• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Any experience of these amps? Kora 100SB

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Hi does anyone have any info or experience of Kora Triode 100SB amps ? they use a pair of 6C33C-B triodes in push-pull, with MOSFET regulated B+ supplies. A customer brought me a pair of mono-blocks, one had blown MOSFETs which had gone short, so the full unregulated voltage was present on the plate of the valves causing them to arc internally. I replaced the MOSFETs and the valves, which has lasted approx. two months and gone again. I have no schematic.
I dislike these valves intensely, having had problems with a BAT amp in the past. The problem is I can't test them easily.
Any thoughts / help appreciated.


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Trace the circuit so you have a workable schematic. Something is causing the mosfets to blow, and simply swapping them as they blow isn't likely to fix the issue. What are the voltage measurements around the power supply? I would get some supporting component has driven or failed, or the heatsink are inadequate.
Thanks for your response, the MOSFETs didn't blow straightaway, the repair lasted two months, when the owner switched the amp off standby, the tubes arced internally, didn't blow the HT fuse but took out the Mosfets. They are IRF840s 5amp / 500volt, on a decently sized heatsink, they are dropping the raw B+ from around 400VDC to 235VDC on the plate, the tubes have a 10R cathode resistor and between -100 to -130 (adjustable) on the grids. I will check out the MOSFET drive components and set the bias so the valves are running colder, and probably drop the size of the B+fuse. I just think these valves are unreliable in this application, so wondered if anyone else has had experience of them?
I've listened to one of these amps before, and found it nice indeed, but that was the extent of my experience with them.

However- in general (not just this design) the 6C33 tubes tend to be problematic without forced cooling due to the enormous amount of heat they subject the sockets to deal with. Check each of the pin holes in each socket to confirm that they still tightly grip each pin, as a loss of connection can cause issues, especially if you lose grid bias with one of the tubes! This could certainly cause overcurrent that would damage the tube and even destroy the mosfet if there isn't adequate means to shed heat.

I wouldn't say the tubes are unreliable in this application, but just that they require some special attention, that may not have been taken into account or maintained.

If I'm not mistaken these use 6BQ7 and EL84 in a cascaded differential design? Maybe a leaky coupling cap is passing voltage where it shouldn't and upsetting bias?

Edit- can't find a schematic but found a decent description of the circuit on page 74 if it helps-

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Thank you for finding the article that was very useful, just one thing is puzzling me the amps that I have a quite a bit older than the ones he tested, and in the article he states that the B+ supply to the plates is unregulated 300VDC, these are definitely regulated by a power MOSFET, to 235VDC, but of course when they go short the full unregulated B+ of 400VDC is presented to the plates and causes the tubes to flash over. Good call on the tube bases BTW, I will check them as the insulation on the connection wires has gone brittle with heat, so will replace them and check the pins. Thanks Dave
Since its such a high heat issue with the filament wiring I would use wire with a silicone jacket if you can find it. It will last a much longer time (practically indefinitely) in this application. That way it doesn't come back with the same issue later. The filament pins conduct heat down through the wiring very well.

If the regulator is dropping that much voltage it would be a good idea to retrofit a large power resistor to help drop some of the voltage (50-70 percent, maybe?) In the feed to the regulator to give it a break. A TO-220 or aluminum power resistor would work, and may fit on the heatsink?

Any way you can get us a few close up shots of the layout, wiring, and board? It would be helpful to see what else may be wrong. Also write down a few voltage measurements in "the usual" places.
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Great minds!, I was thinking exactly the same thing! putting an aluminium chassis mount resistor in series with the B+ line to 'soak up' any power surge associated with the tubes shorting internally. Its clear that the MOSFETs are failing before the fuse has time to react, despite being a fast blow. If you look at the photo you will see where the MOSFETs were mounted (heatsink paste), three of the four have failed. Thanks for your input, Dave


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