Tom Phillips wrote:
>
> Išve seen several 1960šs 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
> havenšt 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
: havenšt 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.

Index:

AB crossover	Amp Classes Defined	Amp Classes Explained	Amp Mods Harmful
Bass Cutoff Frequencies	Bias adjust safeguard	Bias Cathode Resistors	Bias Shift
Biasing methods	Biasing	Channel Switching LDRs	Choke Input Filter
Circuit Memory	circuits	Drop B+	Eyelet board
Fender Response Curves	FET source follower	Filament Cathode Ground	Fixed to Cathode Bias
FM Transmitter Voltages	Grid Blocking	Identify Power Trans Leads	Load Impedance
LV No PC boards	Output impedances	PC Boards	PCB amp problems
Phase Inverter Resistors	Pilot Light Tone	Rectifier voltage drops	Resistor Design
Resistor Max Voltage	Resistor noise	Reverb Tank Types	Reverb tanks
Screen Grids	Standby Switch Ratings	Standby switches	TAME Digest
Tone Circuits	Transformer Design Book	Transformer Info	Transformer Placement
Zener Voltage Drop