Over-Sizing Power Transformers - How much is too much?

[Gaz]

One things no one's mentioned is that over-specing the PT is not always a good idea in some circuits, which are already running tubes at their limits.

For example, an old Marshall, Hiwatt, etc. with nearly 500 volts on the plates and screens with a 1k7 primary is already running the tubes beyond their limits. Even with 1k screen stoppers, the the screens will most likely glow under heavy overdriven conditions. Now the old PTs (and thier clones) allow the B+ to sag enough to bring the voltage down enough to usually run the tubes within their limits. I've even seen modern amps that sag 100vdc with max signal.

If you overrate the PT too much the voltage may not be able to sag enough to keep tubes, especially new production pentodes, from not over dissipating the plates and screens when heavily overdriven.

Now if it's a new design, then you can totally design around a stiff power supply, and you generally have a little more wiggle room if you have a high gain preamp, and are not clipping the power tubes so much.

But don't get me wrong, I totally agree about 'over-specing' PTs for greater life and a punchier tone, just trying to point out there are other factors involved, and since most vintage designs are running tubes past their max limits, there's not much room for stiff supplies. It's not always wise to over spec something just to over-spec it. JM2C.

[rocketeer]

Spot on GAZ ........ totally understand your points on overspecing a PT in an existing design that was meant to sag in order to save components from being pushed too much. I'm with you there. I would think that if you wanted to stiffen up an existing amp one would take into consideration lowering plate voltage to counter the points you brought up.

Mine is a new build and I'm keeping plate voltage down in the mid to lower 4's but wanting it to stay there at clipping to keep the output section stiff.

Thanks for your reply.

[Structo]

Yeah, but if your power transformer has too many amps, won't excess spill out all over inside the amplifier?

[ChrisM]

Yeah, but if your power transformer has too many amps, won't excess spill out all over inside the amplifier?

Yes,
The output section is drawing the most current though. The preamp current is almost negligible compared to the output section. A triode draws what, like 0.5mA-3mA depending where it is being used/biased.
So the sag will really be seen in the output section.

[Gaz]

Spot on GAZ ........ totally understand your points on overspecing a PT in an existing design that was meant to sag in order to save components from being pushed too much. I'm with you there. I would think that if you wanted to stiffen up an existing amp one would take into consideration lowering plate voltage to counter the points you brought up.

Mine is a new build and I'm keeping plate voltage down in the mid to lower 4's but wanting it to stay there at clipping to keep the output section stiff.

Thanks for your reply.

Yeah, I had to learn this lesson the hard way in a high-voltage Marshall clone, but I learned a lot about power sections in the process - makes me wanna stick to master volume amps, and give those power tubes a break!

[TheGimp]

The other issue with overspecing a power transformer is the heater/filament voltage may be over spec.

My current build has 6.6V on the heaters with 124Vac in.

A solution is to add thermistors in series with the heater/filaments to drop the excessive voltage, and help with inrush at the same time.