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On Tue, Oct 17, 2023 at 05:52 PM, W7WRX wrote:

<This whole thing comes up from time to time. Meanwhile this is a completely useless thing to try in a ham radio setting. This information was Intended for broadcast use where they try to stretch life out of the tube. No ham is ever going to <benefit in any way.

## For directly heated, thoriated tungsten tubes like? expensive 3CX-3000 / 6000 / 10,000? triodes, or 4CX-5000 / 10,000 / 15,000? tetrodes,? ?I'm 90% convinced that the grid to cathode shorts are caused by the high inrush current, when no form of step start is used.? Measure the peak current when T=0. You will gag.? ?The fil xfmrs that Henry Radio used are Z limited, such that the max inrush current is no? more than double the tube's rated current....(which is still too much imo).? ?Everybody else's fil xfmr are capable of a lot more than that.? The problem gets worse when the fil xfmr used has a higher current rating than required.? ?Larry,? W7IUV? used a 5 vac @ 30 amp fil xfmr on a single 4-400? amp.? With no inrush protection, that resulted in numerous grid to cathode shorts.? ?

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##? Scott,? KB1SEL? modified a 5.0 vdc @ 200 amp? CCS? server supply, so it outputs 6.3 vdc? @ 160 amps.? ?He's been using that on his 4x15 tube for the last 20 years.?

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##? In the OP's? case, if no room for a 5.0 vdc regulated supply, the? 75 ohm @ 5-10 watt? resistor in one leg of the 240 vac primary will work (or a 0-100 ohm, 25 watt rheostat).? That will not only reduce the fil V on his 8877 to an acceptable level, it will also? provide for step start, and eliminate any inrush. The beauty is.... the resistor stays in the primary at all times.?

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## Some broadcast PA's have used a rheostat in the fil pri..... instead of a variac.? ?The rheostat, once dialed in correctly, provided for the correct heater / fil voltage...and also provided for step start / current limiting on turn on.?

Broadcasting is different in that the PA is run 24/7/365? and down for periodic maintenance.??

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##? IF a variac is used on the bigger, directly heated tubes, it still needs the step start circuit / resistor / relay.? Variac is set, then left alone.? Plan B is what I do, and just dial the variac up each night to desired voltage.? Then the fil current rises slowly from zero? to normal current. ( I have fil V panel meters, and also? a set of jacks for the fluke 87..and also a fil current panel meter.? Fil current meters uses an external 75:1? or? 100:1? xfmr.?

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##? At the cost of these tubes,? I'm not about to be using fil V that's? too high / too low, nor slamming on a ton of inrush current each night.?

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##? between my incoming? 234? vac sagging 8-9 vac on? TX.....and not knowing what it is from hr to hr, or day to day,? ?I had to go the sola route.? On the small? Drake L4B amps,? it's a non issue.? The drake amps get the entire B+? and fil xfmr step started with a 20 ohm resistor in one leg of the mains....that's it.? Done externally.? And the step start relay is manually activated.?

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##? I used a pair of DPST-30 amp relays in a 8x8x4 box...on the floor.? 1st relay applies the 240 vac.? ?Pole #1 of the 2nd relay? shunts the 20 ohm resistor.? ?Pole #2 of 2nd relay? is wired to a pair of RCA jacks...with keyline run in /out? of them.?

A small control cable is run from the 8x8x4 electrical box (with loads of knock outs, home depot)....? to a small mini box on the desk.? Mini box has 2 x mini toggle switches. Toggle #1 turns on relay #1.? ?Toggle #2? turns on relay #2.? ?Amp can not be keyed until? relay #2 is activated. Dead simple mech interlock.? Normal deal is to activate toggle? #1..... then a few secs later, activate toggle? #2.? B+ comes up to 90%..and fils of? 3-500Z's come up to 70% of normal brilliance with the 20 ohms inline.

##? When done for the night,? reverse? sequence is used.? With the 20 ohms back inline, prior to shut down, it kills the back emf 100%.? ? Due to the high C filter caps I used,? I also put the resistor back inline, when switching from CW? (1900 vdc)? to? SSB? (2650 vdc).?

I cant afford a tube failure.? ?As is, the On / OFF? switch on one of my drake amps is fried.? Below the rocker is just a cheap 10 amp rated on / off slide switch.

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Jim? ?VE7RF

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