in):
B+ =
B- =
Grid of OPT to ground =

[jeffx]

I just did step one again, but step 2 is from previous test when resistor heated up.

Step 1:
B+: -59v
B-: -271v
Filament(both): 5v

Step 2:
B+: 428v
B-: Did not check
Grid to Grd: approx +160v on each

Any way to check if the 2SK2563 is good? Thanks.

[Evenharmonics]

Originally Posted by jeffx

I just did step one again, but step 2 is from previous test when resistor heated up.

Step 1:
B+: -59v
B-: -271v
Filament(both): 5v

Step 2:
B+: 428v
B-: Did not check
Grid to Grd: approx +160v on each

Something is wrong with OPT grid to ground voltage. At step 2, it should be negative voltage. Sorry I don't remember which but one of the semiconductors may be damaged or defective. Perhaps one of the gurus here can chime in on this.

[jeffx]

I removed the rectifier tube and checked the grid to ground voltage and the voltage is negative. Put the 5AR4 back in and the voltage is positive with the smell of heating up resistor and a "snap" sound every couple of seconds. Could a bad rectifier tube cause this?

[tubelab.com]

Could a bad rectifier tube cause this?

I doubt it. From your decsription it sounds like the mosfets are shorted allowing the B+ voltage to pass through to the output tube grids. Do not install any output tubes until this is resolved. They could be damaged.

I am not familiar with the particular mosfets that you are using. The specs look OK but the package is different than the TO-220 mosfets that the board was designed for. The pin configuration looks right, but maybe the parts aren't in good thermal contact with the heat sink allowing them to overheat and fail.

If the voltage on the gate pin adjusts OK with no mosfet installed, yet the voltage on the output tube grid doesn't cooperate with the mosfet installed, the mosfer is likely to blame.

[Evenharmonics]

Originally Posted by jeffx

Put the 5AR4 back in and the voltage is positive with the smell of heating up resistor and a "snap" sound every couple of seconds. Could a bad rectifier tube cause this?

If you see a spark inside of rectifier tube when you hear that sound, then the tube is gone. Also, it may blow the power fuse. If the rectifier tube was good but did this now, then something else in the power circuit may be causing it to go bad.

[jeffx]

Thanks for the replies. My bad - the MOSFETs are 2SK3563. I will replace the them and get another rectifier tube.

[jeffx]

I have a new rectifier tube - a JJ GZ34 - and new MOSFETs. I performed the initial tests this evening without the MOSFETs installed. On first test B+ is -0v, B- is much higher at -391v, while filament is good at 5v. With rectifier installed B+ is 474v and B- is -392v. C4 is a Solen 3.3uF.

The gate terminals(without MOSFETS installed) are adjustable from approx -160v to -30v. This range is suppose to be -70 to -10.

I have checked all resistors and they all appear to check good. Suggestions on how to get the voltages lower - or are they in acceptable range with not having the MOSFETs installed?