Biasing by Percentage of Maximum Plate Dissipation

[nuke]

To be honest, I went and double checked my two Fluke meters (87-V and a 289). Both are under 2-ohms for the milli-amp range, and well under 1-ohm for the ampere range and the ampere range is sufficiently resolved for 10's of milliamps on both of them. The error would be minimal, even in the Marshall example.

For instance, 0.83-ohms shunt (measured value on the worse of the two I have), paralleled with 15 ohms, works out to a total shunt of .0.7865 ohms, vs 0.83 ohms, which is ~5% basically.

It really depends how good your meter is. I think 6-ohms is a hang of a lot of shunt resistance for anything that calls it self a test instrument.

[nuke]

double post

[Lauri]

I was bored so I checked to what percentage of maximum plate dissipation Fender biased some of their classic amps. Assuming voltages on schematics are correct and tubes used are close to what datasheet says they should be.

Code: Select all

FENDER:
AMP     MODEL   Pa      TUBE    %PaMax
BASSMAN 5F6A    14W     5881    60%
BASSMAN 6G6     9.5W    5881    41%
BASSMAN AA165   16.5W   6L6GC   55%
DELUXE  6G3     17W     6V6GT   121%
DLXREV  AA763   14W     6V6GT   100%
PRNCREV AA1164  14W     6V6GT   100%
PRO     AA763   15W     6L6GC   50%
SHOWMAN AB763   12W     6L6GC   40%
SUPER   6G4-A   10W     5881    43%
TREMLUX 5E9A    17.5W   6V6GT   125%    CATHODE BIAS
TREMLUX 5G9     15W     6V6GT   107%
TWIN    5F8A    15W     5881    65%
TWIN    6G8A    7.5W    5881    35%
TWINREV AB763   15W     6L6GC   50%
TWINREV AA270   13W     6L6GC   43%

GIBSON:
CRESTLINE       GA77RVT 18.5W   6L6GC   62%     
LES PAUL        GA40    13W     6V6GT   93%     CATHODE BIAS
RANGER          GA55RVT 12W     6L6GC   40%
SUPER 300       GA300   17W     6L6GC   57%
VANGUARD        GA77    16W     5881    70%

[martin manning]

I was bored so I checked to what percentage of maximum plate dissipation Fender biased some of their classic amps.

Nice!
What technique did you use to calculate the plate current?

[Lauri]

What technique did you use to calculate the plate current?

I used LTspice. The models I used are fairly accurate compared to datasheets except screen grid current is a bit higher but in this case it doesn't matter.

[martin manning]

I used LTspice.

That's what I was thinking, good idea!

I made the change to suggest 50-60% for Class AB, and revised the shunt current measurement caveat to "potentially" inaccurate.

[pdf64]

I used LTspice.

...
I made the change to suggest 50-60% for Class AB ...

I suggest to make the acceptable range much wider, eg 40 to 80% depending on circuit specifics, then a nudge to target around 60% if unsure.

I was bored so I checked to what percentage of maximum plate dissipation Fender biased some of their classic amps. Assuming voltages on schematics are correct and tubes used are close to what datasheet says they should be.

I get pretty similar results when ploting the control and and screen grid voltages onto the triode charts, and deriving the resulting anode current (I assume that also includes screen grid current).
So yes, from the available info, Fender seemed to operate 6V6 very hot.
Try running the numbers from the 5E9A Tremolux! https://el34world.com/charts/Schematics ... x_5e9a.pdf

To be honest, I went and double checked my two Fluke meters (87-V and a 289). Both are under 2-ohms for the milli-amp range, and well under 1-ohm for the ampere range and the ampere range is sufficiently resolved for 10's of milliamps on both of them. The error would be minimal, even in the Marshall example.
...

Kudos for checking.
Which ammeter range socket have you been using to undertake these shunt tests?
Which socket do you think most users would select?
Rather than the 10A socket, I think that it would be intuitive to use a more sensitive range when measuring a few 10s of mA.

It really depends how good your meter is. I think 6-ohms is a hang of a lot of shunt resistance for anything that calls it self a test instrument.

I concede all my DMMs are at least 20 years old, but it seems strange to consider that mid to high end Flukes, intended for electronics tech usage, of that age aren't worthy of being considered a test instrument.
What spec DMM do you think most readers requiring that type of document for guidance would be using?
Would they be likely to be equipped with suitable probe clips and experience necessary to undertake a shunt test safely?

[trobbins]

Maybe also worth a note on what to do when the cathode (or anode) currents are not the same, or the amp is a quad. Eg. at what % higher/lower current on one side would it be reasonable to adjust the target and why.

Perhaps also an indication of nominal period to make a repeat measurement.

[martin manning]

Good points. I'll see if I can squeeze them in.

[Lauri]

So yes, from the available info, Fender seemed to operate 6V6 very hot.
Try running the numbers from the 5E9A Tremolux! https://el34world.com/charts/Schematics ... x_5e9a.pdf

Simulation gives about the same results as just calculating the plate dissipation from the voltages. A bit over 17W per tube. I also simulated 5G9 voltages and it has about 15W plate dissipation.

[martin manning]

Updated again.

[Helmholtz]

Additional remarks:

1) The '62 RCA and '63 GE 6V6GT(A) datasheets specifify PDmax as 14W (Desing-Max-Rating).

2) After warming up at idle the OT primary resistance can be 5% or more higher than cold.
A 20°C higher wire temperature means 8% higher resistance.
Temperatures will be higher with the chassis in the cabinet.

[martin manning]

Additional remarks:

1) The '62 RCA and '63 GE 6V6GT(A) datasheets specifify PDmax as 14W (Desing-Max-Rating).

So this might suggest a crude conversion from design max to design center of 12/14, or -14%. This is of course for GE's assumptions and production quality standards for this particular tube type. Assuming something similar for the 6l6GC, it's 30W design max might be derated to 26W design center.

2) After warming up at idle the OT primary resistance can be 5% or more higher than cold.

Noted.

[Helmholtz]

I'd stick to the design-max system as it seems to be most widely used since the 60s.
Derating to design-center is likely to cause confusion.

[nuke]

To be honest, I went and double checked my two Fluke meters (87-V and a 289). Both are under 2-ohms for the milli-amp range, and well under 1-ohm for the ampere range and the ampere range is sufficiently resolved for 10's of milliamps on both of them. The error would be minimal, even in the Marshall example.
...

Kudos for checking.
Which ammeter range socket have you been using to undertake these shunt tests?
Which socket do you think most users would select?
Rather than the 10A socket, I think that it would be intuitive to use a more sensitive range when measuring a few 10s of mA.

It really depends how good your meter is. I think 6-ohms is a hang of a lot of shunt resistance for anything that calls it self a test instrument.

I concede all my DMMs are at least 20 years old, but it seems strange to consider that mid to high end Flukes, intended for electronics tech usage, of that age aren't worthy of being considered a test instrument.
What spec DMM do you think most readers requiring that type of document for guidance would be using?
Would they be likely to be equipped with suitable probe clips and experience necessary to undertake a shunt test safely?

Well, it is incumbent upon the technician to understand the equipment he/she is using and understand its limitations.

I started working on things as a kid when digital multimeters were still expensive and exotic, and the standard high-end equipment was a VTVM, or a FET-Voltmeter, and most of us had at best, a Simpson 260 with 20,000-ohms per volt sensitivity, or more often, a Micronta (Radio Shack) with much worse numbers. I eventually got a Beckman (worth a 40-hour week of pay) LCD digital back in the day.

The "old-man" part of knowing your test equipment is important still today. We've all gotten quite used to even cheap digital meters having very high input impedance and good basic accuracy for voltage. But that may not extended to current, or some of the other functions. Got to read the manual, and if it isn't published, then assume it sucks.

I looked up the manuals for my Fluke meters, the voltage burden in the milliamp range is 1.8mv/ma, which is 1.8-ohms on the milli-amp scale. That is also dependent upon using the correct, specified fuses in the meter. Fluke has a procedure to test the fuses in the manual without opening the case.

I couldn't find the manual for the model you mentioned, perhaps it is just too old to have the manual online.

On the matter of where to set the bias: It is a horseshoes and hand-grenades value for class AB operation. Like I said, plotting the load-lines by first drawing the class-b line, and then the class A line, you can simply use the same slope and move the class-A line up the graph to intercept the class B line. That's actually what is happening when adjusting the bias voltage in a fixed bias circuit.

The bias point in class AB is subjective - we're simply looking to stay in class A until things are "loud enough" that class-B operation and associated distortion is "not noticeable." That holds true for hi-fi and guitar amps. Weirdly enough, very good class-B amplifiers have been built with tubes which provide low-distortion, high-efficiency and very long tube life. (McIntosh with the bi-filar wound transformers for example)

Fender, Marshall, nor Vox ever blessed us with such things.

The advice to bias based on plate dissipation is a rule of thumb based on the notion that whatever pair (quad, sextet, etc) of tubes was chosen and intended to produce maximum power output.

But in many cases, that may or may-not be true. Consider the ubiquitous Blues Deluxe and Blues Deville amps. Basically, the same amp, one is 36 watts, the other is 58 watts, only substantial difference is the B+ and screen supply of about 420v vs. 475 volts. The latter is an example of squeezing the maximum power out of the 6L6GC pair, the other is a more conservative approach. If one were to set 75% of plate dissipation (22.5 watts of idle) in both you'd be looking at 55ma and 47ma. Maybe a fair number for the 60-watt amp, but maybe not necessary for the lower-power 40-watt amp. Perhaps only 15 watts (36ma) of idle is a decent target for the lower-power application. You save yourself some heat, extend the tube life and reduce stress on the transformers while getting good audio performance. (in actual practice, Fender ran these at lower current, yet they barbecue the EL84s in the Blues Junior).

Just something to consider. If you're designing, plot those load lines and understand where you're operating.