Bias tap on b+ or separate winding?

[triplej]

Hello,
I have a VTL compact 100 with a shot power transformer. The replacement transformer from Mercury (OEM) is over $800 landed in AU. I can get two Edcore's with correct voltages for well less than 1/2 that. Problem is the Edcore has a separate bias winding whereas the VTL is tapped off the b+. I understand this is done so bias and b+ track closely. Is it a big deal? wouldn't this happen anyhow on a separate winding as well? (just not as closely).
Thanks

 

[rayma]

Problem is the Edcore has a separate bias winding whereas the VTL is tapped off the b+.
I understand this is done so bias and b+ track closely. Is it a big deal?

You'll be fine. I've regulated the bias voltage on tube amps for years with no problems at all.

 

[triplej]

thanks rayma, that makes sense to me.

 

[DF96]

Provided that all the power transformers see the same mains input voltage they should track each other OK.

Note that the main supply rail HT/B+ and the negative bias voltage should either both be stabilised/regulated or both unstabilised/unregulated. This ensures that either both are stable, or both vary in a similar way.

 

[Enzo]

I think "done so bias and b+ track closely" is more of a marketing rationalization. But if the transformer has the right B+ voltage anyway, what stops you from ignoring the bias tap, and wiring it up just as it was before? Unless the question was more academic.

Without a schematic I can only guess how it is configured. But with changes in the mains voltage, the secondary voltages will also change in ration. But the B+ voltage itself also varies as the current draw does. Winding resistance acts as a series resistor. The bias tap as separate winding would not see that same resistive voltage drop, while a B+ winding tap would. How large an effect that might be is up for discussion.

In the land of guitar amps, the difference is more one of whether the standby switch also kills the bias.

 

[triplej]

thanks all, yes I think it was just a bit of marketing as the comment came from David Manley's book! Unfortunately for me it looks like the edcore's 220ma B+ (360vac) will be too wimpy anyhow.