PT without bias tap

[MickeyB]

Putting together a 5E3 with 6L6's (see schem) and the PT I have does not have a bias tap. Whats the best way to drop the HV 380v winding (380-0-380 PT) down to 50v for the bias circuit?

[defaced]

I know there are ways for taking bias from the HT, but I much prefer taking a small heater transformer and run it backwards off the heater supply. Meaning, take the 6.3v side of the transformer and connect it in parallel with the heaters and take the 120v primary and use it for the bias. You'll need to tweak the values of the resistors in the bias supply to give you the proper voltages, but this way works great.

[Diablo]

You could do it the old Ampeg way. I would use a 25K pot and a 51K resistor in series instead of the 68K resistor, so you can adjust the bias.

http://www.drtube.com/schematics/ampeg/gv22-jp.gif

[Diablo]

I know there are ways for taking bias from the HT, but I much prefer taking a small heater transformer and run it backwards off the heater supply. Meaning, take the 6.3v side of the transformer and connect it in parallel with the heaters and take the 120v primary and use it for the bias. You'll need to tweak the values of the resistors in the bias supply to give you the proper voltages, but this way works great.

I understand you want to source 120 VAC for the bias, but why would you need a transformer to do this? Couldn't you just use 120VAC coming in from the power outlet and the fuse? What is the benefit of going through two transformers to get the same voltage you started with?

[Marc]

how about taking ac off of the HT leads with a pair of 100K 1 -2 W resistors feeding the bias circuit on the schematic? you should use a diode after the 100k resistors and tie them together instead of a single diode. kind of like the Ampeg cap divider but using resistors....

[MickeyB]

Found this on Weber. Looks to be close to the same size PT as mine and the bias circuit runs off the Rect.

[Albert Kreuzer]

I understand you want to source 120 VAC for the bias, but why would you need a transformer to do this? Couldn't you just use 120VAC coming in from the power outlet and the fuse? What is the benefit of going through two transformers to get the same voltage you started with?

You don't want to connect your guitar directly to the power outlet, do you?
The Darwin Awards

[Diablo]

You don't want to connect your guitar directly to the power outlet, do you?
The Darwin Awards

You're correct and I was dangerously wrong. I just looked up transformerless tube amps and isolation transformers. I was thinking the amp would have a three prong grounded cord and be properly wired and grounded to the chassis. The problem is you can't always rely on the wall outlet to be properly wired, so you could have a live chassis.

[defaced]

You can also cheat and use a 12v transformer and you'll get ~60v out of the primary. Didn't think of that earlier.

[Chuck H]

Why not keep it simple. The 5E3 has a CT HV wind so there's no need to cap couple the circuit. This has been done without issue many times.

[Wilder Amplification]

I second the "borrowed" bias supply as well. If there's a CT on the HT winding it's perfectly OK to do (most 50 watt Marshalls do this exact same thing).

Although I think 380-0-380 is a bit high, I think the 5Y3 rec valve is perfect for it. However, I would never run SS rectifiers on that voltage.

[defaced]

Why not keep it simple. The 5E3 has a CT HV wind so there's no need to cap couple the circuit. This has been done without issue many times.

Tomatoes, tomaatoes. Just like in the other recent thread where this came up and you advocated this type of circuit, I prefer something else, which I find much simpler than trying to lump more stuff onto a winding than needed. I don't like circuits that take the bias from anything else. I put all my toys in their own toy box, not in one big one. It does keep things simple, just a different kind of simple.