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Vox Night Train Schematic
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goodtone4u



Joined: 07 Dec 2008
Posts: 8

PostPosted: Tue Jul 28, 2009 1:01 am    Post subject: Vox Night Train Schematic Reply with quote

Very Happy Had this amp for a week to draw the schematic. Happy building!
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ChrisM



Joined: 22 Oct 2008
Posts: 1149
Location: Toronto, Canada.

PostPosted: Tue Jul 28, 2009 1:49 am    Post subject: Reply with quote

Very cool, thanks!
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drew



Joined: 21 Jun 2006
Posts: 222

PostPosted: Tue Jul 28, 2009 2:07 am    Post subject: Reply with quote

Thanks for sharing.
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dave g



Joined: 10 Aug 2007
Posts: 383
Location: Cambridge, MA

PostPosted: Tue Jul 28, 2009 2:20 am    Post subject: Reply with quote

Looks pretty similar to Steve Aloha's blues express. The design "feels" more well-thought out than the Orange Tiny Terror. Interesting!

I bet it would be really cool to replace the PPIMV with VVR.
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Darkbluemurder



Joined: 03 Jan 2006
Posts: 574

PostPosted: Tue Jul 28, 2009 2:12 pm    Post subject: Reply with quote

dave g wrote:
Looks pretty similar to Steve Aloha's blues express. The design "feels" more well-thought out than the Orange Tiny Terror. Interesting!

I bet it would be really cool to replace the PPIMV with VVR.


Yeah, that's also what I thought - a Blues Express or a Princeton without reverb/tremolo.

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txbluesboy



Joined: 02 Apr 2007
Posts: 368
Location: Dallas area

PostPosted: Tue Jul 28, 2009 3:07 pm    Post subject: Reply with quote

What is the purpose of the .01 uf caps in series with all the filter caps?
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John_P_WI



Joined: 15 Aug 2007
Posts: 1078
Location: Wisconsin

PostPosted: Tue Jul 28, 2009 3:44 pm    Post subject: Reply with quote

txbluesboy wrote:
What is the purpose of the .01 uf caps in series with all the filter caps?


The small poly caps in parallel improve the performance of elytics, reducing noise, intermodulation distortion etc. The small polys are much faster responding. Yes, they change the tone of the amp. In my experience the amp is faster / cleaner / brighter sounding. Early in the power supply chain they reduce the diode hash.

John
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Tonegeek



Joined: 13 Nov 2006
Posts: 861
Location: Winston-Salem, NC

PostPosted: Tue Jul 28, 2009 6:29 pm    Post subject: Re: Vox Night Train Schematic Reply with quote

goodtone4u wrote:
Very Happy Had this amp for a week to draw the schematic. Happy building!


I got to poke around one of these recently also. Its a well designed, little amp with a great sound and built like a tank. The Bright/thick switch, and cathodyne phase inverter are pretty slick. And DC heaters! I don't understand the network before the PPIMV, but maybe that is why it works so well and is one of the better MV's I have heard. Great value at $500. It is one amp that may be cheaper to buy than build (I know, that's not why we do it). Thanks for posting the schematic!

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txbluesboy



Joined: 02 Apr 2007
Posts: 368
Location: Dallas area

PostPosted: Tue Jul 28, 2009 8:53 pm    Post subject: Reply with quote

Thanks, Embarassed I really do know the difference between series and parallel but sometimes what Im thinking and what I'm saying (writing) don't always jive. I think its from all that fun I had in the late 60's and 70's.
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hbamp



Joined: 14 May 2005
Posts: 80
Location: The Netherlands

PostPosted: Wed Jul 29, 2009 12:09 pm    Post subject: Re: Vox Night Train Schematic Reply with quote

goodtone4u wrote:
Very Happy Had this amp for a week to draw the schematic. Happy building!

Great work!
Only Mastervolume tandem pot schould be between the caps.

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zaphod



Joined: 07 Jun 2005
Posts: 1

PostPosted: Fri Jul 31, 2009 2:39 pm    Post subject: Reply with quote

That Bright/Thick switch doesn't look right either. In the schematic it appears as if the lower end of the mid pot gets lifted off ground when in the Thick position, which would disable the entire tone stack. Or is the Thick position meant to kill the tone stack? :shock:
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Gee



Joined: 11 May 2007
Posts: 82

PostPosted: Fri Jul 31, 2009 4:16 pm    Post subject: Reply with quote

I believe that the Thick switch does in fact cut out the tone stack.

If the schematic is correct, why is the PPIMV implemented that way? Gut shots of the amp show there to be 6 .1uF caps that match the 6 .1uF caps in the schematic.
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loverocker



Joined: 03 Mar 2005
Posts: 104
Location: England

PostPosted: Mon Aug 03, 2009 10:31 am    Post subject: Re: Vox Night Train Schematic Reply with quote

hbamp wrote:

Only Mastervolume tandem pot schould be between the caps.


That's only required for a fixed bias amp. The NT is cathode biased and should be able to work with a simpler arrangement (simpler even than the one drawn out in the OP's schematic). I have a Vox NT here and I will get around to opening it up soon to check guts against this schematic.

And yes, the tone stack is defeated in Thick mode so you just have Gain and Master in circuit (no Tone control at all except on your guitar). Strange choice - I'd have thought moving the tone stack to a later part of the preamp would have been a more versatile/useful approach... (shrug)

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goodtone4u



Joined: 07 Dec 2008
Posts: 8

PostPosted: Thu Aug 20, 2009 10:57 pm    Post subject: Vox Night Train Design Reply with quote

Hello everyone,

In my investigating the NT amp more, I found a few minor errors in the schematic that was posted last month. Sorry!!!
The amp does work fine with prior error but changing the third gain stage plate resistor brings amp up to exact copy. (and voltages are closer)
Some of the input parts were in error but are mostly for RF suppression.
I checked all my reference pictures and matched audio test results with NT15 amp that my guitar friend kindly loaned me. (And trusted tearing apart!)
And since then, replicated this amp and getting great results. (and sound) Thanks Steve!

Get latest schematic download Rev C.

Have seen some questions what the Bright/Thick switch action cropping up on forums, so here is my take on the subject.
I did detailed audio measurements and found the following from a working NT platform. The separate gain stages could be dissected into "modules" to localize how the preamp feedback was working. Later I will discuss the PI coupling cap question.

The Thick position switch does the following:
1. Lifts the ground tail of mid pot, No tone stack action.
2. 47pf "bright" disabled.
3. Additional signal coupling through 4700pF cap to gain pot. (more tone stack bypassing)
4. Grounds the feedback signal (loop) from V4A through R38 to R24 to V3A cathode, therefore increasing gain.

Now when you place the switch in Bright position:
1. Tone stack operation back in place.
2. 47pf "bright" cap enabled across gain pot.
3. No 4700pF in circuit.
4. Feedback allowed to send signal from V4A to V3A cathode,
therefore reducing gain.

The 1.0uF bypass cap C21 does not short the signal to ground completely, but rather appears like an active EQ circuit. Increasing gain with higher frequencies.
Since the feedback loop is only in Bright position, this affects the tone stack operation and especially range of Bass control. The feedback loop is reducing the low freq gain more than the bass control action.
The mid dip is reduced due to feedback EQ slope is in the same place where the dip would be.
If you have this NT amp, try paralleling cap C21, 1.0uf with a larger 22uf in Bright mode to get more tone control action. Maybe the bass range was too much for amp power output and future cabinet EQ match.

Now on to the four 0.1uf caps past the PI question:
1. It may be a place setting for fixed bias modification.
2. Two pole passive High Pass filter. (grid sticking?)
3. Freeze design and leave parts as is on PCB.
4. Open for further discussion from others.

I have measured the low freq rolloff using the four 0.1uf
cap configuration vs two .022uf. Virtually the same response.
Actually C22, 2200pF rolls off the bass sooner than the PI coupling caps do. This masks any differences between 0.1 x4 over .022uf x2 slope.
Good thing, less parts for home brew.

And finally, variable voltage Mosfet regulator works great on the cathode biased output tubes. The master volume keeps the grid drive fizz down when at low plate/screen voltages on 6BQ5's

That is enough for now.
Enjoy the tone!
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ampdoc1



Joined: 26 Feb 2005
Posts: 636
Location: Tulsa, Oklahoma

PostPosted: Sun Aug 23, 2009 3:04 am    Post subject: Reply with quote

DELETE

Last edited by ampdoc1 on Mon Dec 03, 2012 2:32 am; edited 1 time in total
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