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 Post subject: Switch Voltage Ratings
PostPosted: 23 Jan 2020, 13:47 
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Hello,

I'm interested in integrating a switch that will allow me to switch an amp I'm working on between Pentode and Ultra linear operation. I am thinking of using a 3PDT switch configured as follows (for a single L/R channel of a push pull amplifier output stage):

Throw Position 1:

Pole 1 connects Screen Grid of 0 degree tube to Ultra linear tap of output transformer

Pole 2 connects Screen Grid of 180 degree tube to UL tap of output transformer

Pole 3 connects two current limiting resistors to ground. The other end of these resistors (in parallel to one another) is connected to two zener diodes in series, reverse biased to achieve the screen grid voltage I want in pentode mode. At this node, the connection between the current limiting resistor and the start of the two reverse biased zeners in series, is where the SG voltages are supplied in throw position 2.

Throw Position 2:

Pole 1 connects screen grid of 0 Degree tube to the lower voltage side of one current limiting resistor, with the two reverse biased zeners the sole path to ground.

Pole 2 connects screen grid of 180 Degree tube to the lower voltage side of one current limiting resistor, with the two reverse biased zeners the sole path to ground.

Pole 3 connects the ends of the current limiting resistors opposite the connection with the SG and the two RB zeners, to HT

To summarize, I'd have two parallel current limiting resistors feeding a node that connects to the pentode SG with two reverse biased zeners in series establishing a constant voltage. In throw 1 these two parallel screen grid paths are grounded on one end and open on the other. In throw 2, these two parallel paths are connected to HT by pole 3 and to separate SGs by poles 1 and 2. Now my question is that a typical 3PDT toggle switch datasheet shows a few voltage ratings at a few different currents; I've understood these basically to establish the amount of power the switch can handle. There is also a dielectric voltage rating, sometimes it's different between adjacent contacts and the switch body. My HT in this case is at most 350 VDC and the total current through one switch pole is < 0.015 A. Therefore I'm far below the dielectric strength rating of a switch with a 1 kV dielectric strength. Extrapolating the allowable VA of the switch from the current and voltage ratings identified, the switch should be able to handle the power. I've included a link below to the switch I'm interested in.

https://www.mouser.com/datasheet/2/295/MtogglesBushing-334489.pdf

Am I misunderstanding the switch ratings? Is this application within the allowable range of operation for these switches?
If I were to only throw the switch with the power for the system turned off, is this a safe application?
If someone did throw the switch with the power on, what is the worst that would happen? I've heard that switching between UL and pentode with an energized system is bad for tubes / speakers.


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PostPosted: 14 Feb 2020, 11:12 
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If you use a rotary switch with a plastic shaft you have better isolation between the operator and the voltage.

The most important thing is to not switch while its powered up, there will be a loud thump through the speakers if you do!
I have done it many times switching between Ultra linear/pentode/triode and electron stream, the switch voltage rating is between the contacts, however the contacts are normally at similar voltage between each pole.
So current is normally very low and its just between the enclosure and the operator.

Regards
M. Gregg

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PostPosted: 14 Feb 2020, 13:34 
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To expand on the voltage etc,

The rating of a switch is:
Voltage withstand is the amount of voltage between the contacts>>>this is set by the air gap between the contacts.
The current rating is the cross-sectional area of the contact ie the amount of surface area of each contact which is the same as the current rating of a cable or wire. The thicker it is the more current it can carry based upon material resistance of the conductor and voltage drop across the conductor X current = wattage <<the energy dissipated in the conductor as heat. Which is reflected as insulation melt temperature or melt point of the conductor.<<<this is displayed in a switch as the point at which the contacts weld together. :hot:

Hope this helps

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M. Gregg

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PostPosted: 14 Feb 2020, 13:44 
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So in reference to voltage rating.

The voltage rating of a switch is the voltage difference between switch contacts...ie 10v on each side of a contact means 0V differential between contacts.
So at 300V on each side of the switch also equals 0V. So at this point there is no Voltage difference between contacts. So at this point if the insulation of the switch can withstand the voltage between the contacts and chassis or other metalwork then its all OK. If either the difference between contacts changes due to a fault condition and exceeds the withstand voltage it will flash over or if the insulation withstand voltage is exceeded the same thing will happen.

Life is hard.. LOL :D
So in any system the idea is that if it can fail it fails "safe" ie automatic disconnection of supply takes place <<fusing or circuit breaker etc.
The operation time and energy dissipated during the disconnection time is a directly reflected in the damage caused which must never cause any extraneous metal such as chassis to become live. Sorry I'm boring myself so I'll exit stage left. :D

Regards
M. Gregg

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PostPosted: 14 Feb 2020, 14:30 
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In reference to damage from switching between triode/pentode etc.

One of the main points is that if you have a bypass capacitor on the cathode of an amplifier and you switch between pentode and triode etc.
If the cathode capacitor discharges through the cathode resistor it will suddenly recharge as you energise the switch into the next switch position.
The inrush current will travel through the tube and primary winding of the output transformer this magnetisation of the output transformer gives the thump in the speaker. depending on the current capacity of the transformer and tube it probably will survive "but this is not guaranteed".

In an OTL amplifier this is seen as the speaker cones pulsing in and out as the 0V rail stabilises with the heaters energised and B+ powered on.

Now you can see the effect of heaters at temperature and then applying B+/ht without soft start.
There are lots of "for and against" arguments.

Again these are only my thoughts and I'm sure others will give input.

Regards
M. Gregg

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