On 11/08/2010 04:52, Bill VanAlstyne W5WVO wrote:
> Thanks for the ideas, Bill. OK, so let's say I want to clean this amp up
> once and for all. No more accidents.
>
> For the bias string, I'm going to go with the 1N5408 diodes I already have,
> which have a max forward surge current of 200A. If I put in a glitch
> resistor of 20 ohms, that will limit the short current to 150A, which should
> be good enough to protect the 1N5408s. With a plate current of 1A (more than
> it would really run at), that would be 20 watts, which I would put up to 30W
> for margin, since I run data modes on 6m. I'm assuming since this resistor
> is below the RF choke, it is OK to use wirewound. (If not, what would be
> better? I'm probably going to end up ordering it from Mouser anyway.)
>
I have several HT packs for 3-500Z, 8877 and 4CX1000A amps. All my HT
packs have a glitch resistor that is 25R @ approx 70W comprising 4 x
100R 17W wire-wound resistors in parallel. I use the ones by Welwyn-TT
they are dark green glass-enamel types that are much better at handling
peak/overload currents than the aluminium clad high dissipation types -
the latter are not recommended for high voltage use to their
construction [a smaller resistor in an aluminium heatsink].
In my systems the glitch resistor is in the HT pack just before the
MHV/SHV B+ output connector. If integrating into an existing amp that
has PSU and RF deck combined I would install the glitch resistors in
series with B+ from the HT caps outside the RF deck, ie. between the cap
bank and the HT feed through in to the RF deck.
I also have a 1A fast-blow high rupture current 1 1/4" fuse in line.
The glitch resistor and fuse combination works well and has reduced
several faults that may have killed tubes to a manageable level.
Mike G8TIC
> The bypass cap in the SB-220 is 1,000 pF (0.001 µF). If I want this R-C
> circuit to prevent spikes that could blow things up, is this enough
> capacitance? I don't want to go much more than 20 ohms on the glitch for
> regulation reasons, as well as being a waste of power. The 1,000 pF bypass
> capacitor is a 5 KV stud-mounted doorknob type. I should be able to replace
> it with something bigger if that would help. Advice, please.
>
> Bill W5WVO
>
>
> --------------------------------------------------
> From: "Bill, W6WRT"<dezrat1242@yahoo.com>
> Sent: Wednesday, August 11, 2010 00:04
> To: "[Amps]"<amps@contesting.com>
> Subject: Re: [Amps] Replacing bias string diodes in Harbach rectifier board
>
>> ORIGINAL MESSAGE:
>>
>> On Tue, 10 Aug 2010 20:43:55 -0000, "Bill VanAlstyne W5WVO"
>> <w5wvo@cybermesa.net> wrote:
>>
>>> Maybe the original Heath zener wasn't such a bad idea after all.
>> REPLY:
>>
>> A couple of options:
>>
>> 1. Install a 50 watt stud-mount type zener. Part numbers 1N3305-1N3350
>> and 1N4549-1N4556. Browse the Mouser catalog for the needed voltage.
>> About $10 each.
>>
>> or perhaps even better although more work
>>
>> 2. Replace the diodes with 6A10 type. They have a 400 amp surge
>> rating. Assuming your B+ lead has a glitch resistor which holds the
>> dead-short current below that point, they should last forever. For
>> example, with 3000 volts and a ten ohm resistor, the max possible
>> surge is 300 amps. The leads are a larger diameter and you'd have to
>> drill out the PCB or mount them separately. About 40 cents each.
>>
>> Good luck.
>>
>> 73, Bill W6WRT
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|