B+ protection resistor??

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OK well Iíve spent the last two hours tying to find a mention of this on the AM fone board but no joy.
The reading has been enjoyable.

Iíve even looked in the east and west coast handbooks.

Iím collecting parts for a plate supply, 2000Vdc, legal limit, plate modulated.
What value and wattage should I use for the B+ safety series resistor? The one used incase of a flashover etc.

Is there a rule of thumb? That would be better to know.

Thanks again
Ted / KC9LKE

Lots of good info here:   http://www.somis.org/   

Under the heading "Amplifiers"  Part 2 covers glitch resistors.

10 ohm / 10W wirewound should do ya.

I fully agree, 10 ohms/10 watts.

I would add an AGC1 fuse in series with it. The resistor will let that 250 volt fuse protect tubes from arcs and other glitches.

I have this resistor/fuse combo in my SB-200. The fuse has blown twice, right after I installed new tubes. I feel it saved the tubes.

10/10 is a bit on the weak side if the idea is to limit destructive energy discharges thru the tube. I use 10/10 on 100-200W transmitters.
The value requires knowing how much energy storage is in the PS which can be very high if it has just a lot of C. With a choke input with under 12uF output C, and up to 3KV, 10/10 should be fine.

A SB-220 needs 25/25 at a minimum especially when higher value caps are used and preferably 50W for continued survivability. The stock 220 with the grid chokes should not blow open with enough glitch current limiting, the pie's may slam together and the choke should be replaced. If the grids are directly grounded then you better have a big glitch resistor and even then tube survivability is slim at 25 Ohms.

Ive recently (about 10 months) been using a parallel pair of 50 Ohm 3W NI resistors in place of the chokes but there has been no customer feedback yet. A .01 @2KV disc is installed from each grid pin to ground for improved RF bypassing. This is for normal repairs and 6M conversions.

Around 100A of discharge current is considered safe thru an Eimac 3-500Z. I dont know about Chinese tubes.

At 3000V in the SSB position (I rounded it off for simplicity), 25 Ohms (the choke R) will see 120A....instant Zap with grid now floating at 3000V for a millisecond until it arcs to the filament.
With direct grounded grids and figuring the resistance of the plasma arc at a conservative 1 Ohm we have 3000A....BANG, dead tube, plate choke, diodes, stained undies.

With a 25 Ohm glitch R and 25 more at the socket its 60A and likely everything will survive...maybe.

I didnt toss in the energy stored in that filter bank and this is where it gets tricky. How long will it take the back panel breakers (or fuses in most other amps) to pop and what is happening during that finite amount of time?

Going back to the 220uF replacement filters and 27.5uF total that gives us 123.75W/s which doesnt sound bad but for a .1 second discharge that is now 1237.5 W and a 10 millisecond arc is 12375W. Now you see that a fairly large glitch R can save a lot of hardware....IF the breakers open fast enough. Thats another reason that serious amp builders add a HV fuse and real big commercial amps use even faster methods to dump the PS.


I don't use any series resistors.  But I do have two fuses, one in the CT return on the plate xfmr and one in the HV output from the diode rectifiers.  My B+ runs through a 50hy reactor that has about 250-300 ohms.  I also have a fuse in the primary of the plate xfmr.  I'm only using 30ufd cap in the PS along with two 75ohm filter chokes.  I think I have plenty of series resistance in the supply. Hopefully



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