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Reviews Categories | Amplifiers: RF Power - HF & HF+6M | Johnson Thunderbolt Help


Reviews Summary for Johnson Thunderbolt
Johnson Thunderbolt Reviews: 3 Average rating: 4.3/5 MSRP: $589.50
Description: No longer manufactured. Just a good amplifier.
Product is not in production.
More info: http://
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AJ4FM Rating: 5/5 Jul 25, 2009 12:05 Send this review to a friend
Still fantastic  Time owned: 3 to 6 months
Like the previous review, I've done basically the same modifications. I built a small box that has 2 relays in it for switching the input and output to use this amp with a modern transceiverand for switching the bias. I use SS HV plug-in replacements for the old 866A/3B28 tubes.

What makes this old workhorse such a good amp today is the ability to have continuos operation from 3-30 MHz, a front panel grid tuning control, cutoff and operate bias (regulated), regulated screen supply, full metering of grid, screen and plate current plus HV metering. Dual fans, one under the tubes and one blowing across them. It doesn't do 160 but I don't operate the top band, so I'm happy all around.

I use this amp 99% of the time on RTTY at 500 watts output and it just loafs along. It will do more but 3dB is still only a half of an S unit, get real. Granted it weighs over 120 lbs and this one has a larger Peter Dahl plate transformer in it, so I'd say it is over 135 lbs. My desk doesn't move! The new transformer has a 1 amp secondary so the T-Bolt does really loaf. It is a great amp for what I do, and the price was right...FREE. Go get one while they may still be around. Only 897 were built in the 6 years of production (1958-1964)
 
KD1ZM Rating: 4/5 May 3, 2009 09:39 Send this review to a friend
Great old Amplifier  Time owned: 6 to 12 months
The Johnson Thunderbolt amplifier was introduced in 1958 and it was sold both as a kit for $524.50 and wired & tested for $589.50, (ref. 1/1/61 QST AD).

I just completed restoration of a Thunderbolt and it works great. I made some modification so it would interface with my Icom 781 Transceiver by adding a vacuum relay for antenna switching plus another relay for switching the bias on the 4-400A's when the transceiver is in the receive mode. This mod allow the amplifier to run a lot cooler in the standby linear mode and extends the life of the tubes. One limitation of the Amplifier design is the low plate voltage of 2200 to 2400VDC, one possible additional mod is to replace the 3B28's with solid state rectifiers and or change the HV filter circuit to capacitive input rather than choke. Without modification to the PS the amp puts out 750W CW and between 1000-1200W PEP. The Amp has a lot of gain so it is easy to over drive; I modified the input coupling to gain more control over setting the drive level but you can also add a resistive swamping network.

I had a lot of fun restoring this amp and I enjoy using it, it is now part of my boat anchor AMP collection and I would not hesitate to use it on a daily basis, it is a rugged design. It is also important to note that the 4-400A's tubes are not expensive to replace, typically $35 to $50 for a good used tube and $100 for a new one. I was also surprised to discover that it was not difficult finding original replacement parts for the amplifier; you can even get the original E F Johnson matching maroon and gray paint for the cabinet and front panel etc. to complete the restoration.

I rated the Amp a 4 but it is probably a 4.5 out of 5 now that I have made the modifications, I also have a Collins 30S-1 and Alpha 374A in my Amp collection. If you find a Thunderbolt buy it, it is a low cost KW amplifier that you will enjoy using.
 
VE3FGU Rating: 4/5 Mar 22, 2009 16:03 Send this review to a friend
A good old amplifier.  Time owned: more than 12 months
I have always wanted a Johnson Thunderbolt. I bought mine used about 17 years ago and it is still working just fine. With a matched pair of 4-400C tubes it will easily put out 725 to 750 watts on CW as measured on a Bird 43 meter and probably well in excess of 1000 watts on SSB. If you happen to find one at a flea market or swap meet be aware that this thing weighs about 125 pounds. The only problems I have had is a very nasty resonance in the plate choke on the 18 and 24 Mhz bands and the amplifier gets very hot unless you do a bit of modification to the cooling system. Changing the plate choke for one from a more modern amplifier cured the arcing on the WARC bands. If you are looking for a boatanchor amplifier that covers 3 to 30 Mhz continuously, this is a good one.
 


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