Standby switch on just screen supply... ?

[Gtr0]

Hello.
As is usual at this juncture of building a new amp I am pondering on the standby switch. I do not know enough of the science and theory on it, but it seems to me that amps in general would be better off without a standby switch. But without opening that can fully.... I was looking thorough several schems to see how various manufacturers integrate the standby switch and I came across an interesting one... a Marshall JVM.



It seems the screen supply stems off of the B+ line and is on the other side of the standby switch, where then the preamp in it's entirety is fed it's yummy goodness HT.

Is this a good thing? Seems to me that it is... that the amp more or less benefits from a soft startup via tubes, with exception to the output tube screens of course.

Thanks for your input!

[alexradium]

Hello.
As is usual at this juncture of building a new amp I am pondering on the standby switch. I do not know enough of the science and theory on it, but it seems to me that amps in general would be better off without a standby switch. But without opening that can fully.... I was looking thorough several schems to see how various manufacturers integrate the standby switch and I came across an interesting one... a Marshall JVM.

[ATTACH=CONFIG]56462[/ATTACH]

It seems the screen supply stems off of the B+ line and is on the other side of the standby switch, where then the preamp in it's entirety is fed it's yummy goodness HT.

Is this a good thing? Seems to me that it is... that the amp more or less benefits from a soft startup via tubes, with exception to the output tube screens of course.

Thanks for your input!

that's one reasonable way of doing it,there are others as well,way better than the old Fender way.
It is good practice to ground the screens when in standby mode,otherwise you will hear screeching power tubes when strumming the guitar...

as far as standby utility,in cathode bias amps i'd say no especially with tube rectifiers,HV fixed bias amps,if operated correctly,i'd say yes when the amp is hot and you need to turn it off and on repeatedly,because of the bias circuit nature,which can be very slow in some amps,and can't track the HV values.

[pdf64]

Yes, if standby switching g2, a dual pole switch that clamps g2 to 0V (in standby mode) is a good idea, to avoid it floating and letting blips of class C yuk through should there be a big signal at their g1.

But Merlin shows the best option for a bad job that is HT standby http://www.valvewizard.co.uk/standby.html
Which is a reiteration of the trickle bypass standby of Ed Jahns' PS series monsters https://el34world.com/charts/Schemat...nder_400ps.pdf
I've not seen any earlier versions of the trickle bypass idea?

It seems that HT standby is only of any technical benefit (over just turning the amp's mains power on / off as required) when the amp has silicon rectification and a direct coupled cathode follower (with no other mitigation for excessive g1-k voltage at power up) http://www.valvewizard.co.uk/dccf.html

[Old Tele man]

Something to consider about the "standby" switching event:

1) Switching B+ (between rectifier & first cap) involves highest voltage (B+) and highest current (all tubes)...lots of volt-amps
2) Switching plate voltage (Vp) to power tubes involves slightly lower voltage and only the power tube idle currents...much lower volt-amps.
3) Switching screen voltage (Vs) to power tubes involves roughly same (Vs=Vp) voltage but much lower power tube idle screen currents...even lower volt-amps.

..but...

4) Switching bias voltage (Vg) to VERY negative voltage(*) involves low voltage and virtually NO current...and, thus almost NO volt-amps.


(*) called deep cut-off.

[SoulFetish]

Something to consider about the "standby" switching event:

1) Switching B+ (between rectifier & first cap) involves highest voltage (B+) and highest current (all tubes)...lots of volt-amps
2) Switching plate voltage (Vp) to power tubes involves slightly lower voltage and only the power tube idle currents...much lower volt-amps.
3) Switching screen voltage (Vs) to power tubes involves roughly same (Vs=Vp) voltage but much lower power tube idle screen currents...even lower volt-amps.

..but...

4) Switching bias voltage (Vg) to VERY negative voltage(*) involves low voltage and virtually NO current...and, thus almost NO volt-amps.


(*) called deep cut-off.

I like it. But something to consider, putting the output tubes into cutoff almost puts the amp in a no load condition. This could put nearly the full no load B+ across the filter caps.
Many amps use filter caps which are not rated for the amp's no load condition. So, I would caution someone to make sure the caps are can handle the higher voltages for this type.
But the efficiency and power conservation are attractive features of this style.
A very simple alternative which doesn't change the operating condition, is the one employed by Silvertone in many of their amps. They simply AC short the output tube grids together. This provides an effective mute in push pull amps.

[g1]

The other thing I wondered about that was just how 'deep' into cutoff you need to go, and whether the bias supply would go that far negative. I guess that would depend on the particular amp.

[Gtr0]

Yes, if standby switching g2, a dual pole switch that clamps g2 to 0V (in standby mode) is a good idea, to avoid it floating and letting blips of class C yuk through should there be a big signal at their g1.

But Merlin shows the best option for a bad job that is HT standby http://www.valvewizard.co.uk/standby.html
Which is a reiteration of the trickle bypass standby of Ed Jahns' PS series monsters https://el34world.com/charts/Schemat...nder_400ps.pdf
I've not seen any earlier versions of the trickle bypass idea?

It seems that HT standby is only of any technical benefit (over just turning the amp's mains power on / off as required) when the amp has silicon rectification and a direct coupled cathode follower (with no other mitigation for excessive g1-k voltage at power up) http://www.valvewizard.co.uk/dccf.html

Oh yea, I almost forgot about the first one. I usually implement that actually. Though for one reason or anther (I forget much of what I read in Merlin's books, so I am constantly skimming over again and again) I used to use a 10k 2W resistor in older builds. But on one of my latest amp I moved to a 47k. Actually on the last amp I used that there is a very tiny amount of guitar leaking through when in standby... I assume by this (without looking) that I placed the standby after the reservoir caps... as if I were using tube rectifiers... a habit I suppose since some amps do have tube recs.

As for the second link... I just started doing that mod to new builds for the sake of longevity.

AND there is a question I have been meaning to ask on the forum for clarification, since it is linked here, I will include it....
http://www.valvewizard.co.uk/dccf.html
The opening sentence of the 2nd paragraph says:

The circuit universally consists of an ECC83 gain stage, DC coupled to an ECC83 cathode follower (usually in the same envelope), each with 100k load resistors.

And it is worded exactly the same in the book. Is this a mistake? Only the 1st stage actually has 100k on the plate. The 2nd half draws direct from the B+ node, does it not? It seems obvious to me, but asking because there are LOTS of things that seem obvious to me because I ended up just looking at it wrong or not fully understanding the circuit.... or perhaps in this case reading the text incorrectly?

[pdf64]

I guess that the ‘load’ resistor of a cathode follower is its cathode resistor, as the output is developed across it?

[Gtr0]

I guess that the ‘load’ resistor of a cathode follower is its cathode resistor, as the output is developed across it?

Interesting - good point!!

[Old Tele man]

Yes, I gave you a 'thumbs-up' for that one...which DOES work well IF the PI outputs are nearly balanced so they cancel each other; if not, whatever the difference is, it will want to 'bleed through'...which isn't a problem if the guitar isn't played...but can produce weird sounds. I ran into this problem with my Sears 2x12" Silvertone which used this standby method (i recall one plate resistor had changed value).

The only problem with using "deep cut-off" is being careful NOT to exceed the tube's control-grid to cathode maximum voltage...although there is a tolerance that depends upon the quality of construction. Simple implementation is to grab the highest -voltage on the bias voltage divider and potentiometer stack.

[SoulFetish]

Yes, I gave you a 'thumbs-up' for that one...

Well hell,.. I appreciate it. You're a scholar and a gentleman, Tele.

which DOES work well IF the PI outputs are nearly balanced so they cancel each other; if not. whatever the difference is, it will want to 'bleed through'...which isn't a problem if the guitar isn't played...but can produce weird sounds. I ran into this problem with my Sears 2x12" Silvertone which used this standby method (i recall one plate resistor had changed value).

I haven't run into any bleed through in any I've worked on yet, so there's probably a liberal margin for imbalance. But, there are obviously conditions where imbalance in the output stage would defeat this mute. This could, however, be used to one's advantage, particularly if the symptoms were severe, in that it would be an indication to look for issues which commonly cause one phase of the push pull circuit to drop out (ie: open screen resistor, open plate resistor in the PI, bad tube/tubes, etc).

[trobbins]

Unloading the B+ supply would also be the situation for the switched screen supply option, although any coupling cap to the output stage (if used) would not go through a dc bias change. I guess the degree of B+ unloading would depend on the supply requirements of the rest of the circuitry, and any added current if say the the standby switch had a bypass bleed along with an extra contact for grounding the screen supply.

[Gtr0]

@NoelW - I have to ask... what is the reason for the thumbs down on my post above??