standby switch with full wave bridge rectifier

[joesatch]

i have seen the switch placed in various locations. At the B+ side some at the Ground of the FWB. What's the best placement of the standby switch using a FWB?

[pdf64]

Merlin advises to switch the AC feed to the rectifier http://www.valvewizard.co.uk/standby.html

[joesatch]

great

[tmknight]

Sorry to revive a dead thread, but...

I'm looking into this very standby circuit.  In my trials of this, the 47K "trickle" resistor left 150VDC post-rectifier and full muting was not possible.  100K was better, but 220K offered full muting with a fraction of the voltage post-rectifier.  Which leads me to my question: Wouldn't a much higher "trickle" resistor be preferred (maybe even as high as 1M)?  Seems that would keep some current flowing, though at much lower voltage levels, while still providing muting.  Is someone able to articulate why Merlin used smaller vs higher values?

[tubeswell]

Is someone able to articulate why Merlin used smaller vs higher values?

His resistor is less of a 'mute' resistor, and more of a 'soft start' resistor

[acheld]

To be honest, Merlin does not like standby switches.  I've tried all of the schemes on his website, none of them really satisfactory in operation (I'm not objecting to his reasoning, just how they work).   He has more complicated "delayed soft start" schemes in his recent book, which I haven't tried, and don't plan to.

My opinion, and it is just that, is that standby switches -- as used in Fender amps -- work fine as muting switches, but have no other benefit.  They've been around for decades, and guitarists are used to having them, and normally do not cause problems. 

In practice, I leave my amps on, and rarely use standby.   But if I have to, well then, ya gotta do what ya gotta do . . .

Plugging in was a problem until I discovered Neutrik  Silent Plugs -- a fantastic piece of kit!

[HotBluePlates]

... In my trials ... full muting was not possible. ...

If you want muting, it's much better in every possible way to simply disconnect signal-drive from the output tubes.

[tmknight]

Thanks, all, for the input.  I do not disagree with either Merlin loathing bypass switches or that there are better options for muting.  I was more curious as to whether there was a "mathematical" reason to the 47K resistor "as the compromise" when a bypass switch is present or was it rather arbitrary (I'm lead to believe the latter as true).

[HotBluePlates]

... I was more curious as to whether there was a "mathematical" reason to the 47K resistor "as the compromise" ...

What is you B+ voltage?  Probably not more than 460vdc.

What AC voltage does that 460vdc imply?  460v / 1.414 = 325vac.

How much current can pass through that 47kΩ?  325v / 47,000Ω = ~7mA.

7mA isn't enough to sustain meaningful output from the amp, even if it doesn't kill all sound.

[PRR]

Say the happy power stage is 5k ohms. (500V, 100mA, is 5k.)

50k in series makes the maximum speaker voltage about 1/10th. -20dB. "Softer", no doubt; but not "silent".

If I want to "mute" a TV or Hi-Fi or recording channel, -40dB is significantly softer, but not muted. -60dB is sometimes tolerable. A true mute is lower.

I think Merlin's plan is just another data-point on the futility of a "standby" with no specified performance. Do we want the tubes to live forever, ready for instant action? Quiet the buzz in the silent prayer part of the service? Ease the start-up surge? Raw tungsten heaters? Mercury vapor?

[joesatch]

necropost.  I now have a two phase rectifier and would like to implement a standby that lifts one of the AC secondaries feeding the rectifier diodes. Can someone tell me if this is okay.  My reasoning is this amp has a single pole switch already in place .  please see attachment.  i dont wanna put the standby switch on DC can i do this??

[joesatch]

something tells me with a 2 phase lifting one leg of the ac will still rectify voltage from the connected side (just half the amount of DC?)

[AmberB]

I think you would get most of the voltage through the half of the transformer that's still in the line, but it would be half wave rectification.  I think it would be like taking a secondary that doesn't have a center tap and using half wave rectification instead of a bridge rectifier for full wave. 

[sluckey]

something tells me with a 2 phase lifting one leg of the ac will still rectify voltage from the connected side (just half the amount of DC?)

That's correct but the half wave rectified DCV will be about 0.9 times the secondary ACV.

[SnickSound]

... In my trials ... full muting was not possible. ...

If you want muting, it's much better in every possible way to simply disconnect signal-drive from the output tubes.

I simply ground the PI input, works perfectly. I've seen commercial amps do the same nowadays (so that the FX Send still works when in Standby, allowing you to use it as a preamp)

Muting the amp by using a switch rated for 250V to interrupt 450V+ does not sound like the best solution to me... also you can just half pull the cable from the input (just enough for the shunting switch to engage), bam, muted (personally I just press my tuner pedal).

One place where a standby switch is useful is in amps with a DC Coupled Cathode Follower, until the tube gets hot the grid will be at full B+ while the cathode is at ground potential. Merlin has a solution for that too, a diode from grid to cathode (+ a resistor). I've been doing my amps that way for years, no issue.

[PRR]

redacted

[sluckey]

> ...the half wave rectified DCV will be about 0.9 times the secondary ACV.

WITH capacitor it will charge to Peak, 1.414.

Yep. Just like my attached pic shows.

[PRR]

redacted

[sluckey]

>> 1.414.
> Just like my attached pic shows.

Text says 0.9.

So what! I took the time to clip that pic from the Hammond website. Maybe you could take the time to look at it.   

[pdf64]

something tells me with a 2 phase lifting one leg of the ac will still rectify voltage from the connected side (just half the amount of DC?)

That's correct but the half wave rectified DCV will be about 0.9 times the secondary ACV.

That seems somewhat dubious to me.
Surely the particular average level of V DC will depend on the particular loading?
Of course at some loading it'll be 0.9, but reduce that loading and it'll be higher, increase it and it'll be lower.

[sluckey]

Lot of nitpickers out today!

[pdf64]

Lot of nitpickers out today!

Maybe so, but I don't think it should be perceived as such in this case.
Technical resource info backed by a reputable name like Hammond should be accurate.

Of the equations in that snippet from the Hammond (and other) site, 1 was good, 2 were nonsense.

That seems a rather regrettable scenario to me.

[sluckey]

It'll be OK. I was feeling a bit tender yesterday.