Who's using a variac?

swankmotee

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I'm about to bite the bullet and pick one of these up because my wall voltage at home is 126 and it makes my amps run way to hot! Don't like the sound at all and these babies were designed to run at optimum around 110 to 112. I need to bring the plate voltage back into range since most of mine are in the 550 range right now. Just wanted to ask supafixer and anyone else where they set the voltage for their rigs to get the optimum tone and longer lifespan. Thanx for any commentary on the subject! :)
 

superleadfixer

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I would get a Furman power conditionner first, get the one that has the multi voltage settings, 120, 115, 110volts etc...

As for the variac that could work also but make sure to use a good meter and double check with it the actual voltage out the variac, you could find some are off by 5-10 volts....

What amp do you have?

One other thing is to try to maintain your heaters voltage at 6.3VAC, by reducing your voltage with a variac to 90/100 volts like lots of people do, (don't go lower then that it is not gonna do anything good...) your heaters will drop also below 6.3VAC and tube cathode shredding will occur meaning a tube life reduction.....

You could always use a seperate VAC transformer for your heaters only to keep them at 6.3vac.....

Lowering your B+ or plates will smooth out the tone a little, a nice target is having a B+ of 460VDC, but remember when reducing with a variac everything is, so make sure to rebias your tubes each time you raise OR reduce your variac voltage output......

Make sure your variac is rated at least for 10amp current handling minimum.....
 

swankmotee

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Variacin'

Fixer,
I agree on all points you mentioned but curious minds want to know... where do you like to run your voltage? I'm only looking to bring the voltage back down to the "normal" levels that the amp was designed around, ie 110 to 112, and that should keep the heater voltage well within the proper range. I'm definitely concerned about lenghting my NOS 70's RFT EL34's lifespan because they don't like high plate voltages so bringing in below 500 volts on the plates is my main concern. Also would be nice for allowing them to be baised in a more optimum range so I could get more tone out of them. I have definitely given the Furman piece consideration since I want to run multiple amps in performance off of one unit and that brings up the amount of current it can handle. I was looking into an older variac rated at 15 amps which could probably cruise with a couple of 100 watters hooked up. Also, I do like the flexibilty of slowing bringing up power caps when I install new ones so a variac makes alot of sense. Anyway, any expertise and commentary you have will be greatly appreciated and the heads in question are 3 73's and 68' & 67' Plexi's that all are running over the 500 mark!! Whew! thats alot of juice!!!!Keeps the back room nice and warm in the winter!HA! Thanx!

Tommy Hatcher :)
 

superleadfixer

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You might have a solution within the amps, specialy your plexis.... Witch one(s) has the laydown power tranny? Normaly your 67 and 68 do have them. Those PT have dual tap secondaries normaly so you could just swap the highter AC (you must be using the higher voltage ones now...) to the lower AC ternimals on your PT.. Normaly with a 110-115VAC from your wall outlet and using the higher voltage secondaries your B+ should be around 490VDC, lowering to the lower voltage secondaries your B+ should go to 460VDC, you could go inbetween also (one higher one lower) for a B+ of around 475VDC, so with those posibilities you could choose the one you prefer, you must rebias everytime you do that and in some cases you will have to adjust the bias voltage feed by increasing a resistor value in that line as you could not have enough bias voltage with the lower secondaries from your PT....Simple procedure I'll walk you through it if you need help.....

Your RFT el34 are rated at 450 max plate voltage but many are running them like you do at 500+ without problem, but I agree you should go down at least below 500VDC.

As for the tone well the lower B+ the smoother and more open the fell of your amp will be, I prefer it at 460 even a little lower then that (for me). For a more aggresive somewhat harsher vibes well stay high....

Make sure to rebias and try all variations of B+.....

As for your other 1959s well unless they are equiped with the same type of power tranny as your plexis, the only way to reduce it is to use a variac OR you could always install a power scalling mod from London power, (Kevin O'connors's stuff), that will unable you to adjust any B+ voltage with a pot that you wish, like a volume control...very nice. That mod automaticly rebias the tubes so they stay at there sweet spot, around 70% plate dissipation......
 

Distortedskies

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You have to be very carefull using a variac because of those heater voltages, I would highly reccomend using a seperate filament transformer that is run off the 120V line. You don't want to change the filament supply by more than 10% either way so if you have anywhere between 5.7V and 6.9V you should be alright but if you are out of this range you will drastically reduce the life of your tubes and most likely your tone will suffer. Those RFT's should be able to handle 500V on the plate no problem, the EL34 is rated at 800V on the plate, while alot of new tubes can't really handle this the older ones will and I've had good luck with the new production JJ's which I have run at over 600V without any problems. The problem with going to high on the voltage is not that the tube is run harder its that it will internally short between elements due to the high positive potential. As long as your bias is set correctly a tube run at 500V won't be worked any harder than one at 400V.


-Matt at Triodeelectronics.com
 

swankmotee

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Stepdown tranny

A follow up....... I build a lil' poor man's variac, well just a step down transformer that brought my wall voltage down to 115 and all is well. Everything soundz alot better and much more stable with no glowing from the EL's. Hal-a-lou-ya!!!!!!! I recommend everyone who's owns an old Marshall do the same because the tonal difference is worth it!!
 

superleadfixer

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Re: Stepdown tranny

swankmotee said:
A follow up....... I build a lil' poor man's variac, well just a step down transformer that brought my wall voltage down to 115 and all is well. Everything soundz alot better and much more stable with no glowing from the EL's. Hal-a-lou-ya!!!!!!! I recommend everyone who's owns an old Marshall do the same because the tonal difference is worth it!!

Just check your heaters voltages, at 115v you should be ok, but just in case, if your below 5.7/5.8, I'd increase the wall voltage till you have at least that minimum or your tubes won't have a long life, at 5.5 they won't last a month, below that they won't last a week....if you play dimed of course....saw a 4.9vac :cry: once the tubes didn't last a day...lol :(
 

superleadfixer

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superleadfixer said:
Your RFT el34 are rated at 450 max plate voltage but many are running them like you do at 500+ without problem, but I agree you should go down at least below 500VDC.

The 450vsc on the plates are the manufacterer recomandations (Siemen) specs....all el34 are rated at 25watts and you want to bias them around 60-70% of that.......
 

NewReligion

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Blast from the Past.

Outstanding thread. Much learned.

David :cheers:
 

Micky

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I use 2 variacs.
I have a smaller one like the ebay link shows, and I have a custom built rack mount unit. Digital displays and everything...
 

RickyLee

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I use 2 variacs.
I have a smaller one like the ebay link shows, and I have a custom built rack mount unit. Digital displays and everything...

That one in the ebay link is small? LOL

Wow. Your rack mount one must be quite heavy?

My old Powerstat VariAC's look to be close to the size of that ebay one, and are quite heavy.
 

Quasar-Kid

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My point about standard U.S. voltage being 120 is...
The utility company supplying your home with electricity is attempting to provide 240 Volt utility / spot on 240 volts is the goal...
Your panel will then divide that to a pair of 120 Volt legs

Voltage drop will effect the actual voltage in you home
You can use Ohms Law to determine the voltage drop from the Utility transformer
 

Micky

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Yes, both are heavy.
Except 1 is 18 inches wide.
The smaller one is 8 inches or so...
 


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