K-MAN wrote:I was wondering considering all the great mods people have come up with for the st70 over the years how they sound compared to the original?
kheper wrote:To keep the price of the st70 kit down, the
engineers cut corners with the drive circuitry.
It had very serious flaws. The board is made
of cheap phenolic and degrades over time.
The sound of an un-modded st70 is usually
described as muddy; This muddiness is, no
doubt, the main reason for the existence of
so many after-market boards.
The capacitance in the original design of
the power supply is way too low, which
further contributes to the muddiness.
K-MAN wrote:Is it cheaper buying a junky st70 and salvaging the iron for a new one or to just buy new transformers online?
TomMcNally wrote:Let's keep in mind that the components Dynaco used back
when the ST-70 and MK-III amps were first built were
pretty much the best available at the time ...
The following had not been invented yet:
fiberglass PC boards
gold RCA jacks
1% metal film resistors
gold plated ceramic pcb mount tube sockets
IEC power connectors
boutique audiophile capacitors
silicon diodes were barely on the market
So I must disagree that the Dynacos were cheap
junk ... there wasn't much better available.
The big advantage I see to the Dynaco kits
vs factory assembled brands, was the build
quality was better for a couple of reasons ...
nuts and bolts were used rather than rivets,
and generally, factory workers rushed through
the build because they got paid by the piece.
Much of the old tube equipment is a nightmare
to work on, because it's a mess under the chassis.
I take care of a 1962 vintage RCA radio transmitter,
which was THE BEST in its day ... and yeah, it has
the phenolic pc boards with the same type components
Dynaco used. State of the art back then.
SDS-PAGE wrote:Can't always generalize that cheaper ST-70s on eBay are crappy. As with all other eBay finds, it actually is possible to get a decent ST-70 every now and then. And yes, patience helps. I got mine for around 300 bucks. Irons, chassis, filter caps and other misc. were in great condition. Beside I ended up eBaying the original board with tubes + octals for around 40 bucks, so it was an added bonus. I probably spent around 150 bucks on DIY mods, so I got mine done a whole lot cheaper than if you got everything brand new. Besides, there is something to be said about original Dynaco irons.
The original PA-060 POWER tranny should not be used in a brand new build but the A-470 OUTPUT transformers are fine to use if they have been found to work OK in an older amp OR can be had at a good price.
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