An amplifier for Burning Man

[kstlfido]

Hello all-

I wish to make a stereo tube amplifier for my copper speakers, used in my camp at Burning Man. I would like to share my first steps here as this is the first PP amp I have built. I would very much appreciate your comments. Please go to this thread on the copper speakers. They were outside at Burning Amp 2009.

This amplifier will need to be simple and robust. It will need to generate ~70-100W of audio power per channel. To easily obtain this, I have decided to go Push-Pull.

Output tubes: I can go two basic routes- parallel push-pull using quartets of more common tubes (807, 6550, etc), or transmitting tube pairs (211, 813, GM70, etc).

My camp has power. I can get up to ~6 amps provided to run the amplifier.

I am favoring the transmitting tube route. Thoriated filaments would fit the 'Burning Man' theme. They would look very cool!

Of the thoriated tubes, the GM70 and 813 are easy to obtain and relatively inexpensive. I had some GM70's on hand and built up mono breadboard. B+ set at 900v. I chose cathode bias. I biased the GM70's for -70vdc. Each tube is drawing ~70 mA at this bias point (140 mA total). Total power consumption for this breadboard is ~255 watts.

I am using Jack Illiano's idea of using the TDA2030 amp chip and purchased his driver/splitter transformers he designed for this chip. This driver might not be used in the end, but it was easy to build for experimentation. Attached are the amplifier and power supply schematics. I am using a borrowed Edcore 10K 100W output transformer. No neg feedback yet.

Here are the THD measurements at 1kHz, into an 8 ohm load.

5Vpp (~0.38w) 0.48%
10vPP (~1.6w) 0.54 %
20vPP (~6.0w) .72%
40vPP (~24.8w) 1.9%
55vPP (~47.3w) 2.5% -just before clipping.

It takes a ~2.5v peak to peak signal to take the amp to max output (55V peak to peak) before clipping.

I hooked it up to one of the copper speakers and it sounds OK! I did a quick scope frequency response at mid-power (25w). Flat up to ~16kHz, then slowly tapers off.

The power supply- I got a tip from a local builder (Thanks, John!) to use a bi-polar power supply. It works very well. It also makes for easier parts sourcing. I get ~2vPP ripple under load. I am also using AC on the filaments. Yet hum is negligible in the speaker, thanks to push-pull operation.

Other driver ideas would be to do the normal phase splitter route. I would like more efficiency and power, I might try 813's or 828's in UL mode. Experimentation continues.

Thank you- Kent

[Bas Horneman]

Kent...are you going to make the desert even WARMER than it already is?

I saw a tv program on Burning Man...never knew what it was all about. It looks like fun. But I don't think I could handle such an event, I'm sure I'd just want to go home around 0100 in the am or something.

[nad]

A bit off topic I know;

I've just had a look at your copper speaker thread, 1900's industrial aura, they look great, good stuff!

[Arnulf]

And I bet they'll look even better after 30 days of burn-in

[torrence]

Now that is a breadboard. I say just throw it in the back of the truck as-is and call it good. That picture makes me want to go find some old fruit boxes for my next test setup...

[nad]

Originally Posted by Arnulf

And I bet they'll look even better after 30 days of burn-in

Funny, not really. Childish comment, possibly. Idiocracy, definately.


What's your problem???

[kstlfido]

Hey folks- thanks for the comments!

Yup, I intend to increase the temp on the playa...that will be another project using GM100 tubes!

The speakers worked great- all burnt-in and I am confident they will work well again this year. Getting an amp to drive them properly is another matter.

I am still trying to plot out the AC loadline and taking measurements to see if I can squeeze a bit more power. My goal is at least 70 watts, and not to do class B (I wanna use them for hi-fi down the road). More info as I figure it out.

If it proves too hard to get 70+ watts w/o running the tubes at 1200v+, I might try 828 tubes and run ultralinear at 750V B+. Anyone ever played with that tube?

[wrenchone]

You might want to look at screen driven P-P sweep tubes. The voltages will be easier to deal with. You can get ~100W/channel with 5k iron and 550-600V B+. SY and George will probably have something to say about it. I will be working on something like that some time this year, but I will be building a custom SMPS to power mine, something not easy to duplicate.

[kstlfido]

That could be a viable solution down the road. Yet I want to explore thoriated tungsten filaments for now. More 'Burning Amp' feel for Burning Man!

I have been following George's powerdrive with some interest. I have some 2SK700 about. I dunno if his can drive GM70's directly (I need min 250VPP), but if I go UL then his looks very attractive. I would like some tubes in the front end....

I've sourced some 828's so I hope to play with them soon.

[wrenchone]

It's no doubt that the thoriated filaments look cool, but they are a real power hog - a GM70 P-P rig will be blowing 240W in filament power alone. I'd rather use that power to displace some air. I plan to do a GM-70 amp myself, but a 30W or so SE beast is pretty much the limit of my ambition. I was thinking of going the monoblock route , again with a custom SMPS for power and HV sequencing.