ST-70 Rebuild

I like to see the amplifiers. etc. that others build.  It seems fair I should share one of mine from time to time.  

This is one of my ST-70s.  This one had been in a flood so it needed to be completely disassembled and rebuilt from the chassis up.  The rectifiers are hexfreds.  I think the tubes are obvious.

Enjoy



R

Nice chassis work. How did you do that? Dale

Thanks for your kind words.  I'm not sure what you mean about the metal work, but here's the basics.

The chassis was pretty rusty so I cleaned it up as best I could and took it, the bottom, and the cage to be powder coated.  The 6SN7s are mounted on a small aluminum plate that I had powder coated at the same time as the rest after drilling the mounting holes and cutting the octal socket holes with a Greenlee punch.

I'm pleased you liked the amplifier.  

R

Looks really nice. Like Dale, I really like the chassis work. The driver tubes without a typical circuit board look especially nice. How does one get something like the chassis "powder coated"?

Powder coating is essentially paint, but it is applied without the use of liquid solvents.  The process has been used for a long time for auto parts, appliances and the like.  What one gets from the process is a uniform, robust coating.  It is even more environmentally friendly.  Like traditional painting, the surface should be cleaned and prepped.  I tend to clean things up and then take it to the shop where they may want to do further surface work to ensure the paint will look nice and last.  As you might guess, this is not an inexpensive process, but most shops will work with you, especially if you are using colors that do not require special set up.  The shop I have used helped me pick the textured black finish for the body of the amplifier and the smooth finish for the cage.  They do lots of both so it was easier for them and cost less for me.

Building the amplifier itself was pretty straight forward for me because I have been working on electronics for may years.  When I was learning, vacuum tube amplifiers were what there was and circuit boards were far less common.  Part of the learning process was to get older gear, often old TV sets, and salvage parts from them.  A real bonanza was when we could find a kit someone else had tried to build and given up.  Usually the kit was largely complete, undamaged, and the previous owners were grateful that we were willing to haul it away.

Many of us built audio amplifiers or radios out of the parts we collected.  If we saved up a bit, we got to buy a Knightkit, Dynaco, or Heathkit, but those were usually beyond the means of the most of us so we learned to make do with parts on hand and salvaged chassis.

In my case, the electronics teacher at the local high school let me come in and use the multimeters (all of them were black in those days), soldering irons, etc. and provided hours of advice and quite a few spare parts he had collected over the years.  His name was Ben Stilwell and he was a fine and generous person.

My history made making my own octal driver for the amplifier the natural thing to do.  The circuit design I used is very similar to that used on the octal driver board that is now available so you can get a well designed and supported product (I used the 12AU7 driver board in my other ST-70.)

I hope my longwinded answer helped explain some of where this amplifier came from.  

R

Well done @tubedoc , good rebuild , I'll bet it sounds excellent too .

I would love to see a schematic of your power supply , it looks like you have a bit more smoothing going on there than in regular ST70 .

Here is the schematic of the power supply.  I had never drawn it out so I took the amplifier off the shelf, popped the lid and here goes.  

The cap in the traditional location is a four section 40-40-20-20.  The cap located where the rectifier is usually found is a two section 40-40.

The first capacitor stage following the hexfred diodes is made up of two paralleled 40u sections.  This is followed by the choke and two more 40u sections that supply the B+ for the outputs.  Then there is a 6.8K resistor and a 20u cap feeding the phase splitter followed by a 22K and the last 20u cap feeding the   first stage.  

The power transformer is not the stock PA-60.  I used a beefier transformer I had on my shelf.  The high voltage section was just right for the hexfreds and the 8A 12.6V winding was more than enough for the filaments.  There was no separate bias tap, so I used a small filament transformer to derive an isolated bias section.  By hooking the winding intended to produce  12.6V to the now vacant 5V tap on the original transformer, I was able to get approximately 50V of isolated AC from what was intended to be the the 120V section which to derive the bias.  (The transformer ratio is about 9.5:1 so the calculated AC is about 47V.)

The remaining detail is the DC bias to the tube filaments.  The voltage divider raises the filaments about 20% above ground.  This tends to produce a quieter amplifier.  

As to sound, this amplifier is refined in its presentation.  It is quiet, detailed, and fun to listen to.  

Thanks for asking

Very clever Doc , thank you kindly for taking the time to draw out your circuit and sharing it . I like the clever grid supply too , bonus that it's isolated . I thought the other small iron was also a choke .

Looks great. Tell me more about the preamp.

I had already posted a set of photos of the preamp in a string called PAS3 Pictures.  The preamp came from the same swap meet that the amplifier did and both were flood damaged and corroded.  The preamp has had the power supply updated, but I kept the 12X4 in the supply.  

I replaced the main amplifier board as it was damaged and I would have had to redo it anyway.  The phono board was in relatively good shape so I updated it with replacement parts.  I replaced the switch with a more modern high quality product and replaced the connections with new OFC wire where applicable.  

It works well, sounds nice, and is very quiet.  It is fun to play with and it is nice to save it.

Rob