Self Bias and Fixed Bias for Big Tubes?

[commstech]

Most of the design for the bigger tubes i searched used the fixed biasing, eg. Ongaku 211, Ankoru 845, and many others. Is there any specific reasons why self-bias using high power resistors at the DHT cathode is not use, be it for sonic or safety reasons? can anyone enlighten?

[HollowState]

Large tubes like these require higher voltages to operate correctly. In order to use self (cathode) bias, a large value resistor would be needed which would seriously hurt efficiency. So it's a matter of practicality. It also gived better control.

Victor

[HollowState]

Actually I know of one 845 SE design that uses a kind of cathode bias in a grounded grid arrangement. The cathode is driven from a triode below which is current sourced through a transistor. If I can find the diagram, I'll try and post it.

Victor

[commstech]

thanks for the reply. the design u mentioned sounds interesting...awaits the schematic...

[ray_moth]

the design u mentioned sounds interesting

It makes me dizzy just to think about it

To give a bit more input on fixed vs. cathode bias, as HollowState says, efficiency has a lot to do with it. Another is the class of operation and a third is consideration of B+.

At first glance, cathode bias appears to suit Class A perfectly well; however, it does have its downside. You need a higher B+ because some of it is taken up with the cathode bias voltage. Another disadvantage is the need for cathode bypass caps. These need to be of high capacitance and are usually electrolytic type, which do not give the best sound.

You can avoid using a bypass cap if you use Class A push-pull with a single shared bias resistor (or a CCS); however, the OP tubes must be really well-matched or else you will get a DC offset in the transformer primary, causing some DC saturation of the core. Separate cathode bias resistors give a useful degree of automatic balancing, but then you need bypass caps.

In any (PP) class other than A, fixed bias is called for. Sure, mild Class AB1 (close to Class A) can use cathode bias but separate, bypassed cathode resistors are essential and efficiency is impaired. With deeper Class AB1 and with Class AB2 or Class B, cathode bias is not viable. Fixed bias is the only solution.

[dhaen]

Other aspects:

Self bias normally alows a higher value grid-leak resistor, so driver requirements are relaxed.

Fixed bias gets you more output power.

[mitwrong]

Commstech:
About tube bias, there are 3 types:
1, fixed bias
2, self bias
3, provided bias

Fixed bias is derived from power source, a negative voltage rectifier and trimmed to a desired voltage and feed to the grid leak resistor.

self bias is a positive voltage appears at the cathode when plate current flows, a voltage dropped on the cathode resistor. this voltage with respect to grid, it just turns up side down and become a negative bias voltage. the number of voltage appears at cathode, the same voltage bias.

provided bias is used in transmitter driver or final stage, usually transmitter final stage works on class C, no external bias were supplied, the tube gets it's bias at the driver stage, the coupling resonance coil. at RF positive cycle, the tube flows at max current, and cuts off at negative cycle, then the negative cycle is the bias of the tube. C type amplifier can deliver the highest power than ever been. For details, refer to RF amplifier app. note.

One can shorted the plate to ground, ( with plate load ) and flow up the cathode with a negative voltage supplied, what happen??? The tube still at normal working condition. This is similar to self bias.





we switch around the voltage, then becomes bias

[Miles Prower]

Originally posted by commstech
Most of the design for the bigger tubes i searched used the fixed biasing, eg. Ongaku 211, Ankoru 845, and many others. Is there any specific reasons why self-bias using high power resistors at the DHT cathode is not use, be it for sonic or safety reasons? can anyone enlighten?

Those "big tubes" mentioned there are low u triodes. That means a relatively high negative bias to set a Q-Point. Cathode bias would rob Vpk. The high wattage resistors tend to be more inductive, and really should not be used if fidelity is the design goal. Even if you parallel up a whole bunch of smaller resistors, you still have the heat problem. Besides, fixed bias tends to produce less distortion, sometimes much less.

Other times, the "big tubes" were originally designed as RF finals. In these cases, the amplification factors are so high that they require positive grid bias to set any sort of practical Q-Point. Here, cathode bias isn't possible anyway.

[commstech]

Originally posted by Miles Prower


.....really should not be used if fidelity is the design goal. .
...

Ahhhh......this is the motherhood statement that helped me in the decision for what method of biasing...

[HollowState]

Here's a diagram (I hope) of the (AC) grounded grid SE 845. Although it's not really a cathode bias type as I had thought.
The amp is pictured on the next page.

Victor