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Desktop 1.25W tube amp 7025/ 6JD6
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Author:  roggom [ 12 Mar 2009, 00:06 ]
Post subject:  Desktop 1.25W tube amp 7025/ 6JD6

Hi everyone,
After looking over some of the amps that Magnus linked over from Germany, I was inspired to make a small amp to power cpu speakers. This is another low end build but another easy project for the new builder.

I started with a couple of 6JD6 sharp cutoff tubes, these are found in old stash of tv repair tube kits. I got a couple for $1 each locally. I had the 7025 from a magnavox rebuild, and the great thing is I was able to run the heaters in series because each tube draws 300mA --- the dual triode in series config.

The base is a Levis wallet box, and the output transformers are little Speco T-7010 multi tap matching. For $4 each could not go wrong.

I used a power supply from one of my older projects and running 160v for b+ and a 12v 1A wallwart provides 18 volts for the heaters.
I triode strapped the pentode and used led biasing for all tubes, the sound is rather decent but rolls off at about 150~200 hz, so the low end is not spectacular, but for sitting in front of a computer it does the job. The amplifier portion came in under $20.
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Author:  Soundbrigade [ 12 Mar 2009, 03:13 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Frame by frame ... sings King Crimson and I say: (Picture) Frames are great as amp chassis.

That's a cute piece really.

Btw, I accidently found a schematic for a 5998-amp in my collections. 5998 is a dual triode of 13W (total? or per triode??). Anyway, with a dula smallsignal triode and a 5998 a rather powerful amp was constructed. However the 5998's are EXPENSIVE.
A friend is looking for something to do with a pair of 35L6 ...

Author:  Gio [ 13 Mar 2009, 00:22 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Hi Rogers, that looks great! Great use of the wallet box.

Interesting choice of audio output transformers. I could not find a datasheet, so did you just calculate the impedance of each tap?

How low do you figure they go?

Cheers

Author:  roggom [ 13 Mar 2009, 02:04 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Hi Gio,
Actually the impedance and watt rating are on the side of the box ranging from 8K/ 0.625W to 500/ 10w. These little transformers are popular with spud amp and guitar amp builders.Doug even uses them to tinker around, http://www.ecp.cc/less-pressivo.html.

This one is just an entry level amp, and gives the builder the satisfaction of amplification equal to standard powered CPU speakers. In the future I have my mind around a ECL86 amp, but those tubes are quite pricey. The thought of a few watts with only two tubes is tempting. Might have to budget that one with some nice iron and caps.

Rogers

Author:  CrazzyAbtTubes [ 14 Mar 2009, 16:47 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Cool! another mini tube amp project! :P

Author:  gofar99 [ 14 Mar 2009, 18:54 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Hi, Nice little project. Clever use of low cost parts. BTW What is the current flow in the LED?

Good listening
Bruce

Author:  roggom [ 14 Mar 2009, 23:16 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

I have been tinkering with the amp the last few days and I was frustrated that the sound was not to full potential for a 1 to 2 watt amp. I hooked up a set of old school Superscope 8 ohm speakers and that was a lot better. For testing i bumped the b+ to 300 and the sound really opened up.

With the original b+ of 160v I was drawing 15mA through the led and it sounded decent at low levels but clipped rather easily with 8 ohm 10w speakers. Bumped the b+ to 300 and the current through the leds is 45 mA and sound is way better.
I am going to Bremerton Washington for two weeks of Navy duty, that will be my mission to find a transformer. :xfingers:

Also excuse my ignorance of extension, but I scoped the output and it was max at 17khz and rolled off at approx 200-250 hz.

Author:  CrazzyAbtTubes [ 24 Mar 2009, 15:03 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

I have bought these output matching tranny's from the source, they are 70V 5 watt to 8 or 4 ohm speaker. They have multiple primary taps 5W, 2.5W, 1.25W, 0.62W, 0.31W.

Author:  roggom [ 30 Apr 2009, 23:26 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Hi everyone,
I finally got around to getting an appropriate power supply. I went with a Hammond 269JX and a 6CA4/EZ81 power supply. I calc'd everything on psud2 and came out with 280, real world was 295vdc and no hum. The project is now finished and sounds rather nice. This is not a project for those looking for massive bass, but a bin amp that can fill the average dorm room. The pics are under chassis and a photoshoped one to hide my dirty workbench. Total cost around $75US.
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Author:  Gio [ 01 May 2009, 12:29 ]
Post subject:  Re: Desktop 1.25W tube amp 7025/ 6JD6

Hi Rogers, that looks great! :up: I really like how the LEDs light up the underside of the amp.
Cheers,
Gio

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