The AM Forum

OK well I’ve 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.

I’ve even looked in the east and west coast handbooks.

I’m 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/ (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.

Carl

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

Fred

I think at our energy levels, 10 ohms glitch resistor is good value. Resistor wattage has to be rated according to your maximum mean current (P=RxI²).
Fuses are a bad idea and will not work unless they are rated for the voltage they have to isolate after they break.

73
Geir

I never used a series resistor, but I do have a fast-acting overload relay circuit in each one of my transmitters.  I would have saved myself a tremendous amount of grief (far beyond the mere transmitter - a long story) if I hadn't build my first high power transmitter without one.

Back in my custom 1 and 2 hole 4-1000A building days of the 70's and 80's I always used a HV fuse and a 25/50 glitch resistor. Fuses were plentiful and cheap on the surplus market and since the tubes were always pulls it prevented other damage from a long sitting gassy one. No PS chokes were used and the C was never over about 12uF of series parallel 3-4KV oil filled with 5-6KV of DC. A bit more C for those who wanted 4000-4500V and I bought 200uf/450V SB-220 caps in bulk. Never had the nerve to series them up for 5-6KV!

I learned about the glitch resistor at National which had a 10/10 in the 1963 released NCL-2000. That might have been a first for a commercial ham amp?? It also had a plate overload relay set at 1.2A and the only time that would trip is when the poorly supported 8122 screen leaned into the anode. The tube was toast but nothing else got damaged but a 100 Ohm 1/2W carbon screen "fuse". By late 65 RCA redesigned the tube with ceramic screen spacer supports.

I have always theorized that the inductance of the wirewound resistor would limit the di/dt as does the resistance but have never really looked at the rise time of the current pulse is a flashover. It can't hurt but the resistor voltage may go very high.

I have used a 5 KV fuse also but they are large (approx 6" long )

I use a #30 wire as suggested by a wise man, but what about the 10 Ohm 10W WW resistor? Will it explode? 3500V, 24uF output cap - -maybe it is going to get real hot before the fuse blows.

Ken, W2DTC  has some good comments.  Go to comment 25.

http://w2dtc.com/w2dtc-class-a-rf-amp-page.htm
Al

I've been looking and searching the net for a circuit like that, but haven't found any deatils yet.
Would an overload circuit (for plate and/or screen) be easy to build?

Martin

Ive seen them smoke and crack but not explode. A HB sand fuse isnt hard to make, the main thing is to make it long enough to stop a plasma arc from forming.



Look at the NCL-2000 circuit I mentioned yesterday, cant get any simpler for old tech.

Carl

With a series resistor, a 250 volt fuse can work in a HV line. It may not be the solution to all problems, but it is absolutely worthwhile.

What I did to my SB-200 was based on http://pa0fri.home.xs4all.nl/Lineairs/SB200/sb200eng.htm. I have seen that fuse in the HV blow due to a tube fault, and not arc.

When a 250V fuse and glitch resistor is put in series, the resistor will in the most cases protect your tube.
There is a great chance that the fuse will vaporize its metal and become a “super conductor” until the high current disappear. I have tried this method several times with ceramic cement power resistors (These types of resistors are not ideal because they tend to explode and leave a hell of a cleaning job afterwards) and almost always the resistor will brake as well as the fuse.
And when an adequate dimensioned power resistor brakes it tells me that the fuse is not “fast” acting.

73
Geir

Has anyone tried a thyratron crowbar circuit for protecting tubes and power supplies?

Thyratrons that can sink a few hundred amps and react in 10 microseconds or less are out there on the surplus market.

I basic idea is that when the overload hits, the thyratron turns on and shorts the power supply output to ground blowing the fuse.

I have been working on a crowbar module for a while.
I considered thyratrons but scraped them in favor for SRC’s.
Here is the schematics. The PCB for the circuit is made but I have not put it together and tried it yet.

Oh yes. It will re-route the energy from a 100uF cap @ 4kV

can the capacitor get upset about this?

I think not. There is a 10ohm current limitary resistor in line. If you ignore any ohms loss in leads, connections etc. the peek short current is gonna be in this case maximum: 4000 volts over 10 Ohms, equals 400 Amps (in theory).
I believe that 400 amps in few microsecond will not give this circuit any stress. But as I mentioned above it has not been fully tested yet so it may fail. But i'm certain that it won't.

Any low voltage fuse or for that matter a length of thin wire will arc (a plasma arc)

I would think that a length of thin wire might be asking for trouble as the plasma arc will form and then may jump to something good and burn it up.

The resistor will limit current - and likely the inductance of the wirewound resistor  will help matters as much or more than the 10 ohm resistance.

The best solution is a 5 KV fuse but they are large and are not cheap. The resistor is a good substitute.

This is what I used on my 3500 volt 1 amp supply. I bought it and the holder 20 years ago for $35

http://www.discountfuse.com/A500T1E_1_p/a500t1e-1.htm (http://www.discountfuse.com/A500T1E_1_p/a500t1e-1.htm)

Kinda pricey for a one shot!

Many of the old HV fuses were rebuildable and even a wrong current one can be changed to whatever you want. Ive also rebuilt old style one shots.

Just find anything that is at least 2" long and have at it. A rough rule is 1" per 1000V for the old styles. And still use a resistor.

WOW what a thorough discussion.

Well I plan to use a 15ohm 25Watt I have on hand and make up some 2” fuses. No 30 in aquarium tubing then filled with sand and capped off with ring terms inserted into the tubing.

Well see how it flies!

Thanks again
 
Ted / KC9LKE

I used 1ohm/1watt comps. For hi power linyears I put 2 in parallel.

I disagree completely.
The idea is to rightsize the glitch resistor so it will open without delay....in fact, I like mine right on the hairy edge of talking it open.
I also think a single wirewound resistor such as a tub, sand, power resistor or watever you want to call it....is the way to go because it opens quickly. Multiple paralleled resistors would likely take longer because of the way the current would divide.
Also, in post 25, he does not mention the diode string or the transformer secondary and perhaps choke that could be damaged if the glitch resisistor is slow to open.
I think LA6TPA has it right. That's how I size em'.

the pic below shows the mod B+ and the PA B+ outputs. The green plate is some kind of insulator (the supply chassis is from a gutted Kepco power supply). Next to each I installed fuse holders where I intend to put a 500mA fast-acting fuse for the modulator and a 250mA fast-acting fuse for the PA. These fuses are labeled "250 V."

Are these fuses any different from the "fine wire" fuse many use ? Should I drop them ?

Carl,

    It still works "Slick" to this day, got one on the bench now (NCL2k)
and I went through it for the cust. just before Dayton. It sure was sweet
then but it came back all "Blowed up" on Halloween. R24 a 220 ohm 1W
was burnt in half which would have removed screen voltage and the
11.5k screen bleeder. (Not good) Still one jug is OK and none of the
screen bypass caps ruptured!

    It still makes 500 out on 40 with one jug in there.  I'm sure the other
damage was customer induced. Sweet amp Indeed!

73

/Dan

So did Dentron ;D ::)


Disagree all you want but youre still wrong.

The purpose of the resistor is to limit fault current to a safe level until a fuse opens and survive. An awful lot of nasties can happen if you really think a little resistor will not induce a plasma arc. How many times have you experienced one?

Maybe at 1500V you can get away with it but not with real voltage. OTOH the possibility of a gas arc in a tube at 1500V or less is slim to none. I regetter gassy tubes at 1000V. That fuse can be a real HV fuse, a good HB one, a crowbar that instantaneously dumps the PS caps, or whatever else works safely.




That sort of tells me that the 100 Ohm 1/2W resistor at each socket was replaced by a piece of wire as it should have self destructed long before R-24.

My cure for some customers who refuse to learn how to tune that amp is to limit the screen voltage to 300V in the SSB position as a minimum step or use a current limiter circuit. The RCA 400V spec is too much anyway as its way too easy to get the screen into thermal runaway...its a shitty tube to start with. Use the GS-36B/4CX400A when you run out of 8122's; its a fairly simple conversion.

Ive never seen a blown screen bypass cap except with very early 8122's that had a free standing screen that would lean into the anode when near melting temperature. But the Johnson screen cap rings which I use on 6M conversions sometimes short for unknown reasons. My own NCL-2006 has the same ones I installed in 1964.

Carl