Tubes/Stan By Mode or Off

[BobbySkidz]

Is there more wear on tubes if the amp is left in standyby mode or shut off and then turned back on? Like if I was gonna take an hour break, is there more wear on tubes if turned off/on or left in standby?

Thanks

[Cliff Schecht]

The consensus that I came to after beating this topic to death on a few forums is that standby switches are an unnecessary carryover item from years of blind cloning. The early Fenders had them because Leo was copying from the RCA/Westinghouse schematics pretty blindly (at least the power section topology). Then Marshall copied the 5F6A and kept the standby switch. Vox didn't necessarily copy anybody but they used the standby switch as well. So with all of these popular amps using standby switches, nobody thought to ask why!

It turns out that standby switches are a carryover item from high voltage/powered transmitters and such. If the B+ voltage comes up before the heaters are warm and the bias is achieved then you stress the CRAP out of the power tubes, which are expensive to replace. This is where you see cathode stripping.

In ANY of the tubes that we use for audio (all considered receiver tubes by RCA manuals), there is no need for a standby switch. It's stated right in the tube manuals that receiver type tubes, (especially indirectly heated types) don't require the use of a standby switch.

As it turns out, standby switches in our amps can be a big source of frustration with certain topologies. For exanple, the worse place to put a switch is right before a choke. Since a choke is a big inductor, the current cannot change instantaneously but the voltage can. When you flip the switch, you essentially apply a big step voltage to the choke which can cause it to ring at much higher voltages than the B+ it settles to. This ringing can harm the choke, power transformer, diodes, caps, various resistors, output transformer and/or tubes! While this usually doesn't happen (the Q of the inductor is low usually), certain amps are infamous for dying from the standby switch. I know of the Vox AC30CC2 as a current example of an amp that eventually will fail from using the standby switch (causes the rectifiers to arc!).

There has been a lot of discussion on this. I recommend you do a search for standby switches or sucker one of the search experts into doing so .

[xtian]

Nice summary, Cliff.

[Firestorm]

I don't necessarily disagree with Cliff's analysis, but I don't think Fender was blindly copying RCA manuals as much as responding to players's needs for a way to mute the amps during breaks without having to reset levels because they had turned everything down, or wait for the amp(s) to warm up because they had turned them off. Fender didn't even include a standby switch until the "5D-" models, where they switched the PT center tap (which probably puts a fair amount of hot switching stress on the rectifier tube). The "5E-s" switch the B+, but have a choke in front of the OT center tap which limits the hot switching transient. Once the choke moves between the plates and screens (5F-s), the B+ is still switched, but the first filter is kept charged during standby, which helps protect the rectifier tube, but (as Cliff points out) makes the choke ring like hell. Interestingly, this is the point where Fender first starts using screen resistors -- not to protect the screens, but to shut up the ringing choke.

[BobbySkidz]

Thanks for the replies. This is good to know for amps in general, however the specific amp in question has no choke (Orange Tiny Terror).

[Roe]

vox didn't use standby switchs on the classical amps.

[Reeltarded]

Nope they sure didn't!

I never standby unless I am changing cabinets. If I take a long break, they idle @ volume, or for periods like 3-4 hours the volumes are turned down.

[The New Steve H]

I knew the rectifiers on my church's AC30CC2s blew from time to time, but I didn't know it was a common problem. I bypassed the standby on one of them. Now if there was just a way to bypass the entire amp and substitute a Fender.

[rdjones]

I don't necessarily disagree with Cliff's analysis, but I don't think Fender was blindly copying RCA manuals as much as responding to players's needs for a way to mute the amps during breaks without having to reset levels because they had turned everything down, or wait for the amp(s) to warm up because they had turned them off. Fender didn't even include a standby switch until the "5D-" models, where they switched the PT center tap (which probably puts a fair amount of hot switching stress on the rectifier tube). The "5E-s" switch the B+, but have a choke in front of the OT center tap which limits the hot switching transient. Once the choke moves between the plates and screens (5F-s), the B+ is still switched, but the first filter is kept charged during standby, which helps protect the rectifier tube, but (as Cliff points out) makes the choke ring like hell. Interestingly, this is the point where Fender first starts using screen resistors -- not to protect the screens, but to shut up the ringing choke.

I wonder what, if any, influence the arrival of RDH4 had on this timeline.
This would have been around the time of the 5Dx circuits.

rd

[Structo]

So on the Dumbles with the classic power supply, the standby switch is after the big B+ caps but before the choke.

The B+1 comes off right before the choke.

So does this arrangement stress the choke?

What would be a better solution for the Dumble ODS amp?

I should mention that I have a .01uF ceramic across the standby switch if that makes a difference.