Super Twin: Ultralinear Conversion

[tbryanh]

What is the best way to convert the Super Twin from ultralinear to non-ultralinear?

[Enzo]

Disconnect the OT UL tap wires and connect the screen resistors to a B+ node. You could use the 395v node, or maybe make your own screen node with a dropping resistor and a cap.

[darrellcheng]

what is a node?

[Enzo]

Ih dat ting abub you mouth.


Your B+ supply starts at the rectifier, there is a filter cap right away, then a choke or resistor and then another filter cap, and depending on the complexity of the amp, some additional number of resistor to yet another caps stages. SO the B+ "string" as I sometimes call it, is a series of filter cap stages with a resistor or choke separating them. Each stage, each filter cap really, is called a node.

Look at the AMpeg schematic. Lower right. The B+ supply comes out of the rectifier tube. The B, C, and D in circles are examples of nodes. The very first stage goes up to the center tap of the output transformer. They could have labelled it the A node, but it only went one place, so they didn't bother.

Most of the time, the nodes are all in a row like that. But you could also branch out from some point in the string and create one off to the side. he might prefer to do that for his screens, because the existing second B+ node in the amp we are discussing is substantially lower than the plate node. he could branch off the plate node and create a node specially for his screens.

Node pretty much just means branching point, or potential branching point. Look at the Fender schematic. The B+ supply there has 320v, 305v, and 260v nodes. Note that the first node, the 320v is not used by the amplifier circuit. But it is still there. Fender chose to send power to the output tube from the second node. (They did this to take advantage of the additional filtration)

Can;t recall which, but some Marshall amps have an unused node in the middle of their B+ string.

[darrellcheng]

Thanks Enzo! I always called it the punching bag.

Looks like i should be using the 1st node (for 135w UL SFTR) @ 410v for the screens

off topic question:

Are the filtering resistor and the choke (for RF or LF filtering) interchangable?
what I mean is, Can i just go in there and plug in a choke and take out the resistor without changing anything else (or vice versa)?
Does the choke provide more filtering?

Also, back to UL, same question for the OT's, are UL OT's and standard OT's swappable? (for example, a 100W twin's OT with the 135W twin OT, or vice versa)

Thanks.

One more thing...fender used a full wave center tapped rectifier in the bf, and then switched to the full wave bridge recto, whats the difference?

[Enzo]

A choke has more filtering action than a resistor. The choke has a relatively low resistance and will have a small voltage drop across it. You can replace a choke with a resistor, but you will have somewhat less filtration. You might then have to adjust the values of the dropping resistors in the B+ string to get back to the desired voltages at each node.

That 2700 ohm resistor is dropping 90 volts. Put a choke in its place and you won;t drop anywhere near 90 volts. Your 410 volts becomes 480 or whatever. And all the downstream B+ voltagea will likewise be higher. So you'd have to do some adjustting.

You can use a UL OT without wiring the UL taps, so to that extent they are interchangable. The 135 watt part will be heftier than the 100 watt part, but otherwise would be more or less interchangable.

And the rectification? It served their purposes.