Using 400v cap instead 600v ... repercussions?

[Phil Harmoneeek]

So, I want to try a .02 400v paper in oil cap (that I got for my guitar tone cap) in the first coupling position (after input & before Volume) of my Vicky LP Tweed Twin. The cap in there right now say 600v (as does the schematic). What are the issues with doing this ... ? Don't want to fry anything.

Thanks Randall

[JimiB]

no issues - no way you have more than 400v there.

[Phil Harmoneeek]

That's my thoughts, although I'm just a hack modder ... anyone else care to weigh in ...

Thanks Randall

[JimiB]

Do you have a DMM?
According to my schematic you have 145vdc on that cap. If you have a DMM you can measure the voltage for yourself but I guarantee you you dont have +400v there. If you do the amp isnt working

[Phil Harmoneeek]

I do have one, what points do I hook it to, to measure, just across the cap? (like I said ... hack).

Thanks Randall

[jamme61]

I'm a hack too but it's a good to know this. ground the black lead of the DMM to the amp, (Chasiss is what I do) and then what ever cap you want to measure put the red lead, (from the dmm) to the lead of the cap. If your Dmm has a hold feature on it, use it, it will beep when it gets the reading and hold it for you to read. Just becareful when doing this, high voltage will kill you. make sure your probes on your DMM are rated for the high voltage in your amp, I would think your fine but, make sure. reading the voltages in your amp, is a great way, to make sure it's running good and also for tone tweaking IMHO.

[JimiB]

you would measure dc volts from the cap to ground on the tube side. You can however electrocute yourself with an amp that is turned off and unplugged. Measuring volatges in a live amp is very dangerous. Do so at your own risk and with one hand behind your back.

:agree

[Phil Harmoneeek]

Thanks guys ... got it about the being careful ... I've had a bit of experience in that regard.

Thanks Randall

[Axekisser]

Ah common! You haven't lived until you measure voltage in bare feet on a damp basement floor!! Just kidding! Keep one hand in your pocket and wear safety glasses!!

[Phil Harmoneeek]

Ok, it measured 197v ... looks good, so I'll try out the other cap out later.

Thanks Randall

[Ronsonic]

Here's the other angle: You won't have over 400VDC there when the amp is assembled and running. But I'll bet you do if you power it up with the tubes pulled or if you fail to use the standby switch to allow the heaters to come up to temp before applying power. Something to remember.

That is why you see that sort of voltage derating on caps. The manufacturer has to count on things like that happening.

[Phil Harmoneeek]

Now that is interesting ... so the tubes take the voltage away as the heat so under the conditions you described above more voltage would hit that cap ... that much more? Any more discussion on this?

Thanks Randall

[r8burst]

I guess my question is, why do you want to do this? Seems to me one single cap in a coupling position early in this circuit is not going to make any difference that you 'll be able to detect. In fact, a component that's unlike any of it's brothers in the amp might, if anything, do more harm than good tonally.
I'm as much a tweaker as anybody, but there needs to be purpose to the tweaks. Replacing a single component with one of comparable value seems like just an exercise in soldering.

Just asking:BEER

Tony

[Ronsonic]

Now that is interesting ... so the tubes take the voltage away as the heat so under the conditions you described above more voltage would hit that cap ... that much more? Any more discussion on this?

Thanks Randall

Look at your power supply. Basically a series of resistors that drop the +400V at the power transformer to the lower voltages you see in the preamp - and then finally down to the voltage you saw at the junction of that tube and its plate resistor. How's that work?

We've got Ohm's law telling us that the voltage drop across each resistor is going to be equal to the the current flow through that resistor times the resistance. With the preamp tubes missing or not up to temperature, there is no current through the circuit and therefor no voltage drop.

Go ahead and measure if you like. Caps don't generally just blow up or anything stupid when they are over-voltaged moderately, generally.

it would also be well to measure the voltage not only at the plate of the tube associated with that cap, but also at the B+ side of the associated plate resistor. If you drive that tube to full power it will reach cut-off at which point it won't be conducting, again, no current = no voltage drop across that resistor = the highest voltage that cap will see during normal use.

All that said, your cap will probably be fine, probably.

[Phil Harmoneeek]

Thanks Ron (& all others) ... I will make some measurements over the weekend to see. I did try out the new cap & like it VERY much as it brings in some mids that I was trying to get (was debating different ways to achive this & now it seems I got just what I wanted). I know many people are sceptical about cap changes with the same value ... I have heard it first hand many times, even from the same brand/same value! I know ... crazy ... I'm satisfied by my experiences.

Thanks Randall

[RedMan]

Look at your power supply. Basically a series of resistors that drop the +400V at the power transformer to the lower voltages you see in the preamp - and then finally down to the voltage you saw at the junction of that tube and its plate resistor. How's that work?

We've got Ohm's law telling us that the voltage drop across each resistor is going to be equal to the the current flow through that resistor times the resistance. With the preamp tubes missing or not up to temperature, there is no current through the circuit and therefor no voltage drop.

Go ahead and measure if you like. Caps don't generally just blow up or anything stupid when they are over-voltaged moderately, generally.

it would also be well to measure the voltage not only at the plate of the tube associated with that cap, but also at the B+ side of the associated plate resistor. If you drive that tube to full power it will reach cut-off at which point it won't be conducting, again, no current = no voltage drop across that resistor = the highest voltage that cap will see during normal use.

All that said, your cap will probably be fine, probably.


This is basically the point I was going to make except full power of a tube is saturation not cutoff. The other thing you have to watch for is the voltage on the other side of the cap. In this case it's probably close to zero but in the case of a PI coupler fixed bias it culd be another 50-60 volts that you would need to add to the equation.

[Ronsonic]

True, it won't get quite to cutoff, but close enough that we don't want to ignore it.

How close depends on the amp and how that stage was designed.