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Old 7th January 2017, 03:22 AM   #1
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Default Standby switch buzz?

I have a bud who asked me to make some mods to his Fender Blues Jr. He bought the deluxe Fromel mods. One of the changes was to add a standby switch to the amp. It was wired in the hvac between tge transformer and the rectifiers. When you switch it from standby to operate he amp lets out a short burst of hum or buzz and then it gets quiet after about 2 or 3 seconds.

I also replaced the filter caps. I'm wondering if one of the caps is leaking? Maybe just as it first turns on?

The guy took his amp home, and he was going to call Fromel to see if they have a clue.
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Old 7th January 2017, 04:35 AM   #2
Enzo is online now Enzo  United States
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Nothing wrong with the caps, they are just uncharged. It takes them a couple seconds to charge up, and until they do, the unfiltered DC is nothing but ripple - so HUM. In the normal amp, this goes unnoticed because the tubes have not warmed up enough to make sound as this happens. But with the new and NOT NEEDED standby switch, the tubes are all toasty warm and able to amplify when the switch is flipped, and so you get to hear the charging hum.
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Old 7th January 2017, 08:53 AM   #3
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The standby switch should sit between the output transformer ht feed and the capacitors, this leaves the capacitors charged at all times and turns off the anode supply to the output stage.
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Old 7th January 2017, 11:32 AM   #4
DF96 is offline DF96  England
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Better to remove the standby switch. All it does is reduce the life of components. Exactly which components depends on how it has been wired. Guitarists seem to expect to see a standby switch, but there is no good engineering reason to have one.
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Old 7th January 2017, 12:12 PM   #5
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I have researched the need for the standby switch and totally agree it isn't needed. But it has become a mark of quality in tube amps somehow. I even heard from a boutique tube amp designer who added it to his amp just to be more competitive. But I don't want to insult the guy and tell him it is bull.

I was thinking this was the result of the changes to the circuit too. But I couldn't find anybody else that this happpenee to. It is pretty offensive sounding, so I can't believe nobody else has heard it and complained? The owner is going to call the person who sold him the upgrade kit, Fromel, to see what they say.

I will just wait till he does that.
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Old 7th January 2017, 02:21 PM   #6
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Quote:
Originally Posted by CapnDenny1 View Post
I have researched the need for the standby switch and totally agree it isn't needed. But it has become a mark of quality in tube amps somehow. I even heard from a boutique tube amp designer who added it to his amp just to be more competitive.
Some guitarists like to use the Standby as a muting switch. To reduce any potential damage to the tubes, you can place a fairly high value resistor across the switch (say 47k), so that a small positive voltage is still maintained on the tubes to help draw off their space charge and prevent cathode poisoning in the longer term.

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Old 7th January 2017, 04:55 PM   #7
DF96 is offline DF96  England
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No, you want to maintain the space charge. Keeping some current flowing is needed to stop a cathode interface layer from forming.

Valves last a long time under two conditions:
1. normal heater, normal or somewhat lower anode current (i.e. switched on)
2. cold heater, no anode current. (i.e. switched off)
The two situations to avoid are:
3. normal heater, little or no anode current (cathode interface layer forms)
4. lukewarm heater, significant anode current (cathode bombardment from positive ions)
A standby switch is quite likely to create one of these bad conditions.

Quote:
Originally Posted by CapnDenny1
But I don't want to insult the guy and tell him it is bull.
Telling someone the truth when they believe a lie is not insulting them, but treating them with respect. It is disrespectful to think they are so unimportant that it doesn't matter if they believe nonsense. If more guitarists (and their techs?) were 'insulted' in this way then the standby switch meme might eventually die out and then guitar amp valves might last longer.
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Old 8th January 2017, 05:43 AM   #8
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Quote:
Originally Posted by DF96 View Post
No, you want to maintain the space charge. Keeping some current flowing is needed to stop a cathode interface layer from forming.

Valves last a long time under two conditions:
1. normal heater, normal or somewhat lower anode current (i.e. switched on)
2. cold heater, no anode current. (i.e. switched off)
The two situations to avoid are:
3. normal heater, little or no anode current (cathode interface layer forms)
4. lukewarm heater, significant anode current (cathode bombardment from positive ions)
A standby switch is quite likely to create one of these bad conditions..
I think we're both pretty much on the same page here, and what I'm aiming to avoid by means of the parallel residue is #3. I have always understood (over many years working with valves) that keeping some current flowing means that while the space charge is indeed maintained, it isn't left to remain in a static state close to the cathode, causing an interface layer to form (aka "cathode poisoning").

Quote:
Originally Posted by DF96 View Post
Telling someone the truth when they believe a lie is not insulting them, but treating them with respect. It is disrespectful to think they are so unimportant that it doesn't matter if they believe nonsense. If more guitarists (and their techs?) were 'insulted' in this way then the standby switch meme might eventually die out and then guitar amp valves might last longer.
As I mentioned previously many guitarists like to have the Standby as a mute switch, which I think is reasonable enough. However, there are better ways to achieve that, such as a Standby switch with a bypass resistor, or having a switch which puts a very large bias on the power valves.

BTW your "DF96" handle gives me some great nostalgia, as my first ever valve project, as a kid, was a battery radio, which used a DF96. :-)

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Old 8th January 2017, 02:05 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by AquaTarkus
I think we're both pretty much on the same page here, and what I'm aiming to avoid by means of the parallel residue is #3. I have always understood (over many years working with valves) that keeping some current flowing means that while the space charge is indeed maintained, it isn't left to remain in a static state close to the cathode, causing an interface layer to form (aka "cathode poisoning").
The interface layer forms on the interface between the nickel cathode sleeve and the oxide coating. Nothing whatsoever to do with the space charge. Cathode interface and cathode poisoning are two quite different effects at two quite different places.

Last edited by DF96; 8th January 2017 at 02:05 PM. Reason: typo
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Old 11th January 2017, 07:57 AM   #10
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Originally Posted by JonSnell Electronic View Post
The standby switch should sit between the output transformer ht feed and the capacitors...
No, you don't really want to do that, do you? At least, unless your output stage is of Ultra Linear design, or your finals are triodes. In tetrodes or pentodes the screen grids will melt within parts of a second if you cease draining the cathode currents via the plates. Best regards!
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