Cathode biasing question

[Colas LeGrippa]

Hi buddies 


I have always biased my cathode biased push pull amplifiers at 70% diss and my SE ones at 100%.But lately I read the Rob Robinette's pages and I was a bit confused when reading that a P-P amp could be biased at 100 %, if cathode biased. Many points of view wherever I look.
One thing is sure: my 6550's draw 108mA each at 355V ( plate to K ) and seem happy. I don't want to rely on that though, sometimes my wife seems happy but she feels hell inside....


Colas

[PRR]

Not pushing 99% is like never pressing your car gas-pedal all the way down. Why did you buy a big engine and not USE it?

However 6550 is such a big "engine" that maybe 70% is plenty.

[Colas LeGrippa]

I put in a pair of 6v6 biased at 100% like I did with the 6550's but they lightly red plated so I raised the value of the K resistor by 50 ohm to keep them from self destruction. I am wondering why my 6550's were happy at 108A @ 356 V.....screen current is aprox 5 mA so let s figure out that the plates were drawing 103 mA,  we re over the max diss of 35w, without redplating....uff.....got it ?

[pdf64]

Redplating doesn't tend to be apparent until the plate is dissipating way over its design center / design max / even absolute max limiting value.

Hence the absence of redplating should not be regarded as an indication that the idle point is ok.

But the concept of tweaking bias to a plate dissipation target is invalid; plate dissipation is just a limit, some fraction or percentage of the limit doesn't don't constitute a valid operating point. The 70% upper limit for fixed bias AB amps is put forward purely to avoid the tube overdissipating, it's not suggested as a target http://www.aikenamps.com/index.php/idle-current-biasing-why-70-percent

Unless just copying the operating conditions suggested in tube manufacturer's info, there's no substitute for doing the job properly, using load lines etc, see http://www.valvewizard.co.uk/se.html
Doing the hard work will help to give some insight into tube operation and will tend to yield rather different operating points for 6V6 and 6550 

[Colas LeGrippa]

I played a couple of weeks witj a 6v6 push pull amp that i had built long time ago. This amp had an incredible tone and i shoul have drawn the schematics  at that time...
Looking at the baclk of the amp I saw that both 6v6 's were redplating , obviously causing such a beautiful tone. I changed the circuit    to keep the 6v6 colder but lost the magic of the beast


Colas

[Colas LeGrippa]

Pdf64, a tube has got to work hard and give us all the guts he s got. Biasing a tube cold enough to last a life time is possible. Fine if you prefer a sterile tone. I prefer changing the tubes once a year and getting the best tone a tube can offer.
For me it is like replacing my guitar strings


 


Colas

[jjasilli]

This thread has made me realize that I don't understand biasing, and have to relearn it (assuming I can unlearn what I thought I knew).

[Colas LeGrippa]

I thought I knew my wife until she ran away witn another man.....I'll never understand an amplifier deeply from a to z. Same thing for a woman.
I am a guitarist not a scientific  !

[pdf64]

...both 6v6 's were redplating , obviously causing such a beautiful tone. I changed the circuit    to keep the 6v6 colder but lost the magic...

...a tube has got to work hard and give us all the guts he s got. Biasing a tube cold enough to last a life time is possible. Fine if you prefer a sterile tone...

The above cause and effect analysis is incorrect, and has led you to fall under a misapprehension.
Tube dissipation is not a characteristic or an operational condition.
It is just a limiting value.
Tube characteristics do not change according to plate dissipation.
Rather, as bias voltage is decreased, plate current increases and this causes stage gain and (for an AB amp) conduction angle to increase; it's plate current that matters here, please see any tube info, where it is an operating criteria that is well documented. Also, for cathode self bias, the dynamic change of bias with signal level (described as 'squish' by Aiken*) decreases as the cathode resistor value (and, coincidentally, conduction angle) decreases.
To achieve the preferred plate current operating point but without overdissipating the plates, I suggest to reduce amp's HT voltage. That would be done most simply by replacing the PT to one that had a lower voltage HT winding. Then actually design the amp (rather than just assembling parts and experimenting) using the tube load lines, as per the previously linked Merlin page.
If it is necessary to maintain the power output provided by the previous, higher HT voltage, then it may be necessary to add another pair of 6V6 or move to a beefier power tube.
* http://www.aikenamps.com/index.php/what-is-sag

Alternatively, if you're getting the sound you want and a years hard usage out of redplating 6V6, then as long as your supply of tough 6V6 continues to hold up, then it seems reasonable to leave things as they are.

[jjasilli]

I'm still trying to get my head around plate diss, now realizing that as power amp current draw increases, B+ supply voltage tends to drop, so plate diss might be more or less constant, depending upon the actual circuit in use.  Not to mention that pentode plate voltage is less important than screen voltage.


Still it seems that that the Colas' quoted statements are correct.  The setting of the bias point will affect current draw which will have an ultimate effect on power tube performance -- components exterior to the tube will be affected, such as PT secondary, OT primary and any cathode R.

[pdf64]

...The setting of the bias point will affect current draw which will have an ultimate effect on power tube performance...

Of course but reducing the value of the cathode bias resistor to achieve the desired plate current only need result in overdissipation if the HT is excessive.
My point is that overdissipation is unrelated to all the factors that are probably causing the tone to improve.
Cathode bias amps need a lower HT than is typically used for fixed bias amps with the same tube compliment.
Hence when a cathode/fixed bias switch is added to a fixed bias amp, generally the tubes either overdissipate or the tone goes sound thin and reedy (due to excessive squish / bias shift) when in the cathode bias mode; it ends up next to impossible to find a suitable operating point for cathode bias if the starting point (HT too high) is wrong.

[Colas LeGrippa]

Very interesting stuff thanks

[Colas LeGrippa]

...but tell me have you  built or tweaked amplifiers for decades ( like many of us in this forum) or are you just a theory guy ? As you may know sometings there is a world between books and reality
Thanx again for nice info.
 Colas

[Colas LeGrippa]

...sorry please replace the word something by sometimes in my lasr post

[pdf64]

I've been tweaking, repairing and building tube amps for almost 4 decades, nearly as long as I've been trying to play guitar 
I studied EE at college as I'm a geek and like to try and find out how things work 

[shooter]

it ends up next to impossible to find a suitable operating point for cathode bias

Yups, PRR has about the best method for "close enough", you basically know plate vdc, and OT impedance before you apply power. ohms law gets you I and R.  So now you have your start.  After many Rk swaps, I usually wind up within a 100ohms of math.  (SE amps), PP fixed bias, steal what someone else has proven, save money, time and sanity

[PRR]

> as power amp current draw increases, B+ supply voltage tends to drop, so plate diss might be more or less constant, depending upon the actual circuit in use.

Basic Series circuit. You "should" understand how things vary in a series circuit.

For one thing, it is THE reason your lights dim when you turn on big heaters, or the bassist hits a big boom on a transistor amp.

The condition you are interested in is not "specific" but general. There's a fixed voltage source. There is some resistance in series with the source. Then there is the load. The tube is a variable resistor. You "change bias" and that really changes the tube's effective resistance.

However, many folks here have an animosity to figuring stuff out, and it is a lovely above-freezing day, so the heck with it.