AC30 speaker impedance question

[maynut]

Hi, not really a stompbox question but I'm hoping the community here can help me please.

I have just acquired an old AC30 chassis (I think it's quite an obscure model, the AC30TBR 1007 model, from 1989 Rose Morris period - which I understand to be some of their better amps after shifting production to Precision Electronics - http://www.voxshowroom.com/uk/amp/rmac30tbr_1988_hood.html ) to which I've just fitted valves, but I need to get speaker(s) for it.
On the above link it is the 'issue 3' circuit board.

Now the schematic I found - https://www.korguk.com/voxcircuits/circuits/ac301989.jpg - which I think is the correct one, suggests the output transformer accepts either 4 or 8 ohm impedance speaker load.  However this goes against other AC30 schematics and advice I have seen which would be either 8 or 16 ohm load.

I understand that AC30s are normally 2x8ohm speakers in series for 16ohm, although there is usually an 8ohm tap for using just one speaker, is that correct?

In this case it very stangely looks like my amp should be 2x4ohm speakers in series.  But I have yet to take the O/T out to see what wiring there is or what taps come out, it's kinda hidden within the chassis

But what I am really wondering is if anyone here can tell me if it is safe to put a higher load such as a 16 ohm speaker load (as normal with AC30) on such an O/T - will it get damaged?  Or do they just get damaged if you put too low a load on them - or is there a way to tell from the transformer (no label) or from the wiring etc  if I can find out what the actual output impedance is?

Is it safe to try 16 or even 8ohm just to test it for a few minutes and see if the amp works before I put any more time into it?

Sorry for the long-ish read, but tried to give as much detail about the amp as I could.  Any help or advice appreciated, thanks!!
Roy

[anotherjim]

If that info is correct, you do only have 8 or 4 ohm taps. I'd bet with new tubes you can get away with 16ohm load on the 8 tap just for testing, but in my experience of modern EL84's, they are easily damaged by extreme flyback voltages from the OT primary. It might be better to temporarily rewire your 2x12 to 4ohm. You could fit switching to make it a 4 or 16ohm cab.

[maynut]

Thanks Jim,
I don't really have experience of output transformers so this advice is helpful, thanks.

I'll try soon to have a look on the output trans and see if I can find any more information before connecting anything, might be safer than connecting 16ohms if it's expecting 4 or 8...
I have a mix of old and new valves for testing but kind of trying to do this on the cheap if I can, so I don't want to break anything if I can help it!

Thanks again, I'll post updates if any.
R

[thetragichero]

if you can make out the colors on the output transformer secondary wires, they should be standard: 16ohm grey 8ohm green 4ohm yellow

[PRR]

Is there a part-number on the output transformer?

[maynut]

Thanks Tragichero,

Didn't realise there were standard colours for the wires - are these standard on all AC30's?  All amps/amp heads?  I've only ever had small practice amps up til now.

Also thanks PRR, there seems to be no markings on the OT that I can see, I'm assuming there may have been a label at some point that is long gone...

Thanks all, I was away last night but will dig further into the amp tonight and see about wire colours, OT markings and come back.
Cheers,
R

[maynut]

Failing that, is there any straightforward at to test/measure a transformer to find out the values to be 100percent sure?
Thanks guys

[thetragichero]

color code is meant to be standard across all output transformers. didn't mean you won't run into odballs
the only other way i know of involves disconnecting from the circuit, connecting to mains, taking some measurements, and then doing some math
so yeah we're hoping for colored wires 

[maynut]

Thanks, ok we'll find out colours later - the amp itself seems to be a bit of an oddball haha, perhaps why I hadn't heard of this particular one til I saw it up for sale.
Cheers again,
Roy

[anotherjim]

Wow! I never knew there was supposed to be a colour code standard -  or is this only with Vox?
Waiting for the outcome with interest.

[PRR]

> never knew there was supposed to be a colour code standard

I have never seen an industry standard for the loudspeaker taps. Most mass production only gives one choice. It may be black and green, or whatever they had a bunch of, but does not say 4 8 16. Fancy PA amps can have so many taps they run out of colors.

[thetragichero]

dang maybe I've just gotten lucky with the ones I've found and quick Google searches

[tubegeek]

Failing that, is there any straightforward at to test/measure a transformer to find out the values to be 100percent sure?
Thanks guys

For sure!

Disconnect primary (tubes are connected to the primary) from circuit. (Secondary is already disconnected? If not, disconnect it too. If no speaker is jacked to it, that's fine.) Pulling the output tubes would be sufficient for the primary disconnection.

Connect a modest AC voltage to the primary. 12V, 24V, all ok. No need to go all the way up to line voltage. Any AC wall wart is fine.

Measure the voltage on the primary. Measure the secondary. Ratio of the two is the ratio of the transformer's turns, to a close-enough approximation. For example, 12V in, 2V out, ratio is 6:1.

Now for the tricky part. Impedance ratio is turns ratio squared. In our example, 36:1.  So if the secondary is loaded with a 4 ohm speaker, the tubes will be presented with 4x36 = 144 ohms. If a 16 ohm speaker, 576 ohms to the tubes.

The ratio will be much steeper than this, 25:1 turns ratio maybe, to give maybe a 625:1 impedance ratio. I was just giving an example with easy numbers.

Now all you have to do is research what load the tubes are best designed to work into - bring your results back and I'm sure you'll get some help here. It'll be pretty clear from what the choices are, which speaker is intended.

[maynut]

Thanks everyone,  I haven't had a chance to check this out due suddenly having to work in the evenings.

I'll update you soon, really appreciate all the input on this.

A question about the calculations - I found a data sheet for a replacement AC30 OT (Hammond) https://www.hammfg.com/files/parts/pdf/1750V.pdf which tells me the input and output impedances, along with expected voltages at the 8 and 16 outputs for an input voltage of 346.4v.  Here is also the data sheet which shows the versions of transformer have the same primary impedance for both vintage and reissue AC30 - https://www.mouser.co.uk/datasheet/2/177/1750-736917.pdf - EDIT-this can be found on page 2 of that data sheet.

Tell me if I'm wrong, but I figure I can work it out from this without knowing data about the valves/tubes since it seems any kind of AC30 would have similar output from 4 x EL84 valves, and going to 8 or 16 ohm.

Should be straightforward to know the turns ratio then of a 4 ohm output and calculate which outputs I have on my oddball AC30, does this sound right?

Thanks,
R

[tubegeek]

...an input voltage of 346.4v....

I wonder where they got that number from? Weird!

Tell me if I'm wrong, but I figure I can work it out from this without knowing data about the valves/tubes since it seems any kind of AC30 would have similar output from 4 x EL84 valves, and going to 8 or 16 ohm.
Should be straightforward to know the turns ratio then of a 4 ohm output and calculate which outputs I have on my oddball AC30, does this sound right?

Assuming your amp isn't a SUPER oddball with no relation to the usual AC30 output stage, you are very much not wrong!

[PRR]

>> input voltage of 346.4v....
> I wonder where they got that number from? Weird!

One pair of EL84 loaded in 8k will make over 15 Watts.

So a four-pack of '84 should load in 4k and make 30 Watts.

What is the Voltage of 30 Watts in 4k? SquareRt(30W*4k) is 346.41016 volts.

And yes these voltage ratios will scale down to less-lethal voltages like 6.3VAC@50hz Primary makes 0.28VAC at 8r secondary. (This relies on a good low-voltage AC meter, but cheap DMMs seem to do OK. My old VTVM was well off its best precision at a third-Volt AC.)

[maynut]

Sorry for the delay everyone - work, kids, wife, etc you know how it is  ...

Ok so the primary impedance of the OT must be 4000ohm, this is what replacement transformers for AC30s are rated at, and also on the original schematic of the 1960s AC30/6 - https://www.korguk.com/voxcircuits/circuits/ac301960.jpg.
Basically from the output of the 4 EL84 is the same on all AC30s within a small margin as the circuit hasn't changed much at that stage in the amp.

I've input 20.2VAC (measured across the OT while disconnected from the circuit) from a random PSU, and from the output I get 0.5volt, sadly my multimeter has only one decimal place.
This doesn't seem to tie in with my calculations for 4, 8, or 16 ohm, but is closest to the 4ohm value which I calculate to be 0.639 volt (impedance ratio 1000:1 therefore turns ratio 31.62:1, so 20.2 volt divided by 31.62 gives the 0.639.

Thoughts on this experiment so far? Am I missing anything or does it look like 4 ohm with a margin for manufacturing discrepancy/losses within the wiring of the OT? Or not that accurate a multimeter?

Is there anything else I should try before firing her up with a 4 ohm load?

Thanks for help on this everyone!

[maynut]

Oh also, what could be damaged if I get this wrong?  OT?  Valves? 

[thetragichero]

valves, ot, components and the boards they're on, etc could all be damaged

so let's use 32:1 for turns ratio for that tap. 32*32 = 1024:1 impedance ratio. 1024*4ohm = 4096 ohms primary impedance. we're being rough on the numbers here so yes that would be 4ohm tap if the expected primary impedance is 4k
any other secondary taps to try?

[PRR]

Sure looks like a 6k-6.6k:4 ratio.

6k-ish is commonly used with other tubes in the 30W range, like self-bias 6L6 or EL34.

Are you sure it is the right part? Or a repair picked because it fit the holes?