Can't Bias Amp???

Dennis Detweiler · Post by **Dennis Detweiler** » 10 Oct 2004 5:16 pm

A friend bought new tubes from "The Tube Store" for his Fender 75. The "Store" recommended SED (Russian) Winged "C" 6l6GC power tubes. A local electronics tech said, "I can't bias the amp because the tubes conduct too easily and amp doesn't have enough negative voltage". He also said Sovtec tubes are the only ones that will work. ??? Is this a correct prognosis?

Robert Parent · Post by **Robert Parent** » 11 Oct 2004 3:46 am

I don't know about the bias procedure for your amp, but a 6L6GC is a 6L6GC. It's like saying your car will only run on gas from Shell Oil....

There are quality difference between tubes but to say that other brands won't work is incorrect. I would run from that tech as fast as I could.

Michael Brebes · Post by **Michael Brebes** » 11 Oct 2004 7:03 am

The "75" doesn't have a bias pot, it has a "balance" pot, which is supposed to balance the two tubes in the power stage. Bias is actually fixed on that amp, with the bias being balanced between the tubes. He has two options: either balance the two new tubes and live with the bias where it is, or have the bias circuit rewired to Blackface style specs and turn the balance pot into a true bias pot. the biasing circuit should be able to be changed to the same used in Blackface Bandmaster, Bassman, Super Reverb, Concert, etc.

The "Winged C" tubes are actually made by the original Svetlana company and factory in Russia. Their 6L6 and EL34 tubes sound great. Somehow the distributor had gotten rights to use the name in the US and actually sold that right to another company.

Brad Sarno · Post by **Brad Sarno** » 11 Oct 2004 11:20 am

Sounds like your tech may not know the difference between the balance pot and a bias control. Have a tech add an adjustable bias pot and you'll be in bizness.

Brad Sarno

Ray Minich · Post by **Ray Minich** » 11 Oct 2004 12:30 pm

Gimme a -150VDC and I'll bias that thing!!!

Dennis Detweiler · Post by **Dennis Detweiler** » 11 Oct 2004 4:44 pm

Thanks guys. This forum keeps us on track.
Dennis

Jennings Ward · Post by **Jennings Ward** » 11 Oct 2004 6:42 pm

That tech should go back to school, and take up something besides space the next time........Jennings......

Play steel for the beautifull angelic sound of it.....

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EMMONS D10 10-10 profex 2 deltafex ne1000 pv1000, pv 31 bd eq, +

Ray Minich · Post by **Ray Minich** » 12 Oct 2004 11:27 am

Awful hot CB Linear don't ya think Bob?

Donny Hinson · Post by **Donny Hinson** » 12 Oct 2004 4:54 pm

Rebiasing???

Well, I hope it does more for you that I've ever had it do for me.

I realize I'm gonna take some flak for making this statement, but I'm gonna say it anyway..."Re-biasing an amp is kinda like "presision balancing" a car engine. Yes, it makes a difference, but it's also quite likely you'll never need it or notice it."

To put it simply, biasing does effect the gain, and it does affect the tone, somewhat. But (and this is an emphatic "but"), in normal use, neither the volume nor the tone controls are run in their extreme positions. So...IMHO, it's safe to say that you can get much of the effect of "re-biasing" your amp by just (Heaven forgive us) twisting some knobs.

Of course, it goes without saying that if you're using an amp at it's limits, it may give you a little boost. Likewise, if your bias is way off due to a bad component, you'll need to have the amp repaired, and brought back to spec. But then, you're right back to the old point of "it really doesn't make much difference".

For the vast majority of us, spending money re-biasing an amp every time you change a tube is kinda like spending money re-balancing the tires on your car every time you take a trip. In short, it's usually unnecessary. But if it makes you feel good (and you have the money, or can do it yourself), do it.