hi octane wire gauge

[Marcos - Munky]

Sorry for the dumb question, but I still didn't got the 4*Pi. I think I got the 3D part, that is to put it like this, correct? I'm still bumping my head with the 4*Pi thing...

[thomasha]

Take a look at this transformer orientation>
http://www.tdpri.com/proxy.php?image=https%3A%2F%2Ffarm9.staticflickr.com%2F8575%2F16561199240_09f1acaa2e.jpg&hash=1c2824cc8e5fe81957d7d348b25c1fca

[Marcos - Munky]

Hey Thomas. I can't see the image, but I think it's the same orientation as the one I've linked. I'm just trying to get the 4*Pi part. In my mind I can find only 2 Pi, so there's 2 missing

[thermionix]

You're not the only one who's confused.  4*pi is approximately 12.5.  Of what, I don't know.

[mac]

Sorry to confuse you guys.
2*Pi is the total angle in two dimensions, (XY)
If you rotate in three dimension, space, the angle is 4*Pi, (XYZ)





What I was trying to say is "unscrew the PS and rotate it in space, not only in the chassis plane"
In the picture, 90° xy --> 90° xz

mac

[Marcos - Munky]

Yeah, I got the transformer correct position, I just didn't got the 4*Pi part. And now, thinking about it again, I can only get 2*Pi/2 rotation One 90 degree (Pi/2) in xy and another 90 degree in xz.

Anyway, I can't do this orientation because of the kind of the transformer bracket I've got. But I'm thinking about making another chassis, so I can at least have the correct tubes and transformers positions.

And I'm suspecting my wall outlet ground is not working, didn't tested it yet to confirm this.

[mac]

Anyway, I can't do this orientation because of the kind of the transformer bracket I've got. But I'm thinking about making another chassis, so I can at least have the correct tubes and transformers positions.

Just an idea. Do you have some galvanized steel sheet at hand, or candy can, etc?
You can cut a strip of metal to hold the transformer, and drill two holes in the chassis.

But if you...

And I'm suspecting my wall outlet ground is not working, didn't tested it yet to confirm this.

Test it first.

mac

[Marcos - Munky]

Problem solved! And it was something else that was causing the hum.

The original project calls for an power transformer with no center tap for the filaments supply. Instead, one of the wires are connected to the EL84 cathode, to get an voltage reference. Today I went to the guy who I bought the transformers, and turns out when he made the power transformer he added a center tap for the filaments, but he forgot to tell me that!

So my amp filaments was actually getting two different voltage references at the same time, one from the EL84 cathode and another one from the center tap connected to ground. So we just cut the wire connecting the filaments to the EL84 cathode and left the center tap connected to ground, and all the hum is gone!

[mac]

IIRC most ax84 amps have the filaments connected to the EL84 cathode.
They must have strong arguments to do this. Not an expert, I wouldn't do it.

http://ax84.com/bbs/index.php?id=500257

mac

[Marcos - Munky]

In almost all the amp schematics I've saw, the filaments have a center tap connected to ground. If this is the "better" way or not, I don't know, but surely it's the most common one.

IIRC most ax84 amps have the filaments connected to the EL84 cathode.

I'll keep that in mind next time I build any ax84 amp

[mac]

I'll keep that in mind next time I build any ax84 amp

I'm planning to fly to Natal late March or April. Build another one and we jam together, caipirinhas are on me

mac

[PRR]

Wiring the heater CT (real or virtual) to a small positive voltage reduces heater-cathode leakage. Preamp tubes vary a lot here. If the cathode insulation leaks, 50/60Hz on heater leaks to cathode and thus audio. Back-biasing the parasitic diode reduces the leakage. Maybe 1 of 10 preamp tubes, it matters. In mass production of HI gain amps, this technique is often used so the factory does not have to rotate 10% of tubes (leakers) out of the first hole into higher-level stages where the signal is higher and leak is less of a problem. Unless you love swapping tubes around, you should do it also.

With a power stage that is Cathode Biased, the output cathode is a very good "small positive voltage".

BTW: doing _both_ two-100r to EL84 cathodes AND a grounded CT makes those two-100r resistors "added cathode bias" resistors. I think for typical EL84 bias this raises EL84 current a lot, enough to be a Problem. If the PT winder hard-grounded the 6V CT, then you have to go that way. If it is a lead not shared with other grounds, you could take it to EL84 cathodes for small heater bias.

[Marcos - Munky]

I'm planning to fly to Natal late March or April. Build another one and we jam together, caipirinhas are on me

mac

I've never been to Natal, but I have some friends who did. It's a nice place. Way far from my place, unfortunally. Here in Brazil, sometimes is cheaper to travel to neighbor countries than to travel to brazilian's northwest, where Natal is located.

Wiring the heater CT (real or virtual) to a small positive voltage reduces heater-cathode leakage. Preamp tubes vary a lot here. If the cathode insulation leaks, 50/60Hz on heater leaks to cathode and thus audio. Back-biasing the parasitic diode reduces the leakage. Maybe 1 of 10 preamp tubes, it matters. In mass production of HI gain amps, this technique is often used so the factory does not have to rotate 10% of tubes (leakers) out of the first hole into higher-level stages where the signal is higher and leak is less of a problem. Unless you love swapping tubes around, you should do it also.

I've noticed hum when I turn up the gain. Do you think wiring the heater CT to EL84 cathode helps to get rid of this hum? Heater CT is a lead not shared with other grounds.

[mac]

I've noticed hum when I turn up the gain. Do you think wiring the heater CT to EL84 cathode helps to get rid of this hum? Heater CT is a lead not shared with other grounds.

If you connect on side of te heater transormer to the cathode, disconnect the CT from ground. As Paul said, you might have too much current across the power tube.
What about a rectifier bridge with schottky diodes to avoid adding more turns to the heater transformer.

I've never been to Natal, but I have some friends who did. It's a nice place. Way far from my place, unfortunally. Here in Brazil, sometimes is cheaper to travel to neighbor countries than to travel to brazilian's northwest, where Natal is located.

Same down here.
But Brasil is way cheaper than Argentina. In fact, we are one of, if not, the most expensive country in the world.
I'm planning to move to Natal (or Mallorca) for this reason. Even London, NY or Oslo makes more sense to me

mac

[Marcos - Munky]

If you connect on side of te heater transormer to the cathode, disconnect the CT from ground. As Paul said, you might have too much current across the power tube.
What about a rectifier bridge with schottky diodes to avoid adding more turns to the heater transformer.

Yesterday I've tried to connect the center tap to the cathode, and connect the heater to cathode leaving the center tap disconnected. Both gave me the same result as if I've grounded the center tap: hum. I'm almost sure the problem is in my house electrical system. Anyway, I'm thinking about a rectifier bridge, I'll need something that can handles 1,5A. But I'll try to test it somewhere that have a good electrical system before doing anything else.

Same down here.
But Brasil is way cheaper than Argentina. In fact, we are one of, if not, the most expensive country in the world.
I'm planning to move to Natal (or Mallorca) for this reason. Even London, NY or Oslo makes more sense to me

Here it's not that bad. Well, if you don't consider high taxes and corrupt politicians. Brazil have lots of different places and different landscapes. I wish I can move to somewhere else, it's just that living 30 years at the same place (even the same city) is kinda boring

[Marcos - Munky]

Duplicated post.

[mac]

Yesterday I've tried to connect the center tap to the cathode, and connect the heater to cathode leaving the center tap disconnected. Both gave me the same result as if I've grounded the center tap: hum. I'm almost sure the problem is in my house electrical system. Anyway, I'm thinking about a rectifier bridge, I'll need something that can handles 1,5A. But I'll try to test it somewhere that have a good electrical system before doing anything else.

A simple test.
Get a long wire, connect one side to the chassis, the other to a metal water pipe, water heater tank, etc.
A 20lts paint bucket full of water and some salt sometimes does the job.

mac

[Marcos - Munky]

I've tried to connect it directly to a "real ground" today: a metal bar buried about 2-3m in the ground. The hum is still there. I'll try to isolate the preamp stages to find where is the problem. Also, I'll change the power supply eletrolytics and use 12AX7s for the tubes. If the hum is still there after all of this, I'll give up and build it again from the beginning in january, using the correct transformers/tubes positions.