Class AB1 Bias Current?

[Steve Conner]

Personally I'd rather those resistors didn't act as fuses.

If they do fuse, the heater-cathode insulation could break down. If you use the 100 ohm virtual centre tap thing on your heaters, then those resistors will blow too, and your whole heater string will elevate to some crazy voltage, possibly damaging some other tubes.

[hasserl]

The method measures total Pa. It subtracts out the screen power. Measured total current is about 9% lower than what you'd get with bias probes, assuming screen current is only 10% of plate current. The recommended values, 70%-80% of Pa, when measured with a bias probe, which includes screen current in its cathode current measurement, would be around 43mA to 49mA for EL34s on average. That's up to double the stock value. Assuming +/- 10% variation on the tubes, we can get a tube running at 54mA cathode current. A slightly spunky 30W 6L6GC can be drawing 64ma, and that's recommended. 56mA is borderline too low.

I'm here to get opinions from guys like you. Do you think that this is good advice? They're the highest recommended values I've found.

I think sticking to 70% of max plate dissipation, or less, is good advice for a fixed bias class AB amp. I don't know about the 70 - 80% range when using a probe, I don't know where that came from; but since the probe measures cathode current, not plate current, that might be OK, but I question why push the envelope like that? Unless someone thinks that picking up that extra couple percent really makes a difference, and they are willing to sacrifice powere tube life for that extra bit of tone they think they get from running the tubes hot, than go for it. But IME it does not, particularly in a "high gain" type of amp that generates most of the distortion in the preamp and the power amp is run very linearly.

Something for the Carvin guys to keep in mind is that the tube sockets are pcb mounted. When tubes fail due to age they can sometimes fail in way that causes high currents to be drawn that result in damage to the pcb. Carvin does not provide pcb's to replace a damaged board, it is their policy and that's that. I have a Nomad on my bench now that had a burned up power tube pcb. I'm converting it to chassis mounted sockets and terminal strips. It's a bit of a pain in the @$$. IMO I'd keep the bias current below 70% of max and change tubes out often enough to avoid having one fail in use.

[BackwardsBoB]

Richard -

Thanks for that. You're back in the mainstream. The folks at carvinbbs greatly value your opinion.