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Stand-by Switch for B+ Ratings
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Author:  Danny [ 24 Nov 2020, 18:00 ]
Post subject:  Stand-by Switch for B+ Ratings


I am building a KT88 SE amp and besides the main power switch I would like to add a second switch to manually control the B+ (460V). This way I can let the heaters warm up before applying B+.
I know know there are ways to do this automatically but I want to keep the implementation as simple as possible.
The toggle power switch that I plan to use is rated 10A at 250VAC.
Would it be safe to switch the 460V being generated by the GZ34/5AR4 rectifier tube?



Author:  Les [ 24 Nov 2020, 21:41 ]
Post subject:  Re: Stand-by Switch for B+ Ratings

Since you're using tube rectifiers, it is not necessary to delay power. They'll all warm at about the same rate, and the output off the tube rectifier is a soft rise - voltage rises as it warms. However, a better approach (if you just have to have a "stand-by") would be to separate your powers - use a low voltage filament transformer separate from the HT (2 transformers), then install the "stand-by" switch on the line (mains) side of the HT transformer. Put your main power switch to feed the low voltage transformer and the "stand-by" switch. Another option would be to utilize a relay with 600v rated contacts and use the switch to control the relay.
460+ volts could cause arc over in the switch contacts possibly endangering you (the user). A relay would at least eliminate that danger by isolating the HT from the switch

IMHO :thumbsup:

Author:  M. Gregg [ 25 Nov 2020, 05:22 ]
Post subject:  Re: Stand-by Switch for B+ Ratings

B+ standby switch is a bad idea,
But +1 for Les in the last post.

You can do some real damage using a B+ switch with a SS rectifier.
NB soft start is not a B+ switch.

The problems arise in two main situations.
With Heaters up and running if you have cathode bypass capacitors on the power tubes.
When you switch on the B+ the cathode caps will charge very quickly and current will surge through the power tubes and output transformer winding.
Only as the capacitors charge up tubes slowly go into normal idle current.
So in this situation a soft start is preferred.

With a tube rectifier using standby for B+ causes the same issue as above the B+ comes straight on if the rectifier heater is already running.
You will lose what you gained using a tube rectifier its sort of the same as a SS rectifier except for the soft recovery and volt drop but that's a different issue.
As already posted the tube rectifier is a soft start so B+ rises as the heaters power up.

The problems with B+ on before the tubes conduct is more of an issue if the tubes are direct coupled.
Then you can Get B+ on the grid and a possible flash over to the cathode.
In that case you can put a neon or diode from cathode to grid to protect the tubes.

The other problem with a B+ switch is if you don't turn it on and then switch off the amp the B+ capacitors can stay charged due to no current being drawn.
This creates two problems one is the danger that you might touch the power supply caps and get a shock with them still charged after power off.
The other is that if you have direct coupled tubes and switch the B+ on with the amp off you can over volt the grids and cause a flash over in the tubes.
So In every event a discharge resistor on the B+ supply is a MUST DO!

NB one other point is that a B+ switch stops current being drawn through the tubes so the B+ voltage will be higher with the switch off and can over volt the PSU caps possibly causing them to explode and when you switch the B+ switch on you are connecting an even higher voltage to the tubes than would normally happen= bigger surge through the power tubes.

I remember when I was discussing this with someone at the time I was in favour of the B+ switch and I have fitted them in some amps BUT with precautions to stop all of the above.

This is what was said," TV sets never had B+ switches and the tube was very expensive to replace, over all the decades of TV sets. Can a BILLION tv sets be wrong?". :D I found it hard to argue the point!

M. Gregg

Author:  M. Gregg [ 25 Nov 2020, 05:46 ]
Post subject:  Re: Stand-by Switch for B+ Ratings

Where a B+ switch can be useful,

Is if you intend to Pull out power tubes ect fast and change them, if the glass breaks you are exposed to holding the B+
Ie in equipment used in venues like guitar amps or PA equipment.
Where some fool will have to get gear going fast under pressure from management.
B+ OFF thick glove pull the tube and change.
But in this day and age Its moot.

I suppose someone will tube roll like this but why... :D

I did fit a B+ switch in an OTL amp I built because of the heat and power consumption if I left the room for some reason, the amp was quite small but it dropped the power consumption by 150Watt. But on power up you could see the tubes settle causing the speakers to cone flap for a second.

M. Gregg

Author:  M. Gregg [ 25 Nov 2020, 07:22 ]
Post subject:  Re: Stand-by Switch for B+ Ratings

One other time a B+ switch is useful,

This has no relevance to ordinary tubes. So ignore it.

Is to run "new" NOS tubes with just the heater to reactivate the Getter.
I did this with 6c33c tubes which is another reason why the OTL has a B+ switch.

I ran the 6c33c tubes for 8 hours continuous with no B+ its also supposed to expand the internals into correct working position.
I thought it might be moot but "people that know" have said if you don't do it the tubes fail after a couple of months.

M. Gregg

Author:  gofar99 [ 26 Nov 2020, 12:27 ]
Post subject:  Re: Stand-by Switch for B+ Ratings

Hi, With tube rectifiers I agree that a switch is not needed. If you use SS rectifiers then it is likely a delay is needed. Stuffing the peak DC voltage on you filters with no load on them is not a great thing for longevity and reliability. That would be about 1.4 times the RMS of the transformer. Any transformer RMS over 360 would dump over 500 VDC on the filters. Filters rated over 500VDC is much more costly that ones at or below that voltage. I use a relay between the output of the SS rectifiers and the first filter capacitor. This prevents any build up of DC on the filters until you are ready for it. 30 second delay is fine, but I typically use 60. You can do the timing several ways. I like an old style thermal delay tube for most of my personal builds they look cool and work great. A 6NO30 or 6NO60 are the ones of choice...but they are getting scarce now. I use 555 IC timers in the commercial designs. Either just switches 12 volts to the relay. Getting suitable relays is a topic all in itself. Ones actually rated at about 500VDC is costly and difficult. Strangely most small relays rated at 250 VAC (typically 1 amp) will work fine there. The DC rating for such relays is for high current loads and really for this application can be ignored. As long as you place a X2 capacitor of about 0.1uf with a series resistor of around 50 ohms 1W across the switched contacts as spark suppression. It sounds unlikely, but it works fine. I have not had any failures in my own stuff nor in the many commercial amps out there over the past 10 years. I doubt the manufacturers of the relays (I have used several brands) would approve, but apparently they are much more tolerant than would seem from the specs. I use this arrangement on amps with B+ of 425 at 300ma regularly without any issues. The relays in my very first Oddblcks (KT88 from 2008) are just fine and are in daily use. I would guess many thousands of cycles on the relays. No problems.

So.... no delay for tube rectifiers, but I highly recommend something for ones with SS rectifiers. You could skip the timer and use a switch to activate the relay. Just a switch is IMO not a great idea as mentioned in other posts.

Good listening

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