Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: taddy on March 02, 2018, 11:19:54 am
-
Hi everyone,
I'm planning on building my first amp based around a deluxe reverb.
I'm a bit worried about the standby switch which is going to have over 400VDC running across it. The UK site where I'm getting my parts has a carling toggle spst (https://modulusamplification.com/Carling-toggle-switch-spst-P4475378.aspx) which looks pretty robust but looking at the specs it says voltage rating of 277DC. Should I be looking at this figure or the dielectric strength which I found on a datasheet from Carling and says 1000V. Is this going to work? If not can you recommend something better.
Cheers
Tom UK
-
Why fit a standby switch?
-
looks pretty robust but looking at the specs it says voltage rating of 277DC
No, it says "Voltage Rating AC: 277 V"
That's the same switch Fender has used for standby on thousands of amps. It's fine.
-
I think that's a Carling 110-63, rated to 250V ac/dc due to it's quick make/break action http://www.carlingtech.com/sites/default/files/documents/110-SeriesTog_Details_%26_COS.pdf
Dunno where the 277Vac comes from, maybe a typo?
Yes, they're been used beyond rather their rating for standby since Fender fiendishly introduced the concept of 'standby', in order to use underrated HT caps (notice a theme?). Good luck finding a switch that is rated to 450Vdc and will fit in the chassis.
I don't perceived that standby could have any conceivable technical merit in a DR.
But the customer base learned their particular version of the standby ritual and expect any well specified tube amp to come with a standby switch, so we're stuck with them in perpetuity.
http://www.valvewizard.co.uk/standby.html
-
Thanks everyone. I think I'll just forget about a standby switch but will include an HT fuse. Have a good day/evening!
-
think I'll just forget about a standby switch
another convert :icon_biggrin:
-
As you might guess, I'm heartbroken.
-
That switch is fine. Switching-off a vacuum cleaner is FAR tougher duty than anything a Fender can do to a switch.
Do NOT substitute "mini" switches of the "same rating". They are obviously not built as beefy. They do work for some folks, but none have the 60+ year reputation of the Carlings.
No standby switch is also fine. It is user-preference, not about the amp's health.
-
...Switching-off a vacuum cleaner is FAR tougher duty than anything a Fender can do to a switch...
My understanding is that dc is tougher to switch than ac, as, when once established, a dc arc will sustain whereas an ac arc naturally tends to self extinguish.
Only quick make/break type switches are suitable for dc, and the typical max current rating for these is ~3A
Higher current switches have a slow switching action and with these, a 250Vac switch might only be rated for ~30Vdc, given the same (resistive) current.
The HT standby arrangement used with the DR puts an additional stress on the switch, due to the flyback voltage from the choke when the current is suddenly interrupted. I've seen hypothesis that the 'walking wounded' switch contacts resulting from this can cause weird, otherwise unexplained, ticking noises https://www.thegearpage.net/board/index.php?threads/will-putting-an-amp-in-standby-before-shutting-down-prolong-the-cap-life.1226042/#post-15220533
Lots of switch info here http://www.aeroelectric.com/Reference_Docs/Switches/CH_Switch_Training_Manual.pdf
...No standby switch is also fine. It is user-preference, not about the amp's health.
I like to think of the nice slow ramp up in HT provided by a GZ34 as being akin to being gently woken in the morning, eg by a gradually increasing light level; compared to the instant slam of HT, and the accompanying surge currents in HT components, that a regular standby inflicts, that may be akin to an ice bucket alarm.
I can't provide data or cite references to back my case, but it seems intuitively reasonable that reducing surge currents would reduce stress and so extend service life of affected parts.
Merlin provides some options for improvement if a HT standby is deemed an essential requirement, see link previously provided.
Interestingly, I noticed that Ed Jahns used the standby 'trickle bypass' method in the PS400 http://schems.com/schematicheaven.net/manu/fender/400ps.pdf
In favour of standby, it may be that in the case of solid state rectification, a 'trickle bypass' type HT standby may be the best option for a soft start, though an NTC thermistor somewhere in the HT could do the job as well.
-
> dc is tougher to switch than ac, as, when once established, a dc arc will sustain whereas an ac arc naturally tends to self extinguish.
Yes; but some "AC" loads really don't want to quit. "Universal" motors are really DC motors bodged to accept AC. They typically have very high self-inductance. A vacuum or drill motor may be bigger than an amp choke; it also has a 7,000RPM rotor which dumps energy back into the circuit (which is trying to become a non-circuit).
The 1957 Electrolux here, you can see the turn-off arc through the switch rocker, and it is brutal compared to any amp-choke-arc I have witnessed. This is not the most-used Electrolux I have ever seen (I think it had a 30 year nap before I got it) but it is working fine for now, knock dirt. Yes, switches are a repair-thing on vacuums, but many seem to live a full life on the factory switch. (That's good/old vacuums. The sucker we got in 2011 died in 5 years, not from switch but really cheap plastic parts.)
-
Yes, with very inductive loads, the current and voltage won’t cross zero together so the tendancy for an arc to self extinguish will be reduced.
And yes, a carling will likely give decades of service, ticking or not :dontknow: