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Power conditioner or variac?'s


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I was wondering if the forum could pass on some advice. I tested my wall outlets and I'am getting readings between 123 and 124.5, this seems to be a little high, and I dont want to damage my recently reconditioned tube amp. The sams photofact recommends 105 to 120 volts. Would I be best served by using a variac or and adjustable voltage power conditioner to solve this problem. Or should I not worry about it? Thanks for any help!

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it is something to worry about. if you have , as in your example, +4 volts above your amps recommended maximums, in most tube amps, that translates to 16V or more plate voltage. Your bias settings could become unreliable if you adjusted them when wall voltage was low, and most of the time the wall voltage is high. Options are Variac, or and ISOTAP. ISOTAP is a box with preset voltages. So if your outlet runs 120V and you want 117 volts, you can just plug it into the 117 volt plug. In the 117 volt plug, your amp would always run below wall out let. Just make sure your ISOTAP can handle the power rating of the amp. You might need two for dual mono's. Now the problem with voltage regulator power conditioners is it will be hard to find one that regulates to the wall voltage you need. I have one that costs 1200 when new, but regulates to 120. It would be great if I could adjust that to say 117, but thats not the case. I have two setup's that use a variac to power up. Variac's are cheap .


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Units can vary as to thier tolerance for excess voltage. In some
cases the 5-10 extra volts are an issue, sometimes not. I do tend to
prefer knocking down voltages for these vintage units that are most
comfy in that 110-115V area though.....just seems to be the thing for keeping them comfortable - and they ARE vintage units so keeping them in the sweet spot is probably a good thing.

I use a unit called a "VIZ Isotap" on my McIntosh MX110, because it is a preamp from that earlier era (50's-60's) where units were optimized for 110-117V use. My power can also run a few volts high here (122V or so), and that has my tranny on the 110 running a bit warmer than necessary. The "Isotap" allows me to knock the voltage down to the 110-115V area, where the 110 runs cooler.

The Isotap is great for lower current draw devices. On monoblock tube amps I'm not so sure. I use a variac on my MC30s, but not because of high voltage (there are adjustable taps within for 117 and 125V use so that's solved), it's that I don't like all 120V hitting the amps at the same time - so I prefer to bring them up slowly.

Might be worthy of a bit of investigation to see where your amp is happiest in terms of wall voltage.

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On monoblock tube amps I'm not so sure. I use a variac on my MC30s, but not because of high voltage (there are adjustable taps within for 117 and 125V use so that's solved), it's that I don't like all 120V hitting the amps at the same time - so I prefer to bring them up slowly.

Those Isotaps look like a neat solution, but probably not needed for something vintage that's been rebuilt with a beefed up power supply. Just guessing cause I'm not that technical. I did however, take a glance at the Mac MC30 schematic and it's tube rectified with four caps in the power supply. The tube rectifier should giv e you a "soft start". If you feel you need a more robust solution it can probably be adapted to take a 5ar4 instead of a 5U4, for an even easier start.

I talked with Mike Beaser (Mike BSE2A3) about this several times and he feels starting with a variac at too low (ie below 90 volts setting can actually do more harm than good . Granted I am not sufficient technically proficient to enough to fully understand what he was telling me. I seem to recall he was referring to heater voltages, smoothing caps etc.

Given his experience with this stuff I'm inclined to stay away from variacs to bring an amp up to speed. But for voltage regulation I have mixed feelings. I have one but don't use it because my amp's been rebuilt.

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Excellent post Audible Nectar. Though, I've never experienced any problems with 125v. If the op had his amp recently rebuilt as stated, then it should be completely up to spec which would include operating at 125v with no problems. As an electrician I see varying voltages everywhere, but this is no cause for concern as our electrical devices are designed to operate within these variations. Yes, transformers will be warmer to the touch but the difference is negligible and I doubt it would shorten its life. As a guitar player, I've had many old tube amps that were completely original and had been played in many a honky tonk with questionable electrical services and grounding systems... 60+ years later those amps are just fine. I've never plugged my amps into anything more than a wall outlet, no problems here.

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Thanks guys for all the helpful information, I think I will start off with a variac because I can use it for other things also. Now wheres the local variac store when you need one??? Thanks once again gentlemen!!!

Ebay has a bunch. Variac is the name trademarked by General Radio, which is no longer in business. Other generic terms are "Variable Autotransformer", "Autoformer", "Variable Autoformer", "Autotransformer".... Manufacturers include Staco and Superior Electric. Make sure you get one rated for 50-60 Hz and enough power for the intended load. Some are rated only for 400 Hz operation and will not work at 50-60 Hz. I'd recommend getting one with built-in fusing and oulet(s). It'll save you the hassle of building an enclosure and wiring.

Some things to watch out for if buying used:

  1. They have a brush that slides along a transformer winding. Sometimes the brush needs to be replaced, so buying a unit that is currently manufactured is recommended, so you can get repair parts.
  2. Sometimes, if the variac was overloaded, the winding will get damaged at the spot where the brush was at the time it was overloaded, quite often at the 120V in/120V out position.
  3. They are pretty heavy, so beware shipping costs.
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