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FDP Forum / Fender Amps: Vintage (before 1985) / sf bias probing gadget?

swampblues

south louisiana, usa

get'em jumpin', mbj
Apr 22nd, 2004 05:20 PM   Edit   Profile   Print Topic   Search Topic

what is it? where to get it? how to use it?

also how is the 1/4" "output tube matching" jack on the back of my ultralinear pro reverb used?

thanks.

Blue Strat

USA

Apr 23rd, 2004 06:09 AM   Edit   Profile   Print Topic   Search Topic

Get a Bias Rite from www.webervst.com

The output tube matching jack allows you to match 2 mismatched power tubes. You would need something like the Bias Rite in order to use this control.

You'll need to have you "bias balance" control converted to a "true bias" control by a tech in order to set the bias of your power tubes.

Procedure for Bias Rite:
Remove one of your power tubes, insert the Bias Rite into your amp's power tube socket, insert your power tube into the bias right. Connect the leads from the Bias Rite to digital mulitmeter, set the meter to DC volts. Turn on the amp and let it warm up for a minute, put standby switch in "play" position. Observe the readings on your mulitmeter. Adjust your bias control for the desired reading.



Mike K

KCA NOS Tubes & Amp Repair

(This message was last edited by Blue Strat at 06:16 AM, Apr 23rd, 2004)

jazzguitar

Delicious tone !

Looking for the perfect job
Apr 23rd, 2004 08:48 AM   Edit   Profile   Print Topic   Search Topic

"You'll need to have you "bias balance" control converted to a "true bias" control by a tech in order to set the bias of your power tubes."


I happen to disagree, politely. If the tubes don't need a *rebias* a bias check is all you need (and if it does, a bias set trimpot is easily added).

Just let the amp warm up 1/2 hour and then set for minimal hum. This balance option is quite nice to have.


swampblues

south louisiana, usa

get'em jumpin', mbj
Apr 23rd, 2004 02:41 PM   Edit   Profile   Print Topic   Search Topic

cool - where can i purchase a bias rite and bias trimpot?

Blue Strat

USA

Apr 23rd, 2004 05:37 PM   Edit   Profile   Print Topic   Search Topic

Get a Bias Rite from www.webervst.com

You don't need a bias trimpot, you can use the "matching" pot and rewire it if desired.

Jazz:
I assumed that people would understand that they wouldn't *necessarily* need the conversion if the tubes fell in the correct range.

Mike K

KCA NOS Tubes & Amp Repair

nonsqtr
Contributing Member
**

usa

...whatever remains must be the truth...
Apr 24th, 2004 02:49 AM   Edit   Profile   Print Topic   Search Topic

You can get a bias gadget on eBay for about ten bucks, maybe fifteen. It only does one tube at a time though. Which is fine for most purposes. I have one of those in my arsenal, it's one of the best investments I ever made. Search eBay under "bias probe" or "Fender bias".

One of the good things about the ultralinear amps is that you can set the bias balance by ear. It's really easy, just unplug the instrument and turn all the volume controls all the way down, and adjust the "output tubes matching" control for minimum hum. You'll have to listen pretty carefully, you can put your ear close to the speaker in a quiet room and you'll hear the background hum. Usually there's a well defined position of the control at which minimum hum will occur.

Then turn all the volume controls all the way up and adjust the "hum balance" control for minimum hum (if your amp has one of those).

Done, you're good to go. Those later model Silverfaces are really easy to set up.

swampblues

south louisiana, usa

get'em jumpin', mbj
Apr 24th, 2004 12:44 PM   Edit   Profile   Print Topic   Search Topic

i do have a "output tubes matching" and a "hum balance" jack (1/4") - how do i use those or where are the controls located on my '76 ultralinear pro reverb?

ok - as i seem to understand it now - this amp offers a control to adjust the bias for collectively both tubes (that would assume they are matched). if i needed to adjust the bias for each individual tube i would have to either make said conversion or use a trimpot on each tube.

nonsqtr - you use the bias probe on one tube - adjusting the bias with a control on the amp(still can't locate that) that affects both tubes? assuming the tubes are closely matched they both should be right on afterwards? you double-check with the probe on the other tube adjusting for a compromised setting if needed?

please excuse my ignorance on the subject - your advise here and in other posts has already enlightened me a lot, thanks.

Blue Strat

USA

Apr 25th, 2004 04:01 PM   Edit   Profile   Print Topic   Search Topic

Check my first response above regarding the (missing) bias control.

Mike K

KCA NOS Tubes & Amp Repair

nonsqtr
Contributing Member
**

usa

...whatever remains must be the truth...
Apr 25th, 2004 11:44 PM   Edit   Profile   Print Topic   Search Topic

Hi swampblues, if your amp has the "output tubes matching" control, that means it's designed to run with mismatched output tubes. Okay, let's qualify that, we'll say "slightly" mismatched. But that's the point of the control.

Also, if your amp has one of these, it won't have the ordinary "bias control" (unless it's been modded somehow). This latter control is the one you're probably thinking of, when you think of the usual bias control that adjusts the bias for both tubes simultaneously (and in that case, you're right that you'd need a matched set of tubes).

But the late 70's silverface Fender amps eliminated the "bias" control, in favor of the "bias balance" control. Your amp will either have one or the other, but not both (that's once again assuming it's a stock amp and hasn't been modified).

The bias balance control "is" the bias control, that is, they both do the same thing internally. It's just that, with the old kind (the "real" bias control), you needed matched output tubes, and you could set your aggregate bias anywhere from very hot to very cold. With the new kind, you can no longer adjust the "aggregate" bias, instead it's fixed at a factory preset value. The only thing you can adjust on the newer amps is the "balance" in the biasing, between one output tube and the other.

So, with a UL Super, you probably have just the "bias balance" control, and the "hum balance" control. There's no "real" bias control on that amp. The good news is, that makes it really easy to adjust when you're changing tubes. The downside of that is, you lose some flexibility in terms of your ability to bias the amp hot or cold.

By the way, that thing that looks like a jack, is really your control. If you look inside the hole with a flashlight, you'll see a plastic tab with a screwdriver slot in it. That's the way to adjust it, just find a screwdriver that fits and you're good to go.

There is a mod you can do, that will allow you to use "both" the bias balance control, and a "real" bias control. That way you can have the best of both worlds, you can bias your amp hot or cold, as well as balancing between mismatched tubes. But the mod involves a circuit change, so you'll have to get into your amp with a soldering iron and move a few things around.

But with a stock UL Super, it's really easy to adjust after installing new output tubes. Just follow the procedure I previously mentioned, that should be just as good as any tech can do in the shop. It would be a rare amp that couldn't be adjusted this way.

swampblues

south louisiana, usa

get'em jumpin', mbj
Apr 26th, 2004 06:18 AM   Edit   Profile   Print Topic   Search Topic

ahhhhhhhhhhhhhh - now it all makes sense - cool!


actually every day, when checking my post office box, i expect to find a bill from you, nonsqtr. your replyes contain a wealth of usable info. i truely appreciate it. my tube amps suddenly seem less mysterious.

thanks, swampy

swampblues

south louisiana, usa

get'em jumpin', mbj
Apr 26th, 2004 07:15 PM   Edit   Profile   Print Topic   Search Topic

nonsqtr - i installed a new set of matched tubes and adjusted the amp following your instructions. it now performs loud and clean with virtually no hum. thanks again.

i now will track down the bozo engineer that charged me 90 bucks for nothing and show him how to do it.

i still will get a bias probe just to see what the "fixed bias" is set to.

RockyA
Contributing Member

Quad Cities USA

"Playin straight into the Amp"
Apr 29th, 2004 02:11 PM   Edit   Profile   Print Topic   Search Topic

The fixed bias is normally a 15k ohm resistor with a negative bias voltage of -52V

FDP Forum / Fender Amps: Vintage (before 1985) / sf bias probing gadget?




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