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Reviews For: SB2-LA

Category: Amplifiers: RF Power - HF & HF+6M

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Review Summary For : SB2-LA
Reviews: 4MSRP: $395 in 1965.
Description:
Linear amplifier to match the SB-33 and SB-34 transceivers.
It runs off 110V, uses 6 sweep tubes, self contained, easy to use. 1000Watts input and it produces about 600Watts output. SSB service only. Uses the 6MJ6 tubes, or 6LQ6, whichever available. Nice simple design, rather squeezed inside.
Product is not in production
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# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
1544.5
KS4PC Rating: 2022-12-01
Cool old amp Time Owned: 0 to 3 months.
Got mine in a trade new caps resistors found good set of tubes easy 500w out on 40 and 80m simple to work on all most 60 years old don't think these new amps will last half as long
N8FVJ Rating: 2020-05-19
Powerful with six 6LQ6 tubes Time Owned: 0 to 3 months.
Frankly I just received this amp. It was inexpensive and I have not powered up yet. I may have the buy six 6ME6 (6LQ6 replacement) for $25 each in NOS on ebay.

The power supply capacitors are needing replacement and frankly a limiting factor in peak power output. HV has about 40 volts AC ripple and voltage sag is large with the small original capacitors. Using dual voltage doubler circuits in series, the old 200uF caps should be replaced with 820uF caps to make full power peak output from this amp. 150uF on the dual voltage doubler output will keep HV AC ripple to about 10 volts.

The amp input is a 75 ohm array of resistors providing a resistive load. You could rewire the amp to grounded screen and connect the grid to the cathodes. Then, lift the cathodes bias up with a 24 volt 50 watt zener and inductor to chassis ground. Remove the input resistor array, drive the cathodes and likely get another 100 watts output power with 100 to 150 watts drive. Input impedance match would likely be less than 2 to 1. It is 1.5 to 1 in stock form.

What I like about this amp is the reliable vintage tubes. New Chinese 811A, 572B and the 3-500ZG fail and are more expensive vs NOS sweep tubes in the 572B and 3-500ZG price range.

I suspect with the power supply capacitor change to 820uF and about 150uF on the HV output will make about 700 watts PEP out on 75 meters in stock form with new power supply capacitors.

For the power and reliability, this amp is a winner.
K3MD Rating: 2016-02-15
Rugged Time Owned: 6 to 12 months.
This is my second SB2-LA. Got it on EBay from a radio recycler in Md. I had to replace the filter caps with higher capacitance value units to combat hum.... initial report on the air were T8. I used ALL Electronics 400V 140uFd electrolytic caps. The filter cap board is very easy to get to. The keying line is -140 volts... you must use an Ameritron ARB-704 or homebrew external 12V relay, or you will blow out the keying transistor or relay in your modern rig. You could also put in a Harbach board. I did not put a 12V relay inside the case for rear of messing up the complex bias/cutoff circuit. Drives well to 400 to 600W out. Input circuit is non-inductive resistors, a la Alpha 76, HF-2500DX, etc. Drives well without a tuner in the exciter. Easy to tune... unit has output meter and "tune"(reduced power) tune-up switch. Replacement used/good 6LQ6 tubes run $15-20 each on EBay. The tubes have NO TVI parasitic suppressors, but rig has suppression circuit in grid circuit. I place a nice big 120V muffin fan on the top..... keeps things cool. Unlike some sweep tube amps, runs 800V on plates, not 1200V.... tubes are not as stressed. This unit is very solidly built. Use as amp at summer cabin.
KH6VP Rating: 2015-02-11
Excellent for its time Time Owned: 3 to 6 months.
Very simple excellent linear amplifier, squeezing its parts and an ability to get 1000watts input, using six sweep tubes (6MJ6) which at 33 watts dissipation each, provides for 198 Watts total of dissipation, rest as RF power, mainly for SSB use only. Matches the SB-34 transceiver, also from the mid-sixties.
Interesting simple design, covers 80, 40, 20 and 15 meter bands only.
Can be used with modern transceivers. Power supply built-in; only 110V [no 220V possible]. Includes TX/RX relay (many from that time didn't).
Was designed when sweep tubes were dime a dozen; can use other tubes [6 PL519 can be used with filament connected in series, tied to a 240V line for example].