Capacitors for a Solid State pre-amp, or how high of volts can ya go? ;-)

[Dennis Perusse]

Hello everyone,

At this time I'm slowly putting together a pre amp for a Vox UL710/UL715 that I am building. Now, for those who want to know, this was a hybrid amp that uses a solid state pre-amp but a tube power amp section. This pcb board I got from RG Keen years ago and now I am finally starting to get on with it and put it together. This amp circuit came in many different flavors of wattages and Vox Showroom has all the awesome details for those who want to read about them. Where I am a Beatles fan and love the albums Rubber Soul, Revolver, and Sgt Pepper, this amp is one I really like and have wanted to build.

Now my dilemma is this. RG Keen instructions have capacitors that have some serious low voltages than what I am used to building with apart from guitar pedals. Can, for example, I use a 2.2 uf cap at 25 volts even though it calls for a 2.2 at 16 volts or are solid state circuits in the case of pre-amps more fickle in terms of handling voltage? I say this since I have a lot of caps that have standard values that are over what some of his caps are stating to use but if I have to order exactly what he states than I'll just go on mouser and order what I need. Since I can't get in touch with RG I'd like to have this question answered before I put stuff in onto the pcb. Any help here is greatly appreciated.

 

[Timbresmith1]

You can sub a higher voltage cap where a lower voltage is called for.
There may be size or fitment issues you’ll need to address if lead spacing is off, or if there’s a case/cabinet involved. Otherwise, as Devo said, “Go forward. Move ahead.”

 

[printer2]

There was some controversy about ten years ago on a high voltage cap being used in low voltage. I said there was no problem. I showed that there would not be, used a 350V cap and a 6.3V cap of the same 22uF value and measured gain of a 12AX7 stage with the capacitors used as cathode bypass. Both caps measured 22 uF, the gain was the same.

"I wondered how much I could see if I scope two different capacitors with the same microfarad ratting but different voltage ratings. I wanted to use standard Fender values but the lowest value I had was 25V in 22uF, I also have 22uF at 350V. Now being that 25V is much larger than the voltage generated across a 1.5k resistor with a 12AX7 and a 100k plate resistor I went searching through some of my unused audio equipment and sacrificed one and came up with a 22uF at 6.3V. Now this is not really a scientific test, I had a circuit that I was breadboarding and I just swapped out the capacitors I had and used the 6.3V and the 350V. I switched between the two using a SPDT switch and took some picture of the scope. The pictures are of an 80 Hz signal, the first is the generator at the input jack, the next is the 6.3V and the third is the 350V. I am really surprised at the results as I would have thought the 350V cap would look terrible. Test setup/capacitors"

350V capacitor

6.3V capacitor

 

[Dennis Perusse]

Thank you everyone for both reading this and replying to this. This makes it much better for me in the long run as I was kinda stressing on this. I was worried that the pre amp would have issues if I went too far with some voltages where it uses transistors. The tube power section I'm not worried about as that isn't my first rodeo but working with Solid State pre amp is something that I am slowly learning about.

 

[dsutton24]

Way back in early 1500s they used to tell us that you should run electrolytics at 80 - 110% of rated voltage to prevent them from deteriorating. Modern electrolytics don't seem to be as bothered by working voltage, the only real concern is physical size.

 

[printer2]

Way back in early 1500s they used to tell us that you should run electrolytics at 80 - 110% of rated voltage to prevent them from deteriorating. Modern electrolytics don't seem to be as bothered by working voltage, the only real concern is physical size.

The thought was that the amount of dielectric would be reduced if not run at high voltage. Not sure how the insulating layer will know it is at a low voltage.