Question about Heater PS - Geloso G257 recorder and argentine rock

[kagliostro]

Recently I discovered that the small Geloso G257 has a place in the story of the argentine rock

Grabador Geloso - Rock Nacional Argentino (Parte 1/2)
Grabador Geloso - Rock Nacional Argentino (Parte 2/2)

Here some info as to convert a G257 as guitar amp
http://www.assabbi.com/instrumentos/gelosog257/index.html

Here the schematic


I've think to redraw the schematic eliminating all what wasn't strictly necessary

doing that I was intrigued by a particular feeding the heaters of the 12AX7, the 4R resistor that is in series with 1/2 of the heaters (pin 4 of the 12ax7)



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The question:

Which can be the particular purpose of that 4R resistor in the heater ??

Many Thanks

K

[sluckey]

Assuming the filament winding supplies 6.3VAC, that resistor will reduce the filament voltage for V1A to 5.75VAC. V1A is the input preamp and is susceptible to filament hum. Reducing the filament voltage to 5.75VAC also reduces the filament current to .137A (rather than .15A). Doing so reduces the chances of V1A picking up hum from it's filament.

ValveWizard talks about running a tube filament at less than specified voltage for hum reduction purposes. See the third paragraph titled Voltage Considerations.

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

[kagliostro]

Ciao Steve

Grazie


It will be as you say only referred to V1b (I assume looking to the image I've find)

Franco

[sluckey]

GrazieIt will be as you say only referred to V1b (I assume looking to the image I've find)

The heater connected to pin 4 serves the triode connected to pins 1, 2, and 3. In your schematic that would be what you labeled V1A. In the pic of the pin layout pins 1, 2, and 3 are called triode 2. So, V1A = pins 1, 2, 3 = Triode 2.

[kagliostro]

 

Thanks

Franco