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Problem With 01A Preamp (Losing Rectifiers)
Problem With 01A Preamp (Losing Rectifiers)
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Old 5th December 2016, 06:50 AM   #11
mogliaa is offline mogliaa  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
Hi Abe, the mounting of the DN2540 looks a bit messy with that paste around. Is unclear whether you have proper TO-220 isolation and whether the white paste isn't providing any conductivity to the chassis. There is absolutely no need to mount the DN2540 to the chassis on the 01a preamp. You can get away without any heatsink at all.
Is difficult to desolder the devices from PCB if you aren't experienced in doing so. An easy way is to cut the FET pins at the mid point and solder the new pins/legs on top of the old ones, without touching the PCB at all.

My guess is that the DN2540 are fried (quite common) and Rod's test above should help you to confirm this. If resistance is low, you can proceed to replace them.

You haven't placed a drain to source protection zener across the 2sK170, so is likely to be damaged as well.\
Ale
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Old 5th December 2016, 07:56 AM   #12
Rod Coleman is offline Rod Coleman  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
Yes, it's certainly true: even if the B+ to chassis test measures open-circuit, it is still possible to get arc-overs from the DN2540's package tab to chassis - usually to the mounting screw. At high voltages (>100V), I try to use a TO220 mounting-clip instead of a screw. The TO220-packaged DN2540 can take >1W without a heatsink, so long as the amp is <50C inside, so that should be OK.


You can test a DN2540 with a DMM too: on 200Ω range, the D→S reading should be about 11Ω, and G→S should be open circuit (on 200KΩ range).
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Old 5th December 2016, 08:04 AM   #13
mogliaa is offline mogliaa  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
Hi Rod,
Agree. For this particular circuit, the DN2540 will be running 3mA and even with a voltage drop of 50-100V (VDS), the dissipation is below 300mW.
In my first 01a version (when I didn't have a PCB), I used a TO92 device instead.

Abe
Check as Rod says. If you're lucky the short is between the DN2540 and the chassis. At some point during testing you have connected the SW1 and that may have blown things out. Do you remember when you noticed the issue? If it's just poor isolation, then removing the DN2540 from the chassis will due the trick.
Either way you need to test it carefully before you switch it on again, otherwise you will continue experiencing the same fault again.
Ale
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Old 5th December 2016, 05:34 PM   #14
amandarae is offline amandarae  United States
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Thank you Ale and Rod!

So far, I have this measurements:
Filament 1: + to 0V =3.36MOhms; - to 0V =10.6M
Filament 2: + to 0V 10.6M; - to 0V 3.4M

GYRATOR
Anode and 0V = 110K
Anode and Chassis = 1.5K
0V to OUT = 110K
B+ and Chassis = 0.2 ohms
GND and chassis = 110k

Yes Ale, you are correct. The first thing when it happened was when I flipped the SW1 switch! And looking at the reading(B+ to chassis) above, it seems to confirm that the short is in the DN2540.

I will take the boards out and do the test both of you posted and replace the parts as necessary. Also, I will get that zener diode installed.

Thank you very much!
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Old 5th December 2016, 06:06 PM   #15
andyjevans is offline andyjevans  United Kingdom
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Good luck! You should be getting some good sounds....
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Old 5th December 2016, 06:42 PM   #16
mogliaa is offline mogliaa  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
No worries, Abe. You're not the first one blowing off a DN2540 (I can't remember how many I have sacrificed myself during abuse and experimentation). In future, I will recommend to build and test the gyrator board first and then connect it to the circuit. Also if you are bolting any TO-220 device with metal tag on to the chassis, you have to measure the isolation as the tab is generally connected to drain / +HV. Keep us posted!
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Old 5th December 2016, 07:58 PM   #17
amandarae is offline amandarae  United States
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YEs, it is through the tab of the DN2450 that shorts the B+ to chassis! I took the bolts out from the two boards, lifted the boards, and no more short! I tested the DN2540 just like Rod mentioned. D to S on both is 11 ohms, and G to S is open. But I will replace both when the parts arrive.

Any simple test I can do for the 2sk170?
Thanks!

Thanks Andy, I hope
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Old 6th December 2016, 03:55 PM   #18
mogliaa is offline mogliaa  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
It looks like the fet survived and the short was due to poor isolation on the TO220 tabs (drain). Before replacing the components, remove the boards and test them with a dummy load and see if you can set the output voltage. If the JFET/MOSFET is/are damaged you will get full HT at the dummy load and you won't be able to vary output voltage when adjusting the trimmer.
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Old 6th December 2016, 04:08 PM   #19
stocktrader200 is offline stocktrader200  Canada
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how much voltage is accross an open S1 ?
the grounds may be at different DC potential.
use a nylon screw and mica insulator on D2540

Last edited by stocktrader200; 6th December 2016 at 04:10 PM.
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Old 6th December 2016, 04:20 PM   #20
hpeter is offline hpeter  Slovakia
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Sorry for asking, but i dont understand extra complexity. Why use ccs and some resistor to have bias voltage ? (lnd150)
Goal is to have ~1/2 Usupply (or other voltage) at tube anode, that can be done by just 2 resistors-divider (2*100k~)
Benefits?
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