[Amps] diode junction temps and thermal resistance

Wed Oct 20 08:08:37 PDT 2010

Jim, in using the string of diodes for bias, do you notice any drift in the 
 idle current as they warm up? Purchased in quantity these big diodes are 
way  less expensive than a big zener diode and would be easy to mount on a 
small  perfboard. 

Makes one wonder why some company doesn't offer premade zener substitutes  
along the line of K2AW's hv diode designs.

Is there a downside to this method of generating cathode bias?

73,
Gerald K5GW

In a message dated 10/20/2010 9:52:59 A.M. Central Daylight Time,  
jim.thom at telus.net writes:

Date:  Wed, 20 Oct 2010 08:08:08 +0100
From: Ian White GM3SEK  <gm3sek at ifwtech.co.uk>
Subject: Re: [Amps] diode junction temps and  thermal resistance

Bill, W6WRT wrote:
>If not overtemp, it's  almost certainly arcing inside the tube. Even 
>good tubes arc  occasionally with no harm to the tube, but there can be 
>harm to  components in the HV- to HV+ path and that includes the bias  
>zener(s).
>
>IMO, for a legal-limit amp a ten watt zener  is marginally adequate. 
>Even if it can dissipate the cathode current x  zener voltage, there 
>isn't much left to absorb arcs. I would recommend  a 50 watt stud mount 
>type.
>
>There is a certain amount of  guessing in the rating because you never 
>know how massive the arc is  going to be.

##  a 50 watt zener requires a 50 w heatsink.   The problem I had
with 10 w zeners back in the 70's is..they are the 1st  component to short
out come glitch time.   The problem with both  a 10 w and a 50 w zener's
is they won't handle a surge worth a  damn.   If you use a zener, use a 
50W unit.   

Actually you *can* know something about the arc.

1. The  peak current. The main function of a surge limiting resistor in 
the B+  line is to limit the peak current to a known value, so that Ipk = 
Vb/R.  The actual arc current will be time-varying but it can never 
exceed that  value of Ipk, so you *do* have a known value to design 
against.

###  use a 50 ohm glitch R in the B+ lead, instead of the usual
10-25 ohm unit,  and now you can really limit the max peak current. 
A 50 ohm glitch can  consists  of parallel 100 ohm WW's... or
2 x 25 ohm WW's in  series.   I use a single  50 ohm-50W WW
in my L4B's... zero  problems.  2650 vdc / 50 ohms =  53A max fault
current..... then  the B+ fuse blows open in < 2 msecs.   

2. The  time it will take to shut the HV supply down (by means of a fuse, 
breaker,  relay, crowbar or whatever means you have chosen). It isn't 
easy to get  beyond an initial guesstimate of "a few milliseconds", but 
further detail  *is* knowable if you're prepared to drill deep enough.

##  I  use  a  B+  fuse.....just before the glitch R.   I  also use a 2nd 
HV fuse,
between sec of plate xfmr  and input of diode  board [ one leg only].   Then
a  magnetic hydraulic breaker  in the 240 vac primary.  Now you van cro-bar 
the
B+ to chassis all day  long...and nothing happens....except a blown B+ 
fuse. 
On the 7 kv supply,  the B+ fuse will open off < 2 msecs.

##  I use a FAST 3agc  type  cathode fuse, in the CT of the fil xfmr, with 
a 
pair of 100k-3W  MOF's across it.   For a grid fuse, I used another FAST 
3agc
type  fuse, wired between chassis and neg terminal of grid meter [or grid  
shunt].
Nothing gets wired across the grid fuse.   if grid fuse  blows open, no 
path for dc grid
current, amp can't be driven, and you end  up with zero power out of amp.   
Input
swr  rises to  infinity.  Input wattmeter detects the high swr and shut's 
down  the
XCVR..and latchs it off. [ the xcvr would shut itself down  anyway].        

The big unknown (to me, at  least) is what fraction of the total arc 
current will flow through the  cathode zener or other bias circuit. To 
trace the current flows, see  Figure 2  of:

http://www.ifwtech.co.uk/g3sek/boards/triode/triode-manual.pdf

If  the grid collects 100% of the arc current, then none of it will flow  
through to the cathode. The current will be steered around that part of  
the circuit, flowing through the B-minus clamping diodes instead, and  
the cathode zener (or other bias circuit) will be  unaffected.

##  good point.  Instead of a zener, on the hb  amps I use a series
string of 6A10 diodes. Then with a 12 / 20 pos rotary  switch, every
2nd or 3rd diode is tapped off.....starting at diode  #10.   For a 
real fine adjust, a spst toggle is used across  diode #1. To improve the
dynamic bias regulation, a big 10,000 to  200,000  uf  lytic is wired across
the entire string of  diodes.  Now the bias can be adjusted on the fly. You 
can
suck 6A all  day though it. With it's 400A surge rating, you can't blow it 
up anyway
so  it never needs to be repaired, or replaced.  No heat sink required  
either. If
you are real hardcore, parallel 6A10's work good too. [12A/800A  surge]

##  for simpler stuff, like my L4B.. I used  10 x  1N5408's in series..and 
a 
SPDT-center off  miniature toggle on the  rear apron.   That toggle gives 
me 3 
positions of bias.  

##  If you have to replace zener's in a GG amp, they were not  engineered
correctly to begin with...and ditto with safety diodes between  B- and 
chassis,
and across  plate + grid meter's.    

Jim   VE7RF 

At  the other  extreme, if the arc punches clear through the grid 
structure then a large  fraction of the current will flow through the 
cathode, causing damage to  the cathode and possibly/probably the bias 
circuit. For examples of  cathode damage,  see:

http://www.ifwtech.co.uk/g3sek/misc/gs35-arcs.gif

The GS35b  is a planar triode with a disk-shaped oxide cathode and a mesh 
("tea  strainer") grid. Comparing the left-hand and right-hand images, 
you can  match up the marks on the anode with the corresponding marks on 
the grid.  If you then zoom into the left-hand image of the grid, you can 
see where  the largest arc has punched through to leave a burn mark on 
the cathode  underneath.

These kinds of damage would be much harder to see in other  types of tube 
construction, but I'm fairly sure that similar effects will  exist.

The key to damage limitation is ALWAYS to install that surge  limiting 
resistor in the B+ line! Eimac Bulletin 17 explains exactly  why:

http://www.ifwtech.co.uk/g3sek/misc/bull17.pdf

--  

73 from Ian  GM3SEK
http://www.ifwtech.co.uk/g3sek

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