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DIY variac/line browner

plexikiller

Member
Messages
98
I was interested in building my own safer version of using a variac to brown out the power supply. Rather than having a rotating part that your drummer could kick and send your amp up in flames, I wanted to mount a simple power transformer in a box with taps for 110, 90V, etc... I found this Hammond that has the appropriate taps:
http://www.mouser.com/Search/ProductDetail.aspx?qs=3GbUB62Nf7cdaEx96r5amQ==
My question is, what VA rating should I be looking for? I found a general approximation of using 1.67xpower. Assuming they mean peak power in that equation, and for a 100 watt rms amp, 100x2^(1/2)x1.67=236... So the above transformer should be fine, right??
I don't understand why some people run 10A variacs. Seems like a bit overkill??
 

Ed Reed

Senior Member
Messages
7,516
Just to be saying. I am NOT a tech but as I understand when volts go down current goes up so running a variac or other device at low voltage can fry your amp. When guys really do that there is a mod done to add another power supply to the heaters so that when you crank down the volts the amp will live. Hope this helps.
 
Messages
2,704
For sizing the isolation transformer I think you'd be wiser to use the power rating of the power transformer, not the amplifier output. If your amp doesn't have a plate on it indicating its power draw (not output), then try approximating by taking the fuse rating (in amps), and multiplying by the line voltage (120V). Then go 1.5X that size (to cover losses). At least, that's what I'd do...

Consider a separate heater supply for your tubes...running the voltage too low could lead to cathode stripping, and early tube death.

Cheers

Kris
 

plexikiller

Member
Messages
98
Yeah I wasn't going to run it at 90V on a regular basis, if ever. More like 110, which is supposedly what my amp wants anyways. I will have the option there, if I do decide to go EVH style. 250 watts would be a safe approximation for the rating of a 100w Plexi power tranny. Correct me if I'm wrong. That would mean.... a VA rating of like 420??
 

plexikiller

Member
Messages
98
Ya thats exactly what I had in mind, only a switch for multiple voltage taps. I experimented with an old variac this weekend and did NOT sound like EVH upon lowering the voltage to 90 (only for a minute). I think I'll just build something more along the lines of Keen's design. thanks
 

plexikiller

Member
Messages
98
I've been considering the Weber unit for some time but it only goes down to 105V. Now I'm doubting that I even want to go lower than that so it is definitely a great option. Thanks for reminding me! I hear so many mixed things about low voltages frying amps. I think it really only shortens power tube life, which is not a serious issue to me. However I'd rather not damage my amp somehow. I really wish there was an official reference for this.
 

booj

Member
Messages
172
Just try it at a gig. If you get done at the end of the night, and can touch your transformer for more than 5 seconds, your alright. Any hotter and you might consider a larger transformer.
VA to me means volts x amps. Power means volts x amps. Power consumption of your amp should be available on its pack panel close to the fuse. Or fuse current x 120 wold be a good guess.:horse
 

sampleinajar

Member
Messages
1,258
Another option is the Weber B+ dropper that uses a Zener diode. This, in theory at least, is what you want since it will leave the heater voltages intact, but drop the voltage to the plates.

Steve
 

PremiumPlus

Member
Messages
1,144
If you don't need it adjustable, this would be a lot cheaper:
http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm[/quotetransformer
I did it both ways, the zener on the center tap of the transformer of the amp, and the RG Keen way. I ended up taking the zener back out of my amp, and I built a box with all the parts sourced from Radio Shack. A 12.6 volt transformer with center tap, 3 amps, and a dpdt switch gives me 117 and 110 volts ac, starting with around 125 line voltage. It works for all my old amps. The idea has been around for a LONG time, I remember learning it in school 40 years ago...and this really sounds great.
 

PremiumPlus

Member
Messages
1,144
If you don't need it adjustable, this would be a lot cheaper:
http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm
I did it both ways, the zener on the center tap of the transformer of the amp, and the RG Keen way. I ended up taking the zener back out of my amp, and I built a box with all the parts sourced from Radio Shack. A 12.6 volt transformer with center tap, 3 amps, and a dpdt switch gives me 117 and 110 volts ac, starting with around 125 line voltage. It works for all my old amps. The idea has been around for a LONG time, I remember learning it in school 40 years ago...and this really sounds great.
 

plexikiller

Member
Messages
98
Is a B+ dropper easy to install? I understand the general concept, I think, of using the diode to get a decent voltage drop going to the plates.
 

booj

Member
Messages
172
That would work. Heres a datasheet of 50 watt zeners of varying current capabilities. <Jhttp://www.mouser.com/catalog/specsheets/DO5%20SPEC.pdf> just add an insulatied heatsink. Hmm. That sounds a bit diffucult. What to use for a heatsink? You'll need a heatsink. You'll need spacers. Hey I could design a kit for ya. Oh yeh Weber allready has one. Never mind:jo
 




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