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Old 19th February 2008, 08:37 AM   #1
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Default 48V B+ linestage

I have come into possesion of a somewhat unusual wall-wart.
It will output DC 48V at 0.1A AND 6.9V at 0.8A!

This got me thinking that I could build a simple and safe (this would be my first project) linestage.

I saw the Aikido 24V project by John Broskie and that looks very interesting, however that design doesn't work for me because of the heater requirements.

My setup involves:
- 2 sources (CD and PC) which output 2V.
- power amp with 1.0V input sensitivity and 100K input impedance.

I would like to use the 48V as the B+ and the 6.9V (or lower) for the heaters. However given the 0.8A limit, I could only use 1 tube per channel.

I think cathode follower would be simple enough for me since I don't need gain, but I don't know which tubes to try.
Unfortunately most plate curve charts don't have great resolution at such low voltage/current so it's hard to tell what's linear.

The list so far is 12BH7 and 6DJ8. Any other candidates?

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Old 19th February 2008, 09:34 AM   #2
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Scratch the 12BH7. Concerntrating too much on the B+, missed the heater voltage!
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Old 19th February 2008, 09:42 AM   #3
Nordic is offline Nordic  South Africa
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Greinacher voltage doubler ?
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Old 19th February 2008, 09:43 AM   #4
poynton is offline poynton  United Kingdom
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How about minature (pencil) tubes?
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Old 19th February 2008, 07:15 PM   #5
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ECC86 is the way to go as they sound damed fine.
Heater is only 330mA.
With a bias of -1V you get 2mA anode current at 22V on the plate, and a respectable plate load of 13K. Gain will be a little high at probably about 8x.

Alternatively strip out the cathode follower output buffer from the Aikido (though this would need an input cap). You might be able to do a good job on this if you created a virtual ground and split supply as you could then eliminate the input cap.

Others have had success with the ECC88 down at 24V on the anode, though I would prefer to stay up at 80V myself as grid current and scratchy pots can be an issue at low voltages (as I found out on my gainclone buffer).

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Old 19th February 2008, 07:54 PM   #6
Jaime is offline Jaime  Uruguay
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Wavelenght Audio (Gordon Rankin) made an preamplifier (called SINE) using 6GM8 (ECC86 ) tubes. This tubes was designed to use with car battery
best regards Jaime
I write in English worse than Tarzan
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Old 19th February 2008, 08:11 PM   #7
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The 6H30 behaves very well at low voltages. Gain is OK for a linestage.

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Old 19th February 2008, 09:45 PM   #8
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Thanks for the suggestions!

I thought the Greinacher voltage doubler only works on AC?

Do you have specific submini in mind?

ECC86/6GM8 would be a good choice as it's specifically designed for such low voltages. Let me go investigate availability.

That's a new one on me and those charts have great detail at the op points I need. Unfortunately the heater current is 0.9A! My supply can only do 0.8 total for both channels.

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Old 20th February 2008, 04:03 AM   #9
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I agree with the notion that the 6GM8/ECC86 is the tube to use. However, I think you should set it up as a battery biased and CCS loaded cathode follower. As you stated, you don't need gain. AAMOF, the 1 V. sensitivity of the power amp allows you to drop nearly 1/2 the I/P voltage, all the time. Wire a 47 KOhm resistor in series with a 50 KOhm log. taper control. Connect the control's wiper to a cap. that connects to the grid circuitry. A 510 KOhm grid leak resistor connects via an alkaline cell (AAA) to the cathode. Use a 470 Ohm Carbon comp. grid stopper.

Use the 6GM8 data sheet to pick the idle current for 1.5 V. of grid bias and 18 V. on the plate. The remaining 30 V. of the PSU operate the CCS load. The O/P coupling cap. should be at least 3.3 muF.
Eli D.
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Old 20th February 2008, 08:35 AM   #10
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The 6GM8 is all but linear and I do not think that everyone agrees with its sonic virtues!

It is better to use a tube like 6DJ8 as CF. In the schematic below you use all of the 48V over the tube. Use the 6.9V both for negative and heaters. Clipping will be unsymmetrcal but the circuit can take over 8Vrms (if you use a serial resistor as Eli suggested) before that happens. The circuit is biased at 4mA. Put a 50k potentiometer in place of R1, wired as a volume control with the wiper to the gridstopper. The CCS is simple and can of course be changed to a more advanced one.
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