Bais question, two different readings.

[XevKai]

Hi all,

I was taking a bias reading on my 97 dual rec the today and I noticed that the inside pair is waay off from the outside pair of tubes. I want to run them just above 70 percent (bout 30ma or so for 6l6). But when I take my readings I have 47 on the inside pair and 22 on outside pair.. This is with the bias pot as low as it can go... Any ideas what could be going on?

 

[siggy14]

Are they brand new tubes and a matched quad? Try taking the two center tubes and putting them on the outside and the two outside tubes and putting them on the inside. iF the bias remains the same as before it is something in the amp, if the two inner tubes read the same bias on the outside sockets as they did on the inside then it is something with the tubes.

 

[xxFIXXEDxx]

Tubes are in need of replacement. Also, if you're shooting for above 70% of plate dissipation, you should be going well above 30 ma, over 40 if it's a 30 watt 6L6 such as as JJ 6L6GC's. It all depends on your plate voltage of your particular amp, of course, but 30 ma is pretty low and will not get the tubes out of crossover distortion. The plate voltage on my dual rec is 432, and with a 30 watt tube, 70% is 48 ma, although I like to run my tubes at about 43-44. This is where I like the sound best. 30 x .7=21. 21/432=.048, or 48 ma. If I run 25 watt tubes, it'll be 40 ma for 70%. But the tube difference is the fact that they've drifted so far from each other. Get a good matched set from a reputable vendor, and you should be good to go.
Hope this helps,
Ryan

 

[fluff191]

Tubes are in need of replacement. Get a good matched set from a reputable vendor, and you should be good to go.

+1

 

[94Tremoverb]

If they're not actually worn out, I wouldn't bother. As long as you have the two pairs arranged like you have, so the power stage is still symetrical, it will be fine. Set the bias for the hot pair not the cold pair - that way any crossover distortion from the cold pair will be masked by the hot pair, but if you do it the other way round or try to average it, the hot pair will be running too hot.

In fact, it might even sound better with two non-matched pairs like that - you should get a bit more harmonic complexity, a little like running Simul-Class.

I would say 70% is too hot in a Rectifier if you're intending to push the power stage hard, by the way. Like xxFIXXEDxx says, go by ear and then check it's in the safe range, not the other way round. 60-65% usually sounds better to me in most higher-powered amps, and it's certainly less stressful for the tubes.

 

[domct203]

Hi all,

I was taking a bias reading on my 97 dual rec the today and I noticed that the inside pair is waay off from the outside pair of tubes. I want to run them just above 70 percent (bout 30ma or so for 6l6). But when I take my readings I have 47 on the inside pair and 22 on outside pair.. This is with the bias pot as low as it can go... Any ideas what could be going on?

If the amp is pushing 450V B+ (Plate Volts) the 47mA pair is at 70% MPD already with a 30W 6L6GC. ((450V * .047A = 21.15W)(21.15W/30W = 70.5%)). The 22mA pair is at 33% which is cold, even for a Mesa.

If it sounds good to you and the 47mA pair is not red-plating you should be OK. The thing that would bother me is that with your bias voltage dimed (idle current draw at minimum setting) you have a pair of tubes at 70+%. The 22mA pairs seems to be within normal 6L6GC specs at 33% at the lowest settings. Naturally these are my best guesses as I do not know your exact Plate Voltage and Bias Voltage.

Things to consider:
Are you running the amp at it's highest potential? (i.e. Bold, Diode). If these bias readings are at Spongy and/or Tube Rectified the situation will get worse if you go to Bold/Diode as the voltages in the amp will increase, taking that 70% pair right into "Eminent Failure Territory"

Carry spares just in case :wink:

Dom

 

[XevKai]

Well, I tested my tubes and they all checked out very well. I am using EH 6l6's and I've never had issues with them before. I always get my sets matched and sent direct from EH, I have a tube tester on hand as well to make sure i'm not getting any lame tubes as well. It ends up one of my bias probes was messed up and was causing a short on one side. Tested both sides with the working one and everything checks out.

I got my bias reading by plugging my plate voltage (470) into a bias calculator @ http://www.ax84.com/contrib/biascalc.php for my measurements. Is there a flaw in this calculator? Should I up the bias a little? Right now I am sitting at 30.8 Ma and it's sounding good. But if there is anything else I can do to juice it a bit, im open to it!

Thanks for the quick responses all! Love this board for that reason

 

[domct203]

I got my bias reading by plugging my plate voltage (470) into a bias calculator @ http://www.ax84.com/contrib/biascalc.php for my measurements. Is there a flaw in this calculator? Should I up the bias a little? Right now I am sitting at 30.8 Ma and it's sounding good. But if there is anything else I can do to juice it a bit, im open to it!

Well, using that bias calculator link I have you at less than 60% (48.2% MPD to be exact) at your readings. Are you selecting the 6L6 instead of the 6L6GC? The 6L6 is a 19W tube, and would be at 76% with your readings. The 6L6GC is a 30W tube.

Use this formula: Max Watts X 0.7 / Plate Volts = Amps

So for the 30W 6L6GC in your amp at 470 Volts at the plates it would be:
30W X 0.7 = 21W (21 is 70% of 30)
21W / 470V = 0.0447 Amps, or 44.7mA.

As you adjust the bias you will need to re-check that mA value as your plate voltage will most likely drop as the current draw goes up. If you drop to 460V you'll need 45.6mA to get 70%.

I wouldn't go above 60% in a Rectifier, YMMV.

Dom

 

[xxFIXXEDxx]

Ah yes, I did forget to say anything about settings on the amp. Thanks to Dom for bringing that up. It definitely must be biased to settings. For example, mine is biased to bold/rectifier. If I wanted to switch to bold/diode, I'd have to re-bias, otherwise the bias would go through the roof.

 

[XevKai]

yeah, Im biasing it in the bold and silicon rectifier settings since thats how I use the amp normally. I picked 6l6 since the tube only reads 6l6EH on it. I know the original use of GC was due to the switch to glass case back in the day and I didn't know that they had different wattages. I should look up the specs on these tubes again and check into that cause if thats the case i am pretty cold right now!

thanks again!

 

[domct203]

yeah, Im biasing it in the bold and silicon rectifier settings since thats how I use the amp normally. I picked 6l6 since the tube only reads 6l6EH on it. I know the original use of GC was due to the switch to glass case back in the day and I didn't know that they had different wattages. I should look up the specs on these tubes again and check into that cause if thats the case i am pretty cold right now!

thanks again!

The "GC" in 6L6GC does not stand for Glass Case (or Container). The "G does indeed stand for glass, with the A, B & C being different versions/revisions. The 6L6GC is rated for 30W, where the 6L6, 6L6G, 6L6GA & 6L6GB are rated for (approx) 19W. There is also a difference in max plate voltage with the "C" the highest of the bunch @ 500V.

Dom