Connecting heater-cathode on SE power amp

[jordan86]

I'm chasing down a nasty 60hz hum in an old AX84 build that a friend built me years ago. My amp does not have a CT on the filaments, real or artificial. I was just looking at the schem and layout again to see if it calls for one, and it does not.

Then I noticed pins 7 and 8 tied together on the SE power tube. Never seen that. Seems really odd. 

Is this some sort of elevated heater scenario? I'm wondering if removing the connection from pins 7-8 and placing a humdinger or CT off the pilot lights might clear up my issue.

[echuta13]

Hmm... if you look at connection "A" on the filament line, it connects to the output tubes cathode which should be for dc on filament (which makes me think the 7-8 pin connection is redundant?) 

[jordan86]

Yeah, I've been reading up on directly heated cathodes but I don't think any common power tubes qualify as that. So not sure why you would connect a filament line to a cathode. Perplexed?

[SILVERGUN]

Look at the two [A]s on the schematic. The line from pin 7 to pin 8 is just longer on the schematic then on the layout. They just decided to make the connection at the tube socket.
Yes, this is heater elevation and according to the schematic it would be 30.5VDC in this case.

[DummyLoad]

elevating heaters without a virtual CT? never tried that. seems to me elevating heaters would have lowest hum with DC connected to a virtual CT.


--pete

[mresistor]

I've heard of people elevating the filaments on 60s Champs, although I've never tried it .

[sluckey]

Is this some sort of elevated heater scenario? I'm wondering if removing the connection from pins 7-8 and placing a humdinger or CT off the pilot lights might clear up my issue.

Yes, that's elevated heaters, although not the optimum way to do it. I would remove the jumper between pins 7 and 8. Then put a 100Ω 1/2W resistor between pins 7 and 8. Put another 100Ω 1/2W resistor between pins 2 and 8.

[jordan86]

Yes, that's elevated heaters, although not the optimum way to do it. I would remove the jumper between pins 7 and 8. Then put a 100Ω 1/2W resistor between pins 7 and 8. Put another 100Ω 1/2W resistor between pins 2 and 8.

Thanks, Sluckey. So am I correct to infer from your suggestion that a center tap (another AC reference to ground) is not necessary when elevating heaters?

[sluckey]

The two resistors form the artificial center tap and C5 provides the AC reference to ground.