TRRI With JJ 6L6GC - Is This a Safe Bias?

[esxmac]

I recently got a TRRI, a Weber Bias Rite, and a full set of JJ tubes. The JJ 6L6GCs are rated at 30 watts. Plate voltage is ~440. It appears I can set the bias between 34 and 47 mv (50% - 70%). I haven't settled completely on what bias setting I like, but its somewhere between 38 and 43 mv.

Given that the JJs are 30 watts, is the 38 - 43 mv range/setting really safe? I am a bit paranoid on my TRRI since this is my third one in 2 months.

[John Phillips]

Yes, it's completely safe.

The only risk would be to the tubes, anyway - if you've had problems with amp failures on this model it's not because the bias was too hot.

[hoochiecoochie]

I have the JJs in my Twin RI and have them biased at about 36. It sounds OK. I have had them in there for about a good year. NO probs. I just keep falling back to my Vibro King though. My Twin is serial number 10, 1991. Approaching vintage. Maybe I need to put those JBLs in it and then hire a roady to move it.

[Blue Strat]

Feel free to go lower too. I've never seen a Twin that wasn't happy biased at 35mA. Use your ears, forget about 70%.

[esxmac]

Using the Weber Bias Rite, I set my JJs to ~40 mv--sounds sweet. However, the face plate and back plate seem pretty warm to the touch. In addition, the plate voltage is only 434. Am I running the JJs too hot? The plate voltage seems quite a bit lower than I have read about elsewhere. I tried turning the bias pot up higher and maxed out at 42.5 mv, ie, the bias pot turned as far as it would go. Is my TRRI ok?

[John Phillips]

Sounds OK. Remember that the voltages in any amp are the result of the supply voltage (which can vary quite a lot) and the PT ratios, which are also subject to manufacturing tolerances. The combination of both could result in differences of at least 10%, ie at least a 45V range for an amp with a designed B+ of 450V - ie between around 425 and 475V is completely normal for an amp with a nominal 450V. The variation due to supply voltage may even vary day to day depending on local demand too. Even the probe metering resistors have a 1% tolerance, which could be changing the reading by up to 4.5V in either direction.

The difference in heating between a bias setting of 35mA and 40mA is pretty marginal - 20mA (total for all four tubes) at 450V is only an increase of 9W, and the amp is dissipating around ten times that normally, even at idle. Tube amps are meant to get hot :).

[esxmac]

Cool, thanks! I ask because the transformer (the large black one--I think its the power transformer) is pretty hot to the touch. I am not sure what the temps are supposed to be. It just seems on the hot side to me.