What type of 6L6 are you using- most should cope with around 35ma-42ma bias?

There are two resistors that alter the range of the bias adjustment. Your bias voltage is too high; lowering it will make your tubes run hotter.

There may be a 1.2K 1W resistor on the rectifier board that can be raised (3.3K is the Dumble value here). There is also a resistor that bleeds the bias voltage to ground. There may be a 15K resistor there. The Dumble value for that one would be 27k. Not sure why he changed the bias circuit, but it may have to do with giving people the range to run EL34s.

Your bias voltage is a negative voltage and it 'slows down' your tubes bias current. If I understand correctly, you are getting too much bias voltage. Raising the 1.2k resistor will lower your range of bias voltage and allow you to make your tubes run in the usual range 30-35ma or so. If the highest you can get your bias current is 25ma then your tubes are running cold.

I am sure that ic-racer knows more about this stuff than I do, but...
My amp tech (who is very sharp) insists that a lower bias works fine and gives the tubes a longer life. He biases my Bassman & Super Reverb at between 25-30mA. Both sound very good and are running at a plate voltage around 475 VDC, which is the other part of the equation you need to know to set your bias. Both the Bassman & SR sound exactly the same at 35mA, which I believe is kind of the safe bet when you are biasing 6L6s with out knowing th e plate voltage. I have not checked, but the OTS plate voltage probably runs about the same as the Bassman/SR . You know you are too cold with your bias when your tubes get very hot and may have a red glow. This will shorten the tube life drastically and may cause permanent damage to the amp components if this is done over a longer period of time.

Having said all that, if you cannot bias above 25mA there is something wrong and it is almost sure to be the resistor used to set the bias. If it were me I would have an amp tech check it out. Some of the guys on this forum could talk you through some troubleshooting if you wanted to do it yourself. Nik may actually be the best place to start. Good Luck!

I recently spent some time with 4 sets of 6L6 tubes in my amp and I found there was a little 'grainy' character with the bias low (25ma) and this got better for each tube up to about 35ma. One set even sounded best at 42ma, but another old, old set arced (internal spark) at 42ma. The sound at 25ma was still very acceptable for all the tubes, though, just depends on how critical you are. It is a trade-off between smoothness of tone and heat. I wound up liking from 35ma to 38ma, but this is a home recording amp. For gigging, less heat and more reliability might dictate around 30ma.

In terms of changing your bias circuit, either raise that 1.2k resistor with experimentation, or e-mail Nik to see what he recommends.

I thought you meaSUred the bias current, not voltage.

oh yeah, duh...