Fixed bias and standby switches

[waster]

On my next build I was planning to not have a standby switch but I have read different things about fixed bias.
I have read that without the negative bias already running before the power tube HT is switched on, the tubes can red plate before the bias gets to its proper level.
This seems to say to me that a standby switch is necessary with fixed bias, and that the bias system needs to be included in the standby system.
I will use one if it is necessary but I would rather not. What are people's thoughts on this?

[printer2]

If you have SS diodes in both then if one has a greater time constant than the other it will charge up sooner. If you have a SS diode feeding your bias circuit and you have a tube rectifier I would think the bias might charge up before the plate supply. Even if the bias is not charged up to its full voltage it may be partly charged limiting the dissipation of the output tubes. Mind you I have not worked with a fixed bias amp yet, just theorizing.

[hackworth1]

Gerald Weber recommends the 5AR4 rectifier tube b/c it heats up slower than the tubes do - which - according to him - gives a similar benefit to a standby switch.

[waster]

I am using a rectifier tube, but ss diode for bias, so yeah I guess the bias might charge up quicker than the rest.

[firemedic]

Having -DC bias voltage on yer power tube's grid, before the plate gets its B+ and the tube is flowing current, will not harm the tube.
Flowing current with no bias voltage certainly will.
i.e., leave the bias supply independent of the standby switch.

[waster]

Originally Posted by firemedic

Having -DC bias voltage on yer power tube's grid, before the plate gets its B+ and the tube is flowing current, will not harm the tube.
Flowing current with no bias voltage certainly will.
i.e., leave the bias supply independent of the standby switch.

Yeah that is the way I meant it. I think I am safe enough seeing as the ss negative bias will charge up more quickly than the tube rectified B+

[Platefire]

To each his own, but I love standby switches! They are simple to install and have great advantages:

1-Allows the heaters to warm up your tubes before hitting it with high voltage DC that saves tube life.

2-Allows you to mute your amp while you take a break with the heaters and bias still running so you have instant sound when you come off break.

3-I don't know, there should be a third good reason??? An extra switch makes your amp look more expensive and technical!!!;>}

[Badside]

Originally Posted by Platefire

To each his own, but I love standby switches! They are simple to install and have great advantages:

1-Allows the heaters to warm up your tubes before hitting it with high voltage DC that saves tube life.

There is just as much evidence that keeping tubes warm with no voltage on damages them. Tubes don't conduct until they've reached a certain temperature anyway.

Originally Posted by Platefire

2-Allows you to mute your amp while you take a break with the heaters and bias still running so you have instant sound when you come off break.

Just unplug your input if you wish to mute it, or turn on your floor tuner.
I NEVER set my amp to Standby during a break, because of the aforementionned problems with keeping tubes hot with no current going through, also because even though the filaments are hot other components are still heat-cycling every time you switch to standby and back so you can actually lower life expentancy. Once it's on, I keep it on for the full night.

Originally Posted by Platefire

3-I don't know, there should be a third good reason??? An extra switch makes your amp look more expensive and technical!!!;>}

You can do other things with that switch, like two different B+ (if your PT allows it), or a pentode/triode switch, or a fixed bias/cathode bias switch, etc.

There is a theory that Leo started putting a Standby switch because some rectifier tubes created a voltage spike when turned on which would go above the power cap rating.

Think about it: do you really think your grand father let his radio on Standby for 2 minutes before turning it on? I don't think so

[sjhusting]

Originally Posted by Platefire

2-Allows you to mute your amp while you take a break with the heaters and bias still running so you have instant sound when you come off break.

I stopped putting standy switches in my builds after reading Merlin's take on it; which was shortly after I watched a JJ GZ34 do exactly what Merlin says can happen when you hit a rectifier with a sudden HV spike.

I now put a 'mute' switch in the amps. Routes the signal to ground just before the PI.

steven

[celeste]

Originally Posted by sjhusting

I stopped putting standy switches in my builds after reading Merlin's take on it; which was shortly after I watched a JJ GZ34 do exactly what Merlin says can happen when you hit a rectifier with a sudden HV spike.

I now put a 'mute' switch in the amps. Routes the signal to ground just before the PI.

steven

Yep, the best way to do it. Gibson did it that way 60 years ago. Gibson amps of the time were designed by EE's, not electronic repairmen.

[waster]

Yeah reading merlins site is what put me off standby switches too, made sense to me.

[waster]

Originally Posted by Platefire

3-I don't know, there should be a third good reason??? An extra switch makes your amp look more expensive and technical!!!;>}

Ha ha good call, however I like things as simple as possible, I really liked Fezz's amp he showed the other day with one knob, and it was a tone knob. One day I will build an amp with no knobs at all, just use the ones on your guitar, just for a laugh more than anything.

[Wally]

Originally Posted by Platefire

To each his own, but I love standby switches! They are simple to install and have great advantages:

1-Allows the heaters to warm up your tubes before hitting it with high voltage DC that saves tube life."

There is not cathode stripping possible at the levels of voltage that htese amps use, so there is no reason to use a standby switch with regards to warming up the tube before the votlage is applied.

"2-Allows you to mute your amp while you take a break with the heaters and bias still running so you have instant sound when you come off break."


As badside notes, leaving an amp in STandby mode for extended periods will indeed harm the tubes....the situation is called 'cathode poisoning'.


"3-I don't know, there should be a third good reason??? An extra switch makes your amp look more expensive and technical!!!;>}

Your last sentence is perhaps the only reason for a switch...and it has nothing to do with tube life or operation of the amp....all of the effect is 'external' to the amp, right? (;^)



RE: Gibson and standby switches....I am going to have to study that one. Interesting.

[jefrs]

Long ago, all our domestic electronics had valves.

And not one of them had a standby switch.


Hartley Peavey writes a paper on the evils of standby switches
http://peavey.com/support/technotes/.../chapter_6.pdf
and then builds amps with standby switches because his customers want them...

If your amp has a valve rectifier then a standby switch is about as much use as a chocolate teapot. With a solid state rectifier it is merely a fashion accessory.

[celeste]

Originally Posted by Wally

RE: Gibson and standby switches....I am going to have to study that one. Interesting.

Wally, someone asked me which ones over at AX84, so to give you a head start looking, here was my reply

Gibson Duo Medalist


Cool different way
Gibson GA100
Gibson GA300RVT
Gibson GA30RV

[celeste]

Originally Posted by jefrs

Long ago, all our domestic electronics had valves.

And not one of them had a standby switch.


Hartley Peavey writes a paper on the evils of standby switches
http://peavey.com/support/technotes/.../chapter_6.pdf
and then builds amps with standby switches because his customers want them...

If your amp has a valve rectifier then a standby switch is about as much use as a chocolate teapot. With a solid state rectifier it is merely a fashion accessory.

Leo got trained on transmitters, where the voltages involved are high enough to make standby's worthwhile. He was a repair tech, never an engineer, he was never trained to understand the finer points of why. Clearly he just copied what he was trained on, with very little thought to design.

As for being a fashion accessory, I believe the technical term is FalseFi. Controls of dubious use, or not even connected, added for the sole purposeof impressing people

[printer2]

Originally Posted by celeste

As for being a fashion accessory, I believe the technical term is FalseFi. Controls of dubious use, or not even connected, added for the sole purposeof impressing people

Sounds like the accelerator on my car.

[hackworth1]

Peavey Article is very good.

Peavey does not malign the use of a standby switch for initial tube heating on start up.

Fast-Heating, Directly-heated rectifier tubes (such as the common 5Y3) combined with indirectly heated preamp tubes (12AX7, etc) and power tubes (6V6, 6L6, etc) benefit from a standby switch or an inrush current limiter or both.

Peavey says that a standby switch should not be used on beer breaks longer than 15 to 20 minutes in order to avoid cathode poisoning.

IIRC, Peavey stated that it is preferable to leave the amp fully powered-up during breaks.

Also that the 5AR4 Rectifier (3 amps current draw) is preferrable - due to its indirectly heated cathode - wherever it can be used instead of the 5Y3 (2 amps). Be mindful of current draw.

[onenotetom]

I am new at this so help me out. I use a standby switch so that I can mute the amp when I switch guitars. Since I only play at home and have a number of guitars I often change a number of times when practicing. Is this a good enough reason for a standby switch? Should I be using a different wiring scheme for this such as a mute switch as described above? If so, how do you wire it?

Thanks, Tom

[printer2]

A volume control works wonders at turning off the signal.