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Can I convert self bias to Fixed?

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#21
I'm the dissenting opinion. I've built the lions share of my amps with fixed bias both for me and my clients. Not had any real problems with them, and Dynaco built over 100K ST-70s with fixed bias and these amplifiers have not been particularly problematic until late in life if neglected.

I've never found an electrolytic bypass cap or even large film that had less or an equal impact on the performance than a 1 - 10 ohm resistor and fixed bias. There is a small efficiency/heat argument to make once you start talking about fairly significant current and voltage across the cathode bias resistor as well.
 
#27
Whatever biasing you use, for push pull output transformers it is real important to make sure the plate currents are balanced. Just a couple of mA difference will make for more bass distortion.
Self Bias: use separate self bias resistors for each tube with its own bypass cap.
Fixed Bias: separate sense resistors (i.e. 1 or up to 10 Ohm) in each cathode to check the current in each tube, and separate bias potentiometer for each control grid.
 
#28
An old Dynaco stereo 70 had matched output tubes. But on one channel the cathode current was 60mA, and the other cathode current was 40mA. The culprit was a leaky "Black Beauty" coupling cap to one of the EL34s.

I have noticed other leaky coupling caps in other amps. Check the voltage on the control grids, and you will see if it is bad. Only a bad tube with a leaky grid, or bad coupling caps cause this voltage to be high.
 
#30
I've seen a rebuilt pair of Heathkit W-5Ms that were converted to fixed bias - perhaps in a bid to eek out some more power?

Anyways, the retrofit included the use of a doorbell transformer flipped: so the secondary was attached to the 6.3V filament supply, and the primary was connected to the diode/cap/pot. It was a clever way - at least inexpensive! - to get the negative supply for the bias though I would probably just use a properly rated transformer myself.
 
#32
If you want to go beyond class A1 you definitely need a fixed bias. The only concern may be, G2 dissipation, but 6L6-oids have sturdy second grids.

In one of my last projects I used 100 Ohm resistors on top of 8.6V filament DC that powers driver Compactron double pentode. I got cleaner filament power, lower heat dissipation, and lower values of cathode resistors, while retained some self-bias. 8.6V was derived by rectifying 6.3V using Shottky diodes.
 

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#35
Fixed vs. cathode bias decision depends on one's goals.

If I want simple driver, trouble-free operation, long tube life, and no customer returns, it is cathode bias.

If I want maximum power with minimum open loop distortion, and tubes are cheap (6L6 types are cheap), it is fixed bias.

Changing cathode to fix bias may not be straightforward. In addition to driver problems already mentioned, fixed bias will need different output transformer for optimal performance.
 
#37
higher dcr means more copper losses..
but as to which one runs hotter i am not sure tbh...
i never really took the time to find out....
core heating depends on core quality and operating currents imho...
fwiw, i make my OPT's with dcr's to within one or two ohms of each other...
 
#38
P = i²R

Power (watts) equals current (i) squared times resistance. Derivative off of two two more common equations

P = IE and
E = IR

Sine E = IR, then substitute 'E' in first equation:

P = IE …
P = I (IR)
P = (II)R
P = I²R

So, for a given CURRENT FLOW, a higher DCR coil (primary, secondary, whatever) will definitely waste more power to heat.

Its just math!
GoatGuy
 
#39
sser₂;5071297 said:
Fixed vs. cathode bias decision depends on one's goals.

If I want simple driver, trouble-free operation, long tube life, and no customer returns, it is cathode bias.

If I want maximum power with minimum open loop distortion, and tubes are cheap (⁶L₆ types are cheap), it is fixed bias.

Changing cathode to fix bias may not be straightforward. In addition to driver problems already mentioned, fixed bias will need different output transformer for optimal performance.
Click to expand...

Actually (and also addressed to WAVEBOURN), … capacitor-bypassed resistance-in-series-with-cathode biasing is a darn good, but imperfect solution. Darn good because constant (well chosen) resistance adapts fairly well to tube aging, to tube-maker variability, to production line variability, and to power-supply fluctuations (inherent AND mains-supply related).

But if you “embrace technology” then the CCS (constant current source, capacitor bypassed) is the perfect solution, even better than a constant negative-voltage "fixed bias" solution.

Why? Because instead of “setting the planned quiescent current and looking up the right cathode bypass resistance”, you do it with a CCS like the LM–317 device. The SENSE-to-OUTPUT resistor is a tiny fraction of the resistor one might use for a resistor-capacitor self-bias solution (not that that matters much), but the precision of the LM–317 in turn ensures that "27 milliamps" (choose your own value) will be passing thru the tube almost entirely without regard to manufacturer, lot number, matching, aging, and so on. You bypass that little circuit with a nice fat low ESR capacitor to allow all audio-frequencies to pass thru at the lowest series impedance, yet still keep the quiescent bias-point established. Its like a "self-trimming" fixed bias supply. A "servo" supply.

And its cheap.
And simple.
And good.
Very good.

A "smart resistor on steroids".
GoatGuy
 
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