Bias and Plate Voltage Measurement

[Al up north]

Greetings,

This is my first post but I've read and enjoyed this forum for years mostly for the guitar stuff but I do want to try biasing my amp on my own (1999 Fender DRRI). I think I need new tubes as I'm not getting any sound out of the amp. I have a new matched pair of JJ 6V6s and I thought I might try putting them in and biasing the amp myself.

I'm aware of the dangers so I wanted some opinions first. I intend to connect everything with the amp off and set the meters to the correct measurement settings before turning on my amp.

I was inspired by the video linked below from Guitologist on youtube where he measures plate voltage and current at the same time using two meters (see time stamp 10:00).



To summarize, Guitologist makes a similar bias probe and leaves the soldered joint on pin 3 of the tube exposed. He sets up a second meter clipping the live end to the exposed pin 3 connection and the other to ground.

My setup is similar - the bias probe is one from Nobsound that reads the plate current from pin 3. The meter on the left in setup to read plate current (I hope) and is connected to pin 3 via the piggy back lead and to ground (amp chassis) via the black aligator clip. The meter on the right will read current for biasing the amp.

Once I know the plate voltage I'll use the calc: watts ÷ plate voltage x 70% to achieve my bias current setting/target.

Will this setup work to read plate voltage?

Thank you,

Al

 

[King Fan]

Welcome. I’ll let others comment on the probe thing. That video bugs me stylistically, but your rig looks safer than his.

If you get plate voltage and plate current, I suspect you know the whole drill. Rob's nice how to bias page reviews the details, and of course your amp isn’t cathode biased.

Plate Dissipation in watts = Plate Voltage * Plate Current

The Plate Current must be in amps, not milliamps so if you get 19.4 milliamps convert that to .0194 amps by dividing by 1000. For cathode biased amps use plate-to-cathode voltage, not plate voltage. To measure plate-to-cathode voltage place one meter probe on the plate pin 3 and the other probe on the cathode pin 8.

Plate Dissipation % = Plate Dissipation / Tube Maximum Plate Dissipation

Example: Your 6V6 is rated for 12 watts max, you measure 400 volts of plate voltage and 19.4 milliamps of plate current.

Plate Dissipation = 400v * .0194a = 7.76 watts of Plate Dissipation

Plate Dissipation % = 7.76w / 12w = .65 or 65%

 

[Al up north]

Thanks for the welcome and link to Rob's info.

I'm still undecided if I want to bias hot ~70% or a little cooler at ~65%. I may experiment but my starting point is getting that plate voltage and I'd rather do so without dropping the chassis

Cheers,

Al

 

[SnidelyWhiplash]

For BF style amps, I always start at 60%. See if you like it, if not, go to 65%. I rarely, if ever, biased starting at 70%.

 

[King Fan]

Thanks for the welcome and link to Rob's info.

I'm still undecided if I want to bias hot ~70% or a little cooler at ~65%. I may experiment but my starting point is getting that plate voltage and I'd rather do so without dropping the chassis

Cheers,

Al

Here's the thing. Though you *do* want to think about bias in %MPD, you definitely don't want to bias to a numerical target. Bias by ear. Some folks may like as low as 55%; a few may like something somewhat over 70%, which is actually still safe (as per the maestro, Merlin Blencowe) -- especially with JJs, which are a highly robust 6V6 variant. Measure, play a few chords, listen, adjust, measure, repeat.

EDIT. BTW, definitely calculate your MPD at 14W for those JJs. Rob's example at 12W is conservative for most old and new 6V6s; his nice bias calculator page now uses 14W for a standard 6V6GT.

 

[Lowerleftcoast]

^^^^^
+1

 

[SnidelyWhiplash]

I always bias 6v6s at 12W just to be conservative. Fenders throw a lot of current on power tubes. They are stressed to begin with.

 

[King Fan]

Good point. Assuming you mean 12W (12% being pretty cold) I sometimes do that too, especially on blackface circuits with high plate voltages. But I still bias by ear, and I find I don't usually go over 70% even using a 12W basis. Heck, turn your knob all the way cold, figure out 70% at 12W *and* 14W, and work up until you like the sound. Remeasure and just see where you are.

With JJs, though, that 12W caution isn't needed -- they can handle 14W all day long.

 

[Al up north]

Thanks for the points about using 14w for my JJs. I know and like how my DRRI sounds so I'll definitely bias by ear once I have a ballpark ma range.

Any feedback on my proposed method above for measuring plate voltage?

 

[SnidelyWhiplash]

Good point. Assuming you mean 12W (12% being pretty cold) I sometimes do that too, especially on blackface circuits with high plate voltages. But I still bias by ear, and I find I don't usually go over 70% even using a 12W basis. Heck, turn your knob all the way cold, figure out 70% at 12W *and* 14W, and work up until you like the sound. Remeasure and just see where you are.

With JJs, though, that 12W caution isn't needed -- they can handle 14W all day long.

Yes I meant 12W.

Thanks

 

[dan40]

As long as you built the probe exactly as he described, you should have no problems using it for a bias reading. Just be very careful with the ends of your meter leads as they will have full voltage on them should you accidentally pull it loose from the meter while testing. Many folks shy away from using this method because of the dangers associated with breaking the plate circuit to take the current reading but as long as you are careful your setup should be fine.

 

[Lowerleftcoast]

^^^^
+1

The above method for measuring mA will have the plate current and high voltage *inside* your cheap (I could add other less flattering descriptors) meter. If things go wrong, the meter will fry. To avoid possibility of electrical shock, do not touch the meter during measurements or when high voltage is present. Keep it hands free.

To measure the current with a safer method. A precision resistor can be placed where the probes are connected. Measure the *voltage drop* across the resistor. If a 1 Ohm 1% resistor is used, the voltage drop will read say 20mV which, with Ohms Law, will translate to 20mA. The current and high voltage will be running through the resistor, not through the meter. High voltage will still be present at the tube socket and probe leads so a hands free approach is still suggested. I would definitely prefer this method. There is little chance of damaging even the cheap meter with this method.

Stay safe.

 

[Jon Snell]

The safest way to read the current dissipation of your valve is to insert a 1 Ohm resistor between each cathode and ground point. Measure the voltage drop across the resistors and using Ohms Law, calculate the current and then the Wattage, after deducting 5% for the screen grid current.

 

[Al up north]

Thanks Lowerleftcoast for the observation re: high voltage going through the cheap meter. I'll invest in a less cheap one before proceeding. Would that make my setup safer?

My process would be:
1. Connect everything with the amp off
2. Set both meters to the correct settings
3. Power amp on but leave in standby - do not touch or fiddle with anything while amp warms up
4. Power amp on from standby, record plate voltage
5. Put amp back on standby for a few minutes without touching or fiddling with anything
6. Power amp off
7. Let tubes cool down
8. Disconnect meter on the left (plate voltage) but leave bias probe in for the next step of setting bias which will involve powering the amp back on, etc.

Feedback is more than welcome on the above process.

Re: inserting a resistor across the probes - to which probes in my photo were you referring? I appreciate the alternative method and will do further research on what "voltage drop" means.

Thank you,
Al

 

[SoK66]

What the world needs is a good bias probe. I was lucky to buy a CompuBias from Mission Amps about 15 years ago, super handy and easiest to use "can't miss" meter ever.

 

[King Fan]

To measure the current with a safer method. A precision resistor can be placed where the probes are connected. Measure the *voltage drop* across the resistor. If a 1 Ohm 1% resistor is used, the voltage drop will read say 20mV which, with Ohms Law, will translate to 20mA. The current and high voltage will be running through the resistor, not through the meter.

Re: inserting a resistor across the probes - to which probes in my photo were you referring? I appreciate the alternative method and will do further research on what "voltage drop" means.

LLC, I like your thinking, but I'm not quite sure how/where one would actually do that with his bias probe. Can you do a diagram? Al, to measure 'voltage drop' across a resistor, you just clip one probe to each lead and set your meter to Volts (mV, if it's not auto-ranging).

 

[King Fan]

And a few other thoughts:

1) doesn't anybody sell a dual-reading (current and voltage) bias probe anymore?

3) Al, never tell anyone I said this, but if you're putting JJs (super-tolerant of bias) in a factory DRRI (typically biased cool at the factory) some folks wouldn't bother with bias measurement or adjustment. But I never said that, right?

3) At some point I stopped using my old bias probe and learned how to drop the chassis (takes 5 min), discharge the caps (takes 5 sec) and, with my nice safe mini-gripper probes, actually measure bias safely.

 

[tubedude]

Rarely if ever have I found better tone and feel at max dissipation in fixed bias amplifiers. 60 to 65% max diss has worked for me.
If you do bias hot, with a Fender that has tremolo, make sure your chosen bias doesn't weaken the trem oscillator.

 

[SnidelyWhiplash]

I have a Bias Pro probe & a SRS PV probe. Have had them for years. Have biased my amps plenty of times with both. Take your time & follow instructions & you'll be fine. Buy a good meter. Cheap tools are maybe a step above junk. I wouldn't try to save a few bucks if I were going to bias an expensive amp.

 

[Al up north]

I have a Bias Pro probe & a SRS PV probe. Have had them for years. Have biased my amps plenty of times with both. Take your time & follow instructions & you'll be fine. Buy a good meter. Cheap tools are maybe a step above junk. I wouldn't try to save a few bucks if I were going to bias an expensive amp.

Agree, I want to buy the "Pro One" bias probe from Eurotubes which displays current and voltage at the same time but alas, their online store has been disabled for a almost two weeks.