Variac or Step up transformer?
Moderator: jingle_jangle
Variac or Step up transformer?
Don-I own a few vintage Vox amps that i play through at home and would like to know what you would recommend using based on improved life expectancy and sonic improvements. Please recommend brand and availability also. Thanks!***Mark
Its MexiCAN not MexiCANT
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toneman
Mark; Hi! Well, with old Voxes it's sometimes best to use a variac with the AC-10, 15 & 30. The reason being is those original tranny's were actually designed to run on 110v A.C. and not today's voltages which (in my area vary between 114-122) are higher.
Now, on the AC-50 and 100 it's most likely best to run the amp on 240v and use a step-up tranny. The oringal tranny's like I said above, work on 110v coming in. with most wall voltages being 5-10% higher this increases the internal voltages and usually most componets like filter cans have a 500v rating. I've seen many old 50's & 100's that with 120v coming in read 525-540v after rectification. This is much too high for those old (and new filter cans) componets and stresses them over their limit.
If you use the step-up tranny @240 your internal voltages will operate around 450-480v and your amp will be much happier running at those voltages. Those tranny's were mainly designed to operate on a 240v setting and the secondary voltages work correct to JMI specs on that setting as well. The other primary input voltage settings are not as accurate and can cause the tranny to put out too much on the secondaries.
What happens, and this is the primary reason JMI/Vox got such a bad rap on blowing up, is that when you run say 120v into the supposed 115v tap it causes the mains(power) tranny to operate harder and increases the heat factor.
Vox used Beeswax to pot their transformers back in the fifties and sixties. When the tranny gets hot the wax melts and runs out onto your amp's plynth(the board that it's mounted on). When the wax is no longer isolating the windings the windings will vibrate against each other and eventually cause the insulation to wear off and results in the windings arcing(shorting) and causing the tranny to go up in a big stinky cloud of smoke. The result is your tranny will either have to be replaced or rewound.
Not good either way.
If your tranny does need to be rewound I would seriously consider and recommend having Mercury Magnetis do the work. They have a way of making your old tranny work with modern voltages and the 115v tap will actually be a true 120v tap and the secondaries will operate correct. I have had both of my old AC-30'sa mains transformer's rewound to work on regular wall voltage. It's very nice to know that I needn't worry about using them and having any problems occure due to excesive B+ supply voltages. My old AC-50 though runs at an even 475v with it's original power tranny.
Now, on the AC-50 and 100 it's most likely best to run the amp on 240v and use a step-up tranny. The oringal tranny's like I said above, work on 110v coming in. with most wall voltages being 5-10% higher this increases the internal voltages and usually most componets like filter cans have a 500v rating. I've seen many old 50's & 100's that with 120v coming in read 525-540v after rectification. This is much too high for those old (and new filter cans) componets and stresses them over their limit.
If you use the step-up tranny @240 your internal voltages will operate around 450-480v and your amp will be much happier running at those voltages. Those tranny's were mainly designed to operate on a 240v setting and the secondary voltages work correct to JMI specs on that setting as well. The other primary input voltage settings are not as accurate and can cause the tranny to put out too much on the secondaries.
What happens, and this is the primary reason JMI/Vox got such a bad rap on blowing up, is that when you run say 120v into the supposed 115v tap it causes the mains(power) tranny to operate harder and increases the heat factor.
Vox used Beeswax to pot their transformers back in the fifties and sixties. When the tranny gets hot the wax melts and runs out onto your amp's plynth(the board that it's mounted on). When the wax is no longer isolating the windings the windings will vibrate against each other and eventually cause the insulation to wear off and results in the windings arcing(shorting) and causing the tranny to go up in a big stinky cloud of smoke. The result is your tranny will either have to be replaced or rewound.
Not good either way.
If your tranny does need to be rewound I would seriously consider and recommend having Mercury Magnetis do the work. They have a way of making your old tranny work with modern voltages and the 115v tap will actually be a true 120v tap and the secondaries will operate correct. I have had both of my old AC-30'sa mains transformer's rewound to work on regular wall voltage. It's very nice to know that I needn't worry about using them and having any problems occure due to excesive B+ supply voltages. My old AC-50 though runs at an even 475v with it's original power tranny.
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toneman
Mark; They're all about the same really. Just get one that will work with at least 500 A current draw. My black panel AC-30 is a `60 and the serial number on the plate reads #4290 N. Vox started with #4000 on AC-30 serial numbers. Internal chassis serial numbers didn't start until late `63 and was more for inventory control. Oddly enough I'm beginning to find out that they made both 4 & 6 inputs at the same time. I've seen many 4-inputs that had the chassis stamped for 6 inputs.
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kennyhowes
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Re: Variac or Step up transformer?
Lazarus thread time!
Collin and I were texting about this topic, and while it appears Don Butler is no longer registered here, maybe you all have some insight.
In the interest of maximizing the lifespan and performance of my vintage AC30, I was looking at picking up one of these:
https://www.homedepot.com/p/VEVOR-Varia ... /322865397
Anyone here have any experience, good or bad, with a Variac-esque variable transformer, either in terms of sound quality or performance?
Collin and I were texting about this topic, and while it appears Don Butler is no longer registered here, maybe you all have some insight.
In the interest of maximizing the lifespan and performance of my vintage AC30, I was looking at picking up one of these:
https://www.homedepot.com/p/VEVOR-Varia ... /322865397
Anyone here have any experience, good or bad, with a Variac-esque variable transformer, either in terms of sound quality or performance?
Re: Variac or Step up transformer?
I can't comment on that particular unit, but I can probably tell you WAY more about Variacs than you ever wanted to know.
A variac is a fairly simple device, it's just an "auto-transformer" with a variable tab. An auto-transformer only has one coil instead of separate input and output coils. This means it does not provide any electrical isolation between input & output.
The output will be an AC sine wave like the input, only with the ability to vary the voltage. This means it won't create any extra noise like some other types of converters.
I hope that helps. Let me know if you have other questions...
A variac is a fairly simple device, it's just an "auto-transformer" with a variable tab. An auto-transformer only has one coil instead of separate input and output coils. This means it does not provide any electrical isolation between input & output.
The output will be an AC sine wave like the input, only with the ability to vary the voltage. This means it won't create any extra noise like some other types of converters.
I hope that helps. Let me know if you have other questions...
I have NO idea what to do with those skinny stringed things... I'm just a bass player...
Re: Variac or Step up transformer?
That's it, nothing more?cjj wrote: ↑Wed Feb 07, 2024 8:33 pm I can't comment on that particular unit, but I can probably tell you WAY more about Variacs than you ever wanted to know.
A variac is a fairly simple device, it's just an "auto-transformer" with a variable tab. An auto-transformer only has one coil instead of separate input and output coils. This means it does not provide any electrical isolation between input & output.
The output will be an AC sine wave like the input, only with the ability to vary the voltage. This means it won't create any extra noise like some other types of converters.
Re: Variac or Step up transformer?
FWIW: oddly enough, I may be one of those here with a lot of experience with variacs ... except not in the context of music. In chemistry, they are used to control motors (e.g. stirrers) or heating mantles, as shown in this picture:
In this application, the variac was controlling the voltage applied to the heating mantle under the round-bottom flask. The variac was plugged into a Thermo-Watch unit (made by I2R) to add another layer of control (to avoid overheating). 
It is better, of course, to know useless things than to know nothing. - Seneca
Re: Variac or Step up transformer?
These are the theatre lighting dimmers we had in the auditorium at my high school (I was on stage crew and used them fairly often).doctorwho wrote: ↑Fri Feb 09, 2024 1:54 am FWIW: oddly enough, I may be one of those here with a lot of experience with variacs ... except not in the context of music. In chemistry, they are used to control motors (e.g. stirrers) or heating mantles, as shown in this picture:
P3091467A.jpg
In this application, the variac was controlling the voltage applied to the heating mantle under the round-bottom flask. The variac was plugged into a Thermo-Watch unit (made by I2R) to add another layer of control (to avoid overheating).