• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Anyone built a tube-based (6BE6 or 6HS8) volume control?

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GordonW

GordonW

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Joined 2005
#1
I'm building a preamp... had originally planned to build the Marantz 7 circuit (simplified, no tone controls or loudness/rumble filter etc)... but after thinking and looking around, I'm thinking of making a change in the line stage (leaving the Marantz phono stage as original, though).

Has anyone here built one of THESE (tube-based volume control)?

http://home.netcom.com/~wa2ise/radios/6be6vol.html

My main question is- how quiet is it? Pentode partition noise? Hiss?

Also, how linear does it seem to be, passing sound at different volume levels?

Thanks!

Regards,
Gordon.
 
#2
Hello Gordon,
you started an interesting thread.
At the moment I am making a tube based volume control, too.

I think noise is not the problem here, because the input level at
grid 1 is high and the anode load resistor fairly low to get no gain.
The main problem I see is only 50db attenuation. This is not enough.

In my project I need a phase inverter triode that has ideally unity gain.
I didn't find one, thus I am going to use the 6080 as an phase inverter.
I hope it makes no hum from the filament ...

Kind regards,
Darius
 
#3
I was looking at that... which got me thinking of using two 6BE6s per channel, for a total of 100dB attenuation. I'd think that would do it. This would also put the signal back in-phase...

After considering it further, my question, now, really is, what is the output gain at full-output from these? IOW, can I use two of these cascaded, to form a preamp line gain stage? Or, would I have to get additional gain somewhere else? If I have to add more tubes for more gain, it becomes far less attractive...

Regards,
Gordon.
 
#4
#3

Hi Gordon,

GordonW said:
I was looking at that... which got me thinking of using two 6BE6s per channel, for a total of 100dB attenuation. I'd think that would do it. This would also put the signal back in-phase...
...
Click to expand...

Yes.

GordonW said:
...
After considering it further, my question, now, really is, what is the output gain at full-output from these? IOW, can I use two of these cascaded, to form a preamp line gain stage? Or, would I have to get additional gain somewhere else? If I have to add more tubes for more gain, it becomes far less attractive...

Regards,
Gordon.
Click to expand...

you can increase the gain be increasing the 22KΩ resistor
at the anode.

Kind regards,
Darius
 
#6
Anyone got a handle on how to calculate gain for a 6BE6 wired this way? I've been looking around, but I can't quite find the procedure... I can find data on the tube used as a converter or oscillator, but obviously, not wired like this...

If I just knew how to get the transconductance and effective plate resistance in THIS mode, I could take care of the rest, I think...

Regards,
Gordon.
 
#8
hey,

here is the circuit my used with a pair of chinese 6J2. 6AU6 be able to instead without modify.
 

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#11
Using a variable-mu valve as an audio volume control may mean you have decided to choose between noise and distortion. You would need to do some careful noise estimates.

The fact that these are used successfully in AM receivers for gain control is due to:
1. lower expectations of noise and distortion performance,
2. harmonics and low frequency IMD are filtered away by IFTs.

The 6AU6/EF94 is not a suitable miniature substitute, as that is a straight valve. 6BA6/EF93 or EF92 might be better.
 
#12
6LE8 should also work for a volume control. Seen 'em for $0.50 often. Signal goes in on grid 1, and the differential control goes on the grid 3's (use a dual pot with around 25 V for the complementary control V generation, same pots control both audio channels then). One of the plates gets used for the output. Needs a low R plate load to avoid signal*control*signal distortion effects. ( a constant Beta, PNP, up top for the load would be ideal, and provide considerable current gain. Or a cascode load up top, for no extra current gain)

http://frank.pocnet.net/sheets/135/6/6LE8.pdf
 
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