tube change

[twl]

i recently changed tubes in my bandmaster from gt 6l6 to tungsol 5881 where should i set the bias at for the 5881

[novemberrain]

http://members.shaw.ca/house-of-jim/Htm ... ables.html

Use the 6L6GC table and tweak to taste.

[nomad100]

that's an awesome table next i just need to figure out how to check the bias.

[PCollen]

i recently changed tubes in my bandmaster from gt 6l6 to tungsol 5881 where should i set the bias at for the 5881

Set the bias voltage at the point where quiescent power tube plate dissipation , per tube , is about 65-70% of max allowable plate dissipation for those tubes.

You will have to measure plate voltage, Vp, and plate current, Ip , for each tube, as well as calculate the 70% wattage value of the tubes. If you don't know how to do this, take your amp to a qualified service tech.

[novemberrain]

that's an awesome table next i just need to figure out how to check the bias.

NO math bias:

In a fixed bias amp the cathodes are grounded. If they're not there already lift the ground on each of the power tube cathodes and connect a 1 ohm 1 watt resistor between each cathode and ground. The cathodes are still effectively grounded through the resistor on each you just added. Using Ohm's Law 1 millivolt across a 1 ohm resistor equals 1 milliamp, set your meter to read millivolts and you're actually reading milliamps. Clip one meter lead to the cathode and the other to ground across the resistor. Just twist your bias pot to get your bias in the correct range according to the chart, if you can't get it in range you may have to modify your bias supply.

Leave the resistors in there, they'll be there if you need to set bias again.

With matched tubes the bias of each tube should be the same. To get it exact you may have to do a bunch of fine tweaks skipping back and forth from one tube to the next. If the cathode current of each tube doesn't match your tubes aren't matched.

[NOGE]

Don't forget to add the screen current to the cathode current to get the anode current.

Measure the voltage across the screen resistors for each tube (careful, the screen resistors are connected to HT) and use ohms law to calculate the screen current.

[markh]

Don't forget to add the screen current to the cathode current to get the anode current.

I think he meant "subtract the screen current..."

OTOH, you can just subtract 10% to get you in the ballpark.

--mark h

[dartanion]

Sheesh!!!

Why not just tune the bias pot to taste and check for redplating

No redplates, no problem

[NOGE]

I think he meant "subtract the screen current..."

Doh!

Yes, I got that back to front didn't I. Sorry.

[allgonoslow]

Sheesh!!!

Why not just tune the bias pot to taste and check for redplating icon_razz.gif

No redplates, no problem icon_lol.gif

I read somewhere a similar explanation. Something along the lines of "Crank it up to a nice healthy glow and then back it off just a hair". If you're quick you shouldn't cause any damage.

But, a DMM might be more accurate.

[Aharon]

This is even better ...................a bias calculator.
http://www.webervst.com/tubes/calcbias.htm
Aharon

[maglemer]

And don't forget that the old 5881 (Tung-sol) differ from 6L6GC, and has max Pa 23W instead of 30W as for the 6L6GC.