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AC15HW1 Modifications

I recently made a couple of simple mods to my AC15HW1 amp. The amp sounds fantastic out of the box but with a couple of simple changes it can be amazing and sound closer to the old 1960's AC15. There are four basic changes :-

Reducing the DC filtering capacitor values and using better quality caps.
Changing to better quality signal capacitors.
Adding grid stoppers to the Top Boost and Phase Inverter to avoid blocking distortion.
Reducing R17 to give more gain in the Normal channel.

These simple changes really liven up the amp and remove some the modern "stiffness" associated with too much DC filtering. All components should be replaced with voltage ratings no less than the originals.

WARNING - These amps contain lethal high voltages which can remain present even when the amp is unplugged from the mains supply, do not attempt any modifications unless you know what you are doing.

Signal Cap Changes:-
C13 - Silver Mica 220pF (Bright Switch Cap)
C22 - Silver Mica 100pF (TB Bright Cap)
C21 - Silver Mica 500pF (TB coupling cap)
C23 - Silver Mica 47pF (Treble Cap)
C24 - Mallory 22nF (Middle Cap)
C25 - Mallory 22nF (Bass Cap)
C7, C10, C12, C14, C27 - Mallory 47nF (Stage Coupling Caps original values 100nF)

Filter Cap Changes:-
C1 - F&T 500v 22uF
C2 - F&T 500v 22uF (original value 47uF)
C3, C4 - 630v 100nF (original values 10nF)
C5, C6 - F&T 500v 10uF (original values 47uF)
C19 - F&T 500v 30uF (original value 47uF)

Resistor Changes:-
R17 reduced from 330K to 160K
Add 7.5K 27K resistors to the input pins 2 & 7 of v2 Phase Inverter
Add 7.5K 27K resistor to the input pin 2 of v9 TB channel

UPDATE: I have tested 27K grid stoppers with no detrimental effects to high frequency roll off and these have helped smooth the break-up when the amp starts to overdrive so I would recommend them over the 7.5K.

I used 500v Silver Mica for the small signal caps, 630v Mallory 150M caps for the larger signal caps and 500v F&T caps for the filtering. Resistors are all 1/2 watt. The grid stoppers are soldered directly between the tube base pin and its associated connecting wire. Click on the images below for larger versions, I have highlighted the changes with green and magenta circles.

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The back of a Vox AC15 Hand Wired

My VOX AC15HW1

My Vox AC15 Hand Wired Amp which has replaced my AC30 because it was just too dam loud !

The VOX Hand-Wired Series is a true all-tube amplifier, employing vacuum tubes in the preamp, power amp, and rectifier. The preamp uses three classic ECC83 tubes the power section a duet of EL84 tubes and the rectifier an EZ81. These three elements, when combined, provide the vibrant, organic and unmistakable tone that is the heart of the classic VOX sound.
The HW Series relies on the painstaking technique of turret board hand-wiring – a method requiring a high level of skill and craftsmanship – is used, resulting in wiring that's beautiful in its artistry, ensuring an efficient signal path with minimal loss and no restriction in amplitude.

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