Best Currently Available Express OT in 2022?

[Mark]

Holy cr@p! 10A on the filaments?! I had no idea...

One of my personal favorite pieces of Wreck History...

Somewhere in the archives, my own speculation about this has been published previously. It was based on a discussion with Patrick from Mercury Magnetics (A great guy no longer with us ) Patrick shared stories of working with Ken Fischer and told me of a time when Ken brought in one of his favorite old PT's, taken from a TV set. He wanted to have MM make clones of this tranny for his amplifier biz... It's impossible to know the model or the real story but my speculation was that a PT from a TV set would certainly need to support a bunch of tube heaters. Could this be the origin of the Wreck PT?

It might be kinda cool but otherwise worthless info to study some of those old TV set transformers and see if there was anything close to the Wreck specifications. I've got some old Stancor PT's that are close to the right specs for a Wreck build except for that 10 amp heater tap. It's a bit late to think of salvaging PT's from old TV's for most of us but I think it's safe to say that you could get by with a lot less power capacity on the heater tap.

The transformers would have been from old B&W TV sets. Those are long gone.

[Mark]

I have the MM OT in my clone, and it sounds awesome to me! I agree that the PT probably doesn't have any effect on tone. But you have to have the HP to crank it.

The power transformer does have an effect on tone in that it doesn’t sag much. I built a prototype with a power transformer with low current capability and found the amp started off bright and became more mid focused as I turned the volume up.

[wpaulvogel]

Ken’s first repairs/refinements while still working with Ampeg were for after work customers keeping their Marshalls from failing. He would outfit them with 6V6 tubes and lower the voltage with variac adding a filament transformer to keep heaters at 6.3 volts. With the express amp maybe the overkill was to ensure reliability (definitely). He attained power amp sag with the 1k screen dropper in the power supply vs using saggy power transformers. Genius!

[Roe]

Ken’s first repairs/refinements while still working with Ampeg were for after work customers keeping their Marshalls from failing. He would outfit them with 6V6 tubes and lower the voltage with variac adding a filament transformer to keep heaters at 6.3 volts. With the express amp maybe the overkill was to ensure reliability (definitely). He attained power amp sag with the 1k screen dropper in the power supply vs using saggy power transformers. Genius!

sagging screens voltage only sounds different than sagging both plates and screens.

[Mark]

Ken’s first repairs/refinements while still working with Ampeg were for after work customers keeping their Marshalls from failing. He would outfit them with 6V6 tubes and lower the voltage with variac adding a filament transformer to keep heaters at 6.3 volts. With the express amp maybe the overkill was to ensure reliability (definitely). He attained power amp sag with the 1k screen dropper in the power supply vs using saggy power transformers. Genius!

sagging screens voltage only sounds different than sagging both plates and screens.

When I was experimenting with the Express amp I found the 1K resistor caused the supply to sag by a maximum of 40VDC.
The idea being to cause compression but not boat loads of it.

A low wattage Express needs a larger value resistor. I think 1.8K would work for amps fitted with a duet of EL84’s.

[Roe]

Ken’s first repairs/refinements while still working with Ampeg were for after work customers keeping their Marshalls from failing. He would outfit them with 6V6 tubes and lower the voltage with variac adding a filament transformer to keep heaters at 6.3 volts. With the express amp maybe the overkill was to ensure reliability (definitely). He attained power amp sag with the 1k screen dropper in the power supply vs using saggy power transformers. Genius!

sagging screens voltage only sounds different than sagging both plates and screens.

When I was experimenting with the Express amp I found the 1K resistor caused the supply to sag by a maximum of 40VDC.
The idea being to cause compression but not boat loads of it.

A low wattage Express needs a larger value resistor. I think 1.8K would work for amps fitted with a duet of EL84’s.

Interesting. that indicates a 40mA current draw, with 35mA for the el34s and 5mA for the preamp roughly. That is a lot less than a marshall that can draw 140ma with a quad. But RFTs/Siemens do draw more current than many other el34s. Some of the new chinese ones draw much less screen current