Voltage Corrector

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phorties

Member
Messages
2
Hey guys, first post here.

I've got my hands on an old marshall plexi 100 watt head and I heard through the grapevine that it's better to run those old amps off of 110 volts. When i checked my wall plugs they read 125 volts. I feel like I am going to fry this thing unless I have some way of regulating the power back to 110. Is there a unit I can use to do this? Especially something I can take with me to gigs.

Thanks,
Matt
 
8len8

8len8

Member
Messages
15,222
You won't fry it if the AC power is a few volts off. AC power fluctuates so much across the country and world that if it was harmful there would be amps frying left and right. The 110 VAC just optimizes the sound - has little to do with what's "safe" for the amp.
 
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drbob1

Gold Supporting Member
Messages
30,421
Is that UPS able to provide the necessary amps for a tube amplifier? I'm OK with the Variac, although it needs to be said that you have to measure the output voltage to be sure (it's designed to give the nominal output voltage at a certain input voltage, higher input means higher output).
 
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SBax

Member
Messages
349
Dr. It's not a UPS, just a voltage regulator. It's rated for 10 amps at 120V. My measurements at the 110v setting have been rock solid with the wall voltage at 120-124 . I have been using it for a couple of years with my 67 PR.
 
fiveightandten

fiveightandten

Silver Supporting Member
Messages
7,870
drbob1 said:
Is that UPS able to provide the necessary amps for a tube amplifier? I'm OK with the Variac, although it needs to be said that you have to measure the output voltage to be sure (it's designed to give the nominal output voltage at a certain input voltage, higher input means higher output).
Click to expand...
+1

To explain this in non-technical terms, if you set the variac to 110V at your house, it's not necessarily going to give you 110V at the practice space, or the venue.

However, since you're lowering the voltage (not raising it) any differences will be minimal and aren't going to harm the amp.

As an aside, there are 2 reasons to use a variac:
1) Running at wall voltage will cause internal voltages that are beyond the rated voltage for the components in the amp, or cause plate voltages that are too high.

2) You believe the amp sounds better at the lower voltages.

Weigh your options and do your homework. Decide if it's worth it to lug one around. I have two 60's Vox AC-50 heads. The plate voltage is supposed to be around 465V. They're both over 500V because they were designed to run off 110V from the wall. It's clearly much higher these days.

I replaced the caps and B+ rail resistors in these amps so they're totally safe to run at my wall voltages. I don't carry around my variac simply because they may sound better at lower voltages. They sound great as is.

-Nick
 
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velvetgeorge

Member
Messages
707
You should address the voltage mismatch to keep your vintage amp reliable. Ideally, you should adjust the variac while measuring the filament voltage on your tubes. When they measure 3.15V AC, note the voltage from the variac. Thats your target for proper operation. For example, my '71 100W Marshall needs to be at 112V AC.

That amp has a B+ of 475V with proper AC input. That voltage increases by 4.25V on the tubes for every additional volt AC input. With 120V in that's 509V on the tubes. Far too high for modern tubes. Their relibility plummets over 500V.
Even at 490-500v the screen grid current increases to a point that begs for failure.
In places like Nashville, for example, where line voltage routinely runs up around 125-128V you can imagine the consequences.

If you use a variac, you should measure the actual voltage being output. A variac doesn't lock in a particular voltage. It only steps it down (or up) at the ratio you dial in.

George
 
AXEnGEAR4J

AXEnGEAR4J

Senior Member
Messages
5,898
phorties said:
Hey guys, first post here.

I've got my hands on an old marshall plexi 100 watt head and I heard through the grapevine that it's better to run those old amps off of 110 volts. When i checked my wall plugs they read 125 volts. I feel like I am going to fry this thing unless I have some way of regulating the power back to 110. Is there a unit I can use to do this? Especially something I can take with me to gigs.

Thanks,
Matt
Click to expand...


Owning many different vintage amps I and others have used the Stayco's or alike. Find them on ebay for $70-$100 used.
 
riffmeister

riffmeister

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16,815
Muffdriver said:
Okay, sounds good for the op. But, can you run this anywhere below 110 by disabling the feature that automatically kicks it back up to about the 110 level? If you could would that in effect make it a variable ac transformer? Thanks
Click to expand...

Maybe a voltage regulator followed by a Variac for this.
 
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topboost

Member
Messages
302
I had my tech friend build a step down transformer using a hammond transformer he had lying around. 5 and 10 v drop options. I use it on my filmosound and my old topboost. When I saw all the wax coming out of the transformer in my ac30 I wanted to stop it or at least slow it down. Voltage here is usually around 122V.
 
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SBax

Member
Messages
349
Muffdriver said:
Okay, sounds good for the op. But, can you run this anywhere below 110 by disabling the feature that automatically kicks it back up to about the 110 level? If you could would that in effect make it a variable ac transformer? Thanks
Click to expand...

No.
 
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drbob1

Gold Supporting Member
Messages
30,421
riffmeister said:
Maybe a voltage regulator followed by a Variac for this.
Click to expand...

This would work well, although I'm not a big fan of "browning out" amps, because I'm concerned about the results on relatively expensive tubes (lots of old glass in my amps) of running the heaters at low voltage.

Just to go over it again, a variac outputs the voltage on the dial if the input voltage is the same as it's rated for (so, a 120v variac will output 110v when it's set to 110v). Since wall voltage varies around here from 122-128, 110 may actually put out as much as 118v (measured from my variac). So, you need some sort of way to measure what the actual voltage you're putting out is, or you need a voltage regulator before the variac.

This is most important with a few of my amps: my Orange OR80 goes from the spec 470v on the plates to over 520 on wall voltage: too much for the Siemens EL34s I have in there. Also the 65 JTM45 with Genelux KT66s just sounds wrong any higher than 114v.
 
riffmeister

riffmeister

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sg~guy

sg~guy

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I cant believe this topic keeps coming up,..........OH WAIT!!!!....this is THE GEAR PAGE!!!!!,...(the last post on this subject got deleted),...

<[email protected]&-a-ARACOM-----<<<<<

<----old--&-an-electrician--never-even-heard-of-an-amp-frying-from-(+/- 10/15v)---
 
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