Blues Deluxe Bias Question

Darcy Hoover · January 9th, 2008, 04:48 PM

First off, I should specify that I don't really know what I'm doing, if it isn't blaringly obvious!

I have a 2006 Blues Deluxe.

According to a couple of site's recommendations, I set the bias to 68mV (with the blue plastic adjustment screw on the circuit board, about a year ago).

That's with the voltmetre's ground on the chassis and the positive on the Fender marked Bias Test Point on the circuit board (which I think is the #6 pin of both powertubes but I'm not sure). It wasn't far off that to begin with. Sounded fine.

I recently replaced the powertubes with matched Tung Sol 5881s (on a trusted friend's recommendation, I wanted to get myself a spare set to have on hand anyways) but I checked the bias at the same spot and it's now 46 to 47 mV. Once again, sounds great to my ears, perhaps a tad cleaner.

Perhaps my ears aren't to the point of being able to differentiate between a 22 mV difference in bias, I just want to make sure I am not damaging the amp in any way. Should I crank the amp back up to 68mV? Sounds great as is but.....

Any feedback to point me in the right direction would be apprecitated!

Darcy Hoover · January 10th, 2008, 05:00 AM

Anyone?

I know it isn't supposed to affect bias, but I put a 5751 in the V1, Groove Tubes 12AX7 SAG-MPI matched phase inverter , and a Tung Sol 12AX7 for the middle tube.

I set the bias to 68mV at least a year ago if not more with the stock tubes (and haven't checked it since) and with a different voltmetre, if that would have anything to do with the much colder bias after changing the tubes?

EMan · January 10th, 2008, 10:41 AM

Hello Darcy.

The general rule-of-thumb for setting idle current is to set it for roughly 70% or less of the tube's maximum Plate Power Dissipation rating. This rating can be found by looking that the particular tube's data sheet...easily available on the internet.

I'assume that the bias test point is a 1 ohm resistor connected to the cathodes of both output tubes. Therefore, the voltage you read there with you DMM represents the current through both tubes.

If, originally, the reading was 68mV, that represents 68mA, or 34mA per tube.

To find the % of power at the Plate(s), you multiply that Cathode current reading (34mA) times the DC voltage measured on the Plate of the tube.

%Power = Calculated Power / Rated Power = 70%

For example, with a 5881 rated at 23 Watts maximum Plate Power, 70% of that = 16.1 Watts.

Now you can set the current by . 16.1 Watts / Measured Plate Voltage = Idle Current .

Remember that the number you measure at the bias test point will be TWICE what is calculated, as it is reading two tubes at the same time.

Hope this helps!

Darcy Hoover · January 10th, 2008, 07:05 PM

Thanks for the reply!

Darcy Hoover · January 10th, 2008, 07:56 PM

I found the sheet for the Tung-Sol 5881s, and indeed the maximum plate dissipation is 23 Watts, and plate voltages are listed as either 360 or 400 V for Class AB (400 V for the triode connection), so

40mA X 400 V = 16 W
or
44mA X 360 V = 16 W

Either way, I'll stick with below 70 at the test bias point to be on the safe side...(actually before you replied I called Torres Engineering and he recommended staying below 70, actually felt 67 to 68 should be nearly ideal)

The wife is home now so I can't tell how it sounds till tomorrow, but thanks again for taking the time!

EMan · January 11th, 2008, 08:39 AM

Darcy - You must measure the Plate Voltage on your am to use in the calc. Note also, that when you adjust the idle current, the Plate Voltage will change to some degree. The BD schematic listed on Mr. Gearhead states a B+ voltage of 427VDC.

Most stock Fender amps run well under 70%.

Darcy Hoover · January 11th, 2008, 09:06 AM

Thanks again! That's why the 68mV seems to be the number.

I was quite confused there (if that wasn't obvious) as some of the data I found was measuring the voltage from pin 5 of the powertubes, and some were utilizing the bias test point, and I didn't see the difference, but of course the values are nowhere near the same. Your help has clarified a few of the things I was confused about.

The amp does sound better now, a tad "thicker" if that makes sense.

EMan · January 11th, 2008, 05:39 PM

Good deal, Darcy...the BD is a nice amp...glad it sounds good for you!

January 9th, 2008, 04:48 PM	#1 (permalink)
Darcy Hoover Tele-Meister Join Date: Sep 2007 Location: Halifax, Nova Scotia Posts: 412	Blues Deluxe Bias Question First off, I should specify that I don't really know what I'm doing, if it isn't blaringly obvious! I have a 2006 Blues Deluxe. According to a couple of site's recommendations, I set the bias to 68mV (with the blue plastic adjustment screw on the circuit board, about a year ago). That's with the voltmetre's ground on the chassis and the positive on the Fender marked Bias Test Point on the circuit board (which I think is the #6 pin of both powertubes but I'm not sure). It wasn't far off that to begin with. Sounded fine. I recently replaced the powertubes with matched Tung Sol 5881s (on a trusted friend's recommendation, I wanted to get myself a spare set to have on hand anyways) but I checked the bias at the same spot and it's now 46 to 47 mV. Once again, sounds great to my ears, perhaps a tad cleaner. Perhaps my ears aren't to the point of being able to differentiate between a 22 mV difference in bias, I just want to make sure I am not damaging the amp in any way. Should I crank the amp back up to 68mV? Sounds great as is but..... Any feedback to point me in the right direction would be apprecitated! __________________ http://www.soundclick.com/darcyhoover

January 10th, 2008, 05:00 AM	#2 (permalink)
Darcy Hoover Tele-Meister Join Date: Sep 2007 Location: Halifax, Nova Scotia Posts: 412	Anyone? I know it isn't supposed to affect bias, but I put a 5751 in the V1, Groove Tubes 12AX7 SAG-MPI matched phase inverter , and a Tung Sol 12AX7 for the middle tube. I set the bias to 68mV at least a year ago if not more with the stock tubes (and haven't checked it since) and with a different voltmetre, if that would have anything to do with the much colder bias after changing the tubes? __________________ http://www.soundclick.com/darcyhoover

January 10th, 2008, 07:05 PM	#4 (permalink)
Darcy Hoover Tele-Meister Join Date: Sep 2007 Location: Halifax, Nova Scotia Posts: 412	Thanks for the reply! __________________ http://www.soundclick.com/darcyhoover

January 10th, 2008, 07:56 PM	#5 (permalink)
Darcy Hoover Tele-Meister Join Date: Sep 2007 Location: Halifax, Nova Scotia Posts: 412	I found the sheet for the Tung-Sol 5881s, and indeed the maximum plate dissipation is 23 Watts, and plate voltages are listed as either 360 or 400 V for Class AB (400 V for the triode connection), so 40mA X 400 V = 16 W or 44mA X 360 V = 16 W Either way, I'll stick with below 70 at the test bias point to be on the safe side...(actually before you replied I called Torres Engineering and he recommended staying below 70, actually felt 67 to 68 should be nearly ideal) The wife is home now so I can't tell how it sounds till tomorrow, but thanks again for taking the time! __________________ http://www.soundclick.com/darcyhoover

January 11th, 2008, 09:06 AM	#7 (permalink)
Darcy Hoover Tele-Meister Join Date: Sep 2007 Location: Halifax, Nova Scotia Posts: 412	Thanks again! That's why the 68mV seems to be the number. I was quite confused there (if that wasn't obvious) as some of the data I found was measuring the voltage from pin 5 of the powertubes, and some were utilizing the bias test point, and I didn't see the difference, but of course the values are nowhere near the same. Your help has clarified a few of the things I was confused about. The amp does sound better now, a tad "thicker" if that makes sense. __________________ http://www.soundclick.com/darcyhoover

January 10th, 2008, 10:41 AM	#3 (permalink)
EMan Tele-Holic Join Date: Apr 2003 Location: Dallas, Texas Posts: 939	Hello Darcy. The general rule-of-thumb for setting idle current is to set it for roughly 70% or less of the tube's maximum Plate Power Dissipation rating. This rating can be found by looking that the particular tube's data sheet...easily available on the internet. I'assume that the bias test point is a 1 ohm resistor connected to the cathodes of both output tubes. Therefore, the voltage you read there with you DMM represents the current through both tubes. If, originally, the reading was 68mV, that represents 68mA, or 34mA per tube. To find the % of power at the Plate(s), you multiply that Cathode current reading (34mA) times the DC voltage measured on the Plate of the tube. %Power = Calculated Power / Rated Power = 70% For example, with a 5881 rated at 23 Watts maximum Plate Power, 70% of that = 16.1 Watts. Now you can set the current by . 16.1 Watts / Measured Plate Voltage = Idle Current . Remember that the number you measure at the bias test point will be TWICE what is calculated, as it is reading two tubes at the same time. Hope this helps!

January 11th, 2008, 08:39 AM	#6 (permalink)
EMan Tele-Holic Join Date: Apr 2003 Location: Dallas, Texas Posts: 939	Darcy - You must measure the Plate Voltage on your am to use in the calc. Note also, that when you adjust the idle current, the Plate Voltage will change to some degree. The BD schematic listed on Mr. Gearhead states a B+ voltage of 427VDC. Most stock Fender amps run well under 70%.

January 11th, 2008, 05:39 PM	#8 (permalink)
EMan Tele-Holic Join Date: Apr 2003 Location: Dallas, Texas Posts: 939	Good deal, Darcy...the BD is a nice amp...glad it sounds good for you!