Biasing a Blues Deluxe or Hot Rod Deluxe

[nocaster]

First of all...........

You can die when you play with valve amps. Even if they're switched off. If you aren't confident stop reading. Don't come crying to me when you're dead.

Right, on another thread someone queried about biasing these amps. It's a piece of piss so I said I'd take some pics and post them here. Also, there doesn't appear (as far as I'm aware) to be anything online that's easy to follow and not full of "Yankee" jargon/slang.

This process is correct for Blues Deluxe and Hot Rod Deluxe amps. It may or may not apply to other Fender amps but I don't have any experience of any other models so can neither confirm nor deny this.

Here goes :

Unplug the amp. Lay the amp down flat and take the back off. You need to remove 6 screws and then pull the back cover off :



Ohh scary stuff!

If you are replacing tubes then do so now by simply pulling out and replacing the old ones. Take care to line up the tubes correctly to avoid damaging pins.

The two points you need to know about for biasing are marked below. One is a ground point and one is a special resistor Fender installed to enable you to bias your own amp. We will be measuring the voltage differential between these points and adjust it accordingly. This is called setting the bias............



Here's the ground point in more detail.It's the lower red circle in the pic above :



I personally clamp the black lead of my multimeter onto the little bit of metal screwed to the chassis. Using "clip on" multimeter leads is better in numerous ways. It's a bazillion times safer as you clip the test points on and then your hands are free to adjust bias, pick nose, etc.

Anyway, go ahead and clip your black multimeter lead onto this test point.

Then plug the amp in and switch onto standby. Leave for a few minutes to let the valves heat up, then switch on properly.

From now on be even more carefull then you were being before. Death can occur.

To be continued...........................

[nocaster]

Righty-ho

The next test point you need to be aware of is the special resistor Fender installed. It's the upper red circle above and is near the power tubes :



And more specifically :



That leg of that resistor is important. The voltage differential between that and where you connected your black plug is your "bias".

Set your multimeter to its DC mV setting then go ahead and (carefully) connect your red test lead/clamp to that resistor leg.

The reading on your multimeter is your bias for both tubes combined.

Here's mine :



I have 6L6 tubes installed so the bias should be set between 60mV and 70mV. They can obviously be set slightly above or below this but this is the optimum range. Above this will overheat the tubes and reduce lifespan, below this creates a cold tone. I only use my Blues Deluxe for clean tones so I set mine slightly below to increase clean headroom. For info, Fender supply the Blues Deluxe with the bias set at 68mV.

If you want to install 6V6 tubes (very nice mod!) you need this figure to be significantly lower. 40mV is ideal for 6V6 tubes.

So, how do we adjust bias??

To be continued............

[nocaster]

You need to locate the variable resistor that enables you to increase/decrease the bias voltage. This is bang in the centre of the amp :



And more specifically :



Dead easy - turn it clockwise to increase bias and turn anti-clockwise to decrease.

Turn it a little bit at a time and re-check the bias. A little movement can change the bias a lot. Get it set to where you want it and switch off the amp.

Disconnect your probes and put the cover back on.

Job done.

K

[gettinthebug]

That my friend is one awesome lesson and will prolly save peeps here many Kings shillings that otherwise would be thrown at amp techies who as we all know are masters of both the black arts and smoke & mirrors - well done Kev!

[bertie]

excellent, best thing you've ever posted - certainly makes a nice change from looking at pics of that R08 and Martamps every week

seriously - well done

[nocaster]

excellent, best thing you've ever posted - certainly makes a nice change from looking at pics of that R08 and Martamps every week

seriously - well done

I'm glad you got that off your chest!

K

[bertie]

for this thread, certainly

piss-taking asside, this is most excellent work

[relic245]

Another thanks for that lesson.

Is there any reason why you don't connect the red lead to the resistor until the amp is switched on?

[nocaster]

Is there any reason why you don't connect the red lead to the resistor until the amp is switched on?

None at all. It's just the way I worked through it in my head then typed it out!

I suppose connecting it then switching on would make more sense assuming you multimeter is on the correct setting.

K

[carl999pop]

+1