Single-ended output using fixed bias

[Paul P]

The amp I'm building will have a singled-ended output using either
a 6v6 or 6l6. I'm building the amp in such a way as to be able to
convert to class AB push-pull sometime in the future should I want
more power. To this end I'm using an Allen Amplication power
transformer that has a tap for bias voltage.

So instead of using cathode bias for the output tube, which is the
usual way of doing things I believe, I figure why not use fixed bias
(with an adjustment pot) ?

Is there anything wrong with this idea ? Do cathode bias and fixed
bias sound the same ?

Paul P

[Enzo]

Bias is bias - it means the voltage relationship of the grid to the cathode. You can do it either way or even a combination of both, though that is rare.

Ther are subtle differences in the dynamics of the amp between the two.

[cbarrow7625]

You'll be fine with either. Cathode bias will typically give you a subjectively more "fluid" or "loose" feel to the amplifier and fixed bias will have a "tighter" feel with better "attack" when you pluck a string (as compared to cathode bias).

Which should your amp use? It depends. If you are not sure, I would build a circuit that will allow you to bias it either way. Try each and see which way suits your amp and your playing style best.

On my main amplifier I can switch back & forth at will with a switch and I find myself switching it based more on the mood I'm in than anything. I even have a posiiton (this is a push-pull amp) that allows me to bias one side fixed & the other side cathode. That is my favorite position be cause I get the best of both worlds.

Good Luck.
Chris

[Enzo]

That is true, a simple DPDT switch can select between those choices.

One half of the switch connects the power tube grid to either ground for cathode bias or to the bias supply for fixed bias. And the other side then connects the cathode to either the cathode resistor or to ground. Flip the switch any time.

[Earl]

Didn't Fender make some oddball fixed/cathode bias late 60's transitional amps?

Seems like I read that one of the differences between fullt bypassed cathode bias, and fixed was it's possible to get a the bypass cap to hold a charge under certain overload conditions, offsetting the operating point temporarily/momentarily. I'd expect this would affect to sound when playing full blast.

[Joe Bee]

I think the main difference between fixed and self bias is that the latter will lead to some funny behaviour if the amp is biased very cold. With a big signal there will be significantly more average current through the resistor, so there will be more bias voltage and the gain will drop through the floor. This leads to a lot of compression, probably what Chris describes as "fluid" or "loose".

[Paul P]

Thanks all. I just added both bias circuits and a switch to my design. I think
I'll call my amp the "B&W Amp" for Bells'n'whistles Amp or maybe I should call it
the "EBTKS" :-)

Paul P

[jbefumo]

Check out the 'RMS' control on the old Sundown Amps. They're fixed bias, but the RMS control introduces a variable element of cathode biasing. It's really quite effective, IMO, and makes the response increasingly tactile.

[Amp Kat]

The main difference you'll get out of fixed vs Cathode is efficiency which is what you won't get out of Cathode bias if your Cathode resistor is lower which is what many do because they use the resistor chosen by Fender when these amps were designed like the Champ and Bronco however the voltages have increased since then and the voltage across the Cathode resistor are larger which increases the current draw of the tube. Plate voltage vs the voltage across the cathode resistor are the determining factors of where the bias point is set using cathode bias. The tube can be biased to dissipate 100%+ or less depending on these values so if unaware you could be pushing 110 % and really not know it. Fixed bias allows you to operate the tube at a % pretty much exactly where you want with ease. Not sure which OT you used but the Mercury FTPO-5 is a great choice for a one tube SE design.

[jpfamps]

Didn't Fender make some oddball fixed/cathode bias late 60's transitional amps?

Seems like I read that one of the differences between fullt bypassed cathode bias, and fixed was it's possible to get a the bypass cap to hold a charge under certain overload conditions, offsetting the operating point temporarily/momentarily. I'd expect this would affect to sound when playing full blast.

Yes, Fender did make some fixed/cathode bias amps in the late 60s. Vox also used fixed/cathode biasing in the AC50 from the same era.

Main main gigging amp up until recently was a Fender Pro from 1969 which originally had this circuit.

Both the Fender and VOX amps had individual cathode resistors for each power valve (not sure about the Twin or Showman from this era; I think they had one resistor for each push pull pair.).

I think the idea was to apply a bit of negative feedback to compensate for variable valve characteristics at DC to reduce hum.

Interestingly the Fender circuit has a non-polar 5uF cap between the cathodes of each valve. I think the idea behind this was to allow feedback at DC to reduce hum, whilst tying the cathodes together at AC in order to reduce feedback at AC (well at least during the class A part of the signal), thus maintaining output power.

I'm not sure how good an idea the cap is when the amp is overdriven.

Regardless, the circuit was not well received, and Fender/CBS soon changed it, incorporating a bias balance control which obviously allows hum in the output stage to be eliminated.

Conventional wisdom is that these amps sound "bad" because of the fixed/cathode bias circuit, probably because this is the most obvious circuit change.

When I converted my '69 Pro to the BF circuit I left the 150 ohm cathode bias resistors in situ as the amp sounded great.

[Austin]

I am using fixed bias via battery in a se el84 amp and think it sounds great. It is a very simple and easy to understand way to bias that I learned when I first started. Although I have tried other methods, none seem to be any better sounding to my ears. It is about as simple as you could possibly get.




I just noticed I drew the polarity wrong on the battery in the cathode of the 12ax7, it should be positive on the top so the cathode voltage is raised. Sorry about that.

[EFK]

I took a look at the Sundown schematics and I have to say - neat! However, I don't quite understand how it's working. They look to have a "standard" fixed bias setup, but instead of grounding the cathodes directly, they are run through a 200 ohm 5W pot bypassed by a 1 uF cap. So what is happening to the fixed bias circuit as one winds up the pot from no resistance to 100-200 ohms? It's certainly not a switchable setup, so the cathodes are being elevated above ground while the fixed bias is still connected and supplying negative bias voltage. I'd love to know a little more about this and what - in actual practice - it sounded like...

[Chuck H]

It is about as simple as you could possibly get.

With the possible exception of being limited in design by the 9V bias criteria or the repairs that arise from bias failure when the battery dies. IMHO it's simpler in the design process (because your not designing around a strict 9V bias) and in the long haul (because you don't need to change batteries or deal with the amp losing bias each time the battery gets low) to just build in a bias supply.

JM2C

[jbefumo]

Wow -- just stumbled across this looking for something else -- never received notification of the update -- got to check what email address I'm registered under. In any case, sorry for the delay in responding!

Yes, it's basically a fixed bias amp, and when that "RMS" control is at max, the cathode is effectively connected directly to ground, so the amp develops full power. As you turn the RMS control, you're effectively adding resistance, cutting down on current flow, and introducing a variable amount of cathode bias. Overall, it's sort of like turning the fixed bias more negative, but the reaction is different, since cathode biasing introduces an element of negative feedback, so the net result is not the same as biasing the amp colder. The result is subtle, and more tactile than audio. I use it in almost all of my designs, including cathode-biased ones, in which it has the effect of increasing the value of the bias resistor. (I connect the normal cathode bypass capacitors directly to ground, and run the cathode resistors through the ~250R pot.) Kager's designs have also turned me on to the Baxandall tone stack, which I often combine with Steve Bench's inductor-based mid control.

[jbefumo]

Oh yes, my latest build, the SE-30, utilizes a fixed-bias, single-ended KT-120, with an EF86 driving it (behind a normal 2-stage 12AX7 preamp), and also uses the aforementioned "RMS" control (which I label "touch"). I tuned it for about 27-Watts, and the sound is amazingly complex and quite loud through a pair of 10" Celestions.