6HS8 tubes for volume control - diyAudio
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Old 22nd February 2007, 04:39 PM   #1
jkeny is offline jkeny  Ireland
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Default 6HS8 tubes for volume control

Hi all,
I saw this referenced by Sch3mat1c and wondered why everybody isn't using it http://pw2.netcom.com/~wa2ise/radios/6be6vol.html.

Basically it is a pot free volume control - no more nasty pots screwing up the sound - look at this thread on solid state using LDRs to see reports of the sonics of potless sound: Lightspeed Attenuator a new passive preamp

One advantage tubes have over LDRs is matching - the 6HS8 tube is a dual tube originally used in TVs (in the US) and the two sides are matched.

I intend to use it in a rebuild of a Rogers Amp (changing all of the circuitry) and so I bought some of these tubes.

Has anybody tried this and can you tell us something about it?
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Old 22nd February 2007, 07:43 PM   #2
jkeny is offline jkeny  Ireland
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Any takers? As I bought surplus, I can supply some tubes at Ä1 a piece plus shipping.

Would like to compare build & test & results with others.

I just checked the first link above - seems to be broken - here's the correct one http://pw2.netcom.com/~wa2ise/radios/6be6vol.html look half way down the page

John
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Old 22nd February 2007, 07:57 PM   #3
anatech is online now anatech  Canada
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Hi John,
I would check the THD levels at the different gain settings. Your answer may be found there. I'm not sure why it wouldn't be used otherwise.

-Chris
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Old 22nd February 2007, 08:37 PM   #4
jkeny is offline jkeny  Ireland
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Chris,
Are the THD levels to be found online? or do you mean that inmates have found this to be the case? or are you surmising that this is why they have not been used much before? or I should run the test?

English is so imprecise as a language

Unfortunately, I don't have the equipment or know how to do this.

John
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Old 22nd February 2007, 08:40 PM   #5
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A much more better approach
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Old 22nd February 2007, 08:55 PM   #6
jkeny is offline jkeny  Ireland
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Hi Analog,
I'm aware of this - I posted on the thread! The problem with this approach is that individual tubes are used for each channel & matching between channels is problematic - with the approach here one 6SH8 tube is being used for both channels with perfect matching between channels.

I also seem to remember that John Swenson couldn't live with the microphonics of this tube & put the BDT to one side last year but took it up again recently.

This was one of the lines of inquiry that led me to this tube & see here also http://www.diyaudio.com/forums/showt...184#post887184 for John S views

John
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Old 22nd February 2007, 10:32 PM   #7
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The 6HS8 circuit is likely to have high distortion due to its use of varying output plate resistance Rp (versus G1 controlled current) for gain control. The actual signal will be varying this Rp too, so will cause distortion. This is likely to be OK for a 5 tube radio, but not for HI-FI. You could try to restore the linearity by using active counter- nonlinear loads in place of the plate pullup resistors, as in the long forgotten "voltage mirror" gain stage, or its latest re-incarnation as the "Aikido" input stage.

As for the Beam Deflection Tube approach, this was used by radio Amateurs in matched pairs successfully for SSB generation. There are simple tweek circuits published in QST for matching up the tubes (or eliminating the carrier signal for SSB). Main limitation here is the dynamic range of the signal on the deflectors.

One can also just use the classic two quadrant multiplier circuit (called the Gilbert cell in the four quadrant case). This is just a LTP with current sourced tail. The gain input controls the CCS reference and hence current. The LTP tubes above it would then be pentodes with resistive loads for the output.

Don
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Old 22nd February 2007, 11:05 PM   #8
jkeny is offline jkeny  Ireland
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Thanks Don,
I don't pretend to uderstand all that you say but it seems like the BDT approach is inherently more linear than the the 6HS8 circuit.

I will look into the Aikido input circuit to try understand voltage mirrors

What do you mean by SSB & QST?

John
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Old 22nd February 2007, 11:42 PM   #9
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Hello John,
SSB is the abbreviation for single sideband radio modulation, a more efficient form of AM radio. QST is the name of the US amateur radio magazine, also a Ham radio codeword. Beam deflection tube designs for generating SSB modulation appeared back in the 60's. The balancing tweeks were just DC voltage adjust pots for the deflectors and a DC servo for balancing current partition between the two plates, used NE-2 bulbs in the feedback. (there are other concerns about magnetic fields in the environment and demagnetizing the tubes too. )

The voltage mirror idea is about using non-linear thermionic diodes (typically HV ones) for load resistors that cancel the 3/2's power law distortion of the pentode transfer characteristic. One can get similar results by using fixed or zero bias triodes for load resistors as used in the Aikido. (however the Aikido uses them for loading a triode gain stage, which is unnecessary to linearize)

To linearize the 6HS8 tube, one would want to AC couple the "linearizer" loads to the 6HS8 plates. Just DC coupling them would also correct out the gain control action.

Don
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Old 23rd February 2007, 12:15 AM   #10
jkeny is offline jkeny  Ireland
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Don,
I'm afraid I'm out of my depth again (not an unusual condition for me). You seem to be saying that it may be possible to linearise the 6HS8 tube?

I don't understand how
Quote:
AC couple the "linearizer" loads to the 6HS8 plates. Just DC coupling them would also correct out the gain control action.
Can this be simply explained on the circuit below? Sorry if I'm being a bore with this but I've ordered the tubes & want to try to make use of them

Thanks
John
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