Regarding bias circuits

[pjd3]

Hello,

Just thinking out loud about 2 bias circuits I've used in builds, and curious if one makes for a more cleaner bias signal than the other, overall.

There are bias circuits that simply use say, a single resistor with a 50uF or 100uF cap as seen in some Fenders, and, there are bias circuits that are in more of a "T" configuration with say, a 25uF cap joined to another 25uF cap by a 15K ohm resistor.

I think I may be noticing that the T config of bias power filter is used when it is tapped off an outer leg of the HT coil, and the simpler "L" config is use if there is a separate say, 60 or 70 vac bias tap, off the HT coil.

Has there ever been discussion suggesting that in general you can expect a more clean bias signal from the L config? Because if there was, I would just use an T config everytime if it gave a better quality bias signal. I have heard in a few threads that keeping the bias signal as noise free as possible is conducive to a better sound and tone. I would have to suspect that keeping 60 or 120 Hz power cycles off the bias sure wouldn't hurt.

What are your thoughts and experiences with bias and how far one can go to make an appreciable difference?

Thank you very much for your feedback.
Best,
Phil D

[Stevem]

Do you have a scope?

[pjd3]

I do have a scope.

I hadn't even thought about looking at the bias signal that way (duh). I usually do not have my amps up at the bench, though, that would make it a whole lot easier to do that - they are always in some car or truck for the next gig.

I wanted to hear about how others go about choosing which kind they will use for various amps. Where the simpler RC bias circuit seems to be found in Fenders, I had to wonder if the lower impedance separate tap made for a more steady bias signal with less need for more parts. And then, had to assume that perhaps the higher impedance of tapping off an HT leg would make better with an extra cap after a a resistor as in the T one found in alot of Marshalls.

Next time I have one of my amps up here on the bench I will try to remember to look at the waveshape of the bias on whatever amp is on the bench. If the T is better, I figure why not use one in every amp? Maybe I'll just make a TTTTTTTTT and brag about having the best damn bias signal of anybody. Naw. I won't do that. But, yeah, maybe time to find out for myself.

Thank you,
Best,
Phil D

[Stevem]

Me neither, but just wondering what the idle noise floor looks like in a fixed bias amp that just for the second part of the test you convert to cathode bias, of course with the PI tube pulled out so your just looking at the noise floor of the output section .

Output sections don’t have much gain so noise on the grid will not modulate a audio signal like even a triode preamp tube will.

[pjd3]

I certainly can make some comparisons over time on looking at the bias waveforms.

I'll be bringing my stereo 6V6 Plexi up on the bench soon to see if I can get the right channel working (the Left channel was working and sounding wonderful until there was a "pop", and then nothing).

That amp has the T bias circuit, and it may have a separate tap from the HT coils (Musical Power Supply PT). I'll it a point to look at both of the power sections, even though the Left. Hope the Siglent scope has enough precision to see it. (Its a $499 scope that does everything!!!).

Alright, well more things to see, more things to learn.

Thanks guys,

Phil

[syscokid]

I certainly can make some comparisons over time on looking at the bias waveforms.

I would be very interested in knowing what your results will be!

[Astronomicum]

Nothing wrong with being curious as to the bias power supply wave form (ripple), but you do not need a scope to measure power supply ripple voltage which will change as you adjust the RC filters. Just set your meter to AC and keep in mind you are looking at ACRMS (Peak voltage *0.707).

[Helmholtz]

I think I may be noticing that the T config of bias power filter is used when it is tapped off an outer leg of the HT coil, and the simpler "L" config is use if there is a separate say, 60 or 70 vac bias tap, off the HT coil.

When the bias voltage is derived from a high voltage PT secondary, large value voltage divider resistors are used.
If you would use a (single) 100µ filter cap here, the bias ramp-up would likely be too slow.
Instead 2-stage filtering with smaller value caps is used for sufficient filtering and fast ramp-up.