Hi.
There've been so many ST-70 driver broads of different designs for sale in the marketplace since day one. Most most if not all, of those driver boards are of totally 'alien' design from its orginators: Halfer & Laurent in 1959.
Put aside the sonic aspects of the so called upgraded driver boards so readily available todate, what get into my nerve is: are those new boards designed to RESTORE the original ST-70 sound - to make its orginal sonic signature sound better or to REBUILD it to deliver some alien sound qualities??????
I say so because I own a pair of stock PAS-2 & ST-70 which I heavily UPgraded them. When I say upgrade, I mean RETORING them without rebuilding it with a driver board of alien design in order to reserve ST-70 sonic signature as much as possible.
In fact, there are tons of improvement can be made on ST-70 WITHOUT need of replacing its stock driver borad at all.
I've done it by still keeping its old driver board. It sounded so slow, muddy & noisy which was totolly unacceptable to my picky ears when I first acquired it (original 1959 stock version) - a donation from an audiophile friend of mine about 5 years back. I could have returned them back to the donor if I knew nothig electronics as I could not stand the aweful sound at all.
But now this ST-70 sings like a nightingale - fast, see-thru transparent, ultra detailed, musical & pretty quiet - still retaining its 50-year-young stock driver board.
Very briefly, like many DIYers did, I rebuilt the entire 450VDC HV PS by replacing the old stock composite 'lytic cap with discrete AC caps (10uF 400VAC oil capx1+40uF250AC PP-film motor-run can capsx3 ), SS voltage regulator for the stock driver board, rewiring the entire old-school multi-point signal reference grounding system to one-point star format with RFI earthing device into a 3-wire power cord format. Replacing old coupling caps with PP film caps & 1% metal film resistors on the stock driver board.
The most crucial sonic upgrade is to add a special triode-strapped topology to the EL-34 O/P power tubes so that the power O/P stage can be operable in either UL or triode mode by flipping a toggle switch. I have found its triode-strapped mode sound so so so much better than its stock U-L format though the amp delivers much less power (from 35W to say 15W).
So I've the amp working on triode mode since day one of its upgrade years back as I love triode sound over all.
I reduced its stock 20dB global NFB to 10dB to help boosting a little bit on the lower power O/P.
Mind you, this is not a conventional 100R resistor P-g2 strapping as seen using in all triode strapping powerstage designs so far. It is a special 32V g2 level-shifting device keeping Vg2 potential 32V lower than its plate while operating triode mode. Probably the first time ever used in ST-70!
So do we need to replace a new driver board of some alien designs to get better sound out of a ST-70, my answer is NO if one knows enough the business.
c-J