Buildin a shop amp

[battradio]

I'm in the process of building a shop amp from parts I have laying around .The ST 70 chassis was rusty and had huge holes cut into the to and front , a friend ( Blair ) machined a plate to go on the top , and a wood front panel .
the power transformer is from another ST 70 and the output transformers are from early Fisher console that used four 6V6GT's in PPP , the impeadance is 4.2K , and has a tertiary winding just like the Fisher 80AZ amps .The Fisher output are made by Todd and are about 30% larger than the original ST 70's .

Building it to use 6B4G and 5881's interchangeably , can do this because of the pin out of the 6B4G and the 5881 , are the same except for the cathode .

The 5881 uses pin one for the cathode and the 6B4G has a directly heated cathode , with use cathode bias on both tubes using 820 ohm cathode resistors on the 6B4G's and 250 ohm cathode resistors for the 5881's .

Using a choke input power supply , the B+ will be around 350 volts which will be good for both output tube types , will be using SS rectification for the B+ and will use the 5 volt winding for DC filaments for the driver tubes .


The driver board is a DIY-TUBE ST 70 driver board that I traded for from Shannon for some tubes he needed , the driver tubes will be 12BH7's and the input tube will be a 7025
http://www.diytube.com/phpBB2/viewtopic.php?f=6&t=813 .






Will post some schematics later .

It should be around 10 watts with the 6B4G installed and 20 watts with the 5881's , I'm going for sound quality over power out . No soldering has started , so the transformer will be easy to clean up

[Geek]

Looks like a thin of purple heart

[Blair]

It is Gregg. It's been sitting in the garage for years waiting for use.

I was wondering what you did with that chassis Mark. Looks nice!

[kheper]

It's amazing how a little black paint and some nicely worked wood can turn a hunk of rust into a thing of beauty.

[Blair]

Mark asked about the paint when I sent it to him.

http://mobile.walmart.com/m/phoenix;jse ... k/17011160

It finishes smoother than other texture paints. It dries very quick, hard, and if you look at the front corner where Mark chipped it a bit from rolling it over, it can be touched up easily by spraying a bit on a wood plank and using a broken tooth pick and dabbing it on thick. It will dry to almost mate seamlessly to the older paint.

[battradio]

Got it working with the 6B4G's , going to do a bit more work to it so the 5881's will work





I have a better knob to go on it , but not till it is finished .I have separate cathode resistors for the 6B4G (820 0hms ) and for the 58811's I have (250 ohms ) . The cathode resistor is in the center tap of the filaments for the 6B4's , and the cathode resistor in between ( pins 8&1 tied together ) the cathode and ground . What I have to figure out yet is why the polarity of the voltage across the 820 reverses when the 5881's are in the amp ?

[battradio]

Circuit Board



5881/ 6L6 bias mod




Power supply and 6B4 bias mod

[Blair]

Looks awesome Mark!

Glad you could put that chassis to use. Otherwise, my wife would have yet another thing to pick up and threaten me with if I don't get the tube parts out of my house:)