I am looking to bias a tube via a C battery on the grid. -20V is ideal, but -18V would be within an acceptable range. I have been planning to just use 2 Lithium 9V batteries in series, but thought I'd see if anyone had any other suggestions or thought this might be too noisy. For various reasons, it works out better in this case to simply ground the cathode, so biasing via the cathode is not the best option. Thoughts?
You could use rechargeable cells and simply have a DPDT power
switch, have the charger kick in when the music power is "off"
How could pure battery bias be noisy?
Ni-Hydride hold a very consistent voltage as they drain.
But you gotta recharge them slowly if you aren't keeping track
of individual cell temperatures. Trickle charge is perfect...
Lithium, well it depends which lithium chemistry we are talking...
http://en.wikipedia.org/wiki/Lithium_battery
Probably gonna need a regulator or bias will drift with both
temperature and state of charge.
switch, have the charger kick in when the music power is "off"
How could pure battery bias be noisy?
Ni-Hydride hold a very consistent voltage as they drain.
But you gotta recharge them slowly if you aren't keeping track
of individual cell temperatures. Trickle charge is perfect...
Lithium, well it depends which lithium chemistry we are talking...
http://en.wikipedia.org/wiki/Lithium_battery
Probably gonna need a regulator or bias will drift with both
temperature and state of charge.
I thought lithium due to the long shelf life, but I'll look in to the others too.
The input circuit is this
so it shouldn't really drain at all as there shouldn't be much, if any, current draw ... unless I'm missing something ... which is likely.
The input circuit is this
An externally hosted image should be here but it was not working when we last tested it.
so it shouldn't really drain at all as there shouldn't be much, if any, current draw ... unless I'm missing something ... which is likely.
Hi Doug,
In most normal applications for grid bias conventional alkaline batteries will give you close to their rated shelf life as long as grid current is a couple of uA or less. I have a battery powered pre-amplifier and it is now 5 yrs old and with <50hrs use the original batteries still test virtually as new.
Two fresh 9V alkalines particularly Dur*cell brand will give you close to 10V each in some instances getting you close. Alternately you can use a pair of any 9V alkalines and a single AAA cell getting you very close to -20V.
NiMHs and NiCads aren't worth the trouble for grid bias. Used in cathode circuit they will self charge, just make sure that operating current is no more than about 5% of rated cell current for good long term behavior. NiMH types I have tried generally did not sound as good as a NiCad..
(The story goes that a certain guitar pre-amplifier product I worked on had a 9V charge pump chip in it that suddenly starting blowing every time a fresh Dur*cell was used, and never ever with any other brand - it turned out that the particular lot of Dur*cell alkaline batteries we had just received put out 11V when brand new and the chip was abs max rated at 10.5V - solution was to switch to a pin compatible chip from another vendor rated for slightly higher max voltage.)
In most normal applications for grid bias conventional alkaline batteries will give you close to their rated shelf life as long as grid current is a couple of uA or less. I have a battery powered pre-amplifier and it is now 5 yrs old and with <50hrs use the original batteries still test virtually as new.
Two fresh 9V alkalines particularly Dur*cell brand will give you close to 10V each in some instances getting you close. Alternately you can use a pair of any 9V alkalines and a single AAA cell getting you very close to -20V.
NiMHs and NiCads aren't worth the trouble for grid bias. Used in cathode circuit they will self charge, just make sure that operating current is no more than about 5% of rated cell current for good long term behavior. NiMH types I have tried generally did not sound as good as a NiCad..
(The story goes that a certain guitar pre-amplifier product I worked on had a 9V charge pump chip in it that suddenly starting blowing every time a fresh Dur*cell was used, and never ever with any other brand - it turned out that the particular lot of Dur*cell alkaline batteries we had just received put out 11V when brand new and the chip was abs max rated at 10.5V - solution was to switch to a pin compatible chip from another vendor rated for slightly higher max voltage.)
Thanks Kevin. Grid current should be nil as the tube will be biased to -20V, but never see an input higher than about 2Vrms or so. The tube is a 71-A, at least to start with. I looked at a 31, but the lower rp of the 71 is important here. The biasing via the grid should allow for a single winding to be used for two heaters since one side will be grounded. Full schematic is at: http://www.ecp.cc/images/Active_L'es++.jpg (OPT is a Slagleformer)
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