6 Metre "brick" linear amps ...is there such a thing?

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I know you don't want to build the linear but maybe you could talk someone into building it for you. 

The centrepiece of any "linear amp"  is the amp section itself and there is a kit on eBay that says 3.5 to 30MHz at a price that makes it feasible to contemplate. The kit is listed at https://www.ebay.com.au/itm/DIY-kits-70W...3649910598 and the best part is that it is only about $AUD30 for the basic bit AND is posted from within Australia. There are only a few SMD parts involved with the remainder 'big bits'.

You still need to add the LPF section(s) at the output (it is a very basic wideband amp kit you are buying with this),  typically 7-pole, 2 sections, switched if you want it to cover multiple bands. The probability is that it could work on 6 metres too but as there are no device numbers detectable on the 2 x FETs, so you can't tell.

The description is given as:
Circuit class AB linear amplifier for SSB AM CW FM power amplifier for low-power radio power connection；
Suitable for FT-817 KX3 other small power stations

Power supply voltage: 13.8V；Current: 6-10A；Maximum input power: 5W；Maximum output power: 70W
Cooled: fan forced cooling；Frequency range 3.5--30Mhz (10M - 80M)
WARNING: Output prohibit open, short, prohibit VSWR of 2.0 or more
Color: As shown
Material:metal + PC
Package Contents:
1*70W SSB 

How good / feasible it is to run at 50 MHz - don't know. 
The pricing makes it worthwhile experimenting.