Favorite bias measuring and metering methods?

[audiobot]

Hi All,

I've been trying to decide the best means of measuring and/or metering bias for a DHT PP amp. Would an ammeter and a multipole rotary switch to insert the meter into the cathode circuit of each tube in turn be a good choice. It would be nice if an intelligent person with no tube experience could follow a simple procedure to check and bias the outputs. Any schemes to share? Suggested parts values?

Also, if a person needed to balance the current in two tubes (sections), but the absolute value of the current wasn't important, how to best do that? A zero centering ammeter?

Thanks,
Michael

[fdegrove]

Hi,

Michael,

Yes, that would work.
You could replace the rotary switch by a simple DPDT switch for a single PP.

I have an ammeter in place all the time and if I want a readout I just flick a switch, left hand or right hand side, push a button and the meter gives a readout.

Add some TPs to insert DVM probes and you have a voltage readout too.

Cheers,

[audiobot]

Hi All,

Thanks Frank,

What other methods could you suggest? What differences in setup would there be for fixed bias or cathode bias?

How would you handle the balancing situation? Just flip back and forth until you get the same reading on each?

I have an old piece of equipment that has a meter that reads something like 4 current ranges, one or two voltage ranges and pressure in torr. Something like that might not be a good thing for a person who just needs to bias the outputs, but it would be kind of cool to be able to test the whole amp without a multimeter. How would you implement that? Precision resistors and an ammeter?

Thanks,
Michael

[Frank Berry]

On my amplifiers, I install a 10 ohm resistor between the cathode and ground. I measure the voltage drop across the resistor.
10 ma = .1 volt.
When balancing a PP stage, I measure the voltage differential between the two cathodes (at the high side of the 10 ohm resistors) and trim the bias voltage for zero volts.

[Original Burnedfingers]

Hi Michael

I favor the Quicksilver method. The Quicksilver's have a 1/4" jack in which to plug in the meter and make your adjustment and then unplug.
While I like the looks of a meter on board some amps just don't have the room. Individual bias adjustment for each tube would be a plus.

Joe

[SY]

An ammeter is good, especially if you have provision to switch it out of circuit. And I really mean "ammeter"; use the highest current range meter consistent with being able to read the idle current with some accuracy, since high current range equates to low series resistance. For DHTs where you have some confidence in the tube matching or don't have individual bias adjustments, consider putting the meter in the B+ feed.

If you really want to keep the circuit as pristine as possible, use a very low value resistor (like 0.5 ohm or less) in series with the filament transformer's CT, then have an opamp with a x10 or x20 gain provide a nice, easy-to-measure voltage for a voltmeter.

I very much favor differential measurements to set tube-to-tube idle current balance; nulls are easier to get right.

[audiobot]

HI All,

Frank,
Certainly straight forward for most uses. Wonder if it would be more complicated for DHT, fixed bias, etc. I need to get some wires and connections floating around in my head so I can picture the final product.

Joe,
Do the Quicksilvers have individual tube bias? What sort of meter do you plug in?

Sy,
Opamp idea would be cool, but adds a lot of complications to implement, maybe. Would you use one opamp and vary the input connections or many opamps and vary the output connections? For an ammeter, you favor a large current as opposed to a small current with series resistance, where you are only diverting a small percent of the current? To me it seems that if a person wanted to measure currents in both the volt amp/driver section and the power amp section the small current would be easier. Am I missing something? What would be the mechanics of implementing a differential reading meter?

Anyone,
How would the meter that reads both current in different ranges and voltages be done. I dug into my pile of junk far enough to see the meter I have that does this, but there is no indication on the face even whether it is a voltmeter or an ammeter.

Thanks,
Michael

[fdegrove]

Hi,

Michael,

To give you some idea I dug up Bruce Rosenblits diagram for an OTL amp.

You can apply the same principle to any amp really.

BRUCE ROSENBLIT 6AS7G OTL

Cheers,

[audiobot]

Thanks Frank,

Would the meter be a voltmeter reading across the 1 ohm resistors or an ammeter? Do you think the 100 ohm pot is just to calibrate the meter? I guess that would mean voltmeter?

Thanks again,
Michael

[fdegrove]

Hi,

Would the meter be a voltmeter reading across the 1 ohm resistors or an ammeter? Do you think the 100 ohm pot is just to calibrate the meter? I guess that would mean voltmeter?

I can't remember exactly what BR used, I'm pretty sure it's an ammeter.
I use an ammeter which is built in and test points that are switched around according to the rotary switch position.

The 100R trimmer is to calibrate the meter, they're never spot on.
Whether it's an ammeter or a voltmeter that's put there doesn't really matter.
It's just that we're used to use amps for bias current and this is the reading people expect to see there, otherwise you'd need to the conversion.

Cheers,