[mikeyB]

Can any of the gurus validate (and hopefully resolve) my concerns?

Built 2 DOAC - 1st one I can't get my hands on it as me mate is constantly using it for mastering! Now I've finished the 2nd (Kubi advanced version), I've started to confirm my niggling suspicions re. the power supply caps.
PCB ident is for 400V rating - BOM is for 350V and if like me, any UK builds using 350V may have a problem.
Musikding power transformer has 250V HT secondary. Doing the basic math 250 x 1.414 = 353Volts  :sad: This is before taking into account UK mains  - mine measures a rock steady 240V  :!: Accounting for this increase, DC after rectifier is calculated at 369V :shock:
While valves are warming up - no current is drawn, and I measured 370V after the rectifier. This stayed this high for 15 - 20 seconds until the valves conduct :!:
Is there an easy fix for this (inclusion of a power resistor somewhere) or do I need to take out the PCB and upgrade the caps to 400/450V rating :roll: Will this be the case for all 12 HT caps or just the 3 pre-regulator?
Will do a bit more measuring and observation before reporting back :shock:

[radiance]

Those caps will fail eventually. The question is: WHEN??

Just upgrade those caps for peace of mind....

[CJ]

I think you will be O.K.

Insulation needs a little time to break down, as long as you do not hold the voltage over the max rating for too long, I see no problems.

Probably depends on  the cap.

Take a cap that you are using and subject it to some over voltage tests with a 1 meg resistor in series.

See how much current it draws at 379 vdc.
If none, you should be alright.

Most transformers put out about 1.3 times the AC, as there are a few losses that knock it down from the straight 1.414 number I would imagine.

We have a circuit that uses 10 ohm, 1/4 watt  resistors across a 240
volt mains.
But only for a few milliseconds, long enuff to create a voltage drop that hits the gate of an SCR and pulls it in.
At the onset, 240/10 = 24 amps!
5000 watTs across a 1/4 resistor!


Amazingly, the resistors never blow, and we make a ton of these products over the last 15 years.

But your mileage may vary.

[mikeyB]

Thanks for the reassurance CJ :thumb:
While my 2nd unit is on test bench, I'll more likely change the caps - got enough room to get to underside without having to desolder anything :wink:
for my peace of mind. If the Kubi mods notch the performance up another level, then the first unit will get new caps along with Kubi upgrade :thumb:
Thanks again for advice :sam:  :thumb:

[RAM]

Thanks for the warning mikeyB! I'm planning on building one of these soon myself. I'll make sure I get a higher voltage rating for the caps. Hope yours keeps working fine for you.
Rob

[mikeyB]

Had a bit more time on it - Voltage after Reg is ok - rises up to 300ish V for a few secs then drops back to 250ish V. To be safe, I think i'll change the 3 pre reg caps to 450V rating.  :thumb:

[CJ]

Some Fender amps hit the caps with solid state while the tubes are warming up.
I believe on some models that the voltage actually exceeds the cap ratings til the current starts to flow.

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