No.
#26 has no grid current, so grid leak resistor value is almost irrelevant.
Even if your pot's slider go bust (no grid leak resistor), there is no problem, the tube will survive.
The question of grid leak resistors: we use them so that if the grid leaks current, there will not be a situation where the bias of the tube runs out-of-control. Let's say we have a tube run at low power, and has excellent vacuum. In this case the leakage current out of the grid will be 1µA, or even lower. A grid leak resistor of 100K will cause a positive grid bias of 1µA x 100K = 0.1V.
Many factors cause more grid current: gas (old valves, imperfect seal around the pins), or running at high anode dissipation are common provocative situations. in this case, the bias might well be likely to drift.
A positive increase in grid voltage means the anode current increases, so we must take some care, or the current might increase and elevate the power burned in the anode, leading to more grid leakage.
To decide if you need a 'backup' grid leak resistor (example: in case the volume control pot's wiper loses contact):
- is there a limit on the anode-current? If you use Ale's gyrator, the current cannot get out-of-control - it is limited by the gyrator action. Harder biasing will lead to lower anode voltage instead.
- But if you have a transformer at the anode, there is no such current limiting, and more care is indicated.
The volume control (example: 100K audio taper) will work as a grid leak resistor, so you can rely on that, in most cases: use a high quality pot, and avoid using connectors between the pot and the valve-base..
The 26 or 01A is usually run with only 0.5W or so on the anode, so grid leak values can be higher. But 10Y is usually run with high voltage and high current, so unless it is used with a gyrator, it is probably worth using a data-sheet rated grid leak resistor (500K or lower with autobias, 100K or lower for filament bias or grid bias) for the situation where the volume pot loses contact.
Many factors cause more grid current: gas (old valves, imperfect seal around the pins), or running at high anode dissipation are common provocative situations. in this case, the bias might well be likely to drift.
A positive increase in grid voltage means the anode current increases, so we must take some care, or the current might increase and elevate the power burned in the anode, leading to more grid leakage.
To decide if you need a 'backup' grid leak resistor (example: in case the volume control pot's wiper loses contact):
- is there a limit on the anode-current? If you use Ale's gyrator, the current cannot get out-of-control - it is limited by the gyrator action. Harder biasing will lead to lower anode voltage instead.
- But if you have a transformer at the anode, there is no such current limiting, and more care is indicated.
The volume control (example: 100K audio taper) will work as a grid leak resistor, so you can rely on that, in most cases: use a high quality pot, and avoid using connectors between the pot and the valve-base..
The 26 or 01A is usually run with only 0.5W or so on the anode, so grid leak values can be higher. But 10Y is usually run with high voltage and high current, so unless it is used with a gyrator, it is probably worth using a data-sheet rated grid leak resistor (500K or lower with autobias, 100K or lower for filament bias or grid bias) for the situation where the volume pot loses contact.
In my 10/801 preamp I use (TKD) potentiometer without grid leak resistor.
For safety solution try to use large (for example 1M) grid leak resistor, which is connected parallel to the pot slider and ground point. The resultant grid leak value is depends of the pot position (Re: "lower part of pot" // 1M ), always lower than "lower part of pot".
If pot become foul, the 1M grid leak support fix bias point.
Or you can do what I do and use an input transformer and put the volume control on the output of the preamp. The secondary of the input transformer acts as a "grid choke" to take care of any DC grid leak current, while allowing application of battery grid bias without the need for a DC blocking cap.
The VC on the output also means that circuit noise is attenuated along with the signal. When the VC is on the input you always have maximum circuit noise and attenuate only the signal.
The VC on the output also means that circuit noise is attenuated along with the signal. When the VC is on the input you always have maximum circuit noise and attenuate only the signal.
I tested a lot of coupling capacitors a couple of years ago and I liked the Russian teflon FT-3 best. For Ale's board I use FT-2 because they are smaller and just as good. The 0.1uF fits neatly on the Gyrator PCB and I use two of them to make 0.2uF for the coupling caps. FT-2 is rated 200v but they can take more.
... after 300-400 hours.
Each teflon burning time such a long (I use FT and V-Cap too).
At my age, will I live long enough? But at least my heirs would have a nice pre-amp!
In my opinion the V-cap Cutf have a musicality that the FT's are missing. But almost everybody disagrees with me on this one, so I will try ones more with some smaller caps (I've just have lot of experience with the 0.47uF). My V-caps needed 600 hours with 55V and 10mA, so yes, teflons take time. That's why I bought a capacitor burn in machine. I know. Pathetic.
I think a dutch transformer producer has done it. Was it Tribute or Monolith Magnetics?