[Johnson] Johnson Thunderbolt

Wed Jun 25 19:45:14 EDT 2008

Yep, that's true. Generally if I can (shunt resistance value is
reasonable enough), I use a combination of fixed resistance and length of
wire. As an example of not reasonable, the shunt meter resistance for the
plate circuit in the Apache is 0.1 ohms. A length of wire shunt worked
fine for this amateur application.

Pete, wa2cwa

On Wed, 25 Jun 2008 19:29:55 -0400 "Carl" <km1h at jeremy.mv.com> writes:
> 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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