How to measure bias in a Fender Pro Junior (1ohm resistor)

[Iplayloud]

About adjusting the bias on my Pro Junior, i come from the marshall side so like i always used to do with EL-34s i soldered a 1 Ohm resistor between the cathode and ground. Then measuring the millivolts across it and determining the bias in milliAmps. I put that 1 ohm resistor on pin 3 of one tube and read 33 mV.
So .033 mA X 319V (plates) = 11.56 Watts (for that particular EL-84)
I was happy up to this point, because I read all over the internet that they were biased way too hot at around 17 watts per EL84 and here I see that mine was fine.

Now I check the schematic and R30 seems like the obvious bias resistor, I decided to swap it for a higher value, well actually I lifted one leg and soldered to a 50k pot is series. Now no matter how I raised the value of that pot the current draw never changed. So that raises 2 questions, is R30 the proper bias resistor ? And is this a proper bay to check the bias with EL84 by using a 1 ohm resistor ? Thanks

[Iplayloud]

Another thing that didn't seem right is I used an infrared temperature meter to check how hot the plates were, that gives you a good idea if they are biased too hot if you are too lazy to actually open the amp. And one peaked at 420 degress F. That's can't be right because on my MArshall DSL-401 (4xEL84) thaty only run at at arounf 250 degrees ?

[Iplayloud]

Schematic: http://www.schematicheaven.com/fenderamps/pro_jr.pdf

[stokes]

R30 would not be the resistor to adjust.I would replace R29 with a 30 to 50 ohm cermet pot and make your bias supply totally adjustable.Then any time you change power tubes you can dial in the "best" bias point.

[Enzo]

I am going to way out on a limb here and guess that stokes meant 30k to 50k pot instead of 30 ohm.

Unless he wanted the tubes running REALLY hot...


Reading the temperature of the tubes is not how to determine how "hot" the bias is. Tubes are rated for power dissipation. let us say you measure a tube temperature at 280 degrees. How many watts is it dissipating? How would you decide what temperature is best?

Furthermore, you have to be careful using non-contact thermal measuring devices. Are you measuring the temperature of the glass bulb, or are you "seeing" through the glass to the temperature of the metal structure inside? And even then, are you seeing the plate or the exposed heater or parts of the cathode, just what? The cathode has to be at least 1500 degrees - or something like that, I forget the actual number - to emit electroncs.

Changing R30 wouldn;t change the bias voltage - there is no circuit there to make voltage division. And just changing a series resistor won;t work here because of Ohm's Law. What causes a voltage drop? Current through a resistance. How much current is flowing through those grids? None. No current, no voltage drop. Changing R30 doesn't negate the lack of any current, so the voltage drop across R30 stays the same, regardless of its value.

[stokes]

Oops!Thats a safe limb,Enzo.I swear my mind was thinking k but my hands typed something else.

[Enzo]

Well, that's O...k...

[Robert M. Martinelli]

About adjusting the bias on my Pro Junior, i come from the marshall side so like i always used to do with EL-34s i soldered a 1 Ohm resistor between the cathode and ground. Then measuring the millivolts across it and determining the bias in milliAmps. I put that 1 ohm resistor on pin 3 of one tube and read 33 mV.
So .033 mA X 319V (plates) = 11.56 Watts (for that particular EL-84)
I was happy up to this point, because I read all over the internet that they were biased way too hot at around 17 watts per EL84 and here I see that mine was fine.

hi playloud,
The method you're using is "almost" right....you can't simply multiply the cathode current by the plate voltage, because the cathode current is the sum of plate plus screen grid current. You need to determine what the screen grid current is and subtract this value from the cathode current to get the plate current, this way, by multiplying it by the plate voltage you get the actual quiescent plate dissipation, which will be of course lower than the figure you obtained before ( not a bad thing, if you don't want to spend most of your time and money in replacing the output tubes instead of playing... ).

( To determine the screen grid current, expressed in Amps, measure the voltage drop across each 100 Ohm screen grid resistor, then divide the voltage drop by 100 Ohm ).

Now, about those 17 W, I strongly doubt there are current production EL84/6BQ5s capable of resisting more than a few seconds before melting if they were operated in such conditions....even AC30s, which have a ( well deserved ) reputation of being VERY rough on output tubes, are biased @ some 14.5 W, maybe the 17 W figure has been obtained without taking the screen grid current into account as well.

As to the bias circuit/mod behavior and the other issues, +1 on what Enzo said.

Hope this helps

Best regards

Bob

[Iplayloud]

Thanks guys, priceless info. A good lesson for my amateur self about no voltage drop if current isn't spent. I'm sure there's tons of people like me trying to maximize their tube amps performance even though electronics isnt' our branch.

For the screen current I know for EL-34's it's about 5-6 mA but I had read for EL-84 it's about 1 or less, almost negligible. Thanks for pointing it out though I had not included it in my equation.

Now the IR temp meter is totally not an exact science in terms of tube bias but I find it a very good tool for so many uses like deep frying oil temperature, soup, freezer, overheating stuff, so on... and for tubes, the plate structure is definitely hotter when they are biased hotter so it's a fast guess IMO when you have a reference temp. For example on that PRO Jr. one EL84 reaches 420 while the other only 375, you can tell right there it's not a matched set. And on my Marshall DSL I didn't solder 1 ohm resistors yet but they run at 250-275, identical cheap sovteks so the bias has to be lower. It doesn't tell me a number but I know it's not an emergency to open it and bias it. There might be other factors the I'm not aware of..

[Iplayloud]

... Are you measuring the temperature of the glass bulb, or are you "seeing" through the glass to the temperature of the metal structure inside? And even then, are you seeing the plate or the exposed heater or parts of the cathode, just what? The cathode has to be at least 1500 degrees...

I try to carefully aim it at the middle metal structure, supposing the infrared light must pass thru the glass... (?)

[stokes]

I try to carefully aim it at the middle metal structure, supposing the infrared light must pass thru the glass... (?)

An easier method is to just watch your plates,if they dont glow orange/red you are not in any danger,if they sound good,leave them.If they sound cold or sterile bias them hotter,if they break up too much bias them colder.Bottom line is,if they sound good and arent redplating,you are good,you dont need to know the numbers.

[Robert M. Martinelli]

For the screen current I know for EL-34's it's about 5-6 mA but I had read for EL-84 it's about 1 or less, almost negligible. Thanks for pointing it out though I had not included it in my equation.

Nope, the screen grid current in an EL84 P-P can be ( and usually is ) around 5-8 mAmps , so it's NOT negligible, and that's the reason why I pointed it out.

If it really was 1 mAmp or less I wouldn't have allowed myself to chime in, as I don't like cork sniffers or hair splitters myself .

( check "Ig2" on EL84 datasheets, or better yet, measure it yourself with the method I suggested in my previous post ).

Cheers

Bob

[MWJB]

Iplayloud, your math is dubious, 33mA x 319 = 10.25W, not 11.56W.

15W+ plate dissipation is not uncommon for a Pro Jr, at least from my experience. I would suggest 30mA per tube minimum as below this they can go sterile. So, if you have 33mA per tube I'd say you don't have anything to worry about. Even so, those amps that run 40mA+ seem fairly reliable, so if you have the means to check & adjust plate current, it won't do any harm...but if not, I would mainly be concerned if you are getting unwanted symptoms like hum, heatfade, glowing plates etc.

[Robert M. Martinelli]

Hmmmm....Yes and no....
I've been recently repairing a Fender BJ belonging to a friend, and found that, with the stock bias settings ( "insanely" set @ some 15 W ) the tubes' lifespan was considerably shortened, as a new pair of EL84 ( JJ ) lasted "from Xmas to new year's eve" ( Italian saying ).

I modded the bias circuit to run the plates @ 11 W quiescent, still hot, but I didn't want to go below that to keep the amp from becoming too "sterile", as MWJB correctly noted. After six months the tubes are still OK and the owner's very happy with the amp.

If Iplayloud wants to run the tubes THAT hot, then maybe it's advisable for him to throw in a couple of 7189s ( mil-spec EL84 ) or a pair of Russian 6P14Ps ( IMHO ).

JM2CW

Cheers

Bob