Does 70%(Wattage/Voltage)= Plate current or Cathode current(mV@1ohm resistor)?

[jeff]

If I'm biasing EL34s at 450V I want to set idle current to 38.8mA
70%(25/450)= .0388
 
So do I use a 1ohm resistor from cath to ground and measure 38.8mV across it? Or do I need to go higher to take the screen current into account?


In other words concider these two scenerios


1) If I put my current meter between OT and plate I read 34mA. I read 4.77V across the 1K screen resistor meaning the screen is drawing 4.77mA. So this would give me a reading of 38.77mV across a 1ohm resistor. In this case my PLATE current would be 34mA 61%(Watts/Volts) and my TOTAL current would be 70%(Watts/Volts)


2) I put my meter between OT and plate and bias to read 38.8mA. There's 4.77mA screen current so now my 1 ohm resistor reads 43.57mV. So that means the PLATE current is at 70%(Watts/Volts) but the TOTAL current is 78.4%(Watts/Volts)
 
So that's my question: Does 70%(Watts/Volts)= Plate current or Total current(reading at 1ohm resistor reading)
Do I bias to get 70% at the plate or at the cathode?

[sluckey]

Does 70%(Watts/Volts)= Plate current or Total current(1 ohm resistor reading)

It equals Plate current if you are using plate power and plate (to cathode) voltage in that formula.

Think about it... Where did you get the number for Watts in that equation? Probably from a tube data sheet. That number of watts is usually called P_AMax which means Maximum Anode (plate) Power. This is also called maximum plate static dissipation. So, if you want plate power, you must use plate current. You could put a 1Ω resistor between the tube plate and the OT plate lead and measure the voltage directly across that resistor, just like you do with a 1Ω cathode resistor.

 

[jeff]

Ok
just took readings
Plate current 33mA (***EDIT originally wrote 66mA but forgot to divide for 2 tubes)
Screen current 4.7ma (4.7V across 1K screen resistor)
Plate voltage 437V


What's confusing me is those bias probes. I thought that all they were are male to female sockets where all pins are connected(1to1,2to2etc.) but with a 1ohm in series between pin 8. Basically a 1ohm cath to ground going to two leads. Well to use it, they tell you to use the formula .7(Watt/Volt). That's what was cooking my noodle. Do those probes actually take plate current? 450V on leads??? Or cath current, and using them plus their formula leads to colder bias?


I just googled bias probe and that's what it looks like, a 1ohm resistor cathode to ground. So if this is the case and they are giving you the formula .7(watt/voltage) and assuming that 10% of that reading is actually screen current and, in fact, aiming for 60% plate dissapation?

[sluckey]

Not all bias checkers are the same. Hoffman's checker doesn't use a resistor. He just brings both pins 8 out on test leads and you set your meter to really measure current, not a mV voltage across a 1Ω resistor.

But, all the bias checkers schematics that I've seen are measuring current (or mV) on the cathode. So, the reading you get is always a bit higher than the actual plate current. This causes your calculations to be off a bit, but they err on the side of caution. Your actual plate power will always be a bit lower (cooler) than the calculated power. Less chance of accidentally red plating.

I've always been happy using cathode current in my calculations. I install 1Ω resistors on the cathodes inside all my amps just to make it easy to check. If I ever need to have gnat's ass precision, I'd put a 1Ω resistor on the plate.

[jeff]

Added something to my post(above) as you were posting.


So let me get this straight, when you bias your amp with a 1ohm resistor you use .7(watt/volt)= mV on resistor and call it a day?

[HotBluePlates]

What's confusing me is those bias probes. ... Do those probes actually take plate current? 450V on leads??? Or cath current, and using them plus their formula leads to colder bias?

I'm with Sluckey on this.  I'm not bothered by the inclusion of screen current in cathode current when biasing.

FWIW, I don't bias amps much, even when I built them to have external pin jacks and trim pot adjustment.  I just play the amps as long as there's not obvious overheating.  When I was first starting out I was much more concerned about "perfect bias" than I am now.  My opinion now is you have to get bias very-wrong to have any real impact on the amp's sound, and overheating in use is the biggest issue that bias resolves.  However, I do want a bias supply to function flawlessly.

The recent exception was a vintage amp I acquired recently.  I did check it to assess its overall health, and the 6V6's were running a little hot (B+ was high due to 125vac at the wall instead of 110vac).  The amp has non-adjustable fixed bias, and I brought it to a happy place simply by running it on a variac delivering 110vac.  I then selected a pair of 6V6's from among my stash that idled about-right, rather than modify the bias board.

For this amp, I did use the OT shunt method of plate current measurement because there were no already-installed 1Ω resistors.  Process was power off, clip meter leads in place (looking to see the insulated jaws to prevent contact with adjacent socket pins), power on, read current, power off, move meter lead to other tube, repeat.

I'm not saying anyone else should go that route, but the amp does sound (at least a little) better with 110vac on the power cord, B+ voltages are landing where they should be according to the schematic, and the tubes are running happily.

[jeff]

Ok, that's what I'm doing too because the amps construstion pervents me from using 1ohm resistors. In the past I've always used them but being forced to measure actual plate current got me thinking about this.


 I'm looking at a chart that lists voltages and currents. It shows 70% as hot, 60% as average and 50% as cool. Doing the math, this 70,60,50% of actual Plate Watt Dissapation. So this seems to be saying 60% is average. So that, seem to me, to jive with using a 1ohm resistor and reading 70%(Watt/Voltage) because by reading 70% at the cathode the plate is really at 60% dissapation(assuming ~10% of that reading is screen current)


Does that make sense? Or am I confused?

[sluckey]

(assuming ~10% of that reading is screen current)

That's a mighty broad assumption! Who said that? Someone selling bias checkers? 

[HotBluePlates]

I'm looking at a chart that lists voltages and currents. It shows 70% as hot, 60% as average and 50% as cool. Doing the math, this 70,60,50% of actual Plate Watt Dissapation. So this seems to be saying 60% is average. So that, seem to me, to jive with using a 1ohm resistor and reading 70%(Watt/Voltage) because by reading 70% at the cathode the plate is really at 60% dissapation(assuming ~10% of that reading is screen current)

Does that make sense? Or am I confused?

Maybe.  It depends on the output tube used and how much screen current those have at idle.  Screen current for 6L6, 6V6, 6550, KT66 tends to be small compared to EL34, EL84.

As for % dissipation at idle... It depends.  For Class A you want to be somewhere near 100% dissipation at idle because any less equals less output power (though the tubes might last marginally longer).

For Class AB, the two important factors are keeping it from overheating at max output power, and getting maximum clean output power.  There's a wide range between 100% dissipation (Class A) and 0% dissipation (Class B) which might optimize the two factors.

The only way you really know is by running the amp under load and adjusting for that max output power condition, because plate current rise from idle to max output power could vary amp-to-amp based on OT primary impedance and supply voltage.

But if you're not looking for every last part-watt of clean power (I like output tube distortion), then maybe you have a wide range of "set it so the tubes don't redplate in use".  And that's where I've wound up with my level of concern over bias.

[jeff]

(assuming ~10% of that reading is screen current)

That's a mighty broad assumption! Who said that? Someone selling bias checkers? 

I'm assuming because I'm struggling to resolve a conflict in my mind between two pieces of information. One, a chart that seems to indicate that 60% plate dissapation is 'average' and Two, a 1ohm resistor cath to ground should read 70%(Watts/Voltage) (probe)


Let's assume this condition:
Plate current 34.5mA
Screen current 4.7mA
Plate voltage 437V


Now by taking the actual Plate Dis.
.0345 X 437 =
15.08 is 60% of the EL34s 25watt rating.   Good there.


Now reading across 1ohm resistor we would read
.0345mV + .0047mV= .0392mV
using our formula .7(watt/volt) we get
.7(25/437)=.040
which in nearly the 39.2mA Good there.


So these two things seem to agree. 60% actual Plate dis ~= to 70% total dis.


My understanding, up to this point has been Bias should be set to achieve 70% Plate disapation. But I'm starting to think that the '70% of plate disapation' rule of thumb is to be used with the 1ohm method of current monitoring to arive at an actual 60% plate disapation.


It's the only way I can make sense of:
       1)bias probe = 70%(watt/volt)
and 2) 60% of Actual Plate Dis = 'average'
 

[sluckey]

We've come full circle.

[HotBluePlates]

... I'm struggling to resolve a conflict ... between ... a chart that seems to indicate that 60% plate dissapation is 'average' and ... should read 70% ...

FWIW, I never saw/heard "70% dissipation" was a "rule" until the early 2000's (maybe 2003, 2004?).

Before that, Gerald Weber had a "rule" of "bias to 35mA".  But that doesn't account for plate voltage or even what tube type you're biasing.  Funny enough, Weber also wrote around all that to "bias by ear" and "people always ask for a specific number, so here's a number..."

I think 70% is just an arbitrary "rule".  I bet the 60% on the chart you mentioned is just as arbitrary.  I think the logic behind "idle at 70%" is that for a sine wave, 70% (more precisely, 0.7071 or √2/2) is an average value between 100% and 0%.

I don't think there will be much to be learned in trying very hard to reconcile one arbitrary rule with another arbitrary rule.  The most important thing is that at no time during idle or playing up to maximum output power do the output tubes redplate.  Whatever gets you there while delivering the performance you want is fair game.

[jeff]

Ok thanks for the help. I guess I'm just worried that I just fixed this amp and I'm gonna wreck it by biasing it wrong. I feel better now. I get what you guys are saying about there being a range of bias settings you can safely operate and I'm pretty comfortable that I'm within that range. I'm still unsure if I'm in the center of that range or on the cool side of that range but I feel good now that I'm in the safe operation range. 


It's a 100watt amp so I haven't gone full tilt with it yet but plates show no hint of red.


Here are my actual measurements:
Plate voltage:   438V
Plate current:  34.5mA
Screen current: 4.7mA
(Plt + Scrn = 39.2mA)


Thanks again for your time and help, you guys rock
    Jeff

[Bluemeany]

Bias towards a lower anode dissipation (if you want a number go for 50%) - it'll lower the stress on the power transformer, output transformer & power valves (meaning all three will live longer). It'll also reduce the overall operating temperature inside your amp when playing long sessions (your capacitors will last longer).

In an A/B with 2 identical amplifiers with one biased at 50% & one biased at 70% your ear won't be able to tell the difference - you can measure a slight increase in crossover distortion on test gear but it's negligible at the volume/distortion levels associated with guitar amplification.

Although I've never measured it I have been told that varying the bias does have an effect on output impedance - the colder the bias the higher the output impedance. This will slightly affect the power amplifiers bandwidth and frequency response when used with an inductive load (in theory you'll get a more "resonant" response). I have, however, never tested or A/Bed this myself by ear or test equipment.

[HotBluePlates]

... I have been told that varying the bias does have an effect on output impedance - the colder the bias the higher the output impedance. ...

"It don't work like that."

Internal plate resistance of a pentode or beam power tube is not really a consideration for output stages.  In all cases, the tube's internal plate resistance is many times the OT primary impedance.

When designing triode output stages, the triode's internal plate resistance matters because a load of 2x plate resistance typically produces the greatest output power for a given distortion %.

[PRR]

> worried that ...I'm gonna wreck it

Too-HOT is bad for the amp. In a fixed-bias amp, 90% of plate rating is pretty hot, try to stay below that.

Too-cold is fine for the amp. On some amps, cold means rough sound at low volume. Thousands of amps have been run mighty cool and nobody complained.

I had a Traynor 7189 BassMate idling at 4W per 12W tube. Not A Problem. Didn't try to "fix" it when I re-tubed.

OTOH I had a Fisher hi-fi 7189. Target idle was 18mA (about 8W/plate). It sounded audibly better at 40mA.... but not much better, and this was 16W per 12W plate, so I backed it down.

It may be a small factor in the complex sound of a 5F6. But I don't think Leo worried about it.

"70% Pdiss" seems to be "a number" which works fine in many cases. There is NO exactness here.

Here's your beer analogy. Say you have a beer tap in your man-cave.Being semi-civilized, you fill a glass and drink from that. If you fill your glass to 150%, 1/3rd of your beer is on the floor. If you fill your glass to 100%, it dribbles. If you fill your glass to 90%, it sloshes when you sneeze. OTOH if you fill your glass to just 25%, you make too many trips back to the tap. 70% is a comfortable level in the glass. But 60% or 80% may be just fine also.

[Sonny ReVerb]

That answer made me thirsty! Cheers!