[Johnson] Johnson Thunderbolt
John Lyles
jtml at losalamos.com
Wed Jun 25 17:42:01 EDT 2008
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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