• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Relays for selecting outputs in 300B amp?

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#22
The rule of thumb is "silver(alloy) for current, gold(alloy) for voltage". Though rules of thumb are often not 100% tight in this case the DIYer does a good job memorizing this. Just like using versions with bifurcated contacts when current is switched....and of course sealed versions in all audio cases when it is about either line signals or speaker signal.

Gold is too weak for currents and it will deteriorate fast. Now you know why many of those nice Japanese amplifiers from the eighties/nineties have relay issues. A special kind of DIYers will sand the contacts removing the gold plating altogether 🙂

There are special relays that switch the first inrush current with wolfram contacts and then bridge these with silver alloy contacts. These exist in speaker relay versions.

The second link in previous post deals with all versions and is a good read.
 
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#23
Wolfram is called Tungsten in English.
Check for relays with Tungsten Silver contacts, sometimes Cadmium is also part of the alloy.
And always put a relay intended for capacitor duty, they are designed for back EMF from power factor improvement capacitors. Arcing is not so big an issue with those, they have long life.
Further protect contacts by putting arc quenchers such as MOVs in parallel.
Biggest size, which will fit your housing and budget.
 
#31
Ya I guess it gets lost in translation.

In French it's batterie au plomb...

Kind of like how Pineapple is Ananas in like every other language...
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Back on topic... Let us know which relay you go with and how it works out for you.
 
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#32
#33
Windcrest77 said:
If you get a 4 deck switch you can double up each contact for twice the surface area (twice the current, twice redundant). An extension shaft from the back to the front of the chassis and all the OPT stuff can be wired right next to the binding posts.

https://www.amazon.com/uxcell-Position-Terminal-Channel-Selector/dp/B071WLXR5V?ref_=ast_sto_dp
Click to expand...

Actually this one has 8 poles, so you could quadruple each switch contact area across stereo (4 times the current). I know it has 5 positions, you can just make the extra two positions a doubling up of 4 and 8 ohm speakers. You'd have to click twice through turns to get to the next ohmage, the 5th position can be phones. I couldnt find a 3 position 4 deck switch (at a non-astronomical price) which would be ideal for 4/8/phones setup.

Or make the 5 positions: 4 ohm > 8 ohm > phones low imp > phones med imp > phones high imp (with L Pad resistors for high/med/low matching.

The phones positions have an attenuation resistor (which is the series resistor of the L Pad), that way you blow your ears out less likely. On the phones positions you'd definitely need L Pad resistors so the OPT isn't left floating if the phones are not plugged in too try this calculator for lo/med/hi impedance phone matching maybe 32 / 100 and 300 ohm choices.:

https://robrobinette.com/HeadphoneResistorNetworkCalculator.htm


 
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#34
Some reasons for having remote control of an amplifier's output impedance tap . . .

The amplifier is in a closet, and is hard to get access to it, and change which output tap the speaker wires are connected to.

A speaker is going to be connected to the amplifier, and it is desired to do a quick A / B test of which output tap does the speaker sound best on? . . . 4 Ohm tap? Or 8 Ohm tap?
 
#36
Ananas in Hindi too, maybe in some other Indian languages also..

A tube amp runs hot, you want it in a closet?!

The Japanese used to put a switched jack at the pre amp to main amp connection, diverting the signal to headphones if needed, you could do that, with the additional provision of switching off the main amp when in headphone mode.
 
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