5F6A Biasing ?

[chuckb]

IS there another value of resistor other than 1ohm/3w that I can use for a cathode biasing resistor? I don't have any 1 ohm on hand and I want to get my bias at least in thae ballpark. I have -48 on pins 5 of the power tubes.

Chuck.

[pdf64]

0.5W or 1W is plenty for a 1ohm cathode resistor. 0.5W would take 707mA!
But anything upto 10ohms would be fine - though obviously it will need to be a higher rating the larger the resistance, at 10ohms 3W or 5W would be appropriate.
Just divide the mV reading across the resistor by it's resistance to get the current.
But a rough and ready check, you could measure the OT primary resistances, centre tap to each leg, then measure the voltage across those legs, then calculate the plate current from that.
It's rough and ready because the resistance of the windings changes some with temperature.

[tubeswell]

or use 2 x 2R 1% 0.6W little metal film resistors in parallel on each side - easy to fit it small spaces, and increases the chances of being close to a perfect 1R

[chuckb]

Thanks guys for the info. I'll be picking up 1ohm/3w tomorrow.

[RWood]

Here's what I do for grid-bias; there are certainly a number of favorite methods out there:

I temporarily place an ammeter in series with the power tubes' cathode-to-ground. It sounds like more trouble than it actually is, but it's a simple desolder. I use meter leads with insulated gator clips, too, btw. I set the meter to the 200mA range and adjust the bias to the ballpark I want to be in and let the amp idle for a good long while as everything settles in.

I use a spare volt meter (Harbor Freight - $2.99!) and alligator leads and clip it to pin3 /plate and watch the voltage change as I adjust the bias. While I do this, and let the amp idle, I use a hand calculator to figure out the plate dissipation in watts, tweaking as necessary and playing/listening, too.

When I am satisfied, I use the voltmeter to see what the resultant negative voltage is, on the grids, and make a note of that number. Reconnect the cathode connection and it's good to go.

RWood

[Chuck H]

So for a bias meter you COULD hook up a panel ammeter, as in RWoods example, and leave it in permanently for constant monitoring. Or, also as in RWoods example, have external test points that accept meter probes and a momentary switch that temporarily breaks the cathode path to ground via the test points. When you want to check bias you just plug in your meter and depress the momentary switch. Easy peezy.

Chuck

[chuckb]

Hey Chuck, I've got 1R/5W now and I have them installed on each tube. I was wondering what my bias setting would be for the 5881 tubes I have in the amp.I have 463 vdc on pins #4 of the power tubes and I've set them to 30ma, is this the right bias setting for these tubes? Well I have 30ma on one tube and 29 on the other thereabouts.

Chuck.

[tubeswell]

I have 463 vdc on pins #4 of the power tubes and I've set them to 30ma, is this the right bias setting for these tubes?

Pin 4 is the screen. Pin 3 is the plate. Taking it for granted that the screen is within a volt or two of the plate anyhow, then you are idling at around (463V x .03A =) 13.9W. Depending on which tube datasheet you read, a 5881 has a max dissipation rating of 17W, 19W or 23W, or something in between. So ~14W would be okay for Class AB1. How does it sound to you?

[chuckb]

Hey Tubeswell, The voltage on pin 3 is 463 and the voltage on pin4 is 468. sorry for the mix-up. So when you multiplied 463x .038w where did you get the figure .038 from? was it the data sheet for the 5881? The amp sounds pretty good I must say although at higher volumes I get some farting out of the bass strings. I rechecked all my grounds and got all of the power grounds seperate from the preamp grounds.I'm also using two inputs(bright and normal) instead of four and have made this into an amo head. I'm running it through a 2x12 celestion V030's at the moment because my amp has only a 16R output.

[pdf64]

'(463V x .03A =) 13.9W'

plate voltage x plate current = plate dissipation (power)

0.03A = 30mA