Voltage drop calculation - please check

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Voltage drop calculation - please check

Postby Yazoo55 » 16 Aug 2018, 12:47

I am going to try building the Hughes and Kettner Rotosphere. There is a thread on this already from a couple of years ago. The Rotosphere uses an onboard transformer to provide the grid voltage for the 12AX7. On the original, this is a 15V to 250V transformer. I could only find a 230V to 15V transformer which will be wired up in reverse (again this is suggested in the original thread). In the schematic, 280V goes to the 100K resistors connected to the grids of the 12AX7.

If I have got this right, the 56K resistor highlighted in the schematic drops the approx. 350V after the diode bridge to 280V. My transformer would only put out about 322V after the diode bridge but if I replace the 56K resistor with a 32K resistor this would give me close enough to the original 280V.

Have I got this right?
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Re: Voltage drop calculation - please check

Postby Manfred » 16 Aug 2018, 13:45

Have I got this right?

Yes, you do it right.

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Re: Voltage drop calculation - please check

Postby phatt » 18 Aug 2018, 05:04

Just beware that the schematic is marked wrong. That 12.6V tap is Negative as drawn :scratch:
If you want positive you need to invert D21, C113, C114, C115.
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 18 Aug 2018, 10:09

Thanks, I'll redo that bit. That will save me a lot of grief.
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 18 Aug 2018, 12:20

I have had a look at photos of the board and traced the connections through. The schematic is correct as regards the 12.6V supply. I have to admit I don't understand why this works.
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Re: Voltage drop calculation - please check

Postby marshmellow » 18 Aug 2018, 14:24

Like Phil said, as drawn it will put out a negative voltage. I think it's just mislabelled in the schematic out of habit. The voltge is only used for the tube heaters, but the tube doesn't care if it's negative or positive 12,6V. Both stages are common cathode, so heater-cathode voltage isn't an issue either. I assume the reason they went for negative voltage for the heaters is to balance out the load on the transformer. The high voltage fullwave rectifier is symmetrical, the +-12V is symmetrical, but the +18V loads the positive side more. So to balance that out a bit, they went negative with the heater.
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 18 Aug 2018, 15:23

Great. Many thanks for the explanation. :D
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Re: Voltage drop calculation - please check

Postby phatt » 23 Aug 2018, 14:04

marshmellow wrote:Like Phil said, as drawn it will put out a negative voltage. I think it's just mislabelled in the schematic out of habit. The voltge is only used for the tube heaters, but the tube doesn't care if it's negative or positive 12,6V. Both stages are common cathode, so heater-cathode voltage isn't an issue either. I assume the reason they went for negative voltage for the heaters is to balance out the load on the transformer. The high voltage fullwave rectifier is symmetrical, the +-12V is symmetrical, but the +18V loads the positive side more. So to balance that out a bit, they went negative with the heater.


Arrh yes,, good point. I missed that little subtle detail. :slap:
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 25 Aug 2018, 13:32

I have just about finished doing the boards in Kicad. I would be grateful for some advice on the input sockets though. The way I understand the schematic which I have attached, if the effect is being used in mono only so that a jack plug is inserted in the Right channel but no jack plug is inserted in the Left channel, the input from the Right channel is also routed through the Left channel. It also looks like the same thing is done with the output. I have looked at the Rotosphere thread on this board but none of the DIY versions seem to do this.
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Re: Voltage drop calculation - please check

Postby phatt » 25 Aug 2018, 14:07

Yes that sounds right to me,
Looks like you can use mono in and mono signal will be present on both outputs.
Or Stereo in and mono out. :thumbsup
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 05 Sep 2018, 13:34

I've sent the files off for the boards to be fabricated. I do still have one query if I could again ask for advice. :oops:

The breaker footswitch is connected to P08, P09 and P011 on the main board. I read Thorsten Hillman's guide on replacing the momentary switch with a latching switch and you can clearly see two of the wires are soldered together on one side of the switch. Both P09 and P011 connect to Ground though. What is the the purpose of the extra Ground connection?
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Re: Voltage drop calculation - please check

Postby phatt » 07 Sep 2018, 14:59

Yazoo55 wrote:I've sent the files off for the boards to be fabricated. I do still have one query if I could again ask for advice. :oops:

The breaker footswitch is connected to P08, P09 and P011 on the main board. I read Thorsten Hillman's guide on replacing the momentary switch with a latching switch and you can clearly see two of the wires are soldered together on one side of the switch. Both P09 and P011 connect to Ground though. What is the the purpose of the extra Ground connection?


None, both ways should work. both ways are electrically the same thing. :thumbsup
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 07 Sep 2018, 16:16

Thanks, that confirms what I thought and puts my mind to rest. :D
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Re: Voltage drop calculation - please check

Postby Yazoo55 » 20 Sep 2018, 17:53

OK, I've built it. It still needs to be boxed and I've built a plywood base which I'm going to cover with a bent aluminium sheet. It's always a problem trying to get the right size and shape enclosures for non-standard builds. The 12AX7 seems quite happy. I used a 33K resistor instead of the 56K resistor for the high voltage as I am using a 230V transformer rather than the original 250V transformer.

There are six trimpots. Two set the input bias for the MN3207s, though, as somebody else commented, they don't seem to make much difference. From what I've read, the other 4 set the high and low rotor speeds, 2 for each channel. Any advice on setting these would be gratefully received. And thanks to the original people who tackled this before me.
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