240v to 220v

divad68

Member
2006-10-29 9:56 am
Guys, I've got a Yaqin MC-100B and after having read through a few threads on here I started becoming concerned as I believe my unit is designed to run on 220v, but in the UK our mains supply tends to fall somewhere between 230 & 240v. Mine usually measures around 235v. If it helps, I also tested the voltage across pins 4 and 9 of one of the 12AX7 sockets (with the amp plugged straight into the mains) and it read 6.6v and I think it's supposed to be at 6.3v? Don't for one minute assume I know what I'm on about...because in reality I'm new to it all and just read somewhere that this was a useful measurement to take (I didn't even know how the pins were numbered)!!

With this in mind I bought an APC Line-R 1200 Voltage Regulator, as that can be set to 220, 230 or 240v and the idea (I thought) was if I set it to 220v it will output that level. In reality what seems to happen is that it senses what the incoming voltage is and only if it goes above (or I assume below) a certain level will it actually regulate the output. So, with mine set at 220v it will spend half it's time indicating that the input voltage is 'normal' and won't adjust it at all, but when I test the output it's around 234v. Then I might hear it click and the indicator light on the panel will show the input voltage is' high'. In this circumstance I'd expect the output voltage to measure 220v, but it tends to measure around 205v!

So, to sum up, I don't think that the Line-R 1200 is actually doing what I want it to do because it doesn't continually regulate down to 220v, but only when a threshold level is exceeded and even when it does regulate down, the measured output is significantly lower than it ought to be.

Please can anyone advise what else I can buy that will be able to give me a reliable and (fairly) constant 220v? I've heard mention of 'buck transformers', but I'm not sure what they are and couldn't see any to buy when I put that term into Google.

As a side point, how can I be sure that my particular amp is actually designed to run at 220v? I'm pretty sure it is as they seem to be either 110v or 220v, but would prefer to be certain.
 
It will work fine on that voltage. A buck booster is just a transformer that will make small voltage adjustments depending on how the windings are configured. It will fluctuate with incoming voltage. It is not a regulator. If your incoming voltage is pretty steady at 235 and you want to reduce the voltage with a buck booster then that would work. Common versions will adjust the voltage up or down 12/24 volts or 16/32 volts or 24/48 volts depending on the version and how the booster is wired. The windings are user configurable. You either wire it in series or parallel to get the low or high voltage and feed it in forward or reverse for boost or cut.

The amp should still run fine on that voltage anyway. Electrical appliances are designed to operate within a range without the need for stiff regulation.
 

divad68

Member
2006-10-29 9:56 am
Okay thanks for the responses. Maybe I am worrying too much, but I just want to be sure I'm not stressing-out the internal components (and possibly degrading the sound) when there's something I could do fairly easily to remedy the situation.

NB: Just checked again and the figures are now a little higher - voltage across pins 4 & 9 measures 6.68v and mains input voltage is reading 238v. Still okay?
 

Arnulf

Member
2009-02-02 9:41 am
Line voltage should be 230V +/- certain percentage of this value (+/- 5% minimum) according to EU regulations. Your reading is well within these limits. There is far larger variation among parts in your amplifier than there is in the line voltage. If your amplifier fails at 235V it would just as likely fail at 230V.
 

kevinkr

Administrator
Paid Member
Your filament voltage is about 5% high which is a fair indication that your amplifier was designed for something approaching 220V based on your measured line voltage.

I completely agree with stoolpigeon and suggest you make and use a buck transformer based on stories of overheating and melted power transformers in some Chinese made tube amps here and at other forums.

In this case I'd recommend a 12V/4A transformer installed in a small box with the appropriate mains input and output sockets - I'm thinking IEC 10A types..

The primary of this transformer will be wired in parallel with the incoming mains, the secondary will be wired in series with the hot side of mains between the input and output sockets. The neutral and ground will go straight from one socket to the other. If you use a metal box also connect the ground securely to the box.

Once completed check the output voltage, you have a 50% chance of having wired it correctly, if not just reverse the secondary connections and voila..

Please use a fused mains plug based IEC cordset as is standard in the UK to power the box, use a 4A fuse in the plug. Get one of those IEC cordsets with an IEC male on one end and IEC female on the other to power the amp - make sure it is rated for 240V mains.

You can find all of the parts very cheaply on eBay for this project including the box. For the transformer you can use either a toroid or an EI type which you might want to purchase from RS or Maplins.

I'm assuming you can solder..
 
Last edited:

divad68

Member
2006-10-29 9:56 am
Your filament voltage is about 5% high which is a fair indication that your amplifier was designed for something approaching 220V based on your measured line voltage.

I completely agree with stoolpigeon and suggest you make and use a buck transformer based on stories of overheating and melted power transformers in some Chinese made tube amps here and at other forums.

In this case I'd recommend a 12V/4A transformer installed in a small box with the appropriate mains input and output sockets - I'm thinking IEC 10A types..

The primary of this transformer will be wired in parallel with the incoming mains, the secondary will be wired in series with the hot side of mains between the input and output sockets. The neutral and ground will go straight from one socket to the other. If you use a metal box also connect the ground securely to the box.

Once completed check the output voltage, you have a 50% chance of having wired it correctly, if not just reverse the secondary connections and voila..

Please use a fused mains plug based IEC cordset as is standard in the UK to power the box, use a 4A fuse in the plug. Get one of those IEC cordsets with an IEC male on one end and IEC female on the other to power the amp - make sure it is rated for 240V mains.

You can find all of the parts very cheaply on eBay for this project including the box. For the transformer you can use either a toroid or an EI type which you might want to purchase from RS.

I'm assuming you can solder..
Thank you very much for that advice and those instructions. I'm okay at soldering, but I'll get a mate of mine who is much better to do that for me.
 

divad68

Member
2006-10-29 9:56 am
If you have any steady voltage off the wall in the UK you are very lucky! Mine varies from less than 200 to about 240 on a daily basis! And I am in London.........
Less than 200, that's crazy and I'm sure it must be out of any acceptable limits that may have been set. I'm in London too and the highest I've measured so far is 239 and the lowest about 230, but it's usually around the 235 mark.
 
...suggest you make and use a buck transformer based on stories of overheating and melted power transformers...

You can find all of the parts very cheaply on eBay for this project including the box. For the transformer you can use either a toroid or an EI type which you might want to purchase from RS or Maplins.

I put off building a bucking autoformer because it seemed too much trouble to find the parts. I was also concerned I'd cobble something together on a breadboard and leave line voltage hanging out exposed. I eventually realized I had a dead computer UPS (essentially free parts) that had everything I needed in a neat and tidy box.

After gutting the SLA battery and all the inverter/control circuitry, I found it had a 120:30 volt center tapped transformer inside. It is perfect for the job. It has four convenient outlets on the top of the insulated plastic box, plus two more outlets which bypass the autoformer. It also has a power cord and a circuit breaker too. It took me all of about 15 minutes with a crimping tool to put the insulated spade connectors in the right places and hook the thing up. It drops my rather high 126VAC down to a very comfortable 117VAC, perfect for my vintage US tube gear.

th_P1130484.jpg

http://i69.photobucket.com/albums/i43/Ty_Bower/Transformer/P1130484.jpg
 
David68,

I did a quick check of an RCA tube manual and it states the filament voltage for the tube you noted (12AX7) with filaments paralleled is 6.3 V +/- 10%. Your measured voltage was 6.6 volts which is about +4.5% above nominal well within specifications. I do not think you need to do anything unless your power transformer is running excessively hot (something warmer than you can keep your hand on). If the power transformer is not running excessively hot, I would not loose one wink of sleep over the line voltage being at the level you are seeing. You would have to see a filament voltage in 6.9 volts or more to out of acceptable specs according to RCA.

Mickeystan
 

divad68

Member
2006-10-29 9:56 am
Thanks for all the responses guys. Although it may be debatable whether I actually need to reduce the line voltage I still intend to do so, even if it's just for piece of mind.

The buck transformer is probably the way I'll go, but I've also heard that an alternative solution is to use a variac. I've seen a few on Ebay at varying prices, but am not sure at all what amp rating I'd require - there's ones that output 2A through to ones that do 20A. Can anyone advise what the minimum I'd need would be?
 

Arnulf

Member
2009-02-02 9:41 am
I put off building a bucking autoformer because it seemed too much trouble to find the parts.

What parts ? If you want to drop/raise line voltage by N volts, get a suitable line transformer with N volt secondary and power rating equal to or better yet greater than the product of secondary voltage and expected current draw. Put both windings (primary and secondary) in series in phase and hook it up:

A: for buck operation: mains to outermost wires of your new autoformer, take output from what used to be the primary winding.

B: for boost operation: connect mains to former primary, take output from outermost wires.

Auto-transformer does not isolate output from the input so all safety precautions apply. Since you tube gear already contains a suitable transformer which handles the isolation part, this is not a problem.
 
DIVAD68:
Hi,
You already know your input 6Volts tubes are suffer with a hi supply heater, maybe the output tubes are ''hot'' too, seems all others componets of this amp are being stressed also.

IF you want long live to this amp, you must do the right thing: change the power supply Trafo for a suited 240Volts input, or change/rewind the primary coil of this trafo to 240V.

I could put a new Toroidal transformer If space allow.
I do not like power conditioner for pre-amps and power amps, the current avaliable is too low, the mains outlet amperage avaliable is always higher than in a conditioner.
Hope this help, Gustavo
 

divad68

Member
2006-10-29 9:56 am
Hi
I use an 390VA autotransformer from RS to drop the 240V to 220V for my Music Angel 300B amp. The part number is 293-2656.
Works fine for me
Thanks, I think this may be what I need. From checking the specs on the RS website it looks like the autotransformer you have outputs 1.8A when set for a 240v input and 240v output. I thought that might be too low, but if it's working for you then obviously not. They also sell versions of that line that output 3.2A and 5.0A for not a huge amount more money. I wonder if it would be worth paying the extra?

Edit: The transformers that Zeta4 kindly alerted me to all include VA rating in their specs. They are available in 390, 780 and 1200VA (there are ones that go higher, but that's the point where the price shoots up). I've just done a little research and it appears that the MC-100B is rated at 'consuming' 300W (or some places say 280W). To get from W to VA you multiply by 1.4, so I'm assuming that means I will need a transformer with a VA rating in excess of 420. Is that right?
 
Last edited:
Thanks, I think this may be what I need. From checking the specs on the RS website it looks like the autotransformer you have outputs 1.8A when set for a 240v input and 240v output. I thought that might be too low, but if it's working for you then obviously not. They also sell versions of that line that output 3.2A and 5.0A for not a huge amount more money. I wonder if it would be worth paying the extra?

Edit: The transformers that Zeta4 kindly alerted me to all include VA rating in their specs. They are available in 390, 780 and 1200VA (there are ones that go higher, but that's the point where the price shoots up). I've just done a little research and it appears that the MC-100B is rated at 'consuming' 300W (or some places say 280W). To get from W to VA you multiply by 1.4, so I'm assuming that means I will need a transformer with a VA rating in excess of 420. Is that right?

A transformer with a VA rating of 420VA is a reasonable choice for the application, note though that a 12V 50VA transformer wired as a buck transformer would work as well and probably cost a lot less..