cathode bias confusion

[andrew]

I hope someone could clear this up for me. I have a PP with 6v6s measuring 365 on pins 3/8, 391 on pin 3 to ground, and 25 on pin 3 to ground. the cathode resistor is 450 ohms. I plug these numbers into Weber's bias calculator (the voltage/resistor calculator method) and I get 10 watts per tube and 26ma plate current per tube .
I hooked the tubes to a bias tool and it reads 30ma. The chart that came with the bias tool has a list of plate voltages but does it mean the 8/3 pin readings or 3 to ground?
Should a bias tool be used on cathode biased amps?
These are old Motorolas and are not redplatting. At my voltages what would bias these at?

[martin manning]

Here's the way I see it:
plate voltage Va (pin 3) is 391
cathode voltage Vk (pin 8) is 25
Then, Va-k is 366

Since cathode resistor Rk is 450, cathode current Ik is 25/450 =55.6 mA, or 27.8 mA for each tube
Then plate dissipation Pa (including screen current) is 366 * 0.0278 = 10.17W

The 6V6 is spec'ed at 12W max Pa, so the percent of max is 10.17/12 = 0.847 or 84.7%

This figure is high by 5% or so due to the screen current being included in the calculated cathode current. You can get a better answer by measuring the voltage across the screen resistors and calculating the average screen current, which you would subtract from the Ik calculated above.
It's a little on the high side now, but probably ok, depending on the load impedance. You aren't seeing red-plating, but you might get a bit more life out of the tubes if you reduce the idle current a bit, say 5%.

[andrew]

Thanks Martin. I'll up the resistor to 500 ohms and see what happens.

[andrew]

Using your calculations with a 500 ohm resistor I now get 9 watts and 25ma. plate current which is 75%. Thanks again.

[brewdude]

What size bypass cap are you unsung?

I was under the impression that a cathode biased amp with a smallish bypass cap could be run at near 100% maximum plate dissipation. If using a very large bypass cap it was recommended to run around 70% maximum.

[andrew]

100uf

[martin manning]

There is no reason to run it any hotter than necessary to get a good tone IMO. A trade off against acceptable tube life, or how hot it can be run without immediately destroying the tubes are different questions that depend on several factors including the B+ voltage, how much it sags under load, the load impedance, and the bypass cap.

[pdf64]

My experience is that cathode biased push pull outputs benefit from being run close to class A, in order to avoid the inevitable rise in bias voltage when overdriven causing excessive crossover distortion.
Crossover distortion in cathode biased amps is a tone killer, results in a thin reedy tone.
If the plate voltage is such that excessive dissipation would result in 'near' class A operation, then best to reduce plate voltage somehow or give up on cathode biasing.

To quote from Aiken http://www.aikenamps.com/Biasing.html
'Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.

If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.'

'I was under the impression that a cathode biased amp with a smallish bypass cap could be run at near 100% maximum plate dissipation. If using a very large bypass cap it was recommended to run around 70% maximum'
What difference does the bypass cap value make to dissipation? My experience is that is has to be crazy big (eg >470uF) before it even begins to significantly slow down the ramping up/down of cathode voltage in response to signal level.
Pete

[tubeswell]

What pdf64 said. If they are cathode-biased, then bias them to idle at 100%. It won't do them any harm

[martin manning]

Tubeswell, I don't know how you can make that statement without knowing the B+ and the load impedance.

pdf64, with a bypass cap (and for a given input signal) the lack of negative feedback will cause the power stage to clip sooner, which will affect the time-averaged plate dissipation required.

I'm sticking with bias it no hotter than required to get it to sound good. Why cook the tubes and everything else if you're not getting anything in return?

[guitarmike2107]

suck and see, allot of people won't notice the difference. I run most of my cathode biased amps at 100% and the tubes last years, but if you are happy at 70% then leave it there and the tubes may last decades!

[martin manning]

Recently I had a 1960's 2x 6L6 cathode biased amp in for some work. It was certainly running higher voltages than originally intended, maybe by 40-50V due to higher mains voltages, and sitting right on 100% plate dissipation. Before I installed a larger cathode resistor it would red-plate while idling. Clearly not every cathode biased amp can be biased at 100% without creating problems.

[pdf64]

a bypass cap (and for a given input signal) the lack of negative feedback will cause the power stage to clip sooner, which will affect the time-averaged plate dissipation required

Martin, thanks for that.
I'd be grateful if you could expand on it a little more, or refer me a another thread etc if it's all been done to death before.
For instance, what do you mean by 'clipping sooner' (apart from trivially in that a larger input signal is required when the amp has negative feedback)? Also significance is being put on the value of the bypass cap, which I can't see a reason for.
Pete

[martin manning]

Ok I got to studying this transient behavior with and without a bypass on Rk using a simulation. I'll put a little more time in with it and report back. Interestingly the waveforms (plate current and output voltage) are more distorted (peaks are squashed) if there is no bypass cap (opposite to what I was thinking above), while the distortion shows up at crossover if there is a bypass cap. I don't know if this has been done to death, but I've never looked at it in detail.

[tubeswell]

Very well, what is the load resistance you are using andrew?