Triode VS. UL bias

[gtrmkr]

Need some wise words here. I'm getting ready to wire up a simple EL34 UL amp. Using the triode and UL curves for the EL34 on the Audiomatica site, I get very different -Vg points between the two connections, using the same Va and Ia, and a 5K loadline. For example, at 300V/60ma triode-connected I get about -24Vg. Same conditions but ultralinear-connected, I get ~ -34Vg.

Is this right?

Thanks very much for any insight offered.

Jim

 

[Tom Bavis]

No. Use the triode curves for DC idle state. The UL curve is a dynamic curve, with the screen tagging along at 40% or so of the plate's excursion from idle.

 

[Bandersnatch]

hey-Hey!!!,
The U-L curves depend on where they're started from. What you'll find in practice is that U-L and triode bias conditions will be the same( both have the same voltage on the screens). So as Tom said, for initial design use the triode curves to calculate values.
cheers,
Douglas

 

[gtrmkr]

Thanks Tom and Douglas. So, plotting loadlines based on the triode-connected curves will get me where I want to be with UL connection. Good -- so then there is no meaningful change in the load the tube expects to see going from triode to Ul.

I'm converting a compact little 2a3 I built years ago, which uses a 6688 triode-connected as driver. My plan is to try an E-Linear variant using what's in my junk box. I've never used pentodes as pentodes before, much less ultralinear, so I'm learning somethng new here.

Thanks again for the help.

Jim

 

[Bandersnatch]

Thanks Tom and Douglas. So, plotting loadlines based on the triode-connected curves will get me where I want to be with UL connection. Good -- so then there is no meaningful change in the load the tube expects to see going from triode to Ul.

Thanks again for the help.

Jim

That 'load a tube expects to see' can be taken a few ways. Output impedance will be higher with UL power tubes v. triodes since the plate resistance of the UL connected pentode is *ABOUT* 2.5x that of a triode. If your speakers are OK with that it should be no issue. Proper load line is another; the UL connected pentode will have more 'area' under its g1=0V line at low plate voltage and thus be able to swing more voltage( aka make more power into the same load ). I doubt you'll have any issue with that if you go back and forth between the two connection modes.
cheers,
Douglas

 

[gtrmkr]

Well, got it wired up. It works. Used 6v6's instead of el-34's after deciding my power supply was marginal for the bigger tube. Currently driven by 6J4's, but have 9-pin sockets installed and wired for triode connected 6688's, so I'll give the E-Linear treatment to this soon.

I haven't scoped it yet, but I'm guessing I'm getting about 3 watts into 16 ohms. Big sounding watts, too. I'm a little surprised, frankly.

Thanks for the help again, guys. I learned a few things ...

Best,

Jim