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FDP Forum / Fender Amps: 1985 - Current / Biasing, The Supersonic and the Millivolt v.s. Milliamp Confusion


USA Denver/Chicago

Do What You Love. It's Your Universe!
Feb 4th, 2011 06:20 AM   Edit   Profile  

Supersonic Biasing:

BEWARE If you're gonna bias yer amp: It's millivolts DC (mVDC) not milliamps (ma)! The two are chronically confused all over the forums and threaten to damage your amp, your test gear and your life itself! The Supersonic bias spec is in mVDC!

Speaking of yer life, follow the usual precautions (google 'em if yer not sure) like only put one hand near the electrical parts so you're less likely to make a circuit to ground, especially one that would pass through yer heart - keep yer feet insulated too, and don't touch anything you don't know, even if the power is off, 'cause there are charged capacitors in the amp that can kill you.

You can search for a walkthrough on removing the chassis. In the combo you can pull it at least 3/4ths of the way out without it falling out - you will be looking at the adjustment/test point side of the circuit board, no need to get underneath. The schematics have 2 pages of circuits and 2 pages of physical parts layouts where you can find all the landmarks and surgical locations.

Test and adjust the bias, then play test it to see how it sounds - rinse, lather, repeat.

Testing with a multimeter (if you test with a Bias Probe, follow it's instructions):

Set your multimeter to read hundreds of millivolts (100mVDC or if using a DVM, x100mV or higher) - NOT 'amps' of any kind!

You measure the bias as the voltage between ground and the element of the tube that's being biased (grid, cathode, screen - Note: I assumed the Supersonic is Cathode Biased, it's not that simple, See the following post for an excellent tutorial) - like setting the idle on an engine (for performance, economy, 'smooth' take off - quantifiable, objective values and subjective 'preferences'). Put your black probe (- negative) in contact with the chassis (common ground). Carefully touch the red probe (+ positive) to the bias test point (TP49:r140/Anode D35/V7 pin8 and TP50:r141/Anode D36/V8 pin8 for a Supersonic).

Read the value. In the circuit schematic Fender specifies 32mVDC bias (not ma!).

Adjust the Bias Adjustment Trim Pot (r156) to your desired setting and record the voltage reading. You may wish to set the bias to a particular value (32mVDC perhaps). Most players will want to adjust it until their guitar sounds best.

You may have deduced that you can bias your amp without using a meter. I like to use one to see what the factory set the bias to or the previous owner, or what setting I liked best and what different tubes take.


(This message was last edited by Mercurian at 05:21 PM, Feb 5th, 2011)

Rick Plays The Blues


May I try that again?
Feb 5th, 2011 02:26 AM   Edit   Profile  


the SuperSonic is NOT cathode biased.
Cathode biased means, that the bias current is set by a a resistor between the tube cathode and ground respectively by the value of this resistor. The bias current is the current going from cathode to ground when the amp is in idle. If this bias current is going through a resistor, it creates a voltage, so that the cathode is at a voltage higher than ground, or in the other way, the ground has a negative voltage compared to the cathode - now if the grid is connected to ground, the grid has negative voltage compared to the cathode - the higher the bias current gets, the higher the negative grid voltage gets. Now more negative grid voltage means less cathode bias and it is easily seen, that at some point, the negative grid voltage exactly fits to the cathode current - this is the point for idle. If you now increase the cathode resistor, you get a higher negative grid voltage - cathode current will reduce (and grid voltage as well) until both fit together again at a level of less cathode current.

The SuperSonic - and many other amps - uses a different method: The power transformer and power supply deliver a negative to ground voltage (sometimes adjustable with a pot, sometimes fixed with resistors) that is sent to the grid while the cathode is directly connected to the ground.
Now how to know the cathode current without an amperemeter, respectively without cutting the connection from cathode to ground? Some amps - including the SuperSonic, have a resistor of very low resistance value added between cathode and ground. This resistor should not be mixed up with the bias resistor of a cathode biased circuit - the usual value of only one ohm would be way to small to create roughly -40volts grid voltage from just a few milliampere of cathode current.
No, this resistor is only there to make the cathode current readable as a voltage at this resistor. Some suppliers deliver tube socket adaptors to measure the bias current - these adaptors do exactly the same: they have a one ohm resistor between the cathode connection of the tube and the cathode connection of the amp's tube socket ... the cathode current goes through this resistor creating millivolts of exactly the same value as the cathode current has milliamps, since it is a one ohm resistance.
Your multimeter actually reads millivolts, but you are measuring milliampere actually and you are tweaking the negative grid voltage with the pot.

Contributing Member

That chicken

is WRONG, baby.
Feb 6th, 2011 10:54 AM   Edit   Profile  

"you can pull it at least 3/4ths of the way out without it falling out."

You always want a live amp chassis securely fixed atop your bench when working on it. You never know what might happen.

Here's an example: with the chassis hanging out the back of a cabinet, you might get a small shock that causes you to bump the chassis and knock it completely out of the box. If it falls "butter-side down" in your lap, that may be the last thing you see before your lights permanently go out.



Sep 17th, 2016 03:44 AM   Edit   Profile  

I know this is a rather old topic but hopefully someone will read this. I just got some matched 6L6GC STR tubes and wanted to bias the amp using a multimeter. According to the Weber calculator my max at 425V plate voltage would be 49.4 mA. When I meter at TP 49 and TP 50 The max I can get is 32mV at either resistor. Am I missing something here? FWIW I am in Thailand and using 220V power. Also even though the tubes were matched I am getting readings that differ by 4-5 mA on each tube.Can anyone give me some advice? Thanks.

Contributing Member

MT, USA and Jura, FR

Sep 17th, 2016 11:08 AM   Edit   Profile  


Do you also have a Supersonic as in the original post? Is the power transformer an international 220/240 V version and not the US standard 120 V?

Assuming yes on both questions:

220V wall power will not in itself alter the readings inside the circuit as the AC voltage from the wall is being transformed by the power transformer and converted to DC by the rectifier and filter capacitors.

Did you check to confirm the actual plate voltage? If it is at 425V and you are reading 32mV at the shunt resistors at the recommended test points, your amp is still within the nominal operating range for 6L6's at 56% of max idle plate dissipation

18 Watts is nominal set point, which is 75% of max plate dissipation. Thus, your amp with the maximum setting is biased on the colder side, but if it sounds good, it won't cause any harm, and the tubes will run cooler.

If you are not happy with the sound at that bias setting (e.g. it is harsh or scratchy, sterile), you (or a tech if you are not familiar with the entire process, including safety) could change the bias resistor, a potentiometer, to increase the range of available bias settings.

The 4-5 mA variation between tubes is ok if it sounds ok. Does that mismatch mean that one tube is at 32 mA and one is at 37 mA, or, rather 27mA and 32 mA? If one is at 27 mA, that is biased quite low for a 6L6, and you may need to further investigate the underlying issues.

To follow up on this, I recommend posting a new thread in the Amp Mods, Repairs and Projects section.

FDP Forum / Fender Amps: 1985 - Current / Biasing, The Supersonic and the Millivolt v.s. Milliamp Confusion

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