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 original value 120pF )
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
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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.
Also see AC15HW1 Modifications (Part2)
Hi!
ReplyDeleteI saw your post on the vox forum. I was wonder where you got the schematics? I am looking for the schematics for my AC30HW head. Any idea where i can get that? Thanks in advance!
Jeffrey (stratman1976)
Hi, I emailed Vox and asked them nicely, I didn't expect them to send me anything to be honest but they sent the Service Manual as a secure PDF.
ReplyDeleteBTW... the AC30HW is identical to the AC15 except for two extra tubes in the power section (and a couple of resistors).
ReplyDeleteCool, thanks for your reply. I think i'll try to get the official schemtic from Vox....enjoy your amp!
ReplyDeleteJeffrey
My AC15HW just arrived.
ReplyDeleteIt sounds fantastic but the distortion seems as a little "Buzzy" and I am experiencing ghost tones. Do these mods address anything in these concerns?
The buzziness may be the fact its new and the speaker will take some time to break in. This mod is more likely to produce ghost tones as it reduces the DC filtering. Are you really getting ghost tones though or are you just not used to an amp that gives so much harmonic content ?
ReplyDeleteThanks, Nik. Sorry, for the delayed response.
ReplyDeleteI replaced the original G12M-25 - 55hz with a partially broken-in G12-65-80hz that I had laying around. I have not noticed the Ghost tones nearly as much and cone cry is all but gone. The tone is also "clearer." Possibly, the lower end response of the Geenback made the Ghost tones more noticeable.
Thanks, for the amazing work you have done to present the MOD's! At some point I will experiment with these MOD's but, for now I am enjoying the AC15 as it is +,- a speaker or tube...