Fixed Bias together with cathode bias?

[Balticnoiseforge]

This may be a stupid question, but lets just say that the poweramp would need -30vdc.

Is it possible to apply 15vdc from a fixed bias circuit but at the same time having cathode bias resistor / bypass cap for the rest?

Or would that behave weird since cathode aint grounded?

[SILVERGUN]

This may be a stupid question, but lets just say that the poweramp would need -30vdc.

Is it possible to apply 15vdc from a fixed bias circuit but at the same time having cathode bias resistor / bypass cap for the rest?

Or would that behave weird since cathode aint grounded?

In cathode bias, the cathode still has a path to ground, it just goes through a resistor. That resistor acts as a current limiting device. If you apply plate voltage and connect the cathode to ground with no resistor and no negative bias voltage there is nothing to stop the tube from passing max current and burning up.

Yes, it is possible to have a mixed bias circuit.
why?

(for simplicity I'm assuming you mean -15vdc)
all punctuation matters

[Balticnoiseforge]

In cathode bias, the cathode still has a path to ground, it just goes through a resistor. That resistor acts as a current limiting device. If you apply plate voltage and connect the cathode to ground with no resistor and no negative bias voltage there is nothing to stop the tube from passing max current and burning up.

Yes, it is possible to have a mixed bias circuit.
why?

(for simplicity I'm assuming you mean -15vdc)
all punctuation matters

Ah yes ment -15vdc, but then it is possible.
Is there any amplifiers that uses it?

And is there something else to keep in mind if doing something like that? Etc how many % of it should be cathode biased? Is there a lower limit - like i can imagine.. lets say if you only have 5% cathode bias it could get damaged pretty quick during on/off and such?

{edit-- fixed quote-tags --- PRR}

[sluckey]

I've seen several amps that use both cathode and fixed bias. I have this one...

     http://sluckeyamps.com/magnatone/Magnatone_M10A.pdf

[dude]

I set up a 6V6 Plexi for cathode or fixed bias, each setting has a totally different tone.  In cathode bias the Plexi probably isn't a Plexi anymore but has a lot of sag and gritty tone, great for blues. IMO, having a switch for cathode or fixed bias is a decent mod worth trying at least once...  

[pdf64]

Here’s some mixed bias amps
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab568.pdf

https://el34world.com/charts/Schematics/files/Vox/Vox_ac50_2.pdf

https://www.thegearpage.net/board/index.php?attachments/dsl401-dl40-60-02-pg1-of-2-jpg.656223/

[sluckey]

The cathode resistor on that DSL401 is only 10Ω. Not gonna contribute much to the bias. I'd rather call it a current sense resistor.

[pdf64]

The cathode resistor on that DSL401 is only 10Ω. Not gonna contribute much to the bias. I'd rather call it a current sense resistor.

Bear in mind the high sensitivity of EL84, that the 10ohm resistor is shared by 4 of them, and they don’t need much bias voltage.
eg the AC30 only uses a 50ohm shared bias resistor.
Given the above, I think it’s reasonable to regard that resistor as being integral to setting the output stage operating point, probably adding somewhere between 0.5V and 1.2V to the bias, perhaps 5-10% of the total. So if it was shorted out, the valves would idle a fair bit hotter.
Hence its role in circuit operation is more than just that of a current sensing element.

However, it probably doesn’t facilitate much in the way of self bias regulation, eg if one of the EL84s decides to run hot, the resulting increase in overall bias voltage provided by the resistor won’t help much in correcting that.

[Williamblake]

I set up a 6V6 Plexi for cathode or fixed bias, each setting has a totally different tone.  In cathode bias the Plexi probably isn't a Plexi anymore but has a lot of sag and gritty tone, great for blues. IMO, having a switch for cathode or fixed bias is a decent mod worth trying at least once...

Do you happen to have a schematic to that switching of yours?

[Balticnoiseforge]

Thanks PDF and Sluckey for sharing!

Definently something i will try for this experimental 6v6 build!

[sluckey]

I set up a 6V6 Plexi for cathode or fixed bias, each setting has a totally different tone.  In cathode bias the Plexi probably isn't a Plexi anymore but has a lot of sag and gritty tone, great for blues. IMO, having a switch for cathode or fixed bias is a decent mod worth trying at least once...

Do you happen to have a schematic to that switching of yours?

Here's a circuit I have used...

     http://sluckeyamps.com/november/november.pdf

[HotBluePlates]

... how many % of it should be cathode biased? ...

Cathode-bias is self-correcting, especially when we try to play very loud.

What % of "self-correction" do you want?
0%? ("fixed-bias")
100%? (cathode-bias only)
Something in-between?

... Is there a lower limit - like i can imagine.. lets say if you only have 5% cathode bias it could get damaged pretty quick during on/off and such?

"0%" is fixed-bias.  Fixed-bias is not self-correcting, and risks tearing itself up.
Except there are a lot of fixed-bias amps, and we struggle to get far beyond Class A power unless we use fixed-bias.

Use as-much or as-little cathode bias as you like.  Assuming the cathode resistor is chosen properly & the tube functions within typical limits, cathode bias is safer but delivers less power output.  Fixed-bias is free to tear itself up (it's "less safe") but delivers more output power given the right operating conditions.

[Williamblake]

I set up a 6V6 Plexi for cathode or fixed bias, each setting has a totally different tone.  In cathode bias the Plexi probably isn't a Plexi anymore but has a lot of sag and gritty tone, great for blues. IMO, having a switch for cathode or fixed bias is a decent mod worth trying at least once...

Do you happen to have a schematic to that switching of yours?

Here's a circuit I have used...

     http://sluckeyamps.com/november/november.pdf

I knew, this question was worth asking but i didn't expect a switch that simple.

[PRR]

I knew, this question was worth asking but i didn't expect a switch that simple.

Note that this scheme requires that point D be a source which can be shorted to ground all night.

The way Steve has it, the 100k and 47k can sit shorted forever. There's other bias supplies around 50VAC taps and 470r resistors which would smoke.

[sluckey]

The way Steve has it, the 100k and 47k can sit shorted forever. There's other bias supplies around 50VAC taps and 470r resistors which would smoke.

That's true, but there's an easy fix. A DPDT switch allows you to use this circuit with any bias supply. Like this...

[pdf64]

If considering implementing a switch to select between cathode or fixed bias, by all means go for it, everyone has their own perception of what’s tonally pleasing.
My finding is that a HT voltage and load impedance intended for class AB fixed bias is unlikely to give useful results in cathode bias.
All else being equal, compared to fixed bias, cathode bias may have perhaps twice the bias shift when overdriven, and to me, the tone then gets ratty and thin.
That can be counteracted fairly easily to some degree by doubling the load impedance, which should tend to increase the conduction angle, limit peak current and so reduce the increase in cathode voltage with signal.
Alternatively / also, if using a valve rectifier, a different, lossier type could be substituted to bring the HT voltage down, to help the output valves idle at a happier place.
Other mitigation might include reducing the HT voltage to the phase splitter, in order to reduce the max signal level to the output valve control grids, and so reduce their bias shift.
So I think it can be made to work well and give good results in both modes, just be aware it may take more than just a flick of that switch.

[sluckey]

I put that bias switch in my "November" amp back in 2007. Back then I was experimenting a lot with stuff I read about on the net. I never heard much difference between fixed and cathode bias in that amp and never bothered to try it in any later builds. Not worth the effort IMO.

[dude]

Thanks PDF and Sluckey for sharing!

Definently something i will try for this experimental 6v6 build!

Glad Sluckey posted his schematic of the cathode to fixed bias. I used his schematic and works well, I used a 250 ohm R and 100uf cap (50v), for 6V6's
Oh, I used the DPDT switch as in Sluckey's second schematic he posted

[shooter]

I never heard much difference between fixed and cathode bias in that amp

i've been thinking of changing my 6v6 PP from self to fixed bias.  the amp is a dedicated Bass driver amp, (5" 50W speaker).
the brain says "If I can double the Voltage swing on the primary side, I'll step-up the current on the secondary"
also says "Bass wants big current to move big air"


any thoughts on actually accomplishing enough Umpf at the speaker to be worth the effort?


thanks
dave

[HotBluePlates]

I never heard much difference between fixed and cathode bias in that amp

i've been thinking of changing my 6v6 PP from self to fixed bias.  the amp is a dedicated Bass driver amp, (5" 50W speaker).
the brain says "If I can double the Voltage swing on the primary side, I'll step-up the current on the secondary"
also says "Bass wants big current to move big air"


any thoughts on actually accomplishing enough Umpf at the speaker to be worth the effort?

This seems like a different thread, and needs to have the existing B+ voltage, bias voltage and OT primary impedance noted.

[shooter]

 
..on  sone other site


I'm, just togglin the switch in reply #14 trying to hears a bass boost on my boom-box

[pdf64]

…
I'm, just togglin the switch in reply #14 trying to hears a bass boost on my boom-box

I think whether any difference is apparent may be affected more by other stuff than fixed or cathode bias, eg respective  operating conditions, open loop or NFB around power amp.

[SILVERGUN]

just install one of these and you're good to go

[shooter]

   Thanks SG, I knew you'd have the no math answer


guess i'll just leave well enough alone.
although I can just 1/2 my secondary with another BASS speaker, tubes be damn'd 

[Balticnoiseforge]

Yes plan is to try reach something close to 50-50 Fixed bias / cathode bias, or atleast over 50% cathode bias while delivering the rest through fixed bias.

[HotBluePlates]

Yes plan is to try reach something close to 50-50 Fixed bias / cathode bias, or atleast over 50% cathode bias while delivering the rest through fixed bias.

1.  Make sure the output tube plates & screen voltage is not "too high" (closer to amounts seen in cathode-biased amps than in fixed-biased amps).

2.  Figure out the desired idle current at the screen/plate voltage used.

3.  Figure out bias voltage required (from data sheet curves) to get the desired idle current at the screen voltage present.

4.  When the cathode resistor is shared by 2 tubes:  Bias Volts / Idle Current = Cathode Resistor Value (for half of bias from the resistor)

5.  Add a fixed bias supply to provide the other-half of the target bias voltage.