Use Huge Capacitor for Standby Switch?

[The New Steve H]

I can't find the .047 uF capacitors I bought when I was ordering parts for my 5f6a clone. I need one capacitor between the standby switch and ground. It connects to the filament/cathode pin (#8) on the rectifier.

I don't really know what the purpose of the capacitor is, but from the schematic, it looks like it charges up so you have lots of juice ready to flow when you turn on the output section. If that is the case, is there any reason why I can't use a much bigger capacitor? I happen to have a 0.47 uF lying around.

I also have two 0.022uF capacitors I can parallel.

[diagrammatiks]

wait...i'm not sure if you need that cap.

wait for cliff to chime in.

[ToneMerc]

I don't really know what the purpose of the capacitor is, but from the schematic, it looks like it charges up so you have lots of juice ready to flow when you turn on the output section. If that is the case, is there any reason why I can't use a much bigger capacitor? I happen to have a 0.47 uF lying around.

At first I thought you meant the death cap until I looked at the layout. It's there to suppress switch contact noise(da pop). Generally, .01 to .05 will work.

TM

[martin manning]

Often there is a cap across the standby to prevent arcing, but that isn't what this is doing. It's bypassing the reservoir, so it must be there to bleed HF noise from the power supply and/or plate circuit. It could be left out with no harm done until you get one, or the larger one should be fine. At 0.05uF it won't store enough energy to make any difference... the 40uF reservoir is 800 times as big!

[The New Steve H]

I'm starting to think I should bypass the whole thing. I am trying to understand what Merlin Blencowe says about standby switches. It looks like he thinks they're useless.

On the other hand, my all-knowing DVD guru, Gerald Weber, said something in a video about using the standby switch to prevent some bad thing that happens when you go straight to 120 volts.

[selloutrr]

stripping the tubes .. is "the bad thing"

[The New Steve H]

Now you're bringing it back to me. Blencowe says that's a fairy tale. I went back and found it:

The standby switch usually allows the HT to be turned off while the heater and bias supplies are always on. Contrary to popular belief, the standby switch is not there to prolong valve life-span. The theory is that if the HT is applied while the cathodes are cold they will be 'stripped' by ions crashing into the unprotected cathode. However, this simply does not happen. It is an urban myth borrowed from transmitter and cathode-ray tube technology NOT ordinary 'receiving' valves

[The New Steve H]

Here's more. Makes me wonder if I should have bought bigger caps and omitted the standby switch. I think I have room for bigger ones, but I don't know how big they would have to be to protect the rectifier.

If you're wondering why all those old amps use a standby switch, its because Fender was designing complicated amps on the cheap. In the bigger versions of the Bassman, money was saved by using power supply caps that were rated only for the working voltage, not the peak voltage which occurred before that valves start drawing current.

http://www.freewebs.com/valvewizard1/standby.html

[diagrammatiks]

the standby switch isn't going to hurt anything.

it's not the size of the caps that matters, it's the voltage.

ideally every cap except the cathode bypass caps should be rated 10-25 percent above unloaded b+.

[Phil_S]

Yeah, cathode stripping is the bad thing. Merlin says you don't need the s/b switch. IIRC, cathode stripping is something that applies to old TV transmission equipment, which has a whole lot more riding on it in terms of the tubes and the cost of those tubes, back in the day. Apparently, guitar amps don't have enough juice to rise to that level of concern. After reading several debates on the subject, I concluded that the s/b isn't needed and it's one less thing to deal with in a build. There may be conversations here at AG on this topic. I think you will also find them at ax84.com.

Of course, there's no harm done by have the s/b switch. You choose.

[The New Steve H]

The thing that concerns me is his remark about Fender cheaping out on capacitors. It suggests that if you don't have the switch, Fender's chosen capacitors won't cut it. I don't know what happens when the voltage across an electrolytic is too high. I would think it would just arc, but for all I know, something worse happens.

I have 500V caps in this thing. Am I headed for a smoking mess?

[diagrammatiks]

it's a 50 watt bassman?

merlin's talking how old 50 and 100 watt fenders used 500+ caps for only the first stage...

and then all the coupling caps and the rest of the power supply caps were rated at 400vdc.

without a standby switch all the caps get b+ across them at turn on...which means the caps will eventually fail since they are getting surged every time.

[The New Steve H]

It's a 5f6a, which means two 6L6 output tubes. I think it's supposed to be 35-40 watts.

Schematic for PT: http://www.classictone.net/40-18029.pdf

[boots]

I think the standby switch is a good thing - it can't hurt. I don't know whether cathode stripping is a real issue or not, but hey, being nice to those rare NOS beauties is easy enough to do, so why not use the standby switch?

Has anybody ever tried using a step-start system for the HV? That would be as easy as putting in a thoughfully-selected RC circuit (plus a relay?) with a long enough time constant to allow the cathodes a few seconds of warm-up time.

That would essentially be an "automatic" standby switch, and could be in addition to (in series with) a mechanical standby switch.

[diagrammatiks]

I think the standby switch is a good thing - it can't hurt. I don't know whether cathode stripping is a real issue or not, but hey, being nice to those rare NOS beauties is easy enough to do, so why not use the standby switch?

Has anybody ever tried using a step-start system for the HV? That would be as easy as putting in a thoughfully-selected RC circuit (plus a relay?) with a long enough time constant to allow the cathodes a few seconds of warm-up time.

That would essentially be an "automatic" standby switch, and could be in addition to (in series with) a mechanical standby switch.

pretty much a standard feature in hifi amps.

cathode stripping is bad sure.

but on the other hand if you leave the heaters on for too long with hv you get cathode stripping.

steve you're max unloaded b+ off the 710 winding is 497.