I'd like to keep this relatively simple.. So would prefer avoiding relays to be honest.
I am using rod coleman filament supplies, and the filaments seem to warm up quite quickly as it is.
As for B+ in the amp, there's about a 15 second delay before full B+ is applied to the power tubes, I'm using damper diode tubes for the rectifier, and they warm up very slowly.
Should be fine then, depending on the tube you are using cathode stripping is not likely much of an issue anyway.
And I'm understanding that circuit correctly? 10K pot and 10K R1 value should provide -100V to -200V range?
Yes, very close to it. Really depends on the bias supply output voltage at the given load current.. You could safely change the component values and decrease the load on the supply from the pots and resistor strings. I'm assuming the power drive circuit also requires appreciable current for proper operation.
Grid current for the following power stage is supplied via the drain and source on the fet (separate + and - supplies for the drain and source).. No current is drawn for the grid of the power stage via the bias circuit.
I'm just trying to understand now how that R2 resistor from the wiper to the top of P1 protects in the case that the wiper loses connectivity.
Draw the circuit with the wiper connection omitted, but R2 present. What bias voltage is applied under this condition?
I guess that's like 0 ohm on the top of the divider and 20K on the bottom.. So should be -200V out?
I figure power dissipation in the 10K pots at about 1.1W each -- way too much for 'standard' panel pots. Suggest you move up to 20K or more for the pots and series resistors, assuming that your pots are specified for 1W.
Damn.. That's a good point.. I just had such trouble finding nice pots without shafts with a screwdriver adjustment.
Anyone know of some decent ones? maybe even 50K would be better..
Like these ones (Could only find 10K):
I guess there's these http://www.digikey.ca/product-detail/en/RV4LAYSA503A/RV4LAYSA503A-ND/222797 But they're kinda on the pricey side.
Or these.. http://www.digikey.ca/product-detail/en/53C250K/53C250K-ND/52096 Even more pricey
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Another issue here is that DC current through the pots' wipers ages them pretty rapidly. Might even be better off without the safety resistor in some cases.
All good fortune,
Chris
All good fortune,
Chris
THere's gotta be a universally accepted best way to implement an adjustable fixed bias..
Who one Earth told you that? A few milliamps of DC through a carbon pot that is hardly ever used is nothing. Less than nothing. Yeesh, only on an audiophool forum could people make such a meal out of a bias pot.
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If you use a bias stack , that is a resistor divider from +B to earth of sufficiently high impedance to largely eliminate DC current(think 100K or more), you can insert a pot into that stack and use a MOSFET source follower to supply the actual bias current to your grids. Attach the drain to +B, gate to the pot wiper and the source to the valve grids.
Its not perfect - but it will preserve your pots for a lot longer than using a low impedance voltage divider and pot to do the same job.
Shoog
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For an output stage that draws no grid current a 50k pot is fine. A mA or two of current through it will keep it cool. Use resistors to put the pot adjustment range where you want it. Add a high value safety resistor. Why are we making such a mountain out of a simple matter?
Yea.. I'm going to stick with that schematic I posted most recently, and use 50K pots.
Thanks everyone.
To add some additional discussion to this topic (likely unnecessary), Should I go for the carbon element or conductive plastic element pot?.. They're both 2W pots, and within a few dollars of eachother. 😀
I was under the impression that conductive plastic types would potentially have a longer lifespan.. But not sure if it really even matters.
I was under the impression that conductive plastic types would potentially have a longer lifespan.. But not sure if it really even matters.
thehoj: I use these or similar. Cheap and accurate and I calculate for about 2 mA running through a 10K version.
3006P-1-103LF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey
3006P-1-103LF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey
I'm looking for panel mount since they'll be mounted on the top plate of the chassis of the amp, so I like the idea of no shaft, or small shaft, and Those PEC or honeywell ones are nice because you can actually lock the shaft in place with a nut.
Just trying to decide between these http://www.digikey.ca/product-detail/en/53C250K/53C250K-ND/52096 or these http://www.digikey.ca/product-detail/en/RV4LAYSA503A/RV4LAYSA503A-ND/222797
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