VOX Night Train 15H replacement transformer

ddttyyrr027

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Hello folks,
I'm new to the forum and this is my very first post, I'm a tech but obviously not good enough to understand what's going in my Vox Night Train 15H amp now. And I want to learn something new.
I replaced the output transformer with a different primary impedance than the original one. The VOX's schematics mentions 5200 Ohm impedance on the primary. The new transformer I got isn't the drop in replacement and its primary impedance is 6k or 8k Ohm, depending on the wiring. I used 6k.

Now, after the replacement there's is an annoying oscillation starting around 12kHz and ends somewhere around 20kHz.
The oscillation pitch and volume depends on GAIN, whole tone stack and VOLUME knobs. If the GAIN is at 0, no oscillation. If the guitar isn't connected, no oscillation either. Oscillation pitch and volume can be changed with the volume knob on the guitar.
This all points to V1. The whole system is kinda unstable and when both VOLUME and GAIN are at max, the amp goes to some kind of saturation and it starts motorboating with a lot of noise in the background.

Do you have any idea what should I start with. Things I already tried were swapping leads on the OT, added 100pF in parallel to V1's plate resistor, added 1nF cap in parallel with C19 in the grid circuit of V1, still no change.

Any ideas or tips are very much appreciated.
Thank you
D.

Vox_NT15-1.jpg

Screen Shot 2023-02-01 at 12.48.19 PM.png
 

zeppelinofled69

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Does replacing the top cover of the amp do anything to the oscillation? Meaning the silver one that covers the tubes and transformers. I presume this may act as a faraday cage for everything up top.

Have you tried replacing V1 with a different tube?

Are those wires for the OT you changed anywhere near anything else that could be inducing oscillation? I know those amps are pretty tiny.
 

ChicknPickn

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Hello folks,
I'm new to the forum and this is my very first post, I'm a tech but obviously not good enough to understand what's going in my Vox Night Train 15H amp now. And I want to learn something new.
I replaced the output transformer with a different primary impedance than the original one. The VOX's schematics mentions 5200 Ohm impedance on the primary. The new transformer I got isn't the drop in replacement and its primary impedance is 6k or 8k Ohm, depending on the wiring. I used 6k.

Now, after the replacement there's is an annoying oscillation starting around 12kHz and ends somewhere around 20kHz.
The oscillation pitch and volume depends on GAIN, whole tone stack and VOLUME knobs. If the GAIN is at 0, no oscillation. If the guitar isn't connected, no oscillation either. Oscillation pitch and volume can be changed with the volume knob on the guitar.
This all points to V1. The whole system is kinda unstable and when both VOLUME and GAIN are at max, the amp goes to some kind of saturation and it starts motorboating with a lot of noise in the background.

Do you have any idea what should I start with. Things I already tried were swapping leads on the OT, added 100pF in parallel to V1's plate resistor, added 1nF cap in parallel with C19 in the grid circuit of V1, still no change.

Any ideas or tips are very much appreciated.
Thank you
D.

View attachment 1079637
View attachment 1079638
Not to highjack the thread, but what happens when a transformer goes? I have a dead Vox 15w Night Train head and, while I know the tubes are good, feel no heat coming off the transformer at all. Is it most likely the dead component? If this should be a different thread, no problem.
 

2L man

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Not to highjack the thread, but what happens when a transformer goes? I have a dead Vox 15w Night Train head and, while I know the tubes are good, feel no heat coming off the transformer at all. Is it most likely the dead component? If this should be a different thread, no problem.
There is no voltage output can show that PT has failed that but there can be other more obvious faults which burn mains fuse. If you have a multimeter and schematic it is easy to troubleshoot where the voltage is lost. First measure that there is resistance, 5 ohms ...200 ohms depending amp power, between mains plug hot and neutral when mains switch is On. If no resistance check mains fuse and replace if it show infinite ohms. Chech mains plug resistance again. Insert plug and remove and check resistance again because fuse might have burn again.

If fuse does not burn measure PT output voltages using schematic. HV switch off first measure filament voltage from tube socket pins. If tubes do not get warm most likely there is no filament voltage. When lights are dim there often is orange glow inside tubes when filaments are OK. Please reply your findings when you have done these steps!
 

ddttyyrr027

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Not to highjack the thread, but what happens when a transformer goes? I have a dead Vox 15w Night Train head and, while I know the tubes are good, feel no heat coming off the transformer at all. Is it most likely the dead component? If this should be a different thread, no problem.
In my case the OT was dead only a half. Half of the primary winding, measured from the center tap, showed good resistance of around 100 Ohm, 115 I believe, while the other half showed only around 20 Ohm, so it must have been shot somewhere in the primary winding. The sound was very weak and distorted. Didn't blow the fuse, no red plating either.
 

ddttyyrr027

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Does replacing the top cover of the amp do anything to the oscillation? Meaning the silver one that covers the tubes and transformers. I presume this may act as a faraday cage for everything up top.

Have you tried replacing V1 with a different tube?

Are those wires for the OT you changed anywhere near anything else that could be inducing oscillation? I know those amps are pretty tiny.
For the V1 replacement, I tried different 12AX7s as well as 12AU7s, still the same, more or less.
As for the wires, the OT is different from the original, it has primary's leads on one side, secondary on the other. So I routed the primary wires as they originally where, but wired secondary outside the chassis, just as a test. I tried moving the speaker connectors totally outside of the chassis as well but it behaves still similarly. Also I didn't cut 8k primary wires, just cut their ends and insulated leads, then hid them inside the amp. Could that be an issue? I may have accidentally created a coil just by tightening them together like that. See the pics below.
Regarding the cage, the other and fully functional NT15H I have works without the cage.
 

ddttyyrr027

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For the V1 replacement, I tried different 12AX7s as well as 12AU7s, still the same, more or less.
As for the wires, the OT is different from the original, it has primary's leads on one side, secondary on the other. So I routed the primary wires as they originally where, but wired secondary outside the chassis, just as a test. I tried moving the speaker connectors totally outside of the chassis as well but it behaves still similarly. Also I didn't cut 8k primary wires, just cut their ends and insulated leads, then hid them inside the amp. Could that be an issue? I may have accidentally created a coil just by tightening them together like that. See the pics below.
Regarding the cage, the other and fully functional NT15H I have works without the cage.
 

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zeppelinofled69

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Have you tried moving those extra OT wires that are close to the board around while the amp is on? Those primary wires floating over the board may be inducing some noise into the situation. I'd try to get those into a small bundle and tuck 'em down by that green ground cable between the boards.

Otherwise, could also just be the OT and it's proximity to V1. if that's still unscrewed, you could try moving the OT around while the amp is on to see if that changes the behavior.
 

ddttyyrr027

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These brown and blue wires are the other primary wires, from the 8k winding. I'll try to move them around to see if they affect the feedback.
 

ddttyyrr027

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Have you tried moving those extra OT wires that are close to the board around while the amp is on? Those primary wires floating over the board may be inducing some noise into the situation. I'd try to get those into a small bundle and tuck 'em down by that green ground cable between the boards.

Otherwise, could also just be the OT and it's proximity to V1. if that's still unscrewed, you could try moving the OT around while the amp is on to see if that changes the behavior.
Right on the money! Those extra primary 8k wires were causing the oscillation, when they were anywhere close to the input jack / V1 grid circuit. When I moved those wires closer to the input again, oscillation was back, wow! Looks like the problem is solved, no more oscillations.
So it was an electromagnetic induction afterwards. Now I learned another new thing. Thank you so much, happy days!
 

dogmeat

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for unused pairs I usually cut them short, cap the ends, and twist them together, then tuck. sometimes I put copper foil tape over stuff like that too.

did you check the power tubes? I've seen failed x-formers roach a tube. you have SS rectifier but seen the rectifier tube go too
 

ddttyyrr027

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It's just me, I didn't want to cut the other wires in case the transformer didn't fit or work with the Night Train, so I can either reuse it or sell it.
Power tubes seemed ok on a tube tester, no signs of short or anything out of ordinary.
As you say I'll cut, twist and tuck the wires as far from the input as possible, and as close to the chassis as possible too.

In case others have failed OT on their Night Train, the transformer I used is this one from eBay:
Type: OT20PP VC87 Output transformer 25VA & 8K/6K6 to 4/8/16 ohm
Link: https://www.ebay.com/itm/165915682041

It's primarily intended for Vox AC-15 but obviously works with NT15H as well.
And it was just for $45
 

zeppelinofled69

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Right on the money! Those extra primary 8k wires were causing the oscillation, when they were anywhere close to the input jack / V1 grid circuit. When I moved those wires closer to the input again, oscillation was back, wow! Looks like the problem is solved, no more oscillations.
So it was an electromagnetic induction afterwards. Now I learned another new thing. Thank you so much, happy days!
Hey! Great news.

Glad it was an easy fix.
 

Riff Ron

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Hello folks,
I'm new to the forum and this is my very first post, I'm a tech but obviously not good enough to understand what's going in my Vox Night Train 15H amp now. And I want to learn something new.
I replaced the output transformer with a different primary impedance than the original one. The VOX's schematics mentions 5200 Ohm impedance on the primary. The new transformer I got isn't the drop in replacement and its primary impedance is 6k or 8k Ohm, depending on the wiring. I used 6k.

Now, after the replacement there's is an annoying oscillation starting around 12kHz and ends somewhere around 20kHz.
The oscillation pitch and volume depends on GAIN, whole tone stack and VOLUME knobs. If the GAIN is at 0, no oscillation. If the guitar isn't connected, no oscillation either. Oscillation pitch and volume can be changed with the volume knob on the guitar.
This all points to V1. The whole system is kinda unstable and when both VOLUME and GAIN are at max, the amp goes to some kind of saturation and it starts motorboating with a lot of noise in the background.

Do you have any idea what should I start with. Things I already tried were swapping leads on the OT, added 100pF in parallel to V1's plate resistor, added 1nF cap in parallel with C19 in the grid circuit of V1, still no change.

Any ideas or tips are very much appreciated.
Thank you
D.

View attachment 1079637
View attachment 1079638
You will definitely have to reset the BIAS on the output tubes.
 




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