Fender Dlx Rev build- last Q b4 fire-up.

[Tom Phillips]

...I dont know how I would be able to tell "the bias Voltage at pin(s) 5 of the power tube corresponding to the current readings and the plate voltage readings for each setting you try"

Just measure the voltage at pin 5 with respect to chassis ground.

...I thought thats what I was doing, I thought the bias voltage was ascertained by way of adding the 1r then measuring across, to read for eg 20mV. Using Ohms law the I= 20mV/1R (confusingly 2R.. so what do I do here?).. means 20mA...

That voltage reading across the 1 Ohm resistor is just a means to derive the current. It is not the "Bias Voltage"

[Sea Chief]

These are going to be small ohm and DCV measurements so you do need to know what the default measure is for your meter. The default measure needs to be subtracted from your reading. With the meter set to DCV touch the probes together and wait for the reading to stabilize then record that number Now do the same for a low value resistance setting. Now do rach again. If the readings come out the same (ish) you're good. If they don't, well, some lesser meters will do that and all hope of an accurate measure goes out the window.

First make sure the 1R resistors you bought of ebay are actually 1R. Let's assume they are for the moment. Now, with the amp on, in play mode, plugged into a load and all controls set to zero, measure the VDC across each 1R resistor. Don't forget to subtract your default figure.

The math for the 1R resistor (provided they are indeed 1R) is moot because 1mVDC measured equates to 1mA of current. So the mVDC reading oacross those resistors IS your mA of current. No need to use an ammeter at all.

Multiply the current figure and the plate voltage figure to figure the tubes watt dissipation at idle. You want to see about 7.5 to 8.5 watts.

Watts?? i hadnt even considered or thought watts were involved.. I thought it was the (idle) current I was measuring. My head is nearly bursting I havent a clue whats going on.

Ok how on earth do I multiply 6mV x 478V? even my calculator says 'MEH'!

Ok so 6mV x 478V =

[Chuck H]

6mV is .006 (which is the voltage across the resistor but also REPRESENTS the current through the tube)
478V is 478

Current times voltage is watts.

.006 X 478 = 2.87 watts

A little low

[Sea Chief]

These are going to be small ohm and DCV measurements so you do need to know what the default measure is for your meter. The default measure needs to be subtracted from your reading. With the meter set to DCV touch the probes together and wait for the reading to stabilize then record that number Now do the same for a low value resistance setting. Now do each again. If the readings come out the same (ish) you're good. If they don't, well, some lesser meters will do that and all hope of an accurate measure goes out the window.

I read 0.01mV so Im calling it the same/ no difference.

First make sure the 1R resistors you bought off ebay are actually 1R. Let's assume they are for the moment. Now, with the amp on, in play mode, plugged into a load and all controls set to zero, measure the VDC across each 1R resistor. Don't forget to subtract your default figure.

No, they read 2.2r which is a pita. Each on of the 10 on a strip of resistors 2.2r. And they -are- 1R. The colour codes I checked. So what do I do?

The math for the 1R resistor (provided they are indeed 1R) is moot because 1mVDC measured equates to 1mA of current. So the mVDC reading across those resistors IS your mA of current. No need to use an ammeter at all.

This is what i thought. So I measured, effectively 6mA if current. At max on the pot I menasure, effectively 10 mA of current (But I thought I was looking for ~20mA of current, and not even with the pot maxed?? this is what Im so confused about (let alone adding watts into the equation). Why only 6mV, am I doing s'thing wrong-? is s'thing wrong in the circuit-? but even more confusingly.. someone says this is normal).

And why is it suggested I measure pin 5 and not pin 3?

[Sea Chief]

6mV is .006 (which is the voltage across the resistor but also REPRESENTS the current through the tube)
478V is 478

Current times voltage is watts.

.006 X 478 = 2.87 watts

A little low

I have no idea whats going on- 2.8w??

[Tom Phillips]

Watts?? i hadnt even considered or thought watts were involved...

The basic principle is that you set the idle plate current by adjusting the bias voltage to achieve the correct idle plate dissipation. That's the biasing process. Any one parameter is meaningless without knowing the others.
Bias voltage is the voltage at pin 5 of the 6V6 with respect to ground
Bias current is the plate current flowing through the 6V6. It is derived by using the 1 Ohm resistor method in your amp.
The actual bias setting is the resulting plate power dissipation in Watts.

It's confusing at first and, I believe, confusing to you at this time because you are working a little ahead of your current electronics theory knowledge. What learning materials are you using / have access to? Books / articles at home or on line?

[Tom Phillips]

...No, they read 2.2r which is a pita. Each on of the 10 on a strip of resistors 2.2r. And they -are- 1R. The colour codes I checked. So what do I do?...

Tell us what resistance reading your meter displays when the test leads are shorted together.

[Sea Chief]

Im confused on 4 different levels. I expect to be confused by the 'idle current' which I dont expect to understand/ Im not going to/ and I wasnt ever going to. But that doesnt mean I cannot continue to test A and B and post the figures. Hopefulyy that is where you guys can help by pointing me "thats ok" or "thats not right", as you have been doing.

What I most confused about is ChuckH saying 2.87w. If this is 1/5th of expected figure, then something is seriously wrong: now, I cannot possibly even think what it could be. Adding to the confusion is why no-one seems to think that there is s'thing seriously wrong.

What Im also confused about is the figure I measure of 6mV. Why is this so low?
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What I need to know is simply what to do, to get a basic setting on my DR with regard to the bias. I havent a clue where I am with 6mV. Not an iota of a clue as its so far off "I set my DR bias to 20mA" (of which seems a good general point to aim for).

If you/ anyone could just tell me how I could get a normal figure/ a normal setting Id hugely appreciate it. if something is obvuously completely and utterly wrong (as 6mV seems) then please advise what it could be?

[Sea Chief]

Tell us what resistance reading your meter displays when the test leads are shorted together.

Good point. Ok 1.1 it reads. Good I understand 1 thing. Right so I am going to call my resisitors 1r and can continue.

Do I still need to test the voltage at pin 5-? (or was it meant to be pin 3 you mean?).

[Sea Chief]

V8 pin 5= -42V

[Chuck H]

No, they read 2.2r which is a pita. Each on of the 10 on a strip of resistors 2.2r. And they -are- 1R. The colour codes I checked. So what do I do?

Did you do a default meter reading for the ohm measurement?

So I measured, effectively 6mA if current. At max on the pot I menasure, effectively 10 mA of current (But I thought I was looking for ~20mA of current, and not even with the pot maxed?? this is what Im so confused about (let alone adding watts into the equation). Why only 6mV, am I doing s'thing wrong-? is s'thing wrong in the circuit-? but even more confusingly.. someone says this is normal).

Throw all that out until we know the actual resistance of the cathode resistors.

How on earth do I multiply 6mV x 478v (this is what I measured before on pin 3 of the 6v6).

6mV is .006. 6 milli-anything is .006 for the purpose of the math. And 478V is 478 for the purposes of the math. So the equation is .006 times 478.

And this is NOT what you measured before on pin 3. Pin 3 is the plate and measures 478VDC. Pin 5 is the control grid, which is where the negative bias VDC from the bias circuit is applied. Which brings us to...

And why is it suggested I measure pin 5 and not pin 3?

We need to know that the bias circuit is working. Negative VDC present at pin 5 tells us that bias voltage is present. Turning the bias adjustment pot should change the negative voltage on pin 5. I expect you have negative bias voltage because the power tubes are operating so cold. Whether you have the correct negative bias voltage or the correct amount of adjustment would be determined by checking voltage at pin 5 while adjusting the bias pot. You already measured 50-ish negative volts there before. You should have the ability to adjust that voltage between negative 35ish and negative 50ish. This test is best done with the power tubes removed.

Read everything carefully. There is a lot of information here but we have spelled out everything both rote and with explanation. Take it one step at a time. Pull the tubes and check for negative bias voltage and proper adjustment range on pin 5 first. Then retest your cathode resistors and subtract the meter default number. If it turns out they are indeed 2.2R you can always parallel two of them for 1.1 ohms. BUT BE SURE YOU HAVE AN ACCURATE MEASURE WITH THE DEFAULT SUBTRACTED FIRST!

[Tom Phillips]

...Do I still need to test the voltage at pin 5-? (or was it meant to be pin 3 you mean?).

Yes I want to know the voltage at pin 5 of each 6V6.
Let's do this.

1) Pull out both of the power tubes.
2) Monitor the voltage at pin 5.
3) Adjust the bias pot over its full range and report the minimum and maximum voltage readings that you get at pin 5

This will tell us if your bias power supply circuit is working properly

[Sea Chief]

Read everything carefully. There is a lot of information here but we have spelled out everything both rote and with explanation. Take it one step at a time.

Believe me I have. I spent the whole afternoon on this & the thread reading, re-reading, going over other threads about DRRI bias. I just want to set the bias 'about right' for now (not perfect/ not tuned to my style/ or anything more than just 'about right' is 110% fine with me). Thats all.

I do not understand my 6mV reading, & I do not understand why someone above said this is normal.

Is it normal, or isn't it normal?

[Chuck H]

Good point. Ok 1.1 it reads. Good I understand 1 thing. Right so I am going to call my resisitors 1r and can continue.

Do I still need to test the voltage at pin 5-? (or was it meant to be pin 3 you mean?).

Excellent. Perfect. But I want to iterate that this is exactly what I said you needed to do ten posts ago. Don't rush ahead and skip steps or it will confuse everything and make all three of our efforts wasted.

[Chuck H]

I do not understand my 6mV reading, & I do not understand why someone above said this is normal.

Is it normal, or isn't it normal?

It's not a normal operating range for a guitar amp. It MAY be normal to experience that figure before a proper adjustment. Don't obsess about it anymore. We need to test the other things to see if that figure can be adjusted. That is the whole point of the tests.