[Johnson] Johnson Thunderbolt

[Carl]

That certainly works Pete but be aware that with enough copper wire the 
temperature stability suffers and the readings will change. The idea of 
mostly resistor and fine tune with wire is excellent.

Carl
KM1H

----- Original Message ----- 
From: "Peter Markavage" <manualman at juno.com>
To: <johnson at mailman.qth.net>
Sent: Wednesday, June 25, 2008 7:20 PM
Subject: Re: [Johnson] Johnson Thunderbolt


> If I need a low value of shunt resistance, I generally use a length of 22
> or 24 gauge wire (have lots of that handy). Wire tables will give you
> resistance of a particular gauge generally per hundred feet. Measure the
> current with an accurate outboard meter; then connect the replacement
> meter with shunt; then adjust the homemade shunt (by trimming -when I
> figure out the actual length of wire needed for a particular shunt, I
> generally add several inches so I can trim) with the replacement meter.
> The completed shunt doesn't have to be pretty. I generally wrap the
> length of wire around my finger with the ends protruding for whatever
> length needed and hold it together with tape. Depending on the shunt
> resistance needed, sometimes you can use a combination of a fixed
> resistor and a length of wire. Been making shunts this way since I burned
> out my 5.65 ohm shunt grid resistor in my Apache back in 1961.
>
> Pete, wa2cwa
>
> On Wed, 25 Jun 2008 15:42:01 -0600 "John Lyles" <jtml at losalamos.com>
> writes:
>> Yes, I haven't seen an amplifier with parasitic suppressors on the
>> cathodes, grids and plates, except in very old QST articles on the
>> subject.
>>
>> Its amazing that the Thunderbolt leaves the screen voltage (and
>> bias, of course) on the tubes even when the plate is off. This is
>> very risky, as if the bias supply were to crap out, there is no
>> protection against ruining a good pair of 4-400s.
>>
>> My friend checked all the VRs and they were fine. Thanks for the tip
>> on the 270 K added resistor. Turns out that he believes that the
>> sparks were from dirt, as he did a more thorough wipedown of
>> everything and it hasn't happened anymore.
>>
>> Problem with substituting another meter is that this 1 mA meter is
>> shunted by 1.725 ohm, in the grid I position. If the meter isn't
>> exactly a 50 ohm internal resistance milliammeter, then the
>> calibration is changed. Either change the shunt R, (a special very
>> low value) or get the right meter.
>>
>> Sad thing is that I used a Thunderbolt at work at 5 MHz, in 2002,
>> testing something with high power RF, and it was working well. Now I
>> cannot located where it went, someone either tossed it out, or I
>> misplaced it, or it got stolen.... Drat. If I still had my hands on
>> it, i wouldn't be asking these questions here. However, if anyone
>> has a used meter for the multimeter position for a THunderbolt, will
>> buy it! Thanks and 73,
>>
>> John
>> K5PRO
>>
>>
>>
>>
>>
>> > My oil cap is the same rating.  I understand that oil caps can
>> withstand
>> > more than their rated votage to some extent, but I don't know how
>> much
>> > trust I'd put into that.  But if the arcing was audible, it wasn't
>> the
>> > cap.  The mica capacitors used in the loading padder don't have DC
>> on
>> > them, so that's not it either.  By the way, mine are somewhat
>> lossy as well.
>> >
>> > Check the plate blocking capacitors.  Better yet,just replace those
>>
>> > cheesy TV doorknobs with some real Centralab transmitting
>> capacitors.
>> >
>> > The amp is most certainly capable of parasitics; if you look
>> around,
>> > you'll notice many suppressors in the grid compartment as well.
>> >
>> > It appears that someone swapped out the grid bias regulator tube;
>> it's
>> > supposed to be a VR75 for an early T-bolt with 4-400As, per the
>> book.
>> > The other VR tubes regulate the screen voltage.  Check 'em all.
>> >
>> > Oh, one other thing:  connect a 270K 2W resistor across terminals
>> 5 and
>> > 6 of SW102.  Doing so will prevent the amp from having no bias if
>>
>> > switched to "LINEAR" which will be most kind to your bias supply
>> choke &
>> > transformer.  This change was made for all of the late T-bolts,
>> apparently.
>> >
>> > I'd try to find the source of the problem before looking for
>> another
>> > genuine meter.  And make sure that you bypass them with some big
>> honkin'
>> >   back-to-back rectifiers to (hopefully) keep this from happening
>> again
>> >
>> > Good luck.  Let us know how things go.
>> >
>> > regards,
>> >
>> > Mahlon - K4OQ
>> >
>>
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>>
>
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