6v6 bias with cathodyne

[hpupo]

i am planning on use a cathodyne circuit to drive a pair of 6v6 in pull. Being that i am using a pair of low u triodes (6CG7) as a voltage amp and the actual cathodyne, does it mater why kind of bias i use for the power tubes (cathode or fixed)? If so what advantages and disadvantages, at leas as far as the cathodyne is concerned? what abut sound?

Thanks

[Miles Prower]

Originally Posted by hpupo

i am planning on use a cathodyne circuit to drive a pair of 6v6 in pull. Being that i am using a pair of low u triodes (6CG7) as a voltage amp and the actual cathodyne, does it mater why kind of bias i use for the power tubes (cathode or fixed)? If so what advantages and disadvantages, at leas as far as the cathodyne is concerned? what abut sound?

6V6s aren't a hard load if you operate 'em as pentodes.

The A Number One disadvantage to using a cathodyne as you propose is that it doesn't tolerate very well grid current. The reason the cathodyne stays balanced is that there is just the one current path through the VT. Open another current path (as will happen if the driven Vgk goes positive) and it'll go out of balance.

RC coupling to finals can also lead to problems on overdrive. Turning on the GK parasitic diode will charge the coupling capacitor negative, adding extra unwanted bias that forces operation more towards cutoff. Even if you don't hear the actual clip, this can still cause noticeable sonic degradation until that extra charge leaks off.

It's especially obnoxious when using fixed bias. (Works best if you include a DC coupled grid driver.) In this case (driving the finals directly off the cathodyne) you'd be better off with cathode (auto) bias. What you lose inb terms of linearity, you probably gain back in reduced blocking distortion.

[hpupo]

I probably should have mentioned that before, but this is going to be for a guitar amp, so linearity i less of a concern.
The RC problem you mentioned, is that between the VA that precedes the PI and the PI, or between the PI and the power tube? what is that particular to the cathodyne? Wouldnt that happen also with LTP?

[kevinkr]

Presumably the same thing would happen with an LTP, the only useful comment I have is that LTP are generally preferred for their distortion spectrum in guitar amps amongst other things. It may be that they misbehave in a more "desirable" way when the output tubes draw grid current than the cathodyne does. Pure conjecture on my part, and it's easily checked while you are designing/testing your amp..

[Miles Prower]

Originally Posted by hpupo

The RC problem you mentioned, is that between the VA that precedes the PI and the PI, or between the PI and the power tube? what is that particular to the cathodyne? Wouldnt that happen also with LTP?

If your design is halfway competent, the RC problem should be between the PI and power tubes. You definitely want your finals to clip before any other stage does (unless it's part of an FX subsystem).

The clipping problem isn't particular to the cathodyne, and an LTP driver/splitter would have the same problem. The only difference is that an LTP splitter/driver could source grid current on a transient overdrive. The cathodyne can't.

This kind of blocking distortion isn't wanted in a guitar amp either. It sounds nasty.

[gingertube]

hpupo,
You are on the right track.
One of the best kept secrets for guitar amp, power amp design is to use a cathodyne (split load) phase splitter.
All of the HiFi guys will tell you that the cathodyne works best with with a low mu tube, this is quite correct for a HIFi Power Amp.

Aside:
For equal values of anode and cathode loads Zout at both anode and cathode is approximately = 1/gm
As the loads become unbalanced:
Zout(anode) increases from 1/gm toward RL
Zout(cathode) increases from 1/gm by a maximum factor of RL/u
And that is why the HiFi guys recommend use of a low mu tube.

For a guitar amp you want to emphasize some of the limitations of the cathodyne and you will find that using a high mu tube such as 6SL7, 12AT7, 12AX7 will impart that "warmth" from even harmonic distortion resulting from the asymmetry when over driving.

My own guitar amp runs 6SL7 common cathode amp plus cathodyne splitter into a pair of 6V6.
I mucked about with output tube bias methods, I tried:
- fixed bias
- cathode bias, separate resistors and bypass caps on each tube
- cathode bias , common to both tubescathode resistor with no bypass cap.
The 1st 2 sound very similar.
I finally settled on fixed bias with individual bias adjustment for each tube so I could balance idle currents. That allows low hum with a power supply with only 50uF of HV filtering. That gives a bit of power amp compression due to supply sag when pushed hard (the attack is softened). That suits my mostly folk, blues, vintage rock music taste.

If on the other hand you want to play heavy metal, shreader stuff you will want a stiffer power supply. This sort of sound is developed entirely in the preamp and is best with an almost HiFi'ish Power Amp.

Hope there is something usefull in this "rave".

Cheers,
Ian

[hpupo]

dude, i mean gingertube that was awesome....to tell you the truth i have been looking for that kind of answer for a while (mainly about the "classic" amp the heavy metal part).
I have been reading about the blocking distortion and one thing i dont understand is why is bad if the driver (or the PI in my case) is not able to supply the grid current? this may sound dumb but i just cant see the way this sort of circuit (the imaginary GK diode) would work....

[hpupo]

also, i still dont understand why fixed bias worsens the problem...could someone explain???
thnaks

[gingertube]

hpupo,
Someone has given you bad advice. Fixed bias reduces the problem.

When you drive the amp hard you get high positive signal peaks to the output tube grid. If this peak is high enough such that instantaneous voltage between the grid and the cathode swings above 0V and becomes positive then grid current flows into that GK diode and any capacitor in the current flow circuit charges up (a rectification effect).

That means the coupling caps from the Phase Inverter charge up and if using cathode bias with a bypass cap that cap charges up too. Both these things effectively increase the tube bias and drive the tube toward cutoff. If serious enough it drives the tube into cutoff. This is blocking distortion.

So going fixed bias gets rid of 1/2 the problem - that is fixed bias is LESS susceptible to blocking distortion (since there is no cap in the cathode circuit to charge up).

So then we look at the coupling cap. The amount of charge that cap gets is determined by the circuit impedance. A high Impedance output from the phase splitter limits the amount of current and so limits the problem BUT it also limits the ability of that phase inverter output to bleed away that charge and the amplifiers ability to recover from blocking distortion so we approach this problem in another way.

The trick here is to use a high value grid stop resistor on the output tube grid. That limits the grid current and hence how much the coupling caps charge up. That grid stop resistor also forms a low pass filter with the grid capacitance (Miller Capacitance) of the output tube. For guitar we don't need huge high frequency response so we can use a large resistor here. Use 22K or even 47K (have a look at SY's RLD HiFi Amp he took advantage of this). That reduces the ability of the amp to go into blocking distortion without affecting its ability to recover from blocking distortion if and when it does happen.

I hope I've explained this clearly!!!

Cheers,
Ian

[hpupo]

that's just perfect. thank you sir....