6L6GC bias question...

[Tuberoast]

I have a bias probe, I have put in a set of RCA 6L6GC's (silver lettering on the base). I currently have the bias set at 27ma; Is this too cold? It does sound pretty damn good to my ears right now.

[justonwo]

You have to know the plate voltage for the bias current to be meaningful. What kind of amp is it?

[Tuberoast]

DR with upgraded trannies

[justonwo]

The 6L6GC is a 30 watt tube. Even with a very high plate voltage, this is probably on the cold side. You might try biasing a little hotter to see if you like the sound better. I think you're probably safe to go up to 35 or 40 mA.

[Tuberoast]

I was thinking around 30...but I will goose it up to 35. Thanks for your help

[justonwo]

As I said before, bias current is somewhat meaningless without plate voltage. You need to get a measure of your plate voltage to get an idea where you should set the bias.

[John Phillips]

Despite the GC's 30W rating, I usually find they sound better biased more like a GB (22W-rated) - ie around 15W. (The higher maximum rating just gives you more safety margin and longer tube life.) Even that's 36mA in a 420V amp though, so you probably do have it too cold at the moment. Compare the tone before you assume it will necessarily sound better hotter though.

Remember also to allow a couple of mA for the screen current if you're using a bias probe, since the cathode current includes this. So your target (tone considerations aside) should probably be around 38mA.

Also check that the B+ voltage is 420V - it might not be with different transformers.

[Old Tele man]

FWIW, for nice-clean jazz, I idle the SV-6L6GC's in my '69 SR at 32mA and 480Vdc...that's only about 15.4Watts, or 51% of thier 30W rating.

[John Phillips]

Exactly .

As a general point, IMO far too much is made of running the tubes at a certain proportion of their rating. It doesn't matter - it's the operating point relative to the circuit that counts. The percentage of the rating is only important as an upper limit for reliability and tube life. The best tone may occur at that point, below or even far below it.

If you took an EL34 amp and substituted KT88s, should you then run the tubes at 30W instead of 15? Of course not - you're just wasting an extra 15W per tube and loading the power supply more heavily at idle. You might want to run them a bit hotter, but that should be based strictly on what the amp sounds like as a result of doing so.

It's exactly the same here - just because the tubes have the capability of dissipating more heat, you shouldn't assume that there's always a tone advantage in doing so. If you used Sylvania/Philips 6L6GCs, should you run them even hotter because they appear to be electrically identical to the ones labeled as 7581As (35W rated)? No. It's just that the tubes will take the extra power without harm because they were overbuilt, but it's only actually useful in a circuit which can use it. Otherwise, treat them exactly as 6L6s and enjoy the longer tube life.

[pbradt]

I have a lower B+ because I'm using a 5R4 in my Super. Sounds great. I have the bias circuit tuned to let me bias by tone alone, the range of the bias pot will never be too cold or hot.

With the 5R4, I get about 25 watts, which is *plenty* for any gig regardless of size.

[justonwo]

Very good point, John. My Super Lead sounds pretty mean at 50%, but my Rivera doesn't sound that great until you get up to about 70% (incidentally, that's the bias point recommended by Rivera). It really depends on the circuit.

[BluesHarp]

I have a Fargen 40 watt 6l6gc amp head. With a bias right meter are we talking about 17.5ma per tube ( 35 total ) to about 20ma per tube ( 40 total ) ???

I hate to sound igmo.... but with these nos rca 6l6gc's I have to get it right.

Thanks!!!

[doublee]

Speaking of circuits, I will be borrowing a Deville and wonder if that should also be biased in the mid 30's....? Thanks.

[BluesHarp]

So 38ma ( total of both tubes ) is considered cold? I do use pedals quite a bit so is this the idea? Being able to get the amp to distort a bit at heavy pedal settings is ok but I just want all the avaiable VOLUME that the amp has to offer. Im afraid of biasing so cold as to not get the 40 watts of power/volume. This making any sense?

[VaughnC]

Once you determine what the safe maximum bias ma setting is, use your ears to fine tune it. Most amps I've worked on tended to have a bias setting sweet spot...and I just use my bias meter to make sure I'm not exceeding a tubes dissipation specs.

Here's how I do it: While adjusting the bias control at about gigging volume, I listen for the amp to bloom while playing a choked low E string (while simultaneously watching the bias meter to make sure I'm not exceeding the tubes rating). And I find that increasing the amps bias current beyond the amps bloom point is of little tonal value and only shortens tube life.