st70 standby switch/cathode stripping SS rect

[chopchip]

does anyone have a schematic or some advice about rigging up a standby switch in an st70 to avoid cathode stripping when using a SS rectifier?

thanks

[20to20]

Originally Posted by chopchip

does anyone have a schematic or some advice about rigging up a standby switch in an st70 to avoid cathode stripping when using a SS rectifier?

thanks

Chop, nice to hear from you again. Since you asked, I'd pull that SS stuff and turn it back into a real '70.

[bear]

There isn't much of a problem with indirectly heated tubes...

But you could simply put a low Vdrop tube rectifier in series with the rectified B+, that makes for a slow turn on.

Or you can use a thermistor arrangement too...

_-_-bear

[chopchip]

Originally Posted by 20to20

Chop, nice to hear from you again. Since you asked, I'd pull that SS stuff and turn it back into a real '70.

you're right but i just want to have it for back up and it's another tinkertown project

[pof@]

"...to avoid cathode stripping when using a SS rectifier?"

Can anyone explain?

[20to20]

Originally Posted by chopchip

you're right but i just want to have it for back up and it's another tinkertown project

(1) In the '70 the bias voltage is applied immediately.

(2) the cathodes will just sit there until the heaters come up anyway.

[chopchip]

Originally Posted by pof@

"...to avoid cathode stripping when using a SS rectifier?"

Can anyone explain?

apparently, the B+ voltage comes up so fast in SS rectification that the cathodes havent had time to build up a space charge and so cathode damage results. this significantly reduces tube life i am told. makes sense

[chopchip]

Originally Posted by 20to20

(1) In the '70 the bias voltage is applied immediately.

(2) the cathodes will just sit there until the heaters come up anyway.

i think the cathodes get damaged when there is no space charge
but there is a full B+ voltage.

bias voltage comes up immediately this means that the cathodes are protected from the B+ voltage by the negative bias voltage on the grid?

googling this query there are people who have modded the st70 to accomodate SS rectification with a voltage stabilizer tube or something or a standby switch

thanks very much

[20to20]

Originally Posted by chopchip

i think the cathodes get damaged when there is no space charge
but there is a full B+ voltage.

bias voltage comes up immediately this means that the cathodes are protected from the B+ voltage by the negative bias voltage on the grid?

googling this query there are people who have modded the st70 to accomodate SS rectification with a voltage stabilizer tube or something or a standby switch

The cathods are almost grounded and there will be no conduction until they are hot. There can be no conduction without a space charge.

The negative bias is what keeps the tube from conducting at full max current... It blocks current flow. It is there to regulate immediately. But there will be no conduction until the cathodes are hot anyway. If the cathodes were somehow "blasted" by plate voltage, that would constitute conduction, for any flow to occur.

[chopchip]

Originally Posted by 20to20

The cathods are almost grounded and there will be no conduction until they are hot. There can be no conduction without a space charge.

The negative bias is what keeps the tube from conducting at full max current... It blocks current flow. It is there to regulate immediately. But there will be no conduction until the cathodes are hot anyway. If the cathodes were somehow "blasted" by plate voltage, that would constitute conduction, for any flow to occur.

yeah that makes sense.

joe curcio (manufacturer) says

If you search for research on cathode stripping you will find scientific studies (relatively) that find evidence both that it exists and that it does not and consequently I have concluded that empiric evidence needs to prevail.

Several points:
1. There are several commercial companies with very sharp experienced engineers producing hundreds (if not thousands) of amplifiers that apply B+ to the tubes (using ss rectification) year after year. I'm thinking that their database should support any evidence for the need to delay the HV for so called cathode stripping if needed.
2. I've designed, built and delivered my fair of tube amplifiers and never have I seen any evidence of cathode stripping.
3. I frequently design SMPS power supplies and from that experience it is clear that that current surges stress components severely (as does excessive voltage and excessive power dissipation) and the suggestion to apply B+ after the filaments have reached temperature makes the cure more hazardous than the perceived illness. These pulse currents are very traumatic to all of the components in the chain (including the power supply, regulators if used and the tubes themselves). To that point however, the inrush current limiter (into the cap bank) has real merit.