A little piece of Geeky AC30 history (I hope)

Cirrus

So, it might be absolute nonsense and I've been taken for a ride, but I was curious enough to want to put a bid on this;




It's a Woden Output Transformer, probably one of the first they ever made for vox as the date code puts it in October '62, and the earliest found is from August that year. I'm really excited about it because I do believe there's something in the transformer sound, and really the Output transformer is the only thing in an AC30 signal path that can't be perfectly duplicated with modern equivalents, because you're never quite sure how they were wound inside.

At some point it's been painted black, the last owner tried to clean it up. It's got separate primary halves, for some reason - you need to connect two red wires together to make a centre tap.

It arrived earlier, I've got as far as wiring it into my AC30 sat on a coffee table, with a Jensen P12Q serving as a sacrificial test subject in case some terrible malfunction blew the speaker. It works, next step is getting some stand-up bell covers so I can mount it on the chassis and give it a proper test with some old Alnico speakers.

Funkfingers

Cirrus said:

It's got separate primary halves, for some reason - you need to connect two red wires together to make a centre tap.

To cope with 220/240 and 110/115 volt mains supplies, perhaps?

lustycourtier

Brilliant. Ive had them with Albions and Wodens, and they do act different. 

springhead

Funkfingers said:

Cirrus said:

It's got separate primary halves, for some reason - you need to connect two red wires together to make a centre tap.

To cope with 220/240 and 110/115 volt mains supplies, perhaps?

No it's an output transformer.  

Odd though.

elstoof

Trinity AmpsTC15 (vox/matchless clone) uses a Heyboer output transformer with seperate 5k and 8k primaries,  a toggle at the back of the chassis let’s you switch between them (the crunch/munch switch)

Cirrus

springhead said:

Funkfingers said:

Cirrus said:

It's got separate primary halves, for some reason - you need to connect two red wires together to make a centre tap.

To cope with 220/240 and 110/115 volt mains supplies, perhaps?

No it's an output transformer.  

Odd though.

Having done some reading it sounds like separate primary halves tied together outside the transformer were more common in the old days, and the early wodens did come like this before a revision in '64 simplied them and made them easier/ cheaper to produce.

elstoof said:

Trinity AmpsTC15 (vox/matchless clone) uses a Heyboer output transformer with seperate 5k and 8k primaries,  a toggle at the back of the chassis let’s you switch between them (the crunch/munch switch)

Changing the primary impedance is a useful tool to change the sound, My other AC30 is a 1x12 running at half power with the original output transformer, which means the two remaining EL84s want to see around 8k, but are seeing 4k unless I plug a 16 ohm speaker into the 8 ohm tap. Doing so does make it a bit louder and more bassy, but I also love the sound of an 8 ohm speaker with it too, it's a little quieter, a bit cleaner and tighter, more immediate and the bass has a nice growl without ever getting woofy. I think some of the smaller Matchless amps deliberately ran the same impedance mismatch to get the same effect.

If I can find a suitable AC transformer, I'll run a few volts into the secondary and see if I can work out a rough primary impedance for this woden, I've read they're often on the high side of 4k.

Cirrus

Blasphemy I'm sure but output transformer installed and mounted in standup configuration thanks to @Modulus_Amps providing me with some bell covers.

Tomorrow evening I'll take it to practice and put some proper current through it.

elstoof

Great work there

Modulus_Amps

I would not be surprised if the AC30 OT Marshall used in the 90's is based off the Woden Design. Dagnall, made them and I know they unwound a number of vintage output transformers to get designs for other products. I also know that the winding pattern they used is not just a replica of a typical Marshall winding pattern, which would be the simplest option for a transformer manufacturer to adopt.

The primary impedance for EL84's does make a big difference, in one amp I have here the change in screen current is significant depending on the load, lets you really tweak how much sag you get when fully cranking. Too low and this amp becomes a saggy mess, it doesn't have choke though which amplifies the effect.

Cirrus

Yeah, I really liked the UK-made AC30 I had, and I suspect the OT was part of that.

The stock transformer on the Custom Classic series is a bit odd. It's got smaller laminations but a taller stack of them, and it doesn't sound bad really, just a strange blend of being a tad gutless while also not letting the amp focus in on the mids when pushed. Replacement OTs I've got for ones I've modded before (I've had a MM Woden clone, and one made by Trans-Amp in Chesterfield) have had a better roar to them and more punch.

It's impossible to talk about the sound of an OT without sounding like you are so superstitious an audiophile that you probably believe in witchcraft but ah well.

Cirrus

Right, I finally got to take this amp to practice and hear it alongside drums and bass.

Fucking hell, yes.

GoFish

The knowledge and dedication in this thread brings a smile to my face. Well done chaps, Well done  

stickyfiddle

Cirrus said:

Right, I finally got to take this amp to practice and hear it alongside drums and bass.

Fucking hell, yes.

Awesome

I can't be arsed with full fat AC30s because frankly they're too heavy and too loud when you get them in the sweet spot where everything gets exciting. But playing through a great one is one of my all time favourite experiences - there's nothing like it

Cirrus

stickyfiddle said:

Cirrus said:

Right, I finally got to take this amp to practice and hear it alongside drums and bass.

Fucking hell, yes.

Awesome

I can't be arsed with full fat AC30s because frankly they're too heavy and too loud when you get them in the sweet spot where everything gets exciting. But playing through a great one is one of my all time favourite experiences - there's nothing like it

I'm the same, really. As mentioned above, my other one is a 1x12 with 2 el84s pulled & a lower rated rectifier valve - 15 watts into a single Alnico silver gets me where I want to be in terms of compression etc at practices and gigs, and so far no sound man has had a problem with my volume when using that. It's just nice to have the 2x12 cab with the extra bass for recording, bigger gigs, and just messing around. Plus the full 30 watts means the whole circuit sings and interacts under the current load - Power transformer sag, OT saturation, etc etc

Modulus_Amps

Nice! If you want the Chinese one rewound to see if we can make it better send it up to me. would be interested to see if it can be improved.

Cirrus

That's a very cool and interesting offer @Modulus_Amps , the only thing holding me back: if you did that, I'd end up doing more transformer swappage and after a bunch of modding on two AC30s, plus the recent 5e3 build I should stop tinkering and concentrate on songwriting/ recording for a bit. It's become a running joke at practice - every single time, I've changed something  The singer's started asking as I walk through the door with my amp; "what've you done to it now?"

Cirrus

Realised I haven't actually danced about architecture yet so here goes -

The first thing I noticed with this Woden was that the treble was less in your face - I turned back the cut knob; on all the modern AC30's I've ever had, the cut's lived at about 3 o'clock to stop things getting harsh with distortion pedals. I turned it back to about 1 to get the top to open up, and while those upper harmonics came back they still weren't harsh.

Other than that, it's just thicker overall. More midrange gets through, and the bass had this really lovely compression where it was full but not boomy. I found myself rolling the bass knob back from 2 o'clock to about noon.

It's hard to get scientific about it, because the differences made me set the knobs differently and also subtly play differently. But I'm really happy with the way the voice of the amp's changed

Modulus_Amps

Cirrus said:

Realised I haven't actually danced about architecture yet so here goes -

The first thing I noticed with this Woden was that the treble was less in your face - I turned back the cut knob; on all the modern AC30's I've ever had, the cut's lived at about 3 o'clock to stop things getting harsh with distortion pedals. I turned it back to about 1 to get the top to open up, and while those upper harmonics came back they still weren't harsh.

Other than that, it's just thicker overall. More midrange gets through, and the bass had this really lovely compression where it was full but not boomy. I found myself rolling the bass knob back from 2 o'clock to about noon.

It's hard to get scientific about it, because the differences made me set the knobs differently and also subtly play differently. But I'm really happy with the way the voice of the amp's changed

The offer to rewind was as much for my benifit as it was for yours ha ha, I like looking to see what other winders have done.

Sounds like the Chinese one has a more extended high end frequency response, I would guess they have done a 7 layer interleave pattern, which is easy/cheap to do when labour is so cheap.