Uncle Doug's Bias Test Point on Princeton?

[jordan86]

Anyone know what test point Uncle Doug would be using to read the plate current in mA in this video? I am doing the adjustable bias mod on my Princeton build. 27K resistor and 10K pot. Seems like it would be more convenient to have a "real time" test point to read rather than doing the voltage drop and resistance method on the OT and 6v6 plates after every adjustment.

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Edit: he measures at 10:25 in the video.

Would installing 1ohm/1% resistors from pin8 to ground on the 6v6's be the best way to do that? Just puzzled bc Doug is measuring in mA on his meter. Open to better ideas on how to measure bias for a negative grid biased amp.

Thanks in advance!!!

[sluckey]

No way to tell from that video. But to measure current, the ammeter must be connected in series, so to measure only plate current you must connect the meter between the tube plate and the OT (positive probe connects to the OT lead and negative probe connects to the plate. I doubt U.D. is doing this.

I suspect he may have a bias probe adapter that plugs into the tube socket and the tube plugs into the adapter. The wiring inside the adapter takes care of breaking the plate circuit so the meter can be connected in series with the plate in order to measure true plate current.

Hoffman sells a very similar bias adapter but his breaks the cathode circuit rather than the plate circuit. Much safer because there are no B+ voltages on the meter leads. However, Hoffman's checker measures cathode current which is the sum of plate current and screen current.

For my own amps I find it quicker, easier, and more convenient to install 1Ω 1W 1% resistors between the cathode and ground. I've never used a bias adapter but if I was running a shop and had customers, I would have a bias checker on the bench.

Here's a link to Hoffman's bias adapter in case this method appeals to you...

     https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Tools&ORDER_ID=!ORDERID!


 

[jordan86]

For my own amps I find it quicker, easier, and more convenient to install 1Ω 1W 1% resistors between the cathode and ground. I've never used a bias adapter but if I was running a shop and had customers, I would have a bias checker on the bench.

Thanks Sluckey. I figured that may be the case. Also have some 1 ohm/1% resistors on order from Mouser. Had to pay another $3.50 shipping bc I forgot to get them in my first order  But will be worth every penny for the convenience long term. 

So after installing the 1 ohm resistor, do I put my leads on each end of the 1ohm resistor to get the plate+screen current? (I am assuming that value is all lumped in one, like with Hoffman’s probe).

[sluckey]

That's basically correct. I like to connect my black probe to chassis using a gator clip and my red probe to pin 8 (or the 1Ω resistor if more convenient). Very quick and easy to move the red probe from one tube to the other. No scarey voltages.

Don't forget to set your meter to measure millivolts!

[Willabe]

Also have some 1 ohm/1% resistors on order from Mouser. Had to pay another $3.50 shipping bc I forgot to get them in my first order.

Doug sells them in his on line store. 

[PRR]

> measuring in mA on his meter.

The ideal current meter is ZERO ohms. In practice for our currents, maybe 1 Ohm. The OT winding is a hundred ohms. If you parallel them, about 99% of current goes through meter, 1% through OT. It's no more error than measuring volts on a 100K point with a 10Meg volt meter.

IMHO this is too frikkin dangerous. Your hand is near 400V points. Disconnecting meter diverts that 99% current back through the OT causing a KV voltage spike. And a slip of a lead to chassis (biggest target around) puts a dead short on the power through the meter.

Yes, get 1 Ohm resistors. They do NOT need to be high precision or high power. 10% 1/4W should be enuff for any tube amp. Larger/tighter is fine too.

[EL34]

1% - 1 ohms are here
https://hoffmanamps.com/MyStore/catalog/Resistors.htm

[pdf64]

...The ideal current meter is ZERO ohms. In practice for our currents, maybe 1 Ohm. The OT winding is a hundred ohms. If you parallel them, about 99% of current goes through meter, 1% through OT...

It's worth noting that whilst most decent DMMs may have a negligible resistance when set to higher current ranges, that may not be the case at lower ranges. The latter may perhaps be selected in order to achieve better accuracy. If the meters are fused, some portion of the resistance will be due to the fuse / contact resistance, the rest to the current sensing resistor, which will necessarily have a high value than that used with higher current ranges.
As an example, my ancient Fluke 75 has a resistance of about 7 ohms between its 300mA and common jacks, the Fluke 79iii just over 10ohms  between its 40mA and common jacks; whereas my posh Fluke 189 has 1.8ohms between its 400mA and common jacks; though that rises to 98.6 ohms when a uA range is selected.
The higher resistances will cause a measurement error when used as a current shunt, with the error being on the 'unsafe' side of things (ie actual plate current will be somewhat higher than the measurement indicates).
Yet another reason to avoid this awful method 
Sorry for the delayed response, I meant to do so at the time but must have forgotton 

[PRR]

Thanks for the hard data.

It is also interesting the different voltage burdens.
Fluke 75 ...7 ohms ...300mA ====== 2.1V
Fluke 79iii ...10ohms ...40mA ===== 0.4V
Fluke 189 ...1.8ohms ...400mA ==== 0.8V
Fluke 189 ...98.6 ...a uA range ==== <0.1V