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35W4 Rectifier

10/23/1997 1:29 PM
Craig	35W4 Rectifier I am converting a Sears Radio (35W4, 50C5, 12AU6)into a guitar amp, using the existing tubes, sockets, and basic schematic, with slight variations adapted from several similar 3 watt designs in Pittman's Book. One such variation was adding a 100 ohm, 5 watt resistor between the AC fuse and the heaters of the rectifier. When I turn the amp on, the resistor starts smoking, and gets VERY HOT. Funny thing is, it's retaining it's value, after such a beating. Which brings me to my 3 "burning" questions about the 35W4: 1) I see that some of the schematics that use this tube complement include a power trans. and some don't. The Sears radio design didn't. What allows the 110v AC to produce 6.3v? The only thing in most of these designs that falls into that part of the circuit is the above resistor, and the 2-amp fuse that precedes it. Is there a voltage drop that occurs across all the combined filements? This voltage/amperes theory stuff still really baffles me. I don't get how to look at a schematic, and discern the expected flow of voltage throughout the design. (Aside from the spec'd plate voltages). 2)A 35W4 and a 12AX7 both have 3 'filament' connections; (pins 3,4 and 6 on the 35W4, and pins 4,5 and 9 on the 12AX7). I see in my Sylvania tube manual that in each tube, all 3 pins appear to be connected internally. In Fender designs, I notice that most 12AX7's have pin 9 going to ground, with no other connections. In most of the schematics I've seen for the above type of amp, the 35W4's pin 6 receives the 6.3 feed from the hot side of ther AC source. The switch is on the ground side, the fuse on the hot side. Is it correct to connect the 6.3v feed to pin 6, which is internally connected to pins 3,4? Or do I need to connect them using an external lead? Should pin 6 be grounded, as in the Fender designs for 12AX7's, which would mean I should connect to pins 3 and 4? An Alamo Capri (same circuit, slight variations)I have has the 6.3v feed going into pin 3 of the 35W4, coming out of pin 4, which goes to pin 3 of the 50C5, and so on in the same way. Meaning: The pins must be connected internally, right? 3) What's the deal with using only one side of the 110 AC for the 6.3v? I know this has something to do with the lack of a power trans., but I don't know why.

10/23/1997 1:29 PM

Craig

35W4 Rectifier

I am converting a Sears Radio (35W4, 50C5, 12AU6)into a guitar amp, using the existing tubes, sockets, and basic schematic, with slight variations adapted from several similar 3 watt designs in Pittman's Book.

One such variation was adding a 100 ohm, 5 watt resistor between the AC fuse and the heaters of the rectifier. When I turn the amp on, the resistor starts smoking, and gets VERY HOT. Funny thing is, it's retaining it's value, after such a beating.

Which brings me to my 3 "burning" questions about the 35W4:

1) I see that some of the schematics that use this tube complement include a power trans. and some don't. The Sears radio design didn't. What allows the 110v AC to produce 6.3v? The only thing in most of these designs that falls into that part of the circuit is the above resistor, and the 2-amp fuse that precedes it. Is there a voltage drop that occurs across all the combined filements? This voltage/amperes theory stuff still really baffles me. I don't get how to look at a schematic, and discern the expected flow of voltage throughout the design. (Aside from the spec'd plate voltages).

2)A 35W4 and a 12AX7 both have 3 'filament' connections; (pins 3,4 and 6 on the 35W4, and pins 4,5 and 9 on the 12AX7). I see in my Sylvania tube manual that in each tube, all 3 pins appear to be connected internally. In Fender designs, I notice that most 12AX7's have pin 9 going to ground, with no other connections. In most of the schematics I've seen for the above type of amp, the 35W4's pin 6 receives the 6.3 feed from the hot side of ther AC source. The switch is on the ground side, the fuse on the hot side. Is it correct to connect the 6.3v feed to pin 6, which is internally connected to pins 3,4? Or do I need to connect them using an external lead? Should pin 6 be grounded, as in the Fender designs for 12AX7's, which would mean I should connect to pins 3 and 4? An Alamo Capri (same circuit, slight variations)I have has the 6.3v feed going into pin 3 of the 35W4, coming out of pin 4, which goes to pin 3 of the 50C5, and so on in the same way. Meaning: The pins must be connected internally, right?

3) What's the deal with using only one side of the 110 AC for the 6.3v? I know this has something to do with the lack of a power trans., but I don't know why.

Replies:

R.G. 1) the Sears radio does not produce... -- 10/23/1997 2:32 PM
Bruce CRAIG, CRAIG, CRAIG...Yor scari... -- 10/23/1997 8:28 PM
Doc Just wanted to add a note to all th... -- 10/27/1997 12:01 PM