My Finished ST-70!

[antiquekid3]

Alright, I promised pictures...so here they are!

http://yfrog.com/7bimg0335sufjx

Here's a preview:



Hope ya'll enjoy! Please tell me what you think. This is my first amp, by the way.

Kyle

[Shannon Parks]

Nice work, Kyle - congrats! The chrome and black go together very nicely.

So what did you do to the chassis? It looks new.

[antiquekid3]

Thanks Shannon! The chassis is new. The old one I sold to Don Huebert. He's building a "garage amp" with it.

I was running my amp earlier today and found that my power transformer is getting to 160°F!! That seems WAY too high! I don't have a fan directly on it, but my window A/C unit was running in the room. Just for fun, I found the EL34s are approximately 350°F.

Should I worry about this? I am using (3) 6SN7s and (4) EL34s for a combined current of 7.8A, 3.6A on one winding and 4.2A on the other.

Kyle

[Ty_Bower]

I was running my amp earlier today and found that my power transformer is getting to 160°F!! That seems WAY too high!

Should I worry about this?

I've heard reports of Mark III power transformers running quite a bit hotter than that. I suspect yours is working normally.

[nyazzip]

finned heatsink/12 v muffin fan combo, coupled with thermal paste, operated at 3-4v will prolly drop it 40 degrees yet be (almost)dead quiet. computer geek/gamer stuff

[Ty_Bower]

Panaflo L1A was one of my favorite 80mm "quiet" fans. Even on a full twelve volts they don't make much noise, but they move a good amount of air. They're reliable fans, too, and should last a very long time. Expect to pay $8~$12 for one, but sometimes you'll spot one for less on the 'Bay.

[antiquekid3]

I found a big fan and stuck it behind the ST-70. I put it on low, and now instead of running at a hand-burning 160°F, it is running at a nice warm 105°F! Amazing! I think I'll keep the fan for a while.

What output tubes do ya'll recommend? If something besides EL34s will work, I'd consider them. Especially if they are drop-in replacements!

Kyle

[battradio]

I alwas liked ,the sound of 5881s and US made 6CA7's beter than EL34's .
6L6GB , 7027A ,6550 , 7581, EL 37 and KT66 all will work .

[burnedfingers]

You forgot 6BG6GA's with adapters

[Shannon Parks]

I think your power transformer temp is OK. Amazing to think most of them were hidden away in wood consoles back in the day.

Shannon

[EWBrown]

That is truly impressive, and for a first amp build, even more so! I'm green with envy... I've been building tube stuff for 40+ years, and have yet to make anything nearly so nice looking.

I remember from my College Daze that an ST-70 could very effectively serve as an auxillary dorm-room heater. Add in the fact that most of the dorms were heated by a central "steam plant", the rooms were usually too hot, even during the coldest days of winter. At least we could open the windows when it was needed.

If one wanted to avoid the cold ravages of winter altogether, one could become a "troglodyte" or "tunnel rat" and travel between the various campus buildings through the network of "service tunnels" which connected most of the buildings below ground.

It wasn't all that unusual to see students in classrooms and labs, clad only in t-shirts and shorts, even on the coldest winter days.

Winter, who needs it anyway ;-)

/ed B

[antiquekid3]

Thanks for the compliments! I think I'll stick around! Haha! :-)

As for the 5881/6L6GB, can I safely use those? The plate voltage seems a little low for what the ST-70 puts out. Would the 6L6GC be a better choice?

Any recommendations for where to find the tubes?

Kyle

[battradio]

I know the 5881 work despite there ratings ,ran them for years in my ST70 .

Mark

[antiquekid3]

Okay, cool. Do you think I could even use the original 6L6s? I know a source that has a bunch of those.

Kyle

[Ty_Bower]

The 6L6GB is a 20 watt tube. You might consider biasing it a little lower to try to keep dissipation under that. The stock Stereo 70 biases the tubes to 50mA at 410 volts, for something just over 20.5 watts per tube. I'd like to see the 6L6GB driven around 70% of its max rating, so maybe try for 35mA per tube. Keep in mind that B+ voltages will creep upwards as you bias the tubes lower.

The 6L6 has a maximum plate voltage rating of 360, and a screen voltage rating of 270. You will easily exceed those limits. Although they are beautiful tubes, I wouldn't recommend trying them in a Stereo 70.