Lamp circuit

[Carlin Mitchell]

Is there a reason the lamp is in the heater circuit ? I understand that you know for sure the secondaries are working when it's on that side. Does putting it on the 120V side have any negative affects ?

[UR12]

Is there a reason the lamp is in the heater circuit ? I understand that you know for sure the secondaries are working when it's on that side. Does putting it on the 120V side have any negative affects ?

No and No

[Cygnus X1]

Having the lamp on the heater certainly helped me recently.
Or it may have hurt...

My SEL amp started eating preamp tubes and burning cathode resistors.
I happened to notice the lamp was flickering. Turns out the heater circuit was going awry. It seems a faulty bulb was eating into circuit enough to starve the filaments.

Changed bulb, problem solved.

[UR12]

Having the lamp on the heater certainly helped me recently.
Or it may have hurt...

My SEL amp started eating preamp tubes and burning cathode resistors.
I happened to notice the lamp was flickering. Turns out the heater circuit was going awry. It seems a faulty bulb was eating into circuit enough to starve the filaments.

Changed bulb, problem solved.

I have seen the lamp holders loosen up on some of the old Fender amps and as you rotate the bulb to get it in or out you actually short the lug connections on the lamp holder and short the power to the heater. If this is not caught you have the potential of frying your power tranny. But getting back to OP question, either way will work just fine and has been working fine for decades. One plus in favor of the 120v scheme is that the neon bulbs last a lot longer than the incandesant bulbs ran off of the heater winding. You will save money over the long run by not having to replace bulbs.

You can also use a LED with a large series resistor running off of the last power supply node. This will let you know that you have B+ on your tubes and standby is in the operate mode. Put it on the other side of the standby switch and you have a HV power indicator.

[Phil_S]

Forgive me for stating the obvious, but after your earlier attempt at building, I sense you are new to this. You'll need a 120v lamp if you want to put it on the primary. The heater lamp is a 6.3v lamp. If you already have the 6.3v lamp, just stick with that. There is no practical advantage of one way over the other.

[Carlin Mitchell]

Doesn't the low voltage leg create a lot of noise ? Wouldn't you want as little 6.3 volt wiring in your chassis as possible ? LED's are typically 12 volts .. there probably just as bad ?

[Structo]

The filament winding is for......the heaters.

It is generally left in AC and the tight twisting and the way the heater wires are run is what effects if the AC hum finds its way into the signal chain.

Fender typically ran the heater wires overhead of the sockets and Marshall generally ran them on the chassis floor near the back of the chassis.
Both ways work.
Some amps run heater buss wires as straight wires from socket to socket and sometimes DC is used with that method.

I like the ideal of a neon 120 volt pilot light. The bulb lasts a very long time and does not effect the heater supply.

[Jana]

The typical 6 volt lamp holder in fender amp makes a nice terminal strip for connecting the heater wires from the PT to the rest of the amp. If the amp is laid out correctly, this is all over on the PT side of the amp and there isn't any more danger of hum than having a PT over on that side.

Having said that, I prefer the Marshall style rocker switches, the lighted one for the power. Mainly because I like the double poles and I don't like Carling switches. I have never had a Marshall rocker fail, I have had plenty of Carling switches fail--most often the one for the standby. They just can't handle that high voltage long term.

It's more a matter of the current in the 6.3 volt than the voltage and also the close proximity to the sensitive areas that generates the hum.

[Phil_S]

That little 6v incandescent doesn't put much of a load on the filament circuit. While I agree that the less you put on the filament circuit, the better, as a practical matter, I think this amounts to splitting hairs.

[Tonegeek]

Doesn't the low voltage leg create a lot of noise ? Wouldn't you want as little 6.3 volt wiring in your chassis as possible ? LED's are typically 12 volts .. there probably just as bad ?

LED's are not so dependent on voltage as current. So about anyvoltage will work but you MUST limit the current with a resistor. do not hook an LED up directly to a voltage source without using a limiting resistor. Use OHMS law to figure out the resistor value.

I have used LED's off the heater supply but also like the idea of using them off the B+ as it is more likely the B+ will have a problem before the heater supply will and it is good to know these things.