Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jalmeida on April 17, 2024, 12:10:46 am
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Hello,
I have recently acquired 2 different Vintage Vox amps from the UK. Both wired with Euro 240VAC 50HZ plugs. One has a voltage selector with a 110VAC selection and the other without the 110VAC option. That said itnis my understanding that even running these old Vox JMI amps with the 110VAC primary isn’t optimal either due to the high B+. So some suggest a variac or a step up transformer to run it at 245VAC so they run cooler.
What is the brain trust’s take one this?
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To resume ; Personally, I would operate in the standard way on the 2 Vox at 240 volts and step-up transformers.
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Details :
The Vox selectable at 110 volts ; If it is impossible for you to measure the voltages in the circuit, because they may be within the standards OR if you fear for the transformer, it is better to use a step up transformer and the Vox at 240 vac or a Variac at 110 volts and the amp at 110 vac
Vox 240 Vac ; if the transformer primary cannot be changed to 120 volts ( I did it on a Wem ) OR if you fear for the transformer; Step up transformer.
A used amp; always check that the fuses are the right ones. I've seen 20 amps instead of 3 !!!!
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You'll know you've got it correct when the heaters are very close to 6.3V.
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You are right pdf64
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Thanks for the advise, fellas. As far as the step up transformers go, do they induce noise? Which Step up transformers would you suggest? Of are they all pretty passable? And what about mounting location?
300va should be enough for this i’m thinking.
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Wich Vox ?
AC30 with 240 volts PT wiring use a 3.15 A fuse as I see on schematic
240 X 3.15 = 756 watts. Amp may need only 500-600 watts ?
300VA / 240 volts = 1.25 A
240 X 1.25 = 300 watts
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AC30/6. That math is correct. But should I have enough wattage to blow the fuse? 500watts step ups are all over. If the x 1.25 scale was used for the fuse, then a standard operating current in the 2.35-2.45A range is to be expected. So 600watts would probably be realistic.
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Fuse blow ONLY with amp issue never with input watts or volts
Amp take only the power (watts) it need.
Your home wall outlet can send more than 1500 watts . Did you blow any amp , tv set , toaster .....?
More powerful is the step -up transformer less it will heat and more chance it last longer , Con ; more expensive.
Choose the one in the middle for your use .
One step -up for both amp in same time on one for each ?
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Fuse blow ONLY with amp issue never with input watts or volts
Amp take only the power (watts) it need.
I totally get that. Just trying to aim for what I need.
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Fuse blow ONLY with amp issue never with input watts or volts
Amp take only the power (watts) it need.
I totally get that. Just trying to aim for what I need.
I ask ; " One step -up for both amp in same time on one for each ? "
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Fuse blow ONLY with amp issue never with input watts or volts
Amp take only the power (watts) it need.
I totally get that. Just trying to aim for what I need.
I ask ; " One step -up for both amp in same time on one for each ? "
It would be one for each.
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Wich Vox ?
AC30 with 240 volts PT wiring use a 3.15 A fuse as I see on schematic
240 X 3.15 = 756 watts. Amp may need only 500-600 watts ?
300VA / 240 volts = 1.25 A
240 X 1.25 = 300 watts
The 3A or 3.15 A fuse is appropriate for 115V. ie 350W max. It's far too high for 240V. My AC30 runs fine on a T1A fuse.
I think there wasn't room on the faceplate to note the appropriate fuse for each voltage option.
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Wich Vox ?
AC30 with 240 volts PT wiring use a 3.15 A fuse as I see on schematic
240 X 3.15 = 756 watts. Amp may need only 500-600 watts ?
300VA / 240 volts = 1.25 A
240 X 1.25 = 300 watts
The 3A or 3.15 A fuse is appropriate for 115V. ie 350W max. It's far too high for 240V. My AC30 runs fine on a T1A fuse.
I think there wasn't room on the faceplate to note the appropriate fuse for each voltage option.
Are you running a Step transformer? I would love to know your setup.
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IMO a 350 to 400 watts would do the job
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Are you running a Step transformer? I would love to know your setup.
No. I don't
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Your unit withe 110V winding sounds like a good candidate for a home-brew onboard bucking transformer.
Sluckey's Buckaroo (https://sluckeyamps.com/misc/Amp_Scrapbook.pdf) or RobRob's Bucking Transforme (https://robrobinette.com/images/Guitar/5E3P_Build/Simple_Bucking_Transformer.pdf)r
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If its a 1960 AC30 with 4 x EL84s, 6 x 12A-7s and a GZ34, the PT power consumption at idle will be:
Heater winding = 6.3VAC @ 4.84A
HT winding = 280VAC @ 180mA
Rectifier winding = 5VAC @ 3A
Total secondary V.A = 95.892VA (96W)
So, primary winding current at idle:
For 230V mains = 96W/230V = 0.42A
For 110V mains = 96W/110V = 0.9A
Multiply by a factor of about x 1.5 for absolute max peak draw (but more like x 1.2 is what it'll be in reality due to various inefficiencies)
And make mains fuse about 2 x peak draw in each case.
YMMV (Feel free to check me on my math)
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Your unit withe 110V winding sounds like a good candidate for a home-brew onboard bucking transformer.
Sluckey's Buckaroo (https://sluckeyamps.com/misc/Amp_Scrapbook.pdf) or RobRob's Bucking Transforme (https://robrobinette.com/images/Guitar/5E3P_Build/Simple_Bucking_Transformer.pdf)r
My vintage AC10 has a 115v primary. The AC30/6 has a mark on the voltage selector for 110V, but does not have a lead for the primary. The AC30/6 only has 205, 220 and 240V