SS rectifier with standby switch?

mcentee2

Tele-Holic
Joined
Feb 18, 2011
Posts
541
Location
York
Has anyone got any strong views about using a standby switch with solid state rectifiers ?

I understand about tube rectifiers and standby switches :)

Am interested on what good folks think about them with SS rectifiers from "cold" startup.

Is it just the peak cold voltage/current to the first filter cap that would the main issue ?

(Re tube warm up issues, I am on the side of "they are myths", but am interested in the potential capacitor issues)
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,592
Age
62
Location
Finland
I think one switch might be better and sufficient with tube rectifier.

Two switches are essential with SS rectifier!

When two switches are used with tube rectifier, about 220k 3W series resistor over standby switch terminals keep some charge on filter capacitors and current peak comes less severe.

I think tube rectifier always require (~100R...200R) current limiter resistors to anode circuits what data sheet calls for the AC HV and protective diodes as well. Yes they increase few volts of sag but only reason I use tube rectifier is sag so they are justified.
 
Last edited:

dougstrum

Friend of Leo's
Joined
Oct 6, 2015
Posts
3,659
Location
blu ridge mtn cabin
My builds have ss rectification and I also use a standby switch. When I started building 2 switches seemed to be standard, so that's what I did:rolleyes:
I often find it standby to be handy.

I have a 1st gen Pro Jr, it has no standby and SS rectification~no perceived problem.
 

zook

Friend of Leo's
Joined
Aug 6, 2003
Posts
2,945
Location
Cochise, AZ
Regardless of rectifier type, A standby switch allows the heater to warm up the tubes before the B+ is turned on.
 

Tom Kamphuys

Tele-Holic
Joined
Sep 18, 2018
Posts
928
Age
42
Location
The Netherlands

mcentee2

Tele-Holic
Joined
Feb 18, 2011
Posts
541
Location
York
Sounds like you’ve read Merlin on standby? AFAIK there's no better source. His nice short section on SS at the end looks especially helpful here, showing how to safely and quietly use a single-pole switch with either two-phase or bridge rectifier.

http://www.valvewizard.co.uk/standby.html


Hmmm, good old semantics....

I read that SS section as "if you really had to have a standby switch in your SS rectified amp then this is where it can be placed"

rather than

"You definitely have to have a standby switch in an SS rectified amp",

I couldn't find that latter explicitly stated in his article ?


Merlin's text:

"If you're still convinced that you want a standby switch then you can at least avoid poor implementations..."

And

"When using silicon diodes we don't have to worry about hot-switching..."
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,592
Age
62
Location
Finland
Tom and mcentee2. Standby switch to prevent "cathode stripping".

Interesting read on those link articles. I didn't know there is "cathode poisoning". Obviously it was not known to exists 40 years ago when I began my work which many did use tubes. I guit studying tube technology when I finished school. Perhaps this poisoning comes from HiFi tube builders? :)

Perhaps a resistor to SS rectifier Stby switch terminals, which allows some HV to build and make tubes conduct slightly, is enough to prevent cathode poisoning?

Article did mention that 20 minutes will cause cathode poisoning but was it considered as similar stress to tubes what "cathode stripping" cause? I switch stby On after about one minute so it is worth only 1/20 what "stripping" cause so I will continue installing two switches but I will add a resistor!
 

Wally

Telefied
Ad Free Member
Joined
Mar 17, 2003
Posts
42,157
Location
Lubbock, TX
Cathode stripping cannot occur in our tube amps due to the low level of voltage that is present in these circuits. The engineering papers on the subject I found in the late ‘90s held that it takes 10,000 volts to cause cathode stripping. Cathode poisoning can occur when the t7bes are left in standby operation for long periods of time.
 

mcentee2

Tele-Holic
Joined
Feb 18, 2011
Posts
541
Location
York
Ok, nobody has raised any potential problems with not having a standby switch wrt capacitor voltages.

Seems all good then....
 

dan40

Friend of Leo's
Joined
Aug 19, 2015
Posts
2,837
Location
Richmond Va
I'm in the camp that feels that they are not really needed. I have never had any problems from my builds that did not include a standby switch, both ss rectified or tube rectified.

I recently purchased a PRS MT 15 which is a fixed bias, SS rectified 6L6 amp that does have a standby switch. I thought that the switch was in the B+ line when I first started using it but after doing a little research and circuit tracing, I discovered that the switch was designed to mute the circuit itself and not break the B+ line. I'm guessing that the designers at PRS didn't feel it was a problem to not place the switch in the B+line.
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,509
Location
Salt Lake City
Right, @mcentee2 , I didn’t emphasize Merlin's main point: Standby isn’t needed, cathode stripping isn’t a thing in our amps (as explained above by @Wally ), and misuse of standby can be harmful. After that, he describes 'less bad' ways to implement standby, and then the ways to do that in SS.

I may have easily missed it, but I’m not sure most of our discussion actually addressed your question about filter caps. I’ve seen that discussed in an article somewhere but not sure where… and was it an accurate article? There's a ton of misunderstanding about standby, even among the makers of amplifiers. OTOH I’ve never seen anyone effectively dispute a thing Merlin says. I'd think if absence of standby hurt filter caps, he’d discuss it… but that doesn’t prove much.
 

mcentee2

Tele-Holic
Joined
Feb 18, 2011
Posts
541
Location
York
Right, @mcentee2 , I didn’t emphasize Merlin's main point: Standby isn’t needed, cathode stripping isn’t a thing in our amps (as explained above by @Wally ), and misuse of standby can be harmful. After that, he describes 'less bad' ways to implement standby, and then the ways to do that in SS.

I may have easily missed it, but I’m not sure most of our discussion actually addressed your question about filter caps. I’ve seen that discussed in an article somewhere but not sure where… and was it an accurate article? There's a ton of misunderstanding about standby, even among the makers of amplifiers. OTOH I’ve never seen anyone effectively dispute a thing Merlin says. I'd think if absence of standby hurt filter caps, he’d discuss it… but that doesn’t prove much.


Thanks, I sort of knew that the "pile in" was more likely to happen than directed towards my actual question, but that's fine :)

I can't recall where I read, it might have been in the Peavey article or the London Power one or Dr Z ?

Tbh it all gets a muddle!!

The only thing I took from it was that from a cold start, with no standby switch, the ss rectifier can provide full voltage straight away vs a tube rectifier that has to heat up even if for a short time like a 5y3, but the other tubes aren't conducting yet so the B+ can get high and then drop down, something like that.

A gz34 has slow startup so the above isn't quite the same re max B+, it ramps up/down more slowly with less variance or something.

So it's making sure your caps are rated for that max B+.
 

Wally

Telefied
Ad Free Member
Joined
Mar 17, 2003
Posts
42,157
Location
Lubbock, TX
mcentee2 wrote:……“making sure your caps are rated for that max B+.”

Make sure the voltage rating is a fair amount above the expected B+.
 




New Posts

Top