lGl-2, continuing "hybrid madness" - no GNFB class A

[still4given]

Hi Jeff,

My pleasure. I love doing Valery's designs because he is always right there to see it through. I hope this fix gets you going as well. It's a pretty cool little amp. I'll probably finish this one and build a case for it. With needing three transformers, two PSU and protection there are a lot of wires to connect. If it continues to sound as good as it does right now it will be worth it.

It wasn't difficult mounting everything on the other side of the board.. I recommend it.

Blessings, Terry

 

[jwilhelm]

I've been wanting to hear this one. If I like how it sounds I'm going to redesign the boards to fit in a TubSomo input. This ones too tight in a Slewmaster input size to do a proper layout.

 

[still4given]

I do have a little hum. I can usually find the culprit by moving wires around but that does nothing with this one. Tomorrow I will try setting up a star ground and see if that helps. The music sounds lovey.

Blessings, Terry

 

[still4given]

In the quest to find the answer I left the test speaker plugged and tried unplugging each transformer. Unplugging the rail transformer didn't change anything but unplugging the small 230V/6.3V transformer stopped the hum. So now I'm going to try using a separate transformer for the 6.3V and see if that changes anything. I'll report back.

 

[jwilhelm]

Would these tube inputs benefit from a lifted ground on the input like OS does on his Slewmaster inputs?

 

[vzaichenko]

Cool I will add 470pF caps to my prototype as well. Terry, you're right - it likes the soft-start, connecting the load after the tubes are warmed and the whole thing is up and running.

 

[still4given]

Any idea why unplugging the small transformer kills the hum when disconnecting the 72V transformer doesn't?

Edit: Now that I think about it, there is no ground connection on that transformer and PSU board.

 

[still4given]

Hi Guys,

OK, this is what I have discovered. This morning I decided to hook up only one channel to see if maybe they were reacting with each other. No luck, the hum was still there. However, while moving things around I touched one of the open 6.3V wires with my finger and the hum all but disappeared. This transformer has dual secondaries for both 230V and 6.3V. I am only using one of the 230V pairs but both of the 6.3v, one for each IPS. So next I tried just grounding one side of the open 6.3v leads through a 10k resistor and the hum is now only barely there. Looks like it is the 6.3V circuit that is causing this grounding problem so now I'm asking what is the best way to ground this to do away with the hum without hooking something up wrong.

Thanks, Terry

 

[borys]

Terry
I am using heater connection as bellow.
Regards Peter

 

[still4given]

I just tried using a separate 6.3V transformer and the hum is gone. So now I need to know why I can't use the Antek for both voltages. For some reason the windings are reacting with each other. Any ideas?

 

[borys]

Try to use only one 6,3V ac winding for both channels , heater must have some reference point. Usually it is the earth but here with so high cathode voltage I am not sure.

 

[jwilhelm]

I just tried using a separate 6.3V transformer and the hum is gone. So now I need to know why I can't use the Antek for both voltages. For some reason the windings are reacting with each other. Any ideas?

I've always wondered if the 6.3 could be rectified and regulated to eliminate the chance of hum. Possibly a small cap or inductor may help too.
I think there may be a safety issue if you ground the 6.3V because of the close quarters with the 250V in the tubes. I think it's supposed to float.

 

[still4given]

Ok, using Bory's suggestion above except I used two 10k resistors where he shows 100R and it worked. The ground hum is gone. With my ear right next to the speaker there is a little noise but no ground hum. That little bit of noise may just be because of all the open wiring running all over the place. I'll hook up the other channel now and make sure all is good.

Blessings, Terry

 

[vzaichenko]

I've got no hum issues at my prototype, but Boris's approach with grounding the middle point of two resistors makes a lot of sense. Thanks for bringing it up and testing!

 

[still4given]

I figured you didn't have any hum which is why I've been concerned. The only way I could find to get rid of it was to use a separate transformer for the 6.3v. That is of course, until I tried Bory's approach. I'm not sure why my Antek transformer is reacting differently than yours. I am actually seeing about 6.7V on mine so I may try lowering the values of the two resistors until I get closer to 6.3V. I am really looking forward to A/B'ing this against some of my other amps. I may actually be able to hear some real differences.

Blessings, Terry

EDIT: I just changed the resistors to 100R and there was no change. Still 6.7vac but everything seems nice now so I will just leave the two 100R in there.

 

[vzaichenko]

6.7V AC is good enough. Well, later on, you can use some small (but powerful enough) R in series to make it perfect, as Jeff did earlier.

 

[still4given]

I remember reading that but am having trouble finding it. I will keep searching

 

[still4given]

Hi Valery,

I just read back through the thread and I can't find it. Could you please explain how to reduce the 6.7v to 6.3v?

Thanks, Terry

 

[vzaichenko]

Found it - in >TubSuMo thread<

Put the resistor in series with the paralleled heaters.

Having around 600mA through the heaters (2 tubes together), we want to drop 0.4V, giving us 0.4/0.6=0.67R
Dissipation - 0.4*0.6=0.24W

So, let's do some rounding (R - a bit lower, W - a bit higher), giving us 0.63R / 0.5W
In fact, you can try 0.47R, for example, and see the result. You don't have to try getting 6.3V exactly, just getting closer to it is ok. As I mentioned, even 6.7V is acceptable.

Cheers,
Valery

P.S. Jeff had it at 7.5V initially, which was a bit too high - that's where this point came from...

 

[still4given]

Thanks Valery,

I knew I had read it somewhere. If 6.7v is OK I will probably just leave it alone. The amp is very quiet now. I think everything is the way it should be. I will get it set up on my A/B system today so I can compare it to some other amps today. This was a very interesting build so I am hoping its sound is something special. I may go to a little lower voltage on the rails. I have a few transformers in the +-63v range. That might be better for 4ohm. I'll report back

Blessings, Terry