How to bias the an Express?

[fzfwyv]

Ok team, one last question, although I'm not doing it tonight...too tired.

How do I bias my Express build? Forgive my ignorance but the last few amps I've had were el84 cathode bias amps so I never had to worry about it.

I have a typical multimeter and a matched set of el34's. What voltages do I measure and/or what is the optimum bias setting?

Again, any help is appreciated. Sorry for being such a doofus.

Pics & clips will be coming this weekend if i get the amp fired up tomorrow. I spent tonight hooking up the PT and checking all connections. So far so good.

Thanks again!

-Rich

[paulruby]

First, install 1 ohm resistors between pin 8 and ground for bias measurement. I install the resistor between pin 8 and pin 1 and then ground pin 1. Since V=I*R and R=1 then V=I. The voltage in mV is equal to the cathode current in mA at pin 8.

For a wreck with ~420V B+ and big bottle tubes, you want the current up between 40 and 50mA. You should measure 40 to 50mV on pin 8. If you're runnign 6V6, keep the current down at ~25mA.

[MooreCowbelle]

Check this thread on S2's site:

http://www.websitetoolbox.com/tool/post ... 1&trail=15

[PeteRH]

Measuring plate current is potentially dangerous!! I would not suggest the transformer shunt method and suggest you read Randall Aikens method and also biasing articles at www.aikenamps.com
I can not link direct to the appropriate article but look for this paragraph.

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

Beautiful work Randall!!


I would bias the express at 30V just as Ken suggests. Then work out what the current draw is and if suitable..... use those tubes. If they dont work at 30V stick 'em in ya marshall and try another set.

One thought is that the 30v bias works for both EL34's and 6V6's. The other thought is that the 30 volts works right with the circuit.

Cheers
Pete

[rhinson]

hello, i agree with paul that the 1 ohm resistor method is the best way to be able to SET the bias (use 1% tol. resistors). some people here want to build exact cosmetic clones and won't do that. the post about one of the ways aiken suggests to do it is a very clever way to MEASURE where your bias is at but not that convienent to CHANGE/SET the bias because if you move your bias pot just a little and change the voltage drop you have to grab your calculator, recrunch the numbers, and you may have moved it too much so now you have to move it back again, then recrunch the numbers, and so forth. this is even more cumbersome for newbies because they don't have any sense of what moving the bias pot means in terms of changing the current draw on a given amp. if you have insulated boot/clips for your probe tips doing the transformer shunt method is not that dangerous---just make sure you have a good clip connection on pin 3 of your power tube and you can use the front leg of the big 1k resistor (easy to make a good connection on that one). since you're measuring current you can actually quickly see what you're measuring and make the adjustment. the only "BEWARE" here is that you do not let one probe tip accidently touch anything while removing it until BOTH are unhooked from the amp.
if you're freestyling this circuit forget about this ken said -30v thing. if you use moose's power trannie (around 390 on the plates i think he said) -30v might get you there or it might not (from experience i'd say probably close). if you're using a different trannie that'll give you higher plate voltages, the safest thing to do is with the amp on standby set the neg. voltage to like -36 to -40. turn the amp on check the bias then raise to where you want it to be. most of you guys (esp first timers) will find out you will have to tweak the bias network a little to get it to work for you. i believe a few folks have already posted regarding this. most beginners probably don't have a couple of extra quads of el34's laying around to sub in and out to maybe get one to work. since this hobby is about learning and developing expertise and understanding about tube amps, it's better to be able to adjust the circuit and make it use the tubes you have. the same with the transformer shunt method---it might not be the "safest" relative to the other methods, but in the ongoing learning process it's important to develop your technique and careful attention (ie--watching/thinking about what you're doing) to be able to use it safely if need be. poking around in live tube amps demands this------unless you know a couple of cute paramedics that you don't mind stopping in on a regular basis (non-social call unfortunately). rh

[MooreCowbelle]

rhinson: By "tweak the bias network", what exactly do you mean? I know Nigel just had a situation where he had to sub in a new pot to get the bias range he was looking for - is this something that is expected?

[Legin]

rhinson: By "tweak the bias network", what exactly do you mean? I know Nigel just had a situation where he had to sub in a new pot to get the bias range he was looking for - is this something that is expected?

I changed the 220K resistor to a 180K rather than changing the pot in the end.
Nigel

[PeteRH]

Rhinson,
No arguement here regarding the 1 ohm resistor method of checking bias but as you stated many wreck builders want to build a cosmetic clone and therefore these are out. The next best method is the voltage check.

the post about one of the ways aiken suggests to do it is a very clever way to MEASURE where your bias is at but not that convienent to CHANGE/SET the bias because if you move your bias pot just a little and change the voltage drop you have to grab your calculator, recrunch the numbers, and you may have moved it too much so now you have to move it back again, then recrunch the numbers, and so forth.

You are thinking about this arse about. First you determine the voltages you are looking for. Lets say you want to run 40mA for a set of EL34's at 400V and the resistance of one side of your output tranny is 35ohms. V=IR=0.04 * 35 = 1.4volts You are looking for 1.4 volts and adjust yourbias pot until you reach this.
Lets say you are adjusting by ear but want to keep the valves between 40 and 45mA. The desired voltage for 45mA is 0.045 * 35 = 1.575 volts
I now have a range without running back to the calculator, I can play the amp and listen to what sounds better and doing it a lot safer than the transformer shunt method.

Rhinson, I agree that if you are not using a B+ of 400V then dont use the 30v. If you dont use 400V you aren't building an express.

fzfwyv, I recommend downloading Duncanamps Anode Load calculator at http://www.duncanamps.com/software.html . A very handy excel spreadsheet that helps determine/calculate what plate current to use. I added another three lines to the excel spreadsheet where you enter your chosen plate current and your dc resistance of your output tranny and it then calcs the voltage.

Cheers
PeteRH

[rhinson]

hello pete, thanks for the arse about. i see what you are saying. but quite frankly, if you're talking about clipping both probes live into the circuit (for newbies) you can still fiddle about and get sparks. most meters are fused on the current side and the most damage i've ever done is waste a couple of meter fuses when using the transformer shunt method. the way of using ground reference and subtracting the values is indeed safer but again a little slow to get you there as you can't view what you're doing on the fly. but it's good know all the different ways of biasing----the more ways you know to do a thing the better off you are. i like aiken's site a lot myself----his section on neg. f.b. ought to be a sticky on most amp forums on the web! anyway thanks for the input----i still just wish everyone would use the 1ohm resistors!! rh