Tom Phillips wrote:
>
> I¹ve seen several 1960¹s Ampeg cathode biased designs with the 6.3V
> filament center tap connected to the cathodes of the power tubes.

[...]

> Can anyone comment on the advantages / disadvantages of this design?
> I know that biasing the filaments above ground will reduce hum but I
> haven¹t seen it done this way very often.

Elegant. I haven't seen it often, but it works as well as anything
else. At least in designs where the cathode resistor is bypassed,
because modulating the heater voltage might have a minor effect
on the operation of high-mu tubes, which could be good, bad, or
unpredictable.

From Dr.Distortio--(at)--bs.mhv.net Wed Feb 21 18:14:57 CST 1996
Article: 6660 of rec.audio.tubes
From: Dr.Distortio--(at)--bs.mhv.net (Dr Distortion)
Newsgroups: alt.guitar.amps,rec.audio.tubes
Subject: Re: Filament CT grounded to power tube cathodes
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Date: 21 Feb 1996 17:05:17 GMT
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Tom Phillips (tjto--(at)--est.net) wrote:
: Can anyone comment on the advantages / disadvantages of this design?
: I know that biasing the filaments above ground will reduce hum but I
: haven¹t seen it done this way very often.

Yes, biasing up the heater supply centertap to
a positive voltage helps to reduce heater-cathode leakage induced hum.
I've done this in a couple of my own amps, and it works well although I
wouldn't say the improvement is spectacular. You should not use the
power tube cathodes as a source for the DC heater bias unless they
(the cathodes) are bypassed with a cap.

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Biasing methods	Biasing	Channel Switching LDRs	Choke Input Filter
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