Mojo 5e3 bias question(NOOB ALERT!)

[t-luxe]

Hey guys- recently finished my first build..a MOJO 5E3 kit. Everything went together well and the amps sounds good. It originally came with a Sovtek 5y3, JJ 6v6's, and new production 12ay7 and 12ax7. I 'm currently running NOS 5y3 and NOS JAN Philips 6V6's. With the NOS tubes, according to the
Weber bias calculator, I'm showing 39ma and 14 watts PER tube! At 70% dissapation, I should be at around 24ma per tube. The tubes are not red plating or showing any signs of distress. The kit comes with a 270 ohm cathode resistor. My question- could someone more knowlegable than I chime in here with some suggestions to reduce the plate current? I understand that you can only go so far changing the rcath without incurring crossover distortion- what rcath values would you suggest? The transformer only has one tap so changing that over is not an option. In searching other forums, a few guys say they prefer running the bias that high and have had no problems. What would be the consequences of this? So far(2 months), everything looks OK on mine, but I'd like to get dissapation down somewhat. Here's my voltages/values:

V3/V4
B+= 371
plate to cathode=351v
across rcath=22.2v
rcath value=270ohm

Thanks in advance for any suggestions!

[ThermionicScott]

Cathode-biased amps like the 5E3 are generally biased hotter than 70% (that rule applies more to fixed-bias amps). The reason is that the bias naturally shifts in a cooler direction under large signals, so you need to "start" them hotter, so to speak, in order to avoid crossover distortion.

Those voltages are a little high, considering you're using a real 5Y3. Moving up to a 300-ohm cathode resistor would cool things off. Adding a 100-ohm/2-watt resistor between the 5Y3 and the first filter cap would also help.

Or you could just keep using the JJ 6V6s and they'll last a long time under those conditions, even with no mods.

- Scott

[t-luxe]

Thanks Scott- I'll try a 300 ohm. I like the way the amps sounds and I've had no issues - just want to be sure!

[ThermionicScott]

I just checked the specs on the power transformer included in the Mojo kit. It has a 384-0-384 secondary, which is much higher than the original spec (350-0-350 for the very early tweed Deluxes). That's why your voltages are too high.

Since I'm somewhat OCD and have a lot of free time, if it were my amp, I'd either swap the power transformer for a lower-voltage model, or add resistance to the power supply.

But since your amp seems to behave itself and sound good, I guess I wouldn't advise much tinkering unless it eats tubes.

- Scott

[t-luxe]

ya-i'm gonna leave it alone for now. if it becomes a problem i think i'll swap out the xformer. i appreciate your help,scott. thanks alot.

tom