Coupling caps 400v

[joesatch]

plexi circuit. I ordered some Mallory coupling caps .022 uf . I accidentally ordered them in 400v instead of 630v.  use them?

[sluckey]

no

[joesatch]

well i used them. only up to V2. hopefully no issues.

[EL34]

There is no problem using 400v coupling caps if you are not exceeding the working voltage.
Mallory 150 caps are used in lots of amps


I have sold 1000's of them over the last 25+ years

[joesatch]

it looks like after some mods i only have 3 in place.  i have the V1b  (B+5) 400v coupling cap and two after the slope resistor (tone stack).  no issues?

[pdf64]

... i have the V1b  (B+5) 400v coupling cap ...

Rating may be exceeded if you power up without all valve sockets populated with functioning valves, eg when testing, valve failure.

[joesatch]

[EL34]

... i have the V1b  (B+5) 400v coupling cap ...

Rating may be exceeded if you power up without all valve sockets populated with functioning valves, eg when testing, valve failure.

Yes, if you do not have a load attached to the power supply, the system voltages will be way higher than when current is running through the circuit.
You will have voltage drops across the loads with the valves in place

So take that into account

[sluckey]

Hence my short answer in reply #1. 

[HotBluePlates]

Agreed.  Nothing wrong with 400v Mallory caps, but it is always wise to assume the worst-case will happen & rate parts accordingly.

[jordan86]

I pondered this question recently as well...applicable here...

If I have a coupling cap off stage 1, and then a tonestack before the next stage/volume control/etc....theoretically...there should never be any high voltage B+ Getting in the tonestack section, yes?

Was considering whether to use a 400v cap vs a 600v. I preferred the tone and construction of the 400v orange drop.

[shooter]

yes

correct, the "left side" will be at plate VDC, the "right side" will be at ~~ 0
the problem, when the tubes ain't drawing any current (at power-up) the left-side, might exceed the 400vdc, the cap will fail, then all the left-side stuff will leak to the right-side

[EL34]

It's very easy to see what voltage is across a capacitor with the valves in or out

Just clip on some leads across the cap and fire up the amp
Get ready to shut down the amp if the voltage is too high across the cap

Maybe the phase inverter caps are the only place a 400v cap may not be good to use
Hard to say because every amp is different

This Champ would not be a problem with the valves in and running
https://el34world.com/charts/Schematics/files/fender/Fender_Champ_5E1.pdf

This Twin Reverb looks to have 235 volts with the valves in and operating
https://el34world.com/charts/Schematics/files/fender/Fender_Twin_Reverb_AB763_layout.pdf

[sluckey]

If I have a coupling cap off stage 1, and then a tonestack before the next stage/volume control/etc....theoretically...there should never be any high voltage B+ Getting in the tonestack section, yes?

Was considering whether to use a 400v cap vs a 600v. I preferred the tone and construction of the 400v orange drop.

If you pull all the tubes in the amp (not the rectifier) you will see very high B+ voltages everywhere in the amp, even the tonestack if it is plate driven. Try it and see for yourself. Do you really want that risk?

I always use 630V coupling caps and don't worry. I could get by with just 500V caps in everything I've built but 630V caps are easier to find and cheaper. If you want to play the numbers then multiply the PT HV secondary voltage by 1.414. That is the maximum unloaded B+ that will be inside the amp. Then choose a higher voltage rating for all couplers. You'll always be safe.

[pdf64]

…
This Twin Reverb looks to have 235 volts with the valves in and operating
https://el34world.com/charts/Schematics/files/fender/Fender_Twin_Reverb_AB763_layout.pdf

Bear in mind that the other side of those caps is at the bias voltage, -52V is noted. So 287V across the cap 

[joesatch]

[PRR]

> If you pull all the tubes in the amp

Also when the rectifier warms up before the rest of the tubes, all DC voltage soar. Some of the lazy rectifiers, hardly enough to see. Fast-heat bottles, you get a few seconds at zero, a fats rise to very high for 1 or 2 seconds, and then down to nominal. Silicon rects, FULL over-voltage for 9 seconds.

Saying there is a coupling cap in front just means you have to do tedious analysis on how the charge splits as the B+ rises. If you gonna make a million without losing your shirt, maybe worth the design time. Not for us.

The NEGative grid bias is a very good point often overlooked. Small but may be the camel's straw.

If a grid cap fails the next tube will try to run MAX current. With plate resistor, it may survive (penny-pinch factory 1/4W resistors may smoke). Transformer-loaded the tube will melt-down, unless the transformer smokes first. So it can be unwise to "barely enough" the caps.

[dwinstonwood]

Now I'm concerned about the F&T 50uF/50uF 500V reservoir can in one of my amps that has 472VDC on the 6L6 plates and a SS rectifier.

F&T rates it at 550V "Surge Voltage." Maybe I should have used two 100uF/350v's in series?

https://www.amplifiedparts.com/sites/default/files/associated_files/c-ec50-50-500ft.pdf

[sluckey]

Now I'm concerned about the F&T 50uF/50uF 500V reservoir can in one of my amps that has 472VDC on the 6L6 plates and a SS rectifier.

Well, check the voltage on that cap when you first turn it on from cold start. That'll either ease your mind or send you shopping.

[tmknight]

It seems many amp manufacturers, largely (I've seen some schematics with a mixture (where voltage values are provided)), use 400v preamp coupling caps. Are they cheaping out hoping for the best?  Seems the moral of the story, as has been put forth previously, is it depends on the amp design; use what's appropriate for the aggregate of circuit components.

[joesatch]

i have left the 400v's in the tonestack circuit. cant see them seeing anywhere near that even though the schematic has 630v ones there

[pdf64]

With 5F6a etc based designs, if the valve in the DCCF circuit is bad or not fitted, the voltage on the tonestack caps will be 0V.

So yeah, I didn’t include them in the ‘best to be changed’ list.

[HotBluePlates]

It seems many amp manufacturers, largely (I've seen some schematics with a mixture (where voltage values are provided)), use 400v preamp coupling caps. Are they cheaping out hoping for the best?  ...

You have to provided specifics.  Nothing can be judged on generalized statements.

Which manufacturers?  Which amps?  Got a schematic?

[joesatch]

I've got the amp up and running. all is fine. measuring 175v going to those 400v caps

[sluckey]

I've got the amp up and running. all is fine. measuring 175v going to those 400v caps

Yay! Now pull all tubes (except rectifier) and measure that voltage again.