My Jolida amp connects the center tap of the AX7 filaments to ground and the ES sheild of the power trannie. The filament supply is 12.6V with a center tap connected to ground. I have a pair of 7058s (like the 7057 but with higher mu) tubes in place of the AX7s and they sound glorious in the Jolida. I'm wondering if it is the quality of the tube or the disconnect of the filaments from the common ground that cause the sonic improvement? The latter sounds unlikely but........

Tom §.

 Re: Bias of filaments on Foreplay. - Doc B. 09:08:16 7/16/99 (2)
 Re: Bias of filaments on Foreplay. - Tom §. 09:39:08 7/16/99 (1)
 incessant questions - Doc B. 12:10:38 7/16/99 (0)

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 Re: Bias of filaments on Foreplay. - Doc B. 09:08:16 7/16/99 (2)
 Re: Bias of filaments on Foreplay. - Tom §. 09:39:08 7/16/99 (1)
 incessant questions - Doc B. 12:10:38 7/16/99 (0)

The H-K leakage issue is one that is sometimes brushed aside in designs, because H-K leakage won't necessarily occur immediately, but rather may come on slowly, as a faint but ever increasing background hum. So in that sense, yes, it definitely improves the noise floor, at least over time, to float the heater closer to the cathode's potential. The H-K differential should be kept less than 100V, the lower the better, regardless of the tube. About the only tube I've seen that can take more than 100V difference is the 5965.

In terms of using the 7057 this is not a real problem. A common misconception with applying a DC potential to the heater buss is that it has to be attached at some specific point in the circuit. Think it thru, the heater circuit is really an AC circuit (loop), it just doesn't matter where the DC is attached, it still floats the whole loop at the same potential.
Soo, just attach the DC to pin 4 or 5 instead of pin 9, no problem.

The real challenge ahead of you would be to completely recalculate the operating points of the circuit to optimize the direct coupled cathode follower setup for the 7057. This is a fiddly procedure, change one value and everything else changes. Arm yourself with curves for the 7057 and the math necessary to calculate cathode follower operating points, and keep in mind the limitations in available B+ voltage and current determined by the specs of the power trans. And also the 12.6V secondary's 300 mA max rating.

If the 7057 has a lot more gain than the 12AU7, the volume control will become very hair trigger. We are working up a new stepped attenuator kit that will offer a choice of 0 to -30 db or -20 to -50 dB attenuation to help the 'too loud' situations.

Not to say you can't just plug the tube in and try it. A lot of folks do that with 12AT7s or 12AX7s. It will work, but will it work well?

Question two-

The answer to your question will be found by plugging the 12AX7 back in and lifting the pin 9 ground connection points. This seems to me to be creating a ground loop, since the 12.6V secondary is already grounded at it's center tap.
The best way to do this is with a scope attached to the output of the amp, set to observe the 60Hz and 120Hz in the millivolt range. Watch to see if the noise floor increases or decreases when that connection is disconnected. Of course you gotta listen to it too...

Damn....that is so obvious to me now. I'm smacking myself in the head....

>>We are working up a new stepped attenuator kit<<

Sounds very interesting.

>>Arm yourself with curves for the 7057<<

If I can find any ;(. I did find this:

7057
150V plate @ 10mA will have (5300 Rp)&(36 mu)&(6800 micro mhos)

12AU7
170V plate @ 10mA will have (7200 Rp)&(18 mu)&(2500 micro mhos)

Looks like the 7057 will be a more appropriate sub than a AX7 or AT7.

Thanks for putting up with and answering my incessant questions Doc,
Tom §.