Synopsis of Adjustable Fixed-Bias Measuring Methods, Bias Tools

[gktamps]

I've been helping a friend who is relatively new to amp building with various aspects of his builds. In the course of learning various biasing methods for his amps, some of which are cathode-biased, others adjustable fixed-bias, he has used a couple of online resources which have been useful, but I think can be confusing for new builders. There is a lot of material available on the topic of bias measuring methods both here and elsewhere online and in books, but in discussing this with my friend, and with others who are inexperienced, it can be difficult to synthesize all of the information and put it into context. Specifically, there are differences in the accuracy and safety of each method, in addition to the actual procedures. Also included is a short list of currently available bias measuring tools and a couple no longer in production, so that if a builder chooses to use tube socket-type bias probes, they have some compiled resources.

In my work I produce a lot of tables to convey data and information, so I thought it would be useful to synthesize and contextualize the various bias measuring methods from two sources: Aiken and Robinette. While both do a good job of describing the processes, it seemed to me that a table would be helpful for newcomers to understand the relative merits and shortcomings of each method as detailed by Aiken and Robinette. I have not added any new data or information of my own. This is not a full discussion of the topic of tube biasing, what comprises center-biasing or over/under biasing, etc. - there are many resources available for that information.

Comments and suggestions are welcome. If they warrant a wider scope in the table, I can make those changes. If there are just subsequent comments or observations of the existing material in the table, those would be available to anyone who reads this thread.

Cheers,

Greg

Compiled_Bias_Comparison_Table_01142022.pdf

[aceofbones]

As a first time builder I find this chart really useful. I've come across many of these methods, but rarely do I find decent info about how they compare to other methods, or which would be more suitable for me to use. This is a great way to see all of that in one place!

[gktamps]

Great! You are my target demographic. Thank you for your feedback!

I just cleaned up some text and uploaded current table.

[Phil_S]

First rate. I like the way you discourage the various shunt methods. Just because you can, doesn't mean you should. I've used the transformer DCR and voltage drop to calculate, but, frankly, it made me nervous, and how much does achieving that much accuracy really matter if you might end up...well, in a wood box.

[gktamps]

Absolutely, Phil! Part of my motivation to do this was that it is difficult for a newcomer to understand the relevant risk/reward ratio. Also; a careful, professional tech is likely to be able to safely use methods that would be much riskier for those with less experience.
Thanks for your comments!

[BobSimpson]

Hi gktamps!
Thanks for the table.

I'd like to ask a question.

First, I have a BlueMonkey 88 reverb amp from Brown Note.
I've always taken it to the best tech I know in my area ( IMHO ), Willie Whitaker, otherwise known as Lord Valve.
Sadly, he passed away a few months ago. I need to know how to bias it now.

It has 4x6L6GC output tubes, 2 bias adjustment pots, and 4 bias test points across 1R resistors from pin 8 of each 6l6 to ground.

I haven't done measurements yet, so I'm picking what I believe to be bias voltages from my... er, thin air.

if a pair of 6l6's in class AB would require say -30 volts per tube on the DVM, and was adjusted to that value separately by their individual pots, would each pair of tubes require -60 volts at the pot, or -30 v at each of the 4 test points or -15 volts?

[Phil_S]

Duncan amps is hosting a page with Willie's detailed instructions on how to bias an amp. I'd skip the shunt method.
He was probably among the last of what I see as a dying breed and will be missed for his vast knowledge of vacuum tubes and guitar amps.
https://www.duncanamps.com/technical/lvbias.html

[gktamps]

Thank you for adding this to the discussion, Phil. There is a lot of important practical information in Willie's document. There are several points worth repeating in this thread.

For those new to amp building: tube amp voltages can kill or injure you, damage your amp or property. Learn about the proper safety measures, or take your amp to a tech for biasing/repair.

Also, please review the table for hazards using shunt methods. Read the source materials, including Lord Valve's and you'll see the many warnings. Understand WHY the methods that push the full current and voltage through your meter are much more hazardous.

Just a few important points from Willie's document:
If you aren't yet familiar with biasing, practice with old tubes.
Understand the basing (pin socket positions) for your tubes.
Use a good quality meter - accuracy depends on it (and I'll add that safety does as well).
I agree with the recommendation to use two meters. Read the reasons why this is useful and safer.
Be aware that flux or corrosion will cause incorrect readings (read Willie's advice).
Be cautious about bias socket probes - some interrupt the connections and send the full voltage/current through a wire to your meter, and some use a 1 Ohm shunt resistor. I'll add that you want to make sure the tube, probe, and socket are all making good contact. If you lose bias voltage you will likely have a tube red plate. This happened to me using a commercial bias probe meter many years ago and I lost a really fine pair of SED Winged C tubes.

Also, remember that when you increase or decrease bias current, plate voltage will decrease or increase, respectively. That is why it is helpful to take many measurements of plate voltage as you adjust cathode current. Also, for most adjustable fixed-bias circuits, plate to cathode voltage will be equal to plate to ground voltage because they are connected either directly or through a 1 Ohm sensing resistor. This is different from cathode-biased amps that have a bias resistor of many Ohms.

[gktamps]

It has 4x6L6GC output tubes, 2 bias adjustment pots, and 4 bias test points across 1R resistors from pin 8 of each 6l6 to ground.
If a pair of 6l6's in class AB would require say -30 volts per tube on the DVM, and was adjusted to that value separately by their individual pots, would each pair of tubes require -60 volts at the pot, or -30 v at each of the 4 test points or -15 volts?

Hi Bob,

First, make sure you have a schematic handy, or at least the layout. I found a layout online that shows the cathode pins are connected to the 1 Ohm shunt (aka sensing) resistors. Because of that, you will be reading cathode current and not actual negative grid voltage. Reviewing the table, you'll see that as the first method and not recommended. It's also not how your amp is set up for measurements.

You will be measuring current, but because the 1 Ohm resistors are shunting voltage to your meter, you'll set your DMM to the mV scale and the reading at each red test socket will be the equivalent to the current in mA. So, 30mV displayed on your meter is equal to 30mA of cathode current. You don't divide the reading by 2 because you'll be directly reading the current through each tube.

Looking at the layout, each tube has a red test socket and all are tied to a center, grounded black socket. You'll be measuring the current through each tube but there are only two bias pots - one for each transformer leg and pair of tubes. My suggestion would be to use an online bias calculator to enter the values for plate voltage and cathode current and calculate the plate dissipation. Decide what plate dissipation setting you want to use - 50%, 60%, etc. based on the recommended settings for your tubes. Remember, you'll be reading current for each tube, and they may differ slightly form each other, so my advice is as long as they are pretty close to each other (within a few mA), run your calculations based on the tube passing the higher current. By doing that, one tube will run a little cool, but that's probably better than running the other too hot.

You'll also be measuring plate voltage before each adjustment, so be very careful that your probe makes good contact with the socket pin, and in most cases I recommend using a probe with a hook-type clip, or a really high-quality insulator alligator clip in a pinch, but make sure no metal comes into contact with you or any part of the amp other than the clip at the tube socket. Clip or hang your negative test lead to the chassis where you won't knock it into the amp.

Observe how plate voltage changes with bias setting changes. You'll want to re-enter the plate voltage into the calculator for each test because the dissipation percentage calculations will change. You'll have it dialed in when you reach your target dissipation levels based on plate voltage and cathode current.

With my own amps, once I have bias set for a particular set of tubes, I don't generally bother with reading plate voltage when I want to make minor routine adjustments, I just measure cathode current and make my adjustments. With test sockets and external 10-turn pots like I generally install, adjustments don't require pulling the chassis. When I first bias an amp, change tubes, or lack the relevant data, I open up the chassis so I can measure plate voltage (and all other voltages, since I'm in there).

[BobSimpson]

Thanks gktamps.
I didn't remember if I had to double the negative voltage, halve it, or take it as it was listed for each tube.

I appreciate the response.

[GerryJ]

Thanks for the table, very informative! There are people like me who for both safety and convenience, like to use the devices variously called "swamp probe (mine - company is gone)", or "bias probe" , "bias checker"etc - basically a socket that you plug the tube in, then plug the socket into the chassis socket, and run your two leads to a multimeter and measure plate current.
I believe - but I'm not certain, since they're out of business - the 'swamp probe' was like the 'bias checker' -from another site and my own notes - "The way this Bias checker works is that pins 1,2,3,4,5,and 7 pass uninterrupted through the Bias Checker from the tube base to the tube socket. Pin 8 (cathode) from the tube base is sent out to a Multimeter set to dc milliamps and sent back to pin 8 of the tube socket. You are basically reading the current flowing through pin 8 (the cathode) by putting your meter in line with it. The current flows up from pin 8 (cathode) of the tube base, out through the multi meter and then from the multi meter to pin 8 of the tube socket.The meter is set to DC milliamps and the range is set to under 200 milliamps on the meter."
Looking at Eurotubes, they make a physically identical faux socket with 2 leads for the multimeter, but it seems to work differently - "Our Octal 8 Pin bias probe is a single head unit that must be used in conjunction with a multimeter. This probe takes a direct DC plate current measurement by breaking the plate connection (pin #3). It does not use a reference resistor and it is not in the cathode connection. This means you do not have to compensate for screen current or any other variables."
So bias adjustments based off of the cathode pin 8 ) versus the anode/plate (pin 3).... I'm not sure if there's much practical difference- plenty of people on this board will know the answer, I certainly don't .
BTW, kudos on using the Aiken site, as in the beginning he more or less says that when biasing, there are extremes of over and under-biasing, but between them is a wide range of voltage that if you think it sounds good, it's fine.

[sluckey]

I believe - but I'm not certain, since they're out of business - the 'swamp probe' was like the 'bias checker' -from another site and my own notes - "The way this Bias checker works is that pins 1,2,3,4,5,and 7 pass uninterrupted through the Bias Checker from the tube base to the tube socket. Pin 8 (cathode) from the tube base is sent out to a Multimeter set to dc milliamps and sent back to pin 8 of the tube socket. You are basically reading the current flowing through pin 8 (the cathode) by putting your meter in line with it. The current flows up from pin 8 (cathode) of the tube base, out through the multi meter and then from the multi meter to pin 8 of the tube socket.The meter is set to DC milliamps and the range is set to under 200 milliamps on the meter."

That's how Hoffman's Bias checker works. In fact, your description is basically a word for word quote from his bias checker page...

https://el34world.com/charts/BiasChecker3.htm

[Phil_S]

For those who prefer to use a bias probe, a link to this arrived in my email this morning:
https://www.tubedepot.com/products/tube ... -scout-kit
At $40/pair, seems reasonable to me.

[gktamps]

I really like the looks of that probe kit. It takes both plate and cathode measurements, has excellent instructions and graphics. I'll revise the table to include this kit.

Seems to me one could also connect a jumper between the ground wire banana plug where it enters one DMM and a second DMM then use one for reading plate voltage and another for cathode current, but I haven't got those to test that way.

[gktamps]

Table updated to include TubeDepot probe kit.