VOX: AC4C1-12 Transformer Upgrade

Before I completely rip this Vox AC4 apart and rebuild the amp on tagboard, I thought I'd try swapping the output transformer (OT), just to see if it made any difference to the sound of the amp, as I've read conflicting views online.  

Cutting a very long post short - yes it did make a difference, it sounds much better in my opinion.  I'd already snipped the bright cap (C20) on the master volume and installed a vintage alnico speaker.  The transformer upgrade rounds things out very nicely.  Running 16ohms @ 5k:  No fizz, it sounds bigger and more open sounding.  The amp is cleaner sounding and the breakup is nicer.  It somehow sounds less boxy, which I can't figure out - maybe increased frequency response??  16ohms @ 10k is closer to the stock transformer spec, so some of the original tone returns - I much prefer 16ohms @ 5k.  

VOX AC4 OUTPUT TRANSFORMER SPECS

There's almost no information available on the stock Vox OT apart from the factory part number and that it's made in China.   I did a quick test after I pulled it out, and I found that it has a ~22:1 wind ratio.  With a 16ohm speaker, this puts about 7.4k on the EL84.  An 8 ohm speaker will drop this down to about 3.7k.  

A few forums posts say that it's 10k, I can only go off my own measurements. 

Wattage unknown - safe to say that it has to be at least 4 watts given the rating of the amp.  I'm going to take a guess and say that it's a little over-rated as it's a similar size to the Hammond, which is 8 watts.  Unlike the Hammond, there's no additional taps on the secondary.

HAMMOND 125CSE OUTPUT TRANSFORMER SPECS

I decided to use a Hammond 125CSE as it was available locally at a reasonable price, and the specs looked right.  Hammond transformers are made in Canada and seem to be quite well regarded (completely unrelated to Hammond organs by the way).  

The Hammond 125CSE is an 8w transformer - it has similar dimensions to the stock OT - conveniently the mounting is the same width, so no additional holes were required.  The Hammond felt a little bit heavier and generally speaking seems to be a better build quality.  It certainly sounds better.

There's plenty of information available on Hammond output transformers, the most important of which is on the diagram below.   By way of comparison to the stock OT, the 125CSE can run at 16 ohms with 5k on the EL84.  This is closer to how an EL84 is designed to operate.

VOX AC4C1-12 OUTPUT TRANSFORMER UPGRADE

WARNING:  VALVE AMPS CONTAIN VERY HIGH VOLTAGES - LETHAL VOLTAGES CAN BE PRESENT, EVEN WHEN THE AMP IS DISCONNECTED FROM THE POWER.

I've marked up a few relevant items on the AC4C1 circuit board.  Both the 10" and 12" version are the same.

In very simple terms,  I disconnected and reconnected P2, P8, P10 and P11 on the schematic below, with a replacement transformer and the addition of a rotary switch to allow three options on P10.

The first step is to remove the back of the amp - I like to leave the chassis on the back of the enclosure, as I find it easier to work on this way.  Naturally the amp is not plugged in at this stage, and hopefully has not been used recently (hot valves and potential for stored DC in caps).  Note that this mod involves the B+ supply to the EL84 - we're talking a 320V connection.  

The output transformer is connected to the board using fully insulated 1/4" 6.3mm female spades - these slide off, however some force is required to do this and the circuit boards are flimsy.  If you try this, take it slow and work very carefully.  

The primary connections to the output transformer are directly either side of the EL84 - there's very little room here.  I removed the EL84 before attempting to remove the spades connected to P8 & P2 using a combination of my fingers small pliers - a bit of gentle wiggling and force was required.

Same again for the secondary connections which are attached to the external speaker jack.  This board feels like it wants to snap off, and the spades face the large filter cap on the circuit board - not a great combination.   

Note that there are actually 2 x P10 and 2 x P11 connections on the small circuit board that the extension speaker jack is mounted on - they're linked.  One pair from the OT, the other pair to the speaker.

After the wires were removed from the circuitboard, I removed the stock transformer.  Quick test placement of parts, before drilling the hole for the 3 way rotary switch.  Need to be really careful to make sure no waste from drilling the hole remains, as you don't want bits of metal sliding under that circuit board - boom!   

I crimped new spades onto the Hammond transformer leads and bolted it onto the chassis - I did not trim the leads to the correct size, as I plan on rebuilding this amp at some stage.  I didn't want to cut anything short.  I used heat shrink to terminate the end of the unused orange secondary wire.

The primary windings (blue and brown wires) attach to P8 & P2 on the circuit board.

I'm using a 3-way rotary for the secondaries to increase output options as shown below.  I wired the three secondaries that I needed to a 3 way rotary (white, yellow & green).   The common pin on the rotary connects to P10.  The black secondary connects directly to P11 on the external speaker board. 

I have no plans to detail this process any further - if you can't work out what I mentioned above on your own, it's probably not advisably for you to be playing with high voltages.

I'll add proper labels for the rotary at a later date - while I know what the coloured dots mean, if I ever sell the amp, this will need to be clear to avoid issues.