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Rheostat ratings

3/23/2005 3:40 PM
Rheostat ratings
A buddy of mine offered to sell me a pair of transformers that I'd like to use for an amp. One transformer has taps for 5V and 6.3V. The other tranny is rated 450-0-450 @ 250mA.  
I would like to be able to adjust the incoming AC voltage to the second tranny for different plate voltages. While my variac would be the easiest way, I'd rather the amp have its own rheostat for this adjustment. I checked out Mouser's catalog but I'm confused by the ratings. I'm not sure what is required. Figures are given for watts, ohmic rang, volts, etc. I'm not up to snuff on the required theory/equations to figure out exactly what I need.  
Can anyone help? Thanks.
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3/23/2005 10:01 PM

A rheostat is not a variac. A variac is a brand name for a variable tap autotransformer. It is a transformer that you can adjust the output voltage from.  
A rheostat is nothing more than a variable resistor. Adding it into a transformer circuit would not be a very efficient way to achieve your goal. They do look a bit like variacs.  
There are small variacs. You don't need a 5 amp one for this job.  
You would be better off rectifying the 450 and then adding a regulator to adjust the voltage.
3/24/2005 8:35 AM

Thanks for reply. What would be an ideal regulator (specs) for my application? Can I insert the regulator between ground and center tap to control both legs?
3/24/2005 9:14 AM
Wild Bill

Dynaman, it doesn't sound like there's a simple bolt-in solution to the idea you have in mind.  
If I understand what you've written correctly, you have a separate transformer to supply just the B+. You want to be able to vary this voltage - perhaps to clone Eddie Van Halen's "brown sound".  
A rheostat won't work. In fact, it will blow up! For you to suggest using one implies that you don't have a lot of theory background in electronics. No shame in that - you were smart enough to come here and ask for some tips! :)  
A regulator on the B+ side is probably not for you. There is no bolt-in device, to my knowledge. You would have to build a complete regulator circuit which would probably use at least half as many parts as the whole amplifier. You'd need room to fit the circuit in the chassis, as well. Because it's a custom application there's not really any simple parts made to do the job - the manufacturer might sell 3 units per year so it's not worth his while. You have to "roll your own" solution, designing the circuit and soldering in all the parts. A knowledgeable friend can't give you a 10 second answer. He'd need to spend some hours to figure out a design and then even if you were to build it yourself he's probably have to spend more time teaching you than wiring it himself. That would certainly cost you a lot of beer! :)  
So that leaves using a variac on the primary side of the transformer. A variac rated for 2 amps or better should be beefy enough for your application. it will adjust the input voltage to the transformer and the output voltage will track it accordingly.  
Like always, there's a kicker. Most of the lower range of the variac will not be useful. When the amp B+ goes much below 350 volts the volume will drop like a rock. So the useful range may be only the top 25% of the variac's rotation.  
This is the sort of project that is great for a guy who really knows his electronics and doesn't care how long it takes or how expensive it is. He also won't be disappointed if it doesn't do exactly what he wanted when he was finished.  
A "B+" dial on the front of the amp is not gonna give most players a desirable option. For the cost and aggravation you get the most bang for your buck by using things like variacs during inital design to find the B+ level you like the most and then choosing a transformer and/or playing with the rectifier circuit to run the tubes at that voltage in the finished unit. If you then want a different sound build a different amp! :)  
Not meaning to rain on your parade but that's how things look to me.  
---Wild Bill
3/24/2005 12:30 PM

That's exactly what I had in mind. The amp would basically just be a test mule for me to avoid the hassle and expense of trying different power trannys. The huge ugly purple cab it's going in isn't something I'd want to see on stage. I just thought it might be nice to haul the amp from my basement to the practice spot without messin' with a separate variac.  
You're right! I've absolutely no background in electronics other than assembling a few amps. Thanks for the reply.
3/24/2005 2:06 PM
Dr. Photon

I second wild Bill's suggestions.  
You could use a variac (only needs to be good for a few amps) on the 120 volt input to the plate transformer. The advantage of having a seperate transformer for the high voltage supply and annother one to power the tube heaters (6.3 volts) and the rectifier tubes (if used tube rectification) allows you to diddle with the voltage on the plate transforemr without having to worry about the heater voltages being off. This also means you can use a smaller variac since the heater power isn't going through it.  
The poblem with this is that the control is not very refined. The lower setting of the control will be pretty useless and bad sounding. But it is quite useful to see where you want the B+ for the best sound to your ears. Then you can either make annother amp with this voltage, or take out the varaic and have the amp setup to use this voltage.  
Your other options would be to experiemtn with tube rectifies and diode rectifiers. your transformer has the 5 volt tap needed for tube rectification. SS rectification provides teh most voltage, and the minimum sag. Tube rectifiers are "saggier" (voltage drops under load). The strongest tube rectifier is teh GZ34/5AR4, the 5u4 is in the middle, and the 5Y3 is the weakest with the most sag and voltage drop.  
You can also add resistors and zener diodes. A zener diode acts like a "magical voltage subtractor". It lowers the voltage by an exact amount of volts, and does not introduce any sag. A power resistor could also be used, but the voltage reduction is dependet upon the load drawn, and it adds much sag. Some amplifiers use a lower voltage transformer and a resistor with solid state rectification to emulate a tube rectifier.  
The issue with the tube rectifiers, zeners, and the rresistors is that they are fixed. The most adjustability you have with them would be to select several different ones with a switch.
3/24/2005 4:17 PM

Thanks for the reply. Others have recommended zeners and resistors also but I liked the idea of having the instant adjustment of a variac.

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