Switching AC taps on PT?

[rooster]

I'm building a 5F8-A Twin and I have had Heyboer make up a dual voltage PT, 335-300-0-300-335. I want to be able to run either 6V6s or larger octals (6L6, EL34, etc.) at 400 (6V6s) or 470 plate volts.

Those of you who have built amps with dual taps, are you using this in Class A Rocket type builds or have you come up with something clever to swap voltages in a Class AB amp beyond my method of a terminal strip with 4 gang connections? In my case I have to open the back panel, swap terminals, rebias, put the back panel back on, and play the amp. Which is fine really because I can't imagine a need to do this on the fly...but I do wonder if some of you have devised something more clever. Thanks.

[Mark]

This is how Ceratone does it.

http://www.ceriatone.com/images/layoutP ... iatone.jpg

[rooster]

Mark, I'm familiar with this for Class A. This is just a matter of whether or not the power tubes can deal with the increase in voltage - simple.

What I'm curious to learn is whether anyone here has done something like this (although a more complicated process) with a Class AB amp where the bias point will FOLLOW the power tube voltage swap. In other words, say 28ma (-35 VDC) at 400 VDC to 28ma (-50 VDC) at 470 VDC. All with the flip of a single switch. Just curious.

[Mark]

You could still use that switch but you will need two bias circuits so you are also switching bias.

[martin manning]

Mark, I'm familiar with this for Class A. This is just a matter of whether or not the power tubes can deal with the increase in voltage - simple.

You meant to say cathode bias there.

What I'm curious to learn is whether anyone here has done something like this (although a more complicated process) with a Class AB amp where the bias point will FOLLOW the power tube voltage swap. In other words, say 28ma (-35 VDC) at 400 VDC to 28ma (-50 VDC) at 470 VDC. All with the flip of a single switch. Just curious.

If you take the bias supply from the selected AC HT, then it will track the plate voltage. This should work reasonably well for a change in plate voltage using the same tubes. When changing tube type (6L6 to 6V6 e.g.), you could consider a tube type switch that swaps a resistor in the bias supply, but there is enough variation in the required bias voltage from one set of tubes to the next that this probably wouldn't be practical, and you'd wind up needing to check and reset the bias anyway.

[Clyde]

I use a DPDT switch for the HT and bias, but I don't switch out power tubes.

[M Fowler]

I built an Orange Retro 50 amp which switches from Class A 30w cathode bias to Class AB 50w fixed bias. See schematic and layout.

On other builds I just used a DPDT switch to change HT voltages but don't rebias. I don't find it a beneficial mod rather just use one voltage setting and be done and that is usually the higher HT voltage.

http://ampgarage.com/forum/download/file.php?id=23287

http://ampgarage.com/forum/download/file.php?id=23288

[rooster]

Thanks gentlemen. I appreciate all the input. For what I'm doing I will stick with the internal screw-on/screw-off terminal strip. Eh, it will be a 10-15 minute pit stop but, as you suggest, Mark F, I will probably try a few different quads, find the magic, and keep it there.

Sometimes you start thinking that the Swiss Army knife approach will give you something more useful.