Coupling Cap Voltage Rating Qestion

[silverfox]

In the process of building from schematic, the Clean Channel of a Peavey Road Master. There is an attenuating cap in parallel with a resistor that serves to make up some sort of filter network for the input to the 2nd stage of gain. This network is connected in series with the output coupling cap from stage one. the output voltage, on the Anode, before the output coupling cap is 230 VDC.

My question is: I only have a 100 VDC rated capacitor at the moment to use in the filter network connected after the output coupling cap. Do you think this will be sufficiently rated for this application? From this filter network the next connection is to the grid of the next gain stage.

My thoughts are the output coupling cap on the Anode will reduce the DC voltage to something much less than 100 VDC?

Thanks for any assistance,

Silverfox.

[Gingertube]

http://www.freeinfosociety.com/elect...wPmOqks7zD3-sc

I assume you are talking about C6 = 0.0015uF (15nF).

In this case the 100V rating will be fine because the DC voltage blocking is handled by C4. C4 needs to be at least a 400V rated part but I would err on the conservative side and use a 630V rated part.

Cheers,
Ian

[Chuck H]

And to add to Ians good advice... Since the high voltage DC is blocked by the preceding coupling cap, that second cap in series needs to be rated to handle the AC signal voltage in that circuit. Which won't be 100V

[tedmich]

Ginger's explanation is a good one,another example is seen in the Peavey JSX gain stages 2/3 where the first cap (C3) gets the HV and needs a 630VDC rating while the second cap (C8, paralleled with 470k resistor) can be a 50v NP0 ceramic


The next coupling cap (C4) opts for a more conservative 400v rating likely to trim cost.

[silverfox]

That answers the question

It is actually C44. C44 is in the same position but further down. I'm working on the clean channel and haven't chopped up the schematic yet for posting purposes.

Thanks all that answers the question.

I noticed there are no 68K resistors to filter RF but rather alternate extensive filtering around the preamp sections. Can't wait to hear what this sounds like.

Silverfox.

[bluto]

FWIW something I learned when testing the V1 and V2 plate voltages on my own builds is that when the amp is first powered up you will see close to full B+ on the plates of preamp tubes until the tubes warm up and begin conducting...the voltage then quickly drops off to design level as current flows through the PS and plate resistors (no current flow means no voltage drop)...anyway this prompted be to swap out a 200V rated coupling cap I'd used for a 600V cap. (I understand that the cap in question is for tone but figured what I'd learned was worth a mention anyway)

[R.G.]

Even more pertinent, if a tube heater dies - and it will - or you pull a tube, the plate socket pin is pulled up to B+ solid, while the grid end is pulled down to ground. Worse yet, if all tubes are pulled, which happens, you get unloaded B+. So the right answer is that plate coupling caps need a voltage rating greater than the full B+ at the first filter cap with all tubes pulled, except the rectifier tube.

The currents are limited by the plate and grid resistors, so this may be minimally destructive, especially since modern metallized-foil caps tend to be self healing by oxidizing a small circle in the metallization around the breakover points, but it still isn't good for it on a steady diet. So use maximum B+ with all tubes but the rectifier pulled plus some safety factor you can rationalize.

[Chuck H]

Of course, many vintage amps ignore this and continue working. Many also have leaky caps. While I would agree that for our purposes it's best to design to the max unloaded HV, I would also suggest not using the amp without tubes in it! Sort of equivalent to not driving your car with no tires on it.?.

[R.G.]

... I would also suggest not using the amp without tubes in it! Sort of equivalent to not driving your car with no tires on it.?.

Or letting your dog or your three-year-old on your lap drive your car - both of which I've seen news stories on.

Running the amp without tubes is going to happen in many amps while other problems are being debugged. But yes, pulling your output tubes is an effective mute switch.