Replacement Transformer for Vox AC15?

[CIM]

I just bought a 9 year old VOX AC15 TBR in non-working state - looks like the mains transformer is gone - it pops the line fuse even with all the tubes pulled.

I can get a replacement transformer direct from the makers here in the UK but it will be the same make as the one which blew I expect (Drake), and they are quite expensive - also the older AC15 TB reissues from around the same age of mine seem to have a reputation for unreliable mains transformers so maybe I should look for a different make?

Any alternative suggestions for sourcing an AC15 mains transformer?

I'm not averse to importing one if it makes economic sense - I got a replacement output transformer for my Champ from the USA and it still worked out a good deal - although the AC15 mains transformer is a lot heavier so may not be so economic to get delivered.

Thanks in advance...

[JohnnyCrash]

First, you need to know how much B+ voltage the amp needs... then you need to know if it uses a solid state or tube rectifier.

You can then get a PT based on that info...

Your PTs are usually spec'ed like this:
120VAC Primary, 290V, 0V, 290V.

This means your PT is giving you 290VAC, a solid state rectifier will multiply this amount by 1.3 times, a tube rect by 1.2 times...

Whatever the final Voltage sums up to be after the rectifier is what you're looking at for your high voltage (B+)... so you want a PT that is as close as possible to spec.

Also, you'll need to figure out how much current draw your components will require - the PT must have at least that amount of mA's available.

ALSO,
All of your tube's need heater voltage (6.3VAC), so there is a tap on the PT for this... if you have a tube rectifier it's heater voltage is 5VAC so you'd need a PT that has high voltage taps, then 6.3 heater taps, then 5V rectifier heater taps.

You'll need the 6.3VAC heater tap to have enough current draw for all of the tubes as well (in milliamps).

Confused yet?

[JohnnyCrash]

Now that that is out of the way:

Blowing a mains fuse may not even be a bad PT.

There are tons of reasons for a fuse to keep blowing. It may even be an easy/cheap fix!

When I built my Fender 5D8 Twin clone head, I accidentally wired the PT/rectifier heaters wrong (a dumb mistake, rookies never make!) and kept popping fuses and blowing rectifier tubes :(

You could have a bad Power tube, or a variety of other problems. Also, check your Power tube's sockets (and rectifier sockets, if it uses a tube rectifier) for any chicken fried marks... a short caused here could do the same...

[CIM]

Thanks for the comments, much appreciated.

Confused yet?

Not really, understood most of that!

The amp has a tube rectifier and in fact there are no less than 8 wires coming out of the secondary side. I guess I am hoping that someone who has been there and done it can recommend a tried and tested substitute...

BTW This schematic shows the configuration:

http://www.voxamps.co.uk/downloads/ac1596pa.jpg

Of course I am running on 240V mains so the primary is jumpered accordingly.

Blowing a mains fuse may not even be a bad PT.

There are tons of reasons for a fuse to keep blowing. It may even be an easy/cheap fix!

That's what I was hoping but I have pulled the rectifier tube as well as all the others, plus all the fuses on the secondaries and it still pops the line fuse :(

The mains transformer on the late 90's AC15 reissue seems to have a reputation for blowing so I suspected it anyway before I started but of course I don't want to fit a replacement and blow it up too i.e. I think I need to try and verify that everything else is Ok as far as I can.

You could have a bad Power tube, or a variety of other problems. Also, check your Power tube's sockets (and rectifier sockets, if it uses a tube rectifier) for any chicken fried marks... a short caused here could do the same...

I did check but didn't see any evidence of burn or tracking marks anywhere.

Thanks again.

[Wayne Alexander]

Mercury Magnetics makes the best replacement power and output transformers for that amp. http://mercurymagnetics.com/pages/nav/mainframe.htm call or email them to ask exactly which one(s) you'd want. They aren't cheap, but they're the best.

[CIM]

Mercury Magnetics makes the best replacement power and output transformers for that amp. http://mercurymagnetics.com/pages/nav/mainframe.htm call or email them to ask exactly which one(s) you'd want. They aren't cheap, but they're the best.

Thanks, they look great but $175 + delivery to the UK - a bit too rich for me alas.

[lostpick]

check that tranny out with a meter first??

[JohnnyCrash]

Originally Posted by lostpick

check that tranny out with a meter first??

You gotta have voltage on one side... with the fuse blasting away, it'll be hard to do much...

Well... maybe not... damn I'm sleepy... let me sleep on it and think about it tomorrow...

[zook]

Originally Posted by CIM

I just bought a 9 year old VOX AC15 TBR in non-working state - looks like the mains transformer is gone - it pops the line fuse even with all the tubes pulled.

I can get a replacement transformer direct from the makers here in the UK but it will be the same make as the one which blew I expect (Drake), and they are quite expensive - also the older AC15 TB reissues from around the same age of mine seem to have a reputation for unreliable mains transformers so maybe I should look for a different make?

Any alternative suggestions for sourcing an AC15 mains transformer?

I'm not averse to importing one if it makes economic sense - I got a replacement output transformer for my Champ from the USA and it still worked out a good deal - although the AC15 mains transformer is a lot heavier so may not be so economic to get delivered.

Thanks in advance...

You can use the Weber W025130EU transformer. Use the 270-0-270 taps, the voltages will be fine for a two EL84 amp and there's enough amperage for the amp also.

https://weberspeakerscom.secure.poww...e/magnetic.htm

It's inexpensive and is well made. They should be in stock by now.

Winnie

[CIM]

You can use the Weber W025130EU transformer. Use the 270-0-270 taps, the voltages will be fine for a two EL84 amp and there's enough amperage for the amp also.

Shortened Link

It's inexpensive and is well made. They should be in stock by now.

Winnie

Many thanks for that Winnie, I forgot about Weber - looks like a good candidate. From what I have read, Weber makes good stuff.

One possibly dumb question on wiring it in - on the HT taps that go to the rectifier, the center tap on the Drake transformer is a black wire that comes from 'inside' the transformer. On the Weber diagram:

https://weberspeakerscom.secure.poww...25130eusch.jpg

The HT taps don't show a centre tap wire, only a pair of 45V tap wires on the same secondary - so what would I do with that connection?

Thanks again!

[CIM]

check that tranny out with a meter first??

Well, I pulled the wires off the tags on the secondaries (apart from the HT center tap which is not on a tag) and metered the two sides of the secondary feeding the recifier. Interestingly, one half measures about 3 Ohms, the other half measures about 1 Ohm.

Unless I'm missing something, I would have expected that both sides of the secondary taps would be the same impedance as they are both pumping the rectifier.

So - does that mean I have found the short? i.e. about 2/3rds of one side of the HT secondary is shorted?

And presumably that would be enough to explain the popping fuses.

I suppose the next question is what took the transformer out? It seems possible that it just did it on its own, as the AC15 Drake transformers from that era are known to be unreliable for some reason. I suppose a faulty rectifier tube could have shorted the secondary and fried it, but I am in danger of straying beyond my limit of toob technology knowledge here!

[zook]

Originally Posted by CIM

One possibly dumb question on wiring it in - on the HT taps that go to the rectifier, the center tap on the Drake transformer is a black wire that comes from 'inside' the transformer. On the Weber diagram:


The HT taps don't show a centre tap wire, only a pair of 45V tap wires on the same secondary - so what would I do with that connection?

Thanks again!

The red/yellow wire is the center tap.

The red/white wires are the appropriate HT for your amp. tape, or better yet heatshrink over the ends of the red wires and Blue/red, you won't use them.

Winnie