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Old 28th April 2017, 09:30 PM   #1
jdrouin is offline jdrouin  United States
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Default VTV Octal (6SN7) Line Stage Build --> Mod

I've started receiving the parts to make a 6SN7 line stage inspired by the Wright WLA12 (VA/CF). This will not be a clone, but rather a somewhat open-ended project beginning with the Vacuum Tube Valley (VTV) Octal Line Stage published in VTV Magazine issue #11 (Spring 1999) (schematic attached). The plan is to build the line stage as originally published, listen, and adjust to my system and taste by improving the known-to-be-problematic power supply (it puts out 330V B+ rather than the 370V as published, etc.), tweaking component values and operating points, and eventually adding a 6EM7 high voltage series pass regulator.

I chose the VTV because it's of the same topology and same era as the Wright model and the Bottlehead Foreplay. Confession: this is also a kind of search for Lost Time, back to my first introduction to tube hi-fi in 2001-02, when I heard the Wrights, so the design and sounds "of the period" are part of the curiosity here.

This will also be my first point-to-point and from-schematic build, having previously made two half-kits (Tubelab SE 300B and VTA PH16). My hope is that this thread will be useful to other newbies who want to take on a project like this.

First question: What is the purpose of the 1M resistor between the output coupling capacitor and the output RCA jack? I've simulated the signal section in LTSpice, and the sine wave in the transient analysis looks beautiful without the 1M resistor yet is horribly distorted with it. Is it necessary to use it?
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Old 28th April 2017, 09:43 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by jdrouin View Post
What is the purpose of the 1M resistor between the output coupling capacitor and the output RCA jack?
I've simulated the signal section in LTSpice, and the sine wave in the transient analysis looks beautiful
without the 1M resistor yet is horribly distorted with it. Is it necessary to use it?
Mainly, it provides a path to ground for the output coupling capacitor.
It won't be a source of distortion, so the simulation must be at fault.
Are you sure that the value of the resistor is 1M, and not, say, 1k.
There's another resistor in the amplifier's input, for similar reasons.

Last edited by rayma; 28th April 2017 at 09:46 PM.
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Old 28th April 2017, 09:54 PM   #3
jdrouin is offline jdrouin  United States
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Quote:
Originally Posted by rayma View Post
Are you sure that the value of the resistor is 1M, and not, say, 1k.
Hi rayma,

The schematic I attached reads "1 Meg" at that position, so I assumed it was 1M. I've seen other preamp designs that feature this, even some up to 1M5.

OK, so trying to figure out what wattage rating I'd need for the 1M resistor, the blue line in the transient graph (attached) shows a voltage swing from +15V to -15V on the meter placed just before the 4.7K load resistor (I set up this model as if it were powering a Wright WPA 3.5, which has a 4.7K input resistor).

To calculate the dissipation of a 1M resistor at that voltage, 15^2 / 1,000,000 = .000225W. Or, if we take the full swing, 30^2 / 1,000,000 = .0009W.

So would a 1/4W resistor be OK?
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File Type: jpg vtv-graph-01.jpg (178.4 KB, 560 views)
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Old 28th April 2017, 10:10 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by jdrouin View Post
would a 1/4W resistor be OK?
Sure.

This image shows 4.7k for the output load resistor, which could cause distortion,
as well as reduced LF bandwidth.

It appears that the 4.7k is actually the amplifier's grid stopper resistor, not the one to ground [grid leak].
https://robrobinette.com/images/Guit...ignal_Path.jpg
I think the amplifier's input resistor to ground is at least 100k, or larger. Try it with that value.

Last edited by rayma; 28th April 2017 at 10:27 PM.
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Old 28th April 2017, 10:37 PM   #5
jdrouin is offline jdrouin  United States
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I updated the LTSpice schematic (attached) to reflect the 330V B+, the 1M resistor after the output capacitor, and a 100K load representing the input resistance of the WPA 3.5 as seen by the preamp.

The graph now shows quite bad distortion.

How did you get the 100K load resistor value? The 4.7K in the WPA 3.5 (LTSpice schematic also attached) is indeed the grid stopper of the input 6SN7.

Looking at the 6SN7 datasheet, I don't find a grid resistance value, though there is a grid circuit value of 1-2.2M, which doesn't seem right here.
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File Type: jpg vtv-graph-02.jpg (155.5 KB, 119 views)
File Type: jpg wpa-3.5-ltspice.jpg (146.7 KB, 124 views)
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Old 28th April 2017, 11:26 PM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by jdrouin View Post
The graph now shows quite bad distortion.
This circuit is somewhere in the ballpark, and shouldn't distort badly.
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Old 29th April 2017, 02:30 AM   #7
PRR is offline PRR  United States
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> 1M resistor after the output capacitor

Is that "meg" or "milli"?

In *my* SPICE, upper/lower-case does not matter (FORTRAN guts), so if you want Meg you have to say "Meg". Else you get milli.

Which sure would crap-up a signal expecting many-K loading.

Power dissipation? You are unlikely to run much 1V signal into a power amp. 1V^2/1Meg. This is literally a microWatt. Even with extreme overdrive, hardly a milli-Watt. You pick a resistor big enough for your fingers, or whatever size is lowest-price, or prettiest colors, not by power.

The 1Meg bleeds-off charge on the output cap. Otherwise if left unconnected, cap leakage will bring the output up over 100V DC. If you plug-in at this point, the power amp POPs as it discharges the output cap. This may not be "essential" in hi-fi, where you rarely change connections and never while power is on (tho without the bleeder, the DC may persist a long time). It becomes somewhat essential in music amps and studio work, where plugs are moved often.
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Old 29th April 2017, 05:36 AM   #8
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Yes, there is 1 milli-ohm load now, not 1 Meg.

Concerning the schematic in general, you could remove C1. Then you get lower gain that is maybe more suitable for a line level pre amp, but also better linearity (lowe THD).

Quote:
(it puts out 330V B+ rather than the 370V as published, etc.)
This difference in supply voltage is practically meaningless.

Last edited by artosalo; 29th April 2017 at 05:46 AM.
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Old 29th April 2017, 10:03 AM   #9
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While the difference in B+ rail voltage is (as has been pointed out) probably NBD, a simple way to raise the rail voltage is by using a 5AR4 [low forward drop], instead of a 5Y3 [large forward drop]. Further tweaks are: eliminate the 100 Ω resistor between the rectifier and 1st filter cap., reduce the 1st filter cap. to 47 μF., and increase the paired 100 μF. parts to 150 μF. each. I doubt you will get to 370 V., but you will be close. 270 VRMS is 381.8 V. peak, which leaves little "slack" to take up the inevitable losses.
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Old 29th April 2017, 04:25 PM   #10
Ketje is offline Ketje  Belgium
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Adapt the thing to a lower voltage + some (little) mod's.
Mona
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