ST-70 Rebuild

Hello all - my first post here.

I'm diving into the second re-wire of my ST-70, and thought I'd do a rebuild thread.  I originally bought and refurbished this amp about 20-years ago.    It was my first piece of tube equipment, and in hindsight, I had done several things that have been bugging me.  Most critically, it recently has been suffering from rectifier arc-over.  Searching for answers, I kept getting linked to this forum, and I believe I know the several things I had done wrong, and will correct them this time around.  In tearing down the amp, I also found several wires that had lost a good solder joint.  I'm not looking to hot-rod the amp too much.  I just want to have a solid and reliable amp again.

A bunch of parts are on order - every cap and resistor is being replaced.

I was going to link a few photo's, but it looks like I need to wait a week before I have rights.  I'll post them up as soon as I'm able.

<< I did get a pre-teardown picture in my avatar though

Chad

OK - by the time I could link pictures, the amp was darn near done. Here's a quick rundown of the resto:

Stripped down chassis.


Removed components and bits of clipped wire from the hardware to be re-used.  That's an older version of the Triode driver board.


DIY chassis powdercoat










Rewire - essentially a stock circuit.  The only changes are:
- Diode in lieu of the selenium rectifier
- Diodes added to the rectifier
- Used a 10 ohm resistors for the bias check, instead of the 14.5 ohm.  Bias set to 1v.
The old cloth wires on the transformer were badly rotted and disintegrating.  I spliced new wires as close to the windings as I could.  I used silicone at the splices to keep the wires from moving around too much while tugged on them during wiring.


The amp fired up with no issues.  I set the bias (actually at 0.9v) and everything works great!

Update:  After firing up the amp, I found the bass to be quite weak.  I had hoped that it was new tubes that needed some run-in time.  I think it was the Triode board.  My board was an old version, and the latest documentation is for a board with a slightly different layout.  It's likely that I mixed up some resistor locations.  Rather than track all that down, I ordered a VTA board.  It was shipped very quickly, and I was able to solder it up this weekend.  There was quite a bit of re-wiring required, and I had to clip out a lot of freshly installed components.   It was 100% worth it.  The amp sounds the way it should now.  I'm confident I'll have a reliable amp for many years to come.

.

At first I was "RED?!?!" but then you put on the burgundy, it's unique and gorgeous.

Thank you - I'm very happy with the look.  Just for reference - I used this powder coat:

https://www.powderbuythepound.com/aussies-anodized-red-transparent-flat.html

It's slightly translucent and needs a reflective undercoating.  The nickel plating worked well for this, except some of the darker pitting and scratches still show through (most of the deep scratches were on the bottom.)  I considered coating the transformer covers too, but I got lazy.  Those would have needed to be stripped, then an underlying coat of silver before applying the red.  I also wasn't sure I would get them to exactly match, since the undercoating would have been different.

.

PeterCapo wrote:Transformer covers... I'm facing a difficult decision about mine.  I'm poised to rebuild a Stereo 70, but I am not sure I will be able to manage refinishing the end bells.  They are borderline, cosmetically.

FWIW

Hammertone/Hammerite paint hides a lot of sins, and I think it looks appropriate for vintage equipment.  Depending how bad they are, I'd say straighten out the worst of the dings or whatever,  sand/roughen the stock paint,  maybe even apply a bit of body filler if needed, and hit-em with the hammer paint.

If you look very closely at my pictures (the one with the covers on the table), you can see the hammer finish.