KISS 126
KISS 126
Negative Grid Bias:
On the circuit position of the batteries
by Andre Jute
The purpose of battery bias is to remove the soundshaping element of the bypass capacitor from the cathode circuit of any tube stage. Still, only a straight piece of wire has no sound: the battery itself is a soundshaping element and should be placed so as to do the least damage. Four possibilities are:
A. Batttery in Cathode
B. Battery in Grid Leak
C. Battery in the Earth Leg of the Attenuator
D. Battery in Grid
In considering each possibility, we want to pay attention to component count, longevity of components, effect on other components, and, most of all, the likely change to the sound.
A. Batttery in Cathode
This is the standard ultrafi biasing method. The battery replaces the negative grid bias resistor and its bypass capacitor between the cathode and the ground line. No other parts are required. Since the replaced component is a capacitor, usually a large-value electolytic, which in a simple amp like a potato amp can be heard, it’s removal is an improvement. Since the battery is sized just right for the negative grid bias, and that is the charge voltage flowing over it, this is a superbly self-adjusting circuit.
B. Batttery in Grid Leak
The battery cannot just be shunted across the signal and ground lines. It would form a dead short, creating a most silent amp. The battery therefore requires a resistor in series, and then, to allow for the drop over the resistor, the battery must be made larger. The battery may discharge quickly.
Nor is that all. In this position the battery will put DC on any preceding device, normally the source, which does not have a capacitor coupled output. The additional blocking capacitor required at the signal input is a good reason not to proceed with this version.
C. Battery in the Earth Leg of the Attenuator
This is the same design as “B. Batttery in Grid Leak” above, except that the resistor is now formed by that part of the attenuator to ground. The cap on the signal line is also required to block DC to the source. The current through the attenuator will ruin all but the best components in short order; if you have the best attenuator money can buy, you also paid enough for it not to want to take the chance.
Regardless of the fact that such implementations can occasionally be seen in professionally designed commercial gear, both possibilities B nor C are incompetent because the capacitor in series with the signal and the attenuator (or resistor in the grid leak position or the parallel combination of both) form an unintended filter. If the bias is inserted into these positions, the negative bias on the grid of the tube will in fact not be fixed but will vary with signal frequency. When music is played through the amp the operating point will wander all over the landscape rather than along the intended straight transfer slops. The result may be interesting; high fidelity it won’t be. That subverts the purpose of fixed bias by battery.
D. Battery in Directly in Grid
This is the battery in series in the grid, with the source being protected from DC by a capacitor in series with the signal at the input. In my particular implementation of 417A there is a gridstopper resistor of 55 ohm between the battery and the grid, requiring once more a bigger battery.
Can you do without the DC-blocking cap?
Battery positions B, C and D may in a DIY amplifier perhaps do without the blocking cap, since virtually all CDs have their outputs capacitively coupled, and DIYers anyway have the knowledge to open the source and check for the presence of a blocking cap on the output. Just remember to put the blocking cap in when you pass the amplifier on.
Which batteries should you choose?
I have had no problem with NiCads trickle charging for years on end in the cathode of 417A tubes. In the ultrafidelista community it is generally thought that the life of NiCads in this service coincide with shelf life. If you want to be absolutely safe in case of failure of the NiCads, shunt a 220Kohm resistor from grid to the ground which will protect you against simultaneous failure of the NiCads and the attenuator. If you want to believe the scaremongers, use NiMH batteries which could, at least theoretically, be more reliable.
Conclusion
What works is the battery in the cathode. No other components are required, the implementation does not threaten the operation or integrity of any other component, and it sounds just fine. Any other position causes problems or even trouble. Common NiCad batteries do all the business required.
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP
All text and illustration is Copyright © Andre Jute 2006
and may not be reproduced except in the thread KISS xxx on rec.audio.tubes