Just finished my st70 rebuild

[davygrvy]

Stuff:

diytube input board
triode electronics' PA-060 power transformer
New chassis found on eBay from Scott Anderson
Balancing input transformers from Jensen
All tubes are JJ/Tesla ones from Antique Electronics
white ceramic tube sockets

12AU7s replaced with ECC99s. I'll try an ECC803S inplace of the 12AX7A soon.

The only original parts left are the output transformers which I repainted the laminates and had the bells nickel plated along with the top bell from new power transformer, too. They sure look sweet all shiny like the chassis. A real IEC plug for power with a built-in EMI filter to help keep out some hash noise from the line voltage. Speaker out barrier strips replaced with binding posts. Power switch moved to the front with a big jewel lamp (like a fender) to show me the power is on. XLR connectors put in PAM socket holes on the front.

The sound is awesome. My speakers are Yamaha NS-10MC bookshelfs which aren't that efficient, which is kinda of ashame as I'd like a just a hair bit more SPLs. I love the way the amp distorts evenly and the volume control on my preamp has such an effect on tonality of where you want the program peak to start "squishing". And it's such nice squish, too.

I just love tube amps :)

[davygrvy]

Steely Dan, "Alive in America" I always thought was a bit harsh sounding, but with my st70... silky nice and still wonderfully dynamic.

[Shannon Parks]

Based on my design can someone walk me through the initial power up procedure? (considering that I do not have the front sockets)

Jump back a thread to:
http://www.diytube.com/phpBB2/viewtopic.php?t=789

Verifying that your bias supply is good is the first key to not having your tubes melt down. Then you can attach a pair of cheap alligator clip cables from your DMM to each side of the 10 ohm resistors for the initial biasing. On the original ST70 you could kind of rest the unit on the back of the chassis and the transformers - like a 60 degree angle. I do this when I'm working on one at the bench. It looks like you might be able to do a similar 'flip' on your ST70. I would consider some type of test point plug(s) on the top of the chassis for easy bias maintenance.

For simpler biasing, the 500k pots and the 1M resistors can be omitted. Then the cathodes can be tied together using a single 10 ohm resistor measuring current for both tubes (ie 1V across the 10 ohm resistor at idle).

BTW, your amp looks great! Yellow_Light_Colorz_PDT_19

Shannon

[leadtower]

Nice job DavyG; nice clean wiring. The top plate is pretty also. I am trying to figure out the biasing scheme also. I seem to be stuck on why the 500ohm pots are there. That I think is my problem; but I see Shannon says you don't need them. I am wondering now why they are there?
Shannon if you are reading this; can you say the reason for the 500 ohm pots in the circuit? Thanks. I don't mean to hijack your thread DG I am hoping this is helpfull to you also.

[Shannon Parks]

I seem to be stuck on why the 500ohm pots are there. That I think is my problem; but I see Shannon says you don't need them. I am wondering now why they are there?
Shannon if you are reading this; can you say the reason for the 500 ohm pots in the circuit? Thanks. I don't mean to hijack your thread DG I am hoping this is helpfull to you also.

Those 500k pots are a variable ground reference divider (through the 1M resistor), which is also in parallel with the grid resistors/bias supply. It allows a moderate range of balancing between the pair of EL34s.

The original ST70 let you set the bias per side (or channel). This is a further adjustment to finely tune bias per tube. So you will still start out biasing like in the good old days, biasing the left and right channels to 100mA (ie the combined currents of both front and rear tube of the pair). Then the 500k pot tweaks these tubes to be perfectly matched (ie 50mA each). In my biasing instructions, I think the zero voltage adjustment method works best.

Shannon