Johnson Thunderbolt

[W3AV]

Recently acquired a Thunderbolt amplifier.  I am trying to understand the use of the RESISTOR position on the input of the 4-400 grids.  When running in LINEAR mode, this resistor is used in conjunction with certain attenuators (like when using ranger, viking, etc) as per the manual.  It is my belief that these attenuators provide an output impedance of near the 330 ohms of this resistor, thus matching the tubes input impedance.

Does the "untuned" nature of the input grid work because in the normal operation of this linear, it is pretty much operated in ab1, with no grid current?

If so, is it feasible to actually take the 330 ohm input resistor down to something that would be matched, or easily matched by say the new flex1500 radio.   1-2 watts of am from it should drive the thunderbolt pretty well.

any input on this subject would be appreciated.

tx
bill
w3av

[WQ9E]

Bill,

I have a T-bolt paired with my Johnson Pacemaker. 

If you change the value of the resistor, you will greatly increase the amount of drive needed  Depending upon how sensitive the Flex is to reactive loads, you might be able to use a 4 to 1 balun to get close enough but realize that even if you changed the resistor to a 50 ohm and increased the drive to compensate you the input is not purely resistive.  The Thunderbolt transitions to AB2 when producing the rated input so you will be driving the tubes into the grid current region as you approach the amplifier rated input.

I imagine the easiest solution is to use an auto tuner with the Flex or use the tuned input position of the T-bolt.

[The Slab Bacon]

Congrats on just getting 10lb of szht in a 5 lb bag! ! !    I also have one. the T-bolt is one of the worst power to weight back breakers ever made. If you expect to run it on AM, dont look for much over 100-150W of carrier output. But on the other hand, they're just about indestructable! The biggest problem with them is that the plate voltage is just too low to make a high PEP output, and the plate tank wont allow you to go much higher without arcing over.

If you use the "tuned input" settings it only requires a few watts to drive it to full input. (less than 5w) If you use the resitive input circuit, the bank of resistors also act like a built in dummy load and absorbing just about all of the input drive power. This allows you to just tickle the grids with some voltage for AB1 operation.

Since it was designed to operate with older tube gear, the finals of MOST old gear have the capability of loading into a 300ohm load and it wasn't a problem back then.

Try running it in the tuned input mode with your modern rig, and/or putting a small tuner in line with it. Also try whatever you can to up the screen voltage on the     4-400s, this will allow higher peaks without compromising the plate tank. The plate supply can handle the extra current of the higher peaks easily.

                                                          The Slab Bacon

[KM1H]

Change the resistor to 50-100 Ohms and drive it with more power if using a ricebox or tube rig with a limited Pi-net. It should be capable of 200-300W out while staying in AB1. Use some fixed padding in the tank if necessary

The NCL-2000 has the low power resistive tap and I used to run 300-350W out usning a TS-930 and 940 as the driver on AM. Cap spacing no better than the T Bolt and no Pi Net changes were necessary.

Carl

[The Slab Bacon]

Carl,
       the biggest problem with a t-bolt is it's only got 2000v on the plates. This is just not enough to wake up the 400s. You have to make your powa with plate current. the only way to do that without arcing the final tank is to do whatever you can to up the screen voltage. IIRC, the screens only run around 400v or so. Getting the screen voltage up higher makes a big difference. It made a hell of a difference on my I2K. the plate transfomas are pretty beefy and can handle the extra plate current easily.

Also the T-Bolt has seceral built in inpoot configurations, (its been years since I used mine) but, IIRC, two of them dont use the swamping resistor and require very little drive. I used to use mine with a homebrew 5W piss-weaker and get around 175w out (carrier).
                                                        The Slab Bacon

[Opcom]

The plate transformer runs hot on AM, beware of cooking it.

[Gito]

Hi

If You read carefully page 12 of the Johnson T.B ,it stated that  AM linear operation requires that the Exciter be loaded close to its normal level  with the OUTPUT,then attenuated to prevent over driving the Thunderbolt

In linear operation The Thunderbolt operates in class AB1(Zero grid current) up to a peak power input of 1400 watts ,and it's driven into grid current region,class AB2,to obtain the rated power of 2000 watts.When grid current start to flow,the Thunderbolt grid impedance drops thus presenting a variable load to the exciter.....(Johnson TB manual)

So I believed in linear operation the purpose of the 330 Ohm resistor is a swamping resistor that dominates the input impedance of 4-400 tube ( making a constant load for the exciter).
the input impedance in normal operation must be in kilo ohms order.
The 330 Ohm, is in parallel with this input impedance.Giving a +/- 330 Ohm load.


Gito

[KM1H]

Its a good candidate for a revised PS using modern components if its to be run as a linear.

The Ep is now 2200V and the Esg is 410V. Tossing the chokes and old rectifiers and going with some high C electrolytic caps should bring it up to around 3000 and 500V. Get rid of those low value wirewound equalizers. Do the same with the bias PS and make it adjustable to set the desired idle current and then select the required zeners to keep it regulated. Configure so you have room for a muffin fan for the transformer. If youre feeling flush install some real iron. A Tune cap from a SB-221/HL-2200 (also used as SB-220 upgrade) will give another 1000V spacing and looks to be room to fit.

Id also check the filament voltage at the socket and get it to 4.9V for longevity. A retrofit to 4-500A's is also posssible but will probably require a cooling upgrade if you go for the most power.

Carl

[W3AV]

thanks to all who have replied for the great input.  I think i need to step back, see what i really want this thing to do, and go from there.  Its obvious to me that the first step is to get it on the bench, and see what is really going on inside.  Thanks again ALL!  73