6X4 with EL95 PP

[Shannon Parks]

Do your filaments have a ground reference somehow? Use the center tap to ground if available, or use two 1K resistors as a voltage divider across them to ground. Also be sure your RCA input isn't grounded to the chassis.

Shannon

[spamer]

Thanks for the response Shannon.

No, the filaments do not have a reference to ground or a center tap so that is something I need to do. What should be the rating for these resistors? 1/2W?

Your input ground comment brings up a few newbie questions.

My ground configuration is to one common point which is connected to the chassis. From that same point I connect the mains earth ground.
Is this correct?

If so then I am not sure where to ground the input since I have everything connected to the chassis.

Instead of my current configuration should I have mains earth to the chassis only and then circuit ground to the common point but not connected to chassis or mains ground?

Thanks Again,
Steve

[spamer]

After referencing my heaters to ground as Shannon suggested, and reading many articles about star grounding and hum problems, I tried the following:
reconfigured my star ground such that the phase splitter cathodes and the input are grounded together which is then grounded back to the star.
Filter caps, power tube cathodes, go to the star
earth ground is connected to chassis right next to the input
ran line from star to same chassis point as earth
heater resistors go to the same chassis ground point
rewired my heaters to try and stay away from the circuit and sit flat against the chassis as best I could.
moved the output xfmr to the top of the chassis and turned windings 90 degrees to the power xfmr

There is an intermediate tap on the power transformer that is not exactly in the center. I ran that to the start ground and it sounded better so I left it.

There is a 2nd heater ouput in the power transformer that I am not using. I just taped the ends separately. Should this be referenced to ground?

But it still hums. There are two different sounds that I hear. Without the feedback connect, after power up and the everything is warmed up and stable there is a faint buzz. I am guessing this is the B+ filtering.

When I connect the feedback to the output to I get a strong hum. It sounds like 60hz to me.

Assuming that problem is with the splitter, I tried reflowing the solder in the phase splitter connections but it didnt make a difference.

Any ideas I can try? I know I am in past my knowledge at this point but that is why I wanted to try handwiring. Thought maybe I could learn something. Any chance the transformer could be bad? It is close to 50 years old. I would think that the behavior would be different though.

Thanks for any ideas.

Steve

[DeathRex]

Can I see a schematic of the whole thing? And a new picture of the underside.

Wow usually putting in feedback reduces hum.

Need to see the top of the chassis too.

Goto MIT and steal a oscope. Might be some in their trash bin.

[spamer]

Thanks for the response.

Schematic is here:

http://www.flickr.com/photos/31146530@N06/8222939336/

Pictures

http://www.flickr.com/photos/31146530@N06/8221601429/
http://www.flickr.com/photos/31146530@N06/8221601579/
http://www.flickr.com/photos/31146530@N06/8222677152/

In the configuration shown, I went back to a diode bridge and removed the 6CA4 thinking it might be the problem and the original circuit had a bridge.
Results were the same. The plan was to get it going and then add in the 6CA4.

I have been trolling Craigslist for an oscope. Will probably have to pay a fair bit to get something that works.

Thanks again for any help.

Steve

[20to20]

Is your speaker common post grounded? It's not drawn that way but in all the feedback to driver cathode circuits (GFB) I'm aware of (admittedly limited) the speaker ground to chassis is required.

HTH

20

[spamer]

Bingo! That was it. Everything went quiet when I grounded the speaker common post to the chassis ground.

Many thanks for helping me out.

Steve

[20to20]

Bingo! That was it. Everything went quiet when I grounded the speaker common post to the chassis ground.

Many thanks for helping me out.

Steve

Sweeet. Nice to hit one on the first shot, once in ...forever.

20