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ZKIT1 oddities - missing 0.1uf capacitors? (Read 911 times)
diegopx
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ZKIT1 oddities - missing 0.1uf capacitors?
01/17/22 at 04:21:08
 
Hello!

I recently inherited a prebuilt ZKIT1 amplifier, which Iím in the process of refurbishing into a new case. I tested it for a bit and it sounded great! However, while working on the amp, Iíve found a few oddities which I wanted to run by folks here:

First, it seems like there 2 missing 0.1uf400V capacitors ó marked in PURPLE. Namely, the ones closest to the 6P15P tubes. The 2 capacitors in place seem pretty beefy (marked F104K S 630MEB1), but I was disconcerted to see the two other spots bridging the 6th pins on these tubes empty. Is this okay? While looking for other build examples, it seems like some boards donít have these solder spots at all.

The next oddity has to do with the 1K 5W resistor ó marked RED. This is straightforward enough imo, but my amplifier came wired with a Hammond 158M 10H inductor in place of this component. Iím going to replace this with the originally specíd component, mostly since I donít have room for that DC reactor in my new case; but Iím still curious as to what the logic behind that component choose wasÖ is that a popular mod for this kit?

Lastly, I was reading in some posts (and also the instructions) that one should have a jumper bridge between the circuit ground and ground plane when building in a wood case. However, I canít figure out where that jumper should sit, since there is no label for itÖ

Any commentary on the above would be very much appreciated!
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BWC54
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Re: ZKIT1 oddities - missing 0.1uf capacitors?
Reply #1 - 01/17/22 at 19:34:52
 
The missing capacitors accomplish the Hazen mod by connecting the suppressor (pin 6) to the cathode (pin 3). The Hazen mod is supposed to sound better. Do a search here on Hazen mod or "Christmas Comes Early" to learn more about the Hazen mod.

A choke is used in place of a resistor to make the power supply quieter. You can eliminate the choke and go back to the original design if you want to. If you keep the choke you have to measure what the choke resistance is and you might need to supplement the choke resistance with a smaller additional resistor.

If your board is revision 12 2018 you don't need to add the ground jumper for the ground plane and signal ground. You just need to ground the board where the ground board is marked.
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diegopx
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Re: ZKIT1 oddities - missing 0.1uf capacitors?
Reply #2 - 01/18/22 at 02:09:50
 
Thanks a bunch! Thatís really helpful, Iíll look into that mod. I mostly found it strange to have unpopulated components when the instructions donít really specify anything as being optional during assembly.

So, theyíre basically not hurting anything, but things can be improved further ó great to know.

Thanks also for the explanation of the inductor! My understanding of electronics at this level is pretty rudimentary, and I never thought component types were interchangeable like that.

In regards to the grounding question, my board is actually a revision 9 from 2011. So, the mystery remains there a bit, but Iíll definitely be grounding my cable shields, potentiometer casing, and the board itself to the IEC ground pin. Fingers crossed thatís enoughÖ
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BWC54
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Re: ZKIT1 oddities - missing 0.1uf capacitors?
Reply #3 - 01/18/22 at 12:05:15
 
Here's a thought on why the original builder left those capacitors out. If the original builder intended to use EL84 tubes instead of 6P15 tubes the capacitors aren't required because the suppressor and cathode are connected internally in the EL84 and pins 1 and 6 are not connected to any thing. The Hazen mod only works with the 6P15 tube because the suppressor grid is not connected to the cathode internally. In a 6P15 the suppressor is connected to pins 1 and 6.
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