# Groove Tube 6L6 GE Bias



## ampaholic (Sep 19, 2006)

I've never been a huge fan of Groove Tubes but in a jam I bought a matched pair of their 6L6 GE reissue tubes. The reviews were good on them and it was all I could get locally. These have a #4 rating which is at the low end of their "average" rating.
Here's my concern. I took the Svetlana 6L6's that had been biased at about 32 ma out of my BF Super Reverb, installed the GT's and checked the bias. Without touching anything the bias measured 12.3 ma on one tube and 10.7 on the other. In order to get these biased into the 30 ma range I had to crank the bias pot WAY up, almost to it's max. Plate voltage remained pretty steady at 440 V.
I then pulled a pair of JAN Phillips 6L6's out of another amp, put them in the Super and like the Svetlanas I had to turn the bias control way down to settle in the 30 to 35 ma range. Plate voltage again is riding around 440 Volts.
Does this seem normal with the Groove Tubes? Is it the lower rating (#4) that has them running so cold as compared to the other tubes?
Advice appreciated!


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## Wild Bill (May 3, 2006)

ampaholic said:


> I've never been a huge fan of Groove Tubes but in a jam I bought a matched pair of their 6L6 GE reissue tubes. The reviews were good on them and it was all I could get locally. These have a #4 rating which is at the low end of their "average" rating.
> Here's my concern. I took the Svetlana 6L6's that had been biased at about 32 ma out of my BF Super Reverb, installed the GT's and checked the bias. Without touching anything the bias measured 12.3 ma on one tube and 10.7 on the other. In order to get these biased into the 30 ma range I had to crank the bias pot WAY up, almost to it's max. Plate voltage remained pretty steady at 440 V.
> I then pulled a pair of JAN Phillips 6L6's out of another amp, put them in the Super and like the Svetlanas I had to turn the bias control way down to settle in the 30 to 35 ma range. Plate voltage again is riding around 440 Volts.
> Does this seem normal with the Groove Tubes? Is it the lower rating (#4) that has them running so cold as compared to the other tubes?
> Advice appreciated!


In the words of Tony Soprano:"Fuggedaboutit!"

All tubes vary from the datasheet specs. That's WHY we have bias controls! Tubes today seem to vary even more than they did during the "Golden Years". That's because much of the experience in building them has been lost. The people who used to make them died off and their paperwork was buried with them!

Sometimes I get a pair of tubes that are so far one way or the other that there isn't enough range to set them with the bias pot. So I have to change a resistor in the supply to bring things into range.

It doesn't matter a whit for the performance of the tubes! As long as the bias is set properly they all will perform just fine. What's more, don't worry about a couple of ma. difference. In the hifi world users get all anal about matching tubes as close as possible, to achieve the absolute lowest distortion. Often there will be separate bias controls for each tube.

I keep saying this 'cuz it's important: Why on earth would you want to make your guitar amp as "hifi" as possible? It's an ELECTRIC guitar amp! It's SUPPOSED to have some distortion! That's what makes it sound good! You will never sound like Pat Travers with a hifi amp.

A couple of ma. difference actually makes electric guitar sound better! It adds a little harmonic rich "swirl" to the sound. However, we don't want too much of a difference. The reason we match them in the first place is that an extreme mismatch will have one tube hogging the load, delivering more than its share of power while the other one "loafs". This leads to shorter tube life for one tube and worse yet, an extreme mismatch will cause much more hum and big gobs of unbalanced current flowing through the output trannie, which can damage it! I've seen it happen before.

My personal limit is 4-5 ma. I don't have any theory to back this up as a safe limit. It's just a rule of thumb from my own experience over the years.

Last thing, don't be surprised at the plate voltage staying constant at 440 volts. A guitar amp's power supply doesn't have a lot of regulation. It doesn't need it! A higher idling current will tend to pull the plate voltage down a bit and conversely, a lower one will allow it to rise. If you have the new tubes biased to the same point as the old ones you would expect the plate voltage to be the same!

That's my story and I'm sticking to it!

WB


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## ampaholic (Sep 19, 2006)

Thanks very much for the advice WB! Makes total sense, I'm just surprised at such a vast difference in the tubes.
One thing I will mention is that I recently picked up an Amp Preserver that lets me reduce the line voltage by 6 or 12 volts. The line voltage in my house runs at 123 volts and I always seemed to have higher than spec voltages in all my vintage amps. With the Amp Preserver at -6 I can now run my amps at the intended 117 volts and I typically find all the voltages pretty much bang on.
Another piece of gear to lug but I think it's worth it!
Thanks again.


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## WCGill (Mar 27, 2009)

Ampaholic,
You could easily bias them at 40ma or greater, but unfortunately it seems you'd need to modify your bias supply in order to achieve that. At 30ma you're still biased quite cold.


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## ampaholic (Sep 19, 2006)

Thanks WCGill. I guess most of my tube education came from Gerald Weber books and he always pitched bias settings for 6L6 tubes in the 30 to 35 ma range but I'm starting to realize that it is tough to make such a general statement. With plate voltage at 440 V and bias at 30 to 35 ma I recognize that I'm running the tubes about half throttle. Old habits are hard to break!


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