Combination cathode bias and fixed bias

[hasserl]

The best and easiest way to PA distortion is electronic power scaling.

-g

That's a subjective statement, not objective. It's your opinion.

[stokes]

Tubeswell,I really think you are over complicating this.Get a couple of different value 5-10watt resistors or a suitable adjustable 500 ohm wirewound and stick it between the cathodes and ground,bias it up like the cathode resistors arent there-when you get it to sound like you want,you are there.I found the by-pass cap had very little effect so I didnt use one.

[mooreamps]

That's a subjective statement, not objective. It's your opinion.

Well, it was the easiest way for me to do it.

-g

[tubeswell]

Thanks for al the sagely advice everyone

I have got a variety of 5W and 10W cement and wirewound resistors of different values. I'll try them out this W/E starting with partial fixed/cathode bias, and then full cathode bias, and record the differences and ruminate on them.

Cheers

[hasserl]

Well, it was the easiest way for me to do it.

-g

It can't be easier than just turning up the volume control.

[tubeswell]

Making the bias switchable

BTW what would be the simplest way of putting in a switchable cathode bias/fixed bias?

Would I use a DPDT switch with one of the poles shorting the bias variable ground resistor setup to ground whilst the opposite pole on the same throw opened a low wattage 470R resistor wired in parallel with each of the 10W 470R cathode resistors (assuming separate cathode resistors on each side, or is it better to use a common 5W 250R resistor that can be switched in parallel with a low wattage 250R ?), that otherwise on the opposite throw, when when the resistors were in parallel, reduced the cathode resistance to 1R, commensurate with the fixed bias ground resistor being reinstated by lifting the aforementioned shorting ground? (if you can get your head around that?)

Also I presume there is no danger in having a switch like this and instantarily switching it with the amp under full power as the amp simply ceases to work with the cathode lifted from the ground, and the volatges and current at the cathode are small (or, for some reason that I might be missing, could it damage the OT and would I need to switch the amp off first?)

[hasserl]

You just need to do something like this:


credit to Kevin Oconnor for the original drawing which I just modified

[tubeswell]

THanks Hasserl

Will the 1R 1/2W resistors hack the current under Cathode bias? (I thought the current was depdnet on eth voltage drop, so that if you added 250R then you would also increase teh current throught the 1R, or doesn't it work that way?

[hasserl]

Re the 1/2 watt resistors. I suppose those are just there to measure the bias when in fixed bias mode. The power rating of 1/2 watt means they will dissipate 1/2 watt of power constantly, though it's common practice to use resistors rated at twice the expected duty cycle as a measure of safety. At just 1 ohm these resistors will only be dropping the voltage in the milliap range, regardless of how hot the tubes are run. Half a watt is more than enough. You couldn't flow enough current thru those tubes to exceed the rating with just 1 ohm. The reason you need higher wattage for the 250 ohm resistor is because it will be dropping a lot more voltage across it. Power is a function of volts time current, drop a little voltage and you only need a low power resistor, drop more voltage and you need a more powerful resistor.

Hope that makes sense.

[tubeswell]

Re the 1/2 watt resistors. I suppose those are just there to measure the bias when in fixed bias mode. The power rating of 1/2 watt means they will dissipate 1/2 watt of power constantly, though it's common practice to use resistors rated at twice the expected duty cycle as a measure of safety. At just 1 ohm these resistors will only be dropping the voltage in the milliap range, regardless of how hot the tubes are run. Half a watt is more than enough. You couldn't flow enough current thru those tubes to exceed the rating with just 1 ohm. The reason you need higher wattage for the 250 ohm resistor is because it will be dropping a lot more voltage across it. Power is a function of volts time current, drop a little voltage and you only need a low power resistor, drop more voltage and you need a more powerful resistor.

Hope that makes sense.

Yep thanks hasserl

I guessed that the 1R were there for fixed bias measuring purposes. And I intuitively thought that smaller value resistor dropping less volts would dissipate less power in the form of heat, but I was confused by thinking that total current is what is obtained from the voltage drop across the entire circuit, and therefore the current would be higher for the 1R resistors when they were in series with 250R.

But now that I work this out I see (say for one tube) 17V/471R = .036A, or (similarly for two tubes) 9V/251R = .035mA, therefore you would get .035-.036mA flowing through the 1R (and .036A * .036V = .001W). Okay then, I should have done my sums first. ;-)

[BackwardsBoB]

I see your tremolo problem -

Tubeswell wrote a while ago:

The only question that remains not quite full answered in my view is how changing the circuit to a part cathode-biased circuit would affect the trem in this amp.

Or is the trem only taking the input from the bottom of the output tube grid resistors (which happen to be negatively biased, but which probably benefits the trem anyhow because it puts the trem input at a different ground voltage potential to the rest of the grounds in the circuit?)

Or does having the output stage partially cathode biased (and hence lowering the negative grid bias to compensate for the right tube current) therefore reduce the signal going into the oscillator circuit thereby having some sort of impact on the first oscillatr triode and thereby changing the intensity of the screens switching on and off (possibly meaning that I might need to lower the 1M resistor in series with the depth pot ground to something like 470k to keep a stronger signal in the first oscillatro triode)?

Your understanding of the tremolo circuit is backwards. The screen connection of the tremolo circuit is the power supply of the circuit. the connection to the depth pot is the output. the tube on the right-hand side of the tremolo circuit is a cathode follower. The tremolo circuit is just an oscillator. It doesn't require any signal input.

[tubeswell]

Your understanding of the tremolo circuit is backwards. The screen connection of the tremolo circuit is the power supply of the circuit. the connection to the depth pot is the output. the tube on the right-hand side of the tremolo circuit is a cathode follower. The tremolo circuit is just an oscillator. It doesn't require any signal input.

Thanks Bob I thought the oscialltor output was the output tube screens and that the trem operated by shutting the output tubes on and off. How nuts was that? I see now that the oscillator of course would work better (and it would be better for the tubes) by altering the output tube signal grid. Duh!

[mooreamps]

You just need to do something like this:


credit to Kevin Oconnor for the original drawing which I just modified

With all due respect to Kevin, this is the hard way to do it. If I were to do it, looking at this print, I would just simply throw away the switch on the cathode. Then, what you would have left is "combination bias". Switch in for Class A/B, and switch out for class A.



-g

[mooreamps]

It can't be easier than just turning up the volume control.

It is be....cause electronic power scaling allows you to have the over-driven tones at "lower" volumes. . . . . Also, one side benefit is lowering PA plate current which lowers PA power tube plate dissipation, something an external attenuator does not do.

-g

[hasserl]

It is be....cause electronic power scaling allows you to have the over-driven tones at "lower" volumes. . . . . Also, one side benefit is lowering PA plate current which lowers PA power tube plate dissipation, something an external attenuator does not do.

-g

Ahh, but you're describing your perceived benefits of PS, not it's ease of design/installation.