Choke input hv power supplies

[linearone]

Maybe some greybeards on here remember when choke input power supplies were the norm. Every amplifier had them. The wisdom that was preached is that they offer better regulation. I worked on plenty of amplifiers with them both large and small. The largest supply I've ever used with a choke input was a 1.2 amp supply. To be very Frank I don't think it had any better regulation than a fullwave bridge with a big capacitor. The added cost of a really large iron choke is also a deterrent to building them nowadays. Why did they become so popular.? Was it because large value capacitors for high voltage weren't so readily available?
Did it help strengthen a slightly smaller transformer?

The other thing about running these choke inputs is that they always have a resistor of some high value from b+ to ground, I know that that resistor seems to load the system in some way. Does anybody here know the mathematics for determining what value resistor is needed in what application? From what I understand, the swinging choke is tuned in some fashion by that resistor depending on the load that's going to be applied and the service it's going to be used in. Is this true?

 

[nomadradio]

Choke input is attractive when your rectifier is a tube that can handle high voltages well, but doesn't like current spikes.

Like a vacuum or mercury-vapor tube.

Solid-state rectifiers are the other way around. They laugh off a current spike if it's brief enough. But if you exceed a solid-state rectifier's breakdown voltage, the damage is permanent. And quick.

The choke-input circuit limits peak current. That's what inductors do, along with storing energy to smooth the rectifier ripple. That energy takes the form of voltage spikes back against the rectifier. Biggest drawback IMHO is the need for a minimum current draw. A choke-input filter with no load will deliver almost half-again more voltage than it does under load. The enormous bank of 100-Watt wirewound resistors in an old Henry floor-model power supply must dissipate a hundred-plus Watts on standby to hold down the no-load voltage. No load, as in none from the tubes. The bleeder load must be sufficient to hold the HV down to a safe level on standby.

When Henry's ham linears were first designed, a vacuum or mercury rectifier was your only choice. Solid-state high-voltage rectifiers were finally cheap and reliable enough by 1970 or so.

Henry adopted capacitor-input HV filters with solid-state rectifiers for their later desktop models. That filter choke isn't just heavy, it's big. A single row of eight electrolytic caps takes up a lot less space. No room on a desktop for a filter choke.

And yeah, they kept using choke-input HV filtering long after solid-state rectifiers became better than tubes. Anybody's guess as to why.

73

 

[linearone]

Choke input is attractive when your rectifier is a tube that can handle high voltages well, but doesn't like current spikes.

Like a vacuum or mercury-vapor tube.

Solid-state rectifiers are the other way around. They laugh off a current spike if it's brief enough. But if you exceed a solid-state rectifier's breakdown voltage, the damage is permanent. And quick.

The choke-input circuit limits peak current. That's what inductors do, along with storing energy to smooth the rectifier ripple. That energy takes the form of voltage spikes back against the rectifier. Biggest drawback IMHO is the need for a minimum current draw. A choke-input filter with no load will deliver almost half-again more voltage than it does under load. The enormous bank of 100-Watt wirewound resistors in an old Henry floor-model power supply must dissipate a hundred-plus Watts on standby to hold down the no-load voltage. No load, as in none from the tubes. The bleeder load must be sufficient to hold the HV down to a safe level on standby.

When Henry's ham linears were first designed, a vacuum or mercury rectifier was your only choice. Solid-state high-voltage rectifiers were finally cheap and reliable enough by 1970 or so.

Henry adopted capacitor-input HV filters with solid-state rectifiers for their later desktop models. That filter choke isn't just heavy, it's big. A single row of eight electrolytic caps takes up a lot less space. No room on a desktop for a filter choke.

And yeah, they kept using choke-input HV filtering long after solid-state rectifiers became better than tubes. Anybody's guess as to why.

73

I worked on a swinging choke supply. It was 1.2a and the choke was a peter dahl marked 2-4hy. The choke was literally 70%the size of the plate transformer.