Is my Bias Rite faulty and reading too high?

[Mickey64]

I've just biased my Fender '57 Twin Reissue, with new Tung Sol 5881's at 29.5ma and 32ma, (yes I know they're not that well matched), with my older Weber Bias Rite. The amp sounds a bit cold. So, I searched these pages and found I could test the bias by connecting my meter either end of the bias resistor, which I believe is R45 in this amp. I did that, and it read 48. Does this mean 24ma on each tube? If so, my Bias Rite must be reading too high.

This also explains why, when trying to install a pair of JAN Phillips 6L6WGB's I have as spares, I couldn't use the bias pot to get less than 38ma, using the reading from the Bias Rite. In fact, it seems it could have been around 31ma, which would be OK.

So, is the bias resistor method the more accurate one?

Any replies much appreciated, I really want to get this right!

It also means I may have been biasing my Vibroking too cold for several years, and explains why the tech who serviced that amp a few months ago tweaked the bias up a bit as he said it was running a bit cold. My Bias Rite then read 37ma, so I took it down again!

[RussB]

What is this R45 "bias resistor" you speak of? Is it a 1 ohm resistor from pin 8 of the power tube socket to ground?

[Mickey64]

On the schematic it's a 47k resistor from the bias adjust pot (R46) to ground.

[slider313]

You were measuring negative voltage. -48 The tubes you have are close and if you can bring them up to 36ma it should sound better.

[Mickey64]

Originally Posted by slider313

You were measuring negative voltage. -48 The tubes you have are close and if you can bring them up to 36ma it should sound better.

Sorry slider313, I'm a veteran guitar player but a beginner tech....do you mean the tubes are well matched and that my bias rite is OK after all? Also, 36ma on 5881's seems high, after all I've read.

[slider313]

What's your plate voltage? Running them at 65% mpd at 410 volts would give you about 34ma. If you have one running 29.5 and the other 32 you bring the one at 32 to 36 and the other one will be at 33.5 for an average of 34.75

[r8burst]

This happened to me with my old Weber Bias Rite. Readings were all over the place. I was ready to buy a new meter.

Then someone suggested I change the battery.

Meter now reads perfectly.

Boy did I feel stupid.

Hope this helps.

Tony

[Mickey64]

slider313, plate voltage is currently about 480vdc. Bringing the current up to 36ma works out at about 75%; a bit high?

R8burst, I checked the battery and it reads just over 9 volts. I've put in a new one anyway as it's been there at least 5 years; good idea!

Thanks guys, think I'm going to have to resort to finding a tech, which is not easy in my area.

[RussB]

480v?

That's mighty high. I would not run 5881's with that kinda plate voltage.

To get your 6L6WGB's to bias cooler, replace R45 from 47k to a lower value...33k, 27k, 22k, something around there. The goal is higher negative (grid) voltage on pin 5 of the power tubes.

Many consider the Jan-Phillips 6L6WGB a 6L6GC in a small bottle, so with 480pv 38mA would put you at 60%, just about perfect

[Mickey64]

Thanks again for the responses. Yes, 480v is a bit high, but if my Bias Rite is reading high, then I've biased cold, and the voltage, presumably, will be higher than if I'd biased a bit hotter. I think I'm going to put some 6L6's back in this amp until I know what's really going on.

[slider313]

Bias rites read about 5 volts higher because of the inclusion of the cathode voltage . If you read the voltage on the plates with your meter it will be less.

[Mickey64]

Mine only reads ma, though I'm going to send it to Ted Weber for updating to the new version, which reads plate voltage as well, as soon as I get this issue sorted out.

I've put my Svets back in, and biased to around 33ma, plate 470v, until I get a tech to help me with this. I'm beginning to think I bought the wrong amp; it doesn't break up enough for me. I thought, after reading the TQR and some other owners experiences, that I could put 5881's in to correct this, but if biasing is an issue, and the 5881's can't take the voltage, then I'm stumped.

[jcs]

i own the weber bias rite with plate voltage and cathode current reading,but mine plugs into a multi meter.

i highly suggest getting this model as it allows you to use various multi meters and you read pv and cc at the flip of a switch.

mine has never failed me.

one thing you can do (that i still do) is also use your judgement on how hot the tube actually is and keep an eye on the power tubes plates in a darkened room observing for any hint of a red stripe while playing the amp.

[Mickey64]

Thanks jcs. Actually I just tested the probes by connecting each ends of pin 8 to my meter, and both still have the 1 ohm resistor intact, so looks like the Bias Rite is OK.