Updated Champ amp heater circuit

[jmaf]

Does anyone know where that "virtual center tap" term originated? It's totally wrong, but it seems to have caught on so that people use it anyway. I'm not sure why. Maybe it's just because it looks like a center tap if you draw it that way, but electrically that's not what's going on.

It originated from the fact that the 2x100 ohm resistor trick is simply a poor man's center tap for amplifiers where you didn't have 3.15 + 3.15 VAC. Like almost everything else in guitar amplifiers, the terminology is street based. A lot of the amp modders and builders are self taught and don't have a fancy EE degree from MIT and last I checked they don't deploy tube amps at NASA, so folks use the terms they see which is fine IMO, everyone knows what is being referred to here. I don't understand what the issue is, virtual center tap has a nice ring to it.

[bob p]

Sorry but you are quite wrong
And to make it clear itīs just not a typo, you repeat same wrong concept in different ways

Maybe you're not following parts of the conversation that haven't been explicitly fleshed out with schematics. Maybe the problem is that you jumped to a conclusion when you read post 22 on page 1 and you didn't bother to read posts 39-44 on page 2, which clarified those things that you falsely accused me of not understanding. As if I can't tell the difference between AC and DC! Come on!

It seems that there are a lot of people reading this thread who are wearing blinders that cause them only to think about hooking up resistors to the secondary and wiring the junction to ground, which is actually not such a great way to build your amp. The fact that the two types of secondary windings behave similarly in the case where resistors are attached to them has made some people ask if the windings will behave the same way when something else -- like diodes -- are attached to them. Of course that won't be the case. Current will flow through the center tap. There is no equivalent circuit in that case with non-center tapped secondary, so the simplified banter that the two secondary designs are equivalent in the case of hanging resistors on them is based upon flawed logic as it only considers a narrow subset of transformer behavior. This is precisely why center-tapped and non-center-tapped secondaries have to use different rectifier topologies.

The center tap, whether real (copper wire tap in the middle of the winding) or a halfway point between 6.3V wires created with two same value resistors is NOt just a "DC reference" (even that is wrong, itīs actually an AC reference to chassis, no DC there)

We've already clarified that I never said it was a DC reference in that case -- what I said is that I call them DC offset resistors when I use them, because I provide a DC offset in my builds. Again, it seems that you've made the misteak of imagining a schematic that's different than what I'm talking about. In the posts on page 2 I clarified this -- the resistors provide an AC reference + balancing when you connect the resistor junction to ground, and provide a DC reference + balancing when you connect the resistor junction to an elevated DC source, as the OP asked about the original post, which everyone seems to have ignored. For some reason everyone seems to be ignoring that part of the discussion, and limiting their thoughts to the two resistors to ground model. It's time to take off the blinders.

It seems evident that some people keep reading this thread with one specific schematic in their mind, and are ignoring other topologies that have been discussed. I didn't think I'd have to draw pictures to make it clear, but I guess I was wrong on that. My bad.

[jmaf]

It seems that there are a lot of people reading this thread who are wearing blinders that cause them only to think about hooking up resistors to the secondary and wiring the junction to ground, which is actually not such a great way to build your amp. The fact that the two types of secondary windings behave similarly in the case where resistors are attached to them has made some people ask if the windings will behave the same way when something else -- like diodes -- are attached to them. Of course that won't be the case. Current will flow through the center tap. There is no equivalent circuit in that case with non-center tapped secondary, so the simplified banter that the two secondary designs are equivalent in the case of hanging resistors on them is based upon flawed logic as it only considers a narrow subset of transformer behavior. This is precisely why center-tapped and non-center-tapped secondaries have to use different rectifier topologies.

I've read this 4X (for real) and I have no idea what he's trying to say here. CT and non CT secondaries have to use different topologies due to the limitations of the era, back in the day rectifiers were 2 plate tubes, then we got flooded with cheap silicone diodes. They're completely different circuits but as with any circuit, they obey the same laws.

Dangling two 100 ohm resistors across any power supply is not rocket science and it quiets most hum. What's the big deal here? I couldn't imagine a 2 page thread about something so simple TBH. Are we really discussing OHMs law and 2 100 ohm resistors here?

[Bob M.]

I'm glad that Malcolm pointed out that there's virtue in using 'virtual' in electronics.

I'm glad to see that G1 brought this argument full circle.

Obviously, some people are in the DC offset bias camp; others are just grounding their resistors or pot. I decided to try the resistor grounding idea first and my amp is as quiet as quiet can be (that's an unscientific term) so there's no reason to try anything else. Of course, part of my good luck has to do with shielded grid wires and a very good grounding scheme.

Now, I'm trying to decide if I like the 5V4 rectifier better than the stock 5Y3 and I'm experimenting with a couple of single ended replacement output transformers that I have in stock. For those who have replaced the output transformer on their Champ, VC and/or Bronco amps, what are some successful good choices for an upgraded improvement of this amp's sound?

Thanks all for your input and knowledge,

Bob M.