Biasing a CAT JL2 Sign.


I need your experience and help about biasing safley after retubing a Convergent JL 2 Sign.

I belive that there are better alternatives to the stock tubes and I ordered a set from Kevin at Upscale Audio. It consists of SED 6550C, 6H23 Type 3 for the 6922s, the Mullard CV4003 for the 12AU7s, and the Mullard CV4004 for the 12AX7s. I changed the tubes and since then I have not switched on because I heard on this forum (and belive personally) that this amp is very fussy about tubes. I turned down the bias on the readout knob to 0 but frankly I'm not sure if the 0 on the readout window of this knob is really 0 mV. So my doubts to damage the amp or anything else in my system are quite high.

To make it short - any first hand guidance/help with biasing this specific amp?

Thanks for your help
Helo Hardbop,
First of all, I assume you got matched octet power tubes. All of the CAT amps are easy to bias. I am very fortunate to own both the JL-2 Sig MK2 and the JL-3 Sig Mk2 monos. I have changed tubes on the JL-2 a couple of times. Just make sure you turn down the bias somewhat and turn the amp on. From the manual " Set the selector switch to any of the bias positions and adjust the bias control knob so that the built-in LCD meter reads 37. This indicates an average plate current of 37 milliamps per tube. This control should be adjusted slowly, because the bias control reacts very slowly to changes. Be sure the bias never gois above 50". Repeat the procedure for the other channel.

You also have to adjust the balance. Set the selector switch to balance and adjust the small black knob so that the balance reads 0.0. This knob reacts very quickly. Repeat for the other channel. Be sure to check the bias and balance say 30 minutes later and re-adjust if needed. It might take a day or two to get the tubes to settle down. According to Ken Stevens once the tubes are stable then leave them alone and don't re-bias them. The JL-2 or JL-3 are very stable amps and can be adjusted with or without a load connected to them.

I changed the input tubes on my amps to RAM labs super low noise. They are expensive but do lower the background noise and sound better than stock tubes. I have some of the same tubes you got from Kevin but use them in the preamp and tube DAC. I also got them from Kevin. Good luck and happy listening.