Bias tap question, TW Express

[rooster]

This just occurred to me and have no answer.

If you take your bias voltage supply off one leg on the PT high voltage tap, like Ken did, will the amp sag more or exhibit something that would not occur if you took it from an actual bias tap prior to the finish voltage leg?

Did Ken ever build an Express using a typical bias tap like Fender used in their BF Class AB amps? Has anybody here done this and perhaps A/B'd the differences?

[Mark]

With no signal there is bugger current being drawn by the bias circuit. When signal is applied on a lesser amp the sag caused by secondary resistance MIGHT exhibit some sonic effect, the Trainwreck transformer can deliver a lot of current so it is my belief the current drawn by the bias supply is insignificant. Happy to be proven wrong though.

[rooster]

Sounds reasonable, Mark. All the Marshall Plexi's were done the same way that Kenny's Express was done so I assume he made a choice here based on his experience with those amps.

The reason I ask is that I am having some PTs made for a 5F8-A build and I could have the bias tap added. Eh, I'll skip it.

[Mark]

I had to work on a friends Marshall 2 X 100W power amp, it was one of the first ones that used 5881's, the bias tap just didn't have enough voltage to run the tubes at the correct bias. It would have been fine for EL-34's though. In this instance using one or both sides of the transformer would have been fine and modification would have been easy, the bias taps coupled with how the amp was constructed meant we just had to make do.