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#26 pre amp
#26 pre amp
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Old 25th May 2011, 09:32 AM   #1001
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Could try adding a MOS cap-multiplier to the B+

4.7uF [x1 or x2] MKT (cheap type is OK, as there is no current) and 1,5M resistor [and 12V zener]. Cheap FQPF3N80C is 800V isolated, and can easily do this [1 per channel]

FAIRCHILD SEMICONDUCTOR|FQPF3N80C|MOSFET, N, TO-220F | Farnell United Kingdom
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Old 25th May 2011, 10:07 AM   #1002
massimo is offline massimo  Italy
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I'm altready planning to use Salas HV shunt regs.... one silicon junction in series with B+ it's enough for my taste!
Thank you Rod anyway!
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Old 25th May 2011, 10:46 AM   #1003
andyjevans is offline andyjevans  United Kingdom
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Thus the rectifier tube (#80) is still subject to cathode stripping.>>>

The 80 is directly heated - can you have "cathode stripping" with directly heated filaments?

andy
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Old 25th May 2011, 10:55 AM   #1004
massimo is offline massimo  Italy
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Ops..... #80 and its son 5Y3 are DH. What a noob I'm!
What do you think about a double pwr on switch, Andy?
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Old 25th May 2011, 11:03 AM   #1005
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
"cathode stripping" normally means the electrostatic ripping of cathode layer from the active layer [or DHT filament] in **Transmitter** tubes.

With 600V and lower operation, you don't really risk this happening.

But you do risk the effect described by van der Weyer - active-layer particles being drawn to the grid (where they stick). This is not related to cathode stripping (as applied above).
But over many power cycles, you may end up with more noise and distortion than an unused valve.

The grid, and cathode active layer are present in direct- and indirect- heated tubes, so I would assume the same precautions apply. And since DHTs are usually more expensive to replace, I think the precaution is worth the trouble.

Personally, I think it is (aesthetic) rough-handling to crash the HT on to a valve before it is warmed up, like using a big nailed-boot, or a stick to operate the power switch. DHTs are fragile, and should be treated GENTLY, in all ways.

On the other hand, one may choose to believe that nothing will happen - many here do. That's OK, It's your money, and your tubes. This is DIY!
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Old 25th May 2011, 11:20 AM   #1006
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by Rod Coleman View Post
Felipe, here's a good 6V/50VA:

PRO POWER|CTFCS50-6|TRANSFORMADOR, 50VA, 2 X 6V | Farnell Espaņa

I think Farnell ship with no cost.
Rod with this I only need one tx, right?
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Old 25th May 2011, 11:23 AM   #1007
iko is offline iko  Canada
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#26 pre amp
Question for Andy: have you noticed a significant difference using 2 x 156C plate chokes instead of one? Or I should say, how much of a loss when using just one choke? I happen to have a couple 156C chokes and was thinking of using them one per channel.
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Old 25th May 2011, 11:25 AM   #1008
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Felipe, maybe. It will WORK with one.

But for best performance, one Trasfo for each side will protect the filament in channel L from the power supply current pulses, and diode noise, in channel R.
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Old 25th May 2011, 12:19 PM   #1009
andyjevans is offline andyjevans  United Kingdom
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Originally Posted by ikoflexer View Post
Question for Andy: have you noticed a significant difference using 2 x 156C plate chokes instead of one? Or I should say, how much of a loss when using just one choke? I happen to have a couple 156C chokes and was thinking of using them one per channel.
Yes - there is a significant difference, because it halves the capacitance which is your enemy for the treble. It also doubles the inductance which is useful for a 26. Two are much better sounding.

I wire the two chokes bottom to bottom with the wires coming out the same side. Then connect the right side wire of one to the right side wire of the other (when the normal way up, not upside down!) so you wire them out of phase.

Try getting another two - they're very cheap and surprisingly effective. I'd prefer a really nice anode choke from a good maker, but for now this gets me some good sounds.

Andy
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Old 25th May 2011, 12:38 PM   #1010
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by Rod Coleman View Post
Felipe, maybe. It will WORK with one.

But for best performance, one Trasfo for each side will protect the filament in channel L from the power supply current pulses, and diode noise, in channel R.
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