Fixed Bias suggestions

[dsavitsk]

I am looking to use fixed bias for a DHT with a transformer input. I drew up a couple of options and am wondering if anyone has a preference, or any other options.

The upside to A is that the input signal path is a transformer winding, a battery, and the tube which is pretty minimal. The downside is that because the filament supply is biased up from ground, two channels will require separate filament windings which adds a little bit of hassle.

The upside to B is that both filaments can be powered froma single winding. The downside is the addition of a large resistor (~1M or so?) to the input signal path which may or may not be a problem.

The upside of C is that the input signal path eliminates (or at least limits) the battery and the large resistor. The downside is the addition of a capacitor, and being cap-phobic this seems like trouble.

I also thought about using something like tubelab does in his SE amp: http://tubelab.com/TubelabSE.htm my concern is that if you trace the current loop, the bias PS caps are in the signal path, which seems to just trade one electrolytic for another.

[sajti]

One vote for "B".

sajti

[Yvesm]

Another vote "B" !

But without any resistor in serie with the bias source that should have the lowest possible internal impedance AND resistance.

Rem:
The signal must pass thru the bias source and it must be able to accept some current sourced by the grid while still maintening a stable voltage.

Yves.

[kevinkr]

Count me in for B as well, incidentally the resistor in series with the battery is not required and not recommended if battery bias is to be used. Put a small film cap across the battery to assure that the supply impedance remains low at high frequencies.

[oshifis]

What is better: a capacitor or a battery (maybe shunted by a capacitor) in the input signal path? I wote for C unless there are "audiophile" batteries.

[kevinkr]

Originally posted by oshifis
What is better: a capacitor or a battery (maybe shunted by a capacitor) in the input signal path? I wote for C unless there are "audiophile" batteries.

I have experimented quite a bit with battery bias and have used all three topologies and have generally found "A" and "B" to be preferable. "A" gives very good results with NiCads, (Panasonic/Matsushita) but not such good results with NiMHs, "B" works great with alkaline batteries which may have source impedances of a few ohms or less. "B" in my experience is the most transparent.

"C" adds a lot of additional and IMHO unnecessary components in the signal path. Results can be very good, but the quality of the coupling capacitor is important and hence it will be expensive.

[dsavitsk]

Originally posted by kevinkr
Count me in for B as well, incidentally the resistor in series with the battery is not required and not recommended if battery bias is to be used.

Somehow I got it in my head that I needed a large resistor to keep the battery from draining -- not sure where I thought the current was going to go. Sometimes you need someone else to tell you the obvious.

Originally posted by oshifis
What is better: a capacitor or a battery (maybe shunted by a capacitor) in the input signal path? I wote for C unless there are "audiophile" batteries.

I don't know. There aren't "audiophile" LED's either, but they still sound better in a signal path than any cap I've tried, and I've tried a few. LEDs are just not practical here as the bias is too high -- I'll be using a pair of 9V batteries if that information is important.

That said, I do think it is worth the experiment to see what works the best. I was just looking for a starting point, and to see if there was anything I forgot.

[ErikdeBest]

Originally posted by Yvesm
Another vote "B" !

But without any resistor in serie with the bias source that should have the lowest possible internal impedance AND resistance.

Rem:
The signal must pass thru the bias source and it must be able to accept some current sourced by the grid while still maintening a stable voltage.

Yves.

Hi Yves

I do not completely understand your reminder, but am I correct assuming that one can not get away by using the standard voltage divider (with a potentiometer) to set the bias, but instead one needs a adjustable voltage regulator*, with the output straight to the IT?

many thanks, Erik

* A regulator as the one proposed by Morgan Jones on p335 of Valve Amplifiers?

[rdf]

Hi Doug, just to throw a wrench into the works. I'm playing with it on the bench with an A2 814 SE biased around -90VDC, ~20 ma through a 4.7K grid resistor. Effectively no caps in the bias grid circuit, I use these handy-dandy IXYS boards I picked up somewhere.

[dsavitsk]

Wrenches are appreciated -- is the high impedance of the CCS essentially like a grid choke? Probably not as the signal should go through the resistor, but I guess the CCS allows it to bypass the PS?

What do you you use to supply the current? Just a basic floating dc supply?