In a HF amplifier, a +HV arc to ground sometimes accompanies an
intermittent VHF parasitic oscillation. The reason for the arc is that
a HF tank acts as a low-pass filter, so it does not allow VHF energy to
get pass the Tune-C. As a result, the VHF energy has no way out so it
runs amuck and sometimes causes arcs. The SB-220 and the TL-922 are
examples of amplifiers that occasionally do this. In a stock TL-922,
the voltage buildup is impressive because the 6000V Tune-C can arc, yet
the anode supply is only 3000VDC
On Mar 5, 2005, at 9:45 AM, Tomm Aldridge wrote:
> As I understand this, I really should be adding a very high speed
> clamp to ground from each of my GS35B cathode connections in the form
> of a triggered device or a spark gap. Correct? What I don't
> understand is what failure or operational modes cause thsi to happen
> in a well designed amp?
>
> Tomm
>
> Paul Hewitt wrote:
>> The gap I'm using is a Bourns 2035 series, 90 volt, 20KA (single
>> pulse),
>> hope the glitch resistor holds !! The gap through Mouser is $3.22
>> single
>> piece price
>> Arc to ground or internal arc to cathode, same problem for the
>> filament. If
>> the filament secondary is not grounded the cathode to filament
>> breakdown
>> rating becomes 150v plus the transformer winding breakdown rating, (or
>> feed-thru cap breakdown rating, connecting wire insulation, etc). The
>> 90-150 volt spark gap still saves the filament.
>> I have had unintentional spark gaps between the B- buss and ground of
>> .5"
>> fire,(with catastrophic results) while a 1N5408 "glitch" diode sat
>> there and
>> smiled at me. The spark gap is faster than the diode, race them
>> sometime.
>> I now use a spark gap paralleled with the "glitch diode plus a spark
>> gap at
>> the cathode and all is good in river city.
>> Paul
>> PAUL HEWITT
>> WD7S PRODUCTIONS
>> QRO HOMEBREW COMPONENTS
>> http://home.earthlink.net/~wd7s
>> ----- Original Message -----
>> From: "R. Measures" <r@somis.org>
>> To: "Paul Hewitt" <wd7s@earthlink.net>
>> Cc: "AMPS" <amps@contesting.com>; "Paul Christensen" <w9ac@arrl.net>
>> Sent: Saturday, March 05, 2005 8:14 AM
>> Subject: Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke
>>> On Mar 5, 2005, at 7:57 AM, Paul Hewitt wrote:
>>>
>>>
>>>> Hi Rich
>>>> For the 8877 at HF you can save the filament choke, a simple gas
>>>> discharge
>>>> spark gap at the cathode will fire long before an internal arc to
>>>> cathode
>>>> will jump to the filament.
>>>
>>> What is the peak-I rating of the gas-discharge device, the V-drop
>>> across it at the peak-I rating, and what is its cost?
>>>
>>>
>>>> The discharge tube insures the cathode stays no
>>>> more than the gaps voltage rating above ground potential during a HV
>>>> arc to
>>>> cathode
>>>
>>> The problem is not a HV arc to cathode, the problem is a +HV arc to
>>> chassis ground -- which in turn tries to elevate the cathode to the
>>> neg
>>> amount of HV. Since this is typically several neg. kV, and the max-V
>>> rating between the heater and the cathode is c, 150v, there's trouble
>>> in river city.
>>>
>>>
>>>> and will fire long before a "glitch diode" will turn on and conduct.
>>>
>>> Glitch diodes are thoroughly on at 1v per junction.
>>>
>>>
>>>> Eimac recommends this on the 8877's bigger brothers. Un-grounding
>>>> the
>>>> filament secondary is a good idea also, no sense having the filament
>>>> looking
>>>> like a lower impedance path to ground than the spark gap !!
>>>> Cheers
>>>>
>>>> PAUL HEWITT
>>>> WD7S PRODUCTIONS
>>>> QRO HOMEBREW COMPONENTS
>>>> http://home.earthlink.net/~wd7s
>>>>
>>>>
>>>>
>>>> ----- Original Message -----
>>>> From: "R.Measures" <r@somis.org>
>>>> To: "Paul Christensen" <w9ac@arrl.net>
>>>> Cc: "AMPS" <amps@contesting.com>
>>>> Sent: Saturday, March 05, 2005 12:55 AM
>>>> Subject: Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke
>>>>
>>>>
>>>>
>>>>> On Mar 4, 2005, at 6:30 AM, Paul Christensen wrote:
>>>>>
>>>>>
>>>>>> Whoops - my mistake. The 77Dx's 8877 does not tie the cathode and
>>>>>> filament
>>>>>> together. One side of the filament is grounded. Would it still
>>>>>> be
>>>>>> advisable to add the common-mode filament choke anyway?
>>>>>
>>>>> Not if you have a free supply of 8877s.
>>>>> - I would add a bifilar RFC, unground the filament/heater winding,
>>>>> and
>>>>> wire the cathodes to one side of the heater.
>>>>>
>>>>>> -Paul, W9AC
>>>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>>>
>>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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