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Re: [Amps] 3-500Z glitch resistor

Subject: Re: [Amps] 3-500Z glitch resistor
From: Paul Whatton <>
Date: Mon, 08 Sep 2008 23:39:23 +0100
List-post: <">>
Hi Carl

Carl wrote:
> The bigger the better up to the point of no improvement.
No disagreement with that.
> However the purpose of the resistor is not to absorb 100 % of the 
> energy BUT to limit the current to a managable level until the primary 
> breaker trips.
If I understand what's going on during a flashover correctly the first 
job is to hold the current down to a level that doesn't damage the tube 
or any other components in the amplifier until the breakers trip. The 
first problemm is the energy discharged from the capacitors. After the 
glitch resistor has dealt with that it needs to hang on in there until 
the breaker, or fuses, operate to protect the rectifiers and transformer.

I guess it's a question of amp design philosophy. Do we want the glitch 
resistor to survive a flashover intact? If we don't mind it failing and 
choose, or have, to use a component that isn't rated for the job, how 
reliable is the failure mode?  I don't want a glitch resistor that arcs 
or goes short circuit, so the volts the resistor can survive under 
overload are also important. Does the time needed to get the amp back on 
the air matter? It does to me in a contest. I really ought to use a 
G3SEK control board but being a cheapskate I trade off protection & cost 
against recovery time.
> In a SB-220 for instance there is no room for a 50W or larger enamel 
> resistor and a 25W will survive. The fact that it works is that the 
> original PS diodes have survived.
The comments I made about the size of a glitch resistor were about 
building from scratch rather than retrofitting. I'd stick by my comment 
that a 50W+  wirewound is far better than any thick-film. I have just 
the same problem of limited size with my AEA LA30 which I want to fit a 
glitch resistor in, I'd love to use a 50W WW but there isn't enough 
room. So I'll use the biggest, best, vitreous wirewound I can fit and 
pray a bit. But in home-made amps there isn't an excuse and we can 
design in the space for a decent well-rated sized wirewound.

Guess we might all agree on the following? Use a vitreous ceramic 
wirewound. If at all possible use one that can sink the energy and HT 
volts under a flashover within its ratings. If not use the highest 
power, highest voltage, ceramic wirewound that will fit in the box. 
Thick-film resistors, even big ones, are pants in this application. 
Wirewound resistors, even big ones, are a lot cheaper than tubes or HT 

73 Paul G4DCV


----- Original Message ----- From: "Paul Whatton" <>
Cc: <>
Sent: Monday, September 08, 2008 2:08 PM
Subject: Re: [Amps] 3-500Z glitch resistor

> For Vic who originally asked this question, I've seen the figure of
> about 40A suggested as the current limit so for a 3-500Z with 3kV, 75R
> should do. It isn't the normal power rating of the glitch resistor that
> is the problem but as David points out, it's the resistor's ability to
> survive the energy pulse from the power supply during flashover.
> In my 2m amplifier I use a modest 2kV supply with about 50uF. When I
> built the amp I used a 47R 250W thick-film as the glitch resistor (5
> times bigger than the 50W RCH series). It "felt" and certainly looked
> like it should be more than big enough. But it failed, fortunately open
> circuit, on a flashover. Hindsight is a wonderful thing and checking the
> datasheet I'd exceeded the overload ratings. BTW such a thick-film
> resistor costs about GBP45 new from RS in the UK, that's an expensive
> fuse. Fortunately I'd only paid a couple of quid at a hamfest.
> IMHO a 50W+ ceramic wirewound is a much better and cheaper bet than the
> biggest thick-film in this application.
> 73 Paul G4DCV
> David G4FTC wrote:
>>>> Everyone seems to agree that a glitch resistor between the plate choke
>>>> and the HV supply is a good idea, to limit huge current surges due to
>>>> arcs caused by gas, etc. But recommendations for the value seem to 
>>>> vary
>>>> all over the map.
>>> I like the look of Vishay RCH series or similar (Tyco do an
>>> equivalent). The 50W version is rated at 5.5W without heatsink and
>>> is specified to handle 2500Vrms (that's 3500V peak) short term
>>> overload. If you want to bolt it to a panel for heatsinking, the
>>> insulation is rated for 3500Vrms (5kV peak).
>> Checking page 4 of the datasheet for the RCH50 
>> shows a maximum overload capability of about 50 Joules.
>> A capacitor bank for a linear of 50uF charged to 3kV will deliver 
>> about 225 Joules and
>> under glitch conditions the resistor will be absorbing most of this 
>> power, more if
>> the power supply doesn't have glitch detection/protection and remains 
>> connected
>> to the mains.
>> In order words to be safe you'll need about five RCH50 resistors in 
>> series.
>> I don't know what energy the tube can absorb under flash over 
>> conditions without
>> damage and this will also influence the selection of the resistor.
>> Regards
>> David G4FTC
>> __
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