Marshall says to switch off the Standby first?

[Humbucker]

I've always heard that when switching off a tube amp, you should simply just turn off the Mains Switch and leave the Standby Switch in play mode to allow filter caps to discharge. I believe this is what I read in Gerald Weber's 'A Desktop Reference To Hip Tube Amps' book too.

I just made a deal to buy a used Marshall JCM2000 DSL50 head, so I downloaded the User's Handbook and when I came across the instructions for the Stanby Switch, this is what was said:
"On switching off, the Standby should always be switched before the Power Switch."

What do you make of that? Maybe it really doesn't matter a whole lot when it comes to powering down?

[Enzo]

It doesn't matter a whit which order you turn them off.

As with ANYTHING else in this biz, if there are two things, someone will go crazy trying to finalize some sort of procedure to relate them, complete with rationalizations for any part of it.

The ONLY difference I could imagine is that one way might pop a little less than the other way. ANd I certainly don't care if the amp pops a little at power down.

[Humbucker]

"It doesn't matter a whit which order you turn them off."

That's what I figured. Thanks!!!

[Rob Mercure]

HB, Enzo, I suspect that the reason recommending that the amp be returned to "standby" mode before the power is turned off is so that when you turn the amp on again the standby switch is in "standby" mode and not in the "on" position.

Rob

[Humbucker]

Good point Rob,

That's probably exactly what Marshall was thinking when they wrote that. Thanks!

[Enzo]

OK, fair enough, but the question was what ORDER to turn them off. Either way they will both be off when you are done. And a lot of MArshalls the standby switch breaks the PT line to the rectifiers. Unless the diodes have a lot of leakage, they will not be discharging the caps any faster than the rest of the circuitry.

I'll restate it: Assuming you will turn both off, it doesn't matter which one is first and which second.

[galibier_numero_un]

Hi all,

you should simply just turn off the Mains Switch and leave the Standby Switch in play mode to allow filter caps to discharge.

This is a provocative comment.

My electronics experience is more in the world of hi-fi amps (I'm having a blast in this forum, btw), where common practice is to install a bleeder resistor to drain the p-s caps on shutdown.

Bleeders don't seem to be as commonplace in guitar amps - unless I've been looking in the wrong place

One common circuit location for standby switches is between the c-t of the p-s secondary and ground (lifting the ground to go into standby). In this position in the circuit I can see that a standby switch left in its on (play) position would help to drain the caps eventually, although I wouldn't bet my life on it

Perhaps someone can comment further on bleeder resistors in the context of guitar amp design? Is their absence this just another holdover from 1950's commercial designs?

I look back at the last century, and I'm amazed at how few people were electrocuted and how few houses burned down.

Thanks,
Thom

[R.G.]

I think that the right way to say it is that the order of power and standby switchoff is determined by second and third order considerations, not primary ones. The quirks of how a standby is done and the rest of the amp circuit make a bigger difference than the standby itself.

Marshall's oddity with the bias and standby is one of the issues. Bleeddown of the power filters is another. There are probably others.

I'm with Thom on the power supply though - for the life of me, I don't understand why amp makers don't put in bleeders automatically. It's just not that expensive. In my previous life designing power supplies for A Major Computer Company we had to have the power caps below 10J of stored energy in under 10 seconds.

That was a little severe. But it's easy to make power filter caps get down to safe (i.e. under 42Vdc) in maybe 15-20 seconds. Usually you can blindly stick in a 100K resistor and be done with it. A 100K is like maybe one additional 12AX7 load on the cap, and it's just not enough to siphon off enough power to worry about.

I'm sure there are magic mojo believers who will think that power filter bleeders ruin tone, but then they also believe that they can hear the difference in the copper wire's chemical composition.

Just put in bleeders. Put them in every amp you service, for free, and don't even mention it to the owners. The life you save just could be yours when you reach into the chassis after switching off with no tubes in and the caps are still hot.

[Steve Conner]

Guitar amps don't have bleeders? Typically in an amp with series electrolytics as the main power supply filter, the voltage balancing resistors on these are also the bleeders. They bleed all the preamp filter caps too, because the voltages on these drain back through the dropper resistors.

On older or low-powered amps that didn't use these series stacks of electrolytics, I guess all bets are off If I were rebuilding one of these, I would add at least one bleed resistor somewhere.

[galibier_numero_un]

Hi Steve,

Typically in an amp with series electrolytics as the main power supply filter, the voltage balancing resistors on these are also the bleeders. .

Let's use the 5F6A Bassman circuit as an example. I don't see a bleeder resistor in the circuit anywhere. It looks like a C-L-C-R-C to me, and of course, these resistors are in series.

Bleeder resistors I've installed are in parallel (positive side of cap to ground), and typically valued at 50K to 100K as previously mentioned. I should look at some more modern designs of course - designs that don't have a death cap.

Am I misunderstanding you about this series / parallel thing? I can't visualize a series resistor behaving like a bleeder.

Cheers,
Thom

[Enzo]

You are not yet familiar enough with the guitar amp to accept our jargon. Steve refers to some circuits where the primary filter cap is in reality two caps of lower voltage rating wired in series. Instead of finding one cap with a 600v rating, they use two caps of 350v wired in series for a net result of a 700v rating. To make sure the voltages equalize across the two series caps, they usually add a high value cap in parallel with each cap. 100k ohms for example. THOSE resistors wind up in series across the caps. And they then act as bleeders for the system. Your 5F6A Bassman not only has no bleeders, it has no SERIES caps either.

But in lower voltage systems with a single main filter cap, there will not be those resistors. RG wisely suggest we add them.

I would say the lack of bleeders in guitar amps is not so much a result of some philosophy - your holdover from 1950 designs - as it is the desire to be cheap with parts, and the fact that the amp doesn't care if its caps discharge. They assume the technicians will act safely and discharge the circuit when working in it.

I am all for safety, in my shop we even have a rule that you can't work on high voltage gear when you have a cold. COld medication can slow down your thinking. But we also would howl like wounded animals if the factories all got the safety bug and started adding things like the old Ampeg HV lockout on the speaker connector everywhere. Or maybe an AC mains cutout that pulls out of the chassis when it is removed from the cabinet.

[galibier_numero_un]

Thanks Enzo,

Steve refers to some circuits where the primary filter cap is in reality two caps of lower voltage rating wired in series.

I fee better (and smarter?) now.

Cheers,
Thom

[galibier_numero_un]

Thanks again Enzo

... they usually add a high value cap in parallel with each cap

And yes, I caught the bolded typo above as it was intended to be written:
"high value resistor"

I've seen this voltage equalizing technique, but it's use here hadn't dawn on me until you clarified it

Cheers (again),
Thom

[Enzo]

You are correct, that was my typing error Obvously the parallel element needs to be a resistor. As long as we both wound up in the same place, we are OK.

[d95err]

Fender amps don't have bleeders because of one simple reason - cost. Even the cost of a single resistor was worth saving, especially in the days when everything was handwired PTP.

Marshall and other amps don't have bleeders because they copied from Fender...