Can someone school me on Bias Taps?

[grindliner]

Can someone school me on Bias Taps?

I'm working on modding a Jackson JG2 50w head using a Metroamp JTM45 board to mod it to JTM45/50 specs, and then on to a 1987 to see which I like best.

The PT is unmarked, with uncommon wiring colors on the primary side, it has red 325v HT, yellow 6.3v filament, and black 50v Bias tap., no center taps. one of the bias tap wires is/was grounded in it's original configuration.

Is there an advantage or disadvantage to having a bias tap?

Since I have it Should I use it, or just tape it off?

If I use it, do I just relocate where I connect the B+ to the board, or do I need to change anything else on the board (ie; remove a jumper or anything)?

Right now I have it ready to go as a JTM45, but with the Metro 50w bridge rectifier

[flemingmras]

Since it's there, I'd use it. But I'd omit the 150K resistor shown on the JTM45 layout and just connect the diode directly to the bias tap since it's only 50 volts. Having a resistor there would limit your bias voltage too much and you wouldn't have enough negative voltage to bias your tubes, which would overheat them.

Ground one of the black wires from it (doesn't matter which one since it's AC current) and connect the other black wire to the diode on the side of the diode that has the silver band/stripe on it. The rest of the bias supply should be the same.

[SDM]

May need a smaller resistor there, definitely not 150K, but perhaps 1K-10K or thereabouts depending on tubes and B+ you actually get, so I'd leave provisions for it on the board. 50VAC measured with an RMS meter is about 70 volts peak, so - voltage may be too high with resistor completely omitted.

As to the lack of CT for the HV secondary, and heaters, do you have an artificial CT in place for the heaters? Or is there a heater CT? Will also need to wire the diodes as you would in a 100 watter, as a full wave bridge, or ground - terminal if using a rectifier block instead. Just mention the latter things in case they haven't already come up elsewhere.

[flemingmras]

May need a smaller resistor there, definitely not 150K, but perhaps 1K-10K or thereabouts depending on tubes and B+ you actually get, so I'd leave provisions for it on the board. 50VAC measured with an RMS meter is about 70 volts peak, so - voltage may be too high with resistor completely omitted.

As to the lack of CT for the HV secondary, and heaters, do you have an artificial CT in place for the heaters? Or is there a heater CT? Will also need to wire the diodes as you would in a 100 watter, as a full wave bridge, or ground - terminal if using a rectifier block instead. Just mention the latter things in case they haven't already come up elsewhere.

Yeah he already has the bridge rectifier for the HT secondary.

Also...the "artificial center tap" that SDM speaks of is basically a 100 ohm resistor to ground on each leg of the heater secondary. Some Fender amplifiers did this with a pot by hooking the two outside terminals of the pot to the heater secondary wires and the center leg to ground for an adjustable "hum balance". Can't remember what value pot they used for that though.

[Structo]

Usually the Fenders used a 10K pot there but it has some tail resistors there as well.

You can use a 100 ohm pot with each outside leg connected to the heater wires with the center lug on the pot to ground.

[rockgod212]

i am doing something similar to this but only im using the a tube rectifier. would i do the same connections? where i connect the bias tap on the board there should be a wire going to pin 6 on the rect tube. where do i now put this wire?

[flemingmras]

i am doing something similar to this but only im using the a tube rectifier. would i do the same connections? where i connect the bias tap on the board there should be a wire going to pin 6 on the rect tube. where do i now put this wire?

If your power transformer has a dedicated bias tap, remove it. That wire is only used when there is no dedicated bias tap and you're constructing a bias supply via "borrowing" from one side of the HT winding on a tube rectifier or a full wave grounded center tap SS rectifier.

[rockgod212]

yeah i figured it out, i got the amp running and it sounds good. its an old hilgen that i just built a marshall preamp for it and re wired for 6l6's. its a good sounding 30 or so watt amp. the hilgen in its original form wasnt that great of an amp but now its finally a usable amp. building amps is becoming my new addiction and its fun to do. by the way flemingmras, do you have any insight as to how to install the weber switching relays for the other amp i built. they should be here today sometime.

[flemingmras]

yeah i figured it out, i got the amp running and it sounds good. its an old hilgen that i just built a marshall preamp for it and re wired for 6l6's. its a good sounding 30 or so watt amp. the hilgen in its original form wasnt that great of an amp but now its finally a usable amp. building amps is becoming my new addiction and its fun to do. by the way flemingmras, do you have any insight as to how to install the weber switching relays for the other amp i built. they should be here today sometime.

What is it that you're using them to switch? Also, you'll need a separate low voltage DC supply to power the relay coils as well. I've got a layout for one I use that runs off the heater supply. What's the coil voltage rating on the relays you bought?

[rockgod212]

i bought the weber 4 independent dpdt switch relays on the one board and they need 6 volts or 3 volts plus ground. i will be switching the gain stages on the afd style amp. i have 3 dpdt switches on the amp total. if you have a layout of how to hook it up that would help me greatly. i already assembled the board. now i just need to get it in the amp.

[flemingmras]

i bought the weber 4 independent dpdt switch relays on the one board and they need 6 volts or 3 volts plus ground. i will be switching the gain stages on the afd style amp. i have 3 dpdt switches on the amp total. if you have a layout of how to hook it up that would help me greatly. i already assembled the board. now i just need to get it in the amp.

A DPDT relay is the exact same thing as a mechanical DPDT switch as far as the switching goes. You have two moving contacts that each switch between two stationary contacts just like a DPDT switch does. The only thing that makes it different is wiring up the coil to its power supply and footswitch jack. The footswitch just turns the relay coil on and off, which makes the relay's internal DPDT switch operate.

[rockgod212]

could or would the toggle switches still be operational when the footswitch cable is not plugged in? or is it one or the other as far as what switching system i use? have you ever wired one up? could you explain which wire goes where on the switching board and where i connect the female jack on the board for the footswitch cable?

[flemingmras]

could or would the toggle switches still be operational when the footswitch cable is not plugged in? or is it one or the other as far as what switching system i use?

Yes. You'd hook your toggle switches up to the foot switch jack. If you use a switching jack (like the Cliff jacks), you could hook the switch up from sleeve to the switch contact on either tip or ring so that the jack will completely override the toggle switch when you plug in the footswitch.

have you ever wired one up? could you explain which wire goes where on the switching board and where i connect the female jack on the board for the footswitch cable?

Not that specific board no. I have my relays directly on my turret boards, then wire their switches to where they need to go.

[rockgod212]

do you have a layout of how you wire your relays you could send me? just so i have something to reference.

[flemingmras]

do you have a layout of how you wire your relays you could send me? just so i have something to reference.

I don't, but I could make one if I knew which exact relays you were dealing with (make and part number) as well as the circuit they're going in.