Tnx, Mike.
A few years ago I ran some tests on the 30-ohm, 0.5W resistors we
were using for grid fusing and concluded that a 3-500Z two-holer amp
would have to produce over 350mA of grid current per tube in order to
blow them , so we switched to 0.25W units.
Has anyone else who bought one of our suppressor retrofit kits
experienced consistent grid fusing R failures with 0.25watters?
On Jul 21, 2006, at 5:07 AM, Mike Schatzberg wrote:
> I imagine, because this is some years ago now, that I was seeing
> not more
> than approximately 100 mils per tube, or a total of 200 mils for
> the pair
> grid current.
>
> When I run the amp, take it from my amp museum and fire it up
> periodically,
> I still run it the same way today. The resistors are still in
> there, but at
> 1/2 watt for equalization purposes.
>
> I have receive several emails from other list members indicating
> that they
> too had nuisance failures with their 1/4 watt resistors, and have
> changed
> the component to 1/2 watt as well, with no more failures. It is
> interesting
> how many folks will not offer their experiences on the reflector,
> but only
> directly.
>
> I believe that in general both resistors blew at the same time, but
> it might
> not have been so in every case. It did happen numerous times before I
> changed the wattage.
>
> This was with Chinese tubes, ones for which I have never been able
> to obtain
> spec sheets. Although my wattmeters are not traceable to NITS,
> they all
> read the same thing here, and the amp was loaded to 1.5 KW output
> (which I
> have been told here is impossible) with less than 100 watts drive,
> when the
> resistors opened in SSB.
>
> By the way, I have also received emails from list members who have
> stated
> that they always see about a 10% increase in output for given drive
> with the
> Chinese tubes. I guess the combination of all my wattmeters
> reading exactly
> the same amount higher than they should, and the 10% greater output
> might be
> an explanation for the 1.5 KW readings. One day I'll get a laboratory
> instrument here on loan, and check their accuracy.
>
> Regards,
>
> Mike
> W2AJI
> ----- Original Message -----
> From: "R L Measures" <r@somis.org>
> To: "Mike Schatzberg" <cherokeehillfarm@earthlink.net>
> Sent: Friday, July 21, 2006 5:59 AM
> Subject: Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)
>
>
>>
>> On Jul 20, 2006, at 9:18 PM, Mike Schatzberg wrote:
>>
>>> Richard:
>>>
>>> They just blew open suddenly, nothing else amiss. I would replace
>>> them, and
>>> be right back on the air with full output. I estimated that under
>>> normal
>>> conditions, the 1/4 watt resistors must have been carrying pretty
>>> much their
>>> rated current, and must have been relatively warm during transmit.
>>
>> Thanks, Mike. How much grid-I were you running when the grid-R
>> blew? Did one or two blow.
>>>
>>> I guess one of the issues I always thought about is that the time
>>> to burn
>>> out must have been proportional to the duration of the
>>> transmission, as well
>>> as to the degree of over drive which the grid experienced. I also
>>> wondered
>>> if the resistors (film flame proof) had a temperature memory so to
>>> speak,
>>> and would remember how much time was spent at elevated temperature,
>>> slightly
>>> changing value over a period of operation.
>>>
>>> I didn't have 30 ohm resistors in the application, but I wouldn't
>>> have
>>> thought the 27 ohm resistors would have made a great deal of
>>> difference in
>>> the power they had to dissipate.
>>>
>>> 73,
>>>
>>> Mike
>>> W2AJI
>>>
>>>
>>>
>>>
>>> ----- Original Message -----
>>> From: "R L Measu
>>> res" <r@somis.org>
>>> To: "Mike Schatzberg" <cherokeehillfarm@earthlink.net>
>>> Cc: "Gary Schafer" <garyschafer@comcast.net>; "'Tom W8JI'"
>>> <w8ji@w8ji.com>;
>>> "'Paul Christensen'" <w9ac@arrl.net>; <amps@contesting.com>
>>> Sent: Thursday, July 20, 2006 11:42 PM
>>> Subject: Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)
>>>
>>>
>>>>
>>>> On Jul 20, 2006, at 7:07 AM, Mike Schatzberg wrote:
>>>>
>>>>> Hello Amp Fans:
>>>>>
>>>>> I have experimented with "grid fuse", while investigating grid
>>>>> equalization
>>>>> and trying to eliminate the need for using matched pairs of
>>>>> triodes.
>>>>>
>>>>> I believe what I tried at the time was about a 27 ohm 1/4 watt
>>>>> resistor.
>>>>>
>>>>> What I found was that the resistor was very sensitive to grid
>>>>> current peaks.
>>>>> The value that I selected for use with a pair of 3-500ZG triodes,
>>>>> in an SB
>>>>> 220 amplifier, seemed to be very sensitive to slight over
>>>>> drive, or
>>>>> so it
>>>>> seemed.
>>>>>
>>>>> The resistor would open circuit perhaps once in 3 months, while I
>>>>> tested
>>>>> this system on the air in daily use. I can only assume that high
>>>>> current
>>>>> peaks caused it to open, I never heard any arcing or other anomaly
>>>>> from the
>>>>> amp.
>>>>
>>>> Mike S.: We use 30-ohm, 0.25W resistors as fuses and they will
>>>> carry
>>>> normal SB-220 grid current without changing value. With serious
>>>> mistuning, they will darken but remain intact.
>>>> Were your failed resistors slow-roasted, or did they just blow open
>>>> suddenly?
>>>>>
>>>>> ...ilman/listinfo/amps
>>>>>
>>>>
>>>> R L MEASURES, AG6K. 805-386-3734
>>>> r@somis.org
>>>>
>>>>
>>>>
>>>
>>>
>>
>> R L MEASURES, AG6K. 805-386-3734
>> r@somis.org
>>
>>
>>
>
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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