loftin white 801 amp

[Sheldon]

I got inspired by Darius's musings on Loftin White amps, so I modified my 801 resistor loaded amp into a Loftin-White amp, but using a triode for the input tube, as I don't need the high sensitivity of the original.

The power supply noise nulls out with the pot set at 10k (37k total), which is very close to the predicted value. The resistor chain divided by u (37k/20) should equal the resistance of the input cathode resistor (18k). Seems very quiet. I get about 0.9W output at 2.6vRMS input. Frequency response on the low end is about 100Hz before the transformer saturates. The -3db point on the upper end is 41kHz. Square waves look good. Good enough to go ahead and build it out. I won't use it full range, so I wound the transformer for the top end. I'll probably go with something like 12-14k for the final pair of OPT's.

Amp

[Sheldon]

Supply

[oldeurope]

Hello Sheldon,
congratulations for making worlds first 801 Loftin White amp
and sharing it with us here in the forum.

Clever to use an indirectly heated rectifier tube to
get a soft start for the 801.

Don't forget this cap: post 13

I recommend to make the 6SN7 AC heater voltage symmetrical.
(Two resistors to 26VDC rail.)

Kind regards,
Darius

[korneluk]

Originally posted by Sheldon
The power supply noise nulls out with the pot set at 10k (37k total), which is very close to the predicted value. The resistor chain divided by u (37k/20) should equal the resistance of the input cathode resistor (18k).

Sheldon,

The schematic shows a 6SL7, which has a u=70, not 20, and the cathode resistor is 1.8k not 18 like in your description. Something does not jive in your calculations.

-- josé k.

[oldeurope]

Hello josé k.,
just ensure to get unity gain from cathode of the output triode
to the anode of the driver triode.
This makes the driver signal floating on the cathode level of
the output tube.

Kind regards,
Darius

[Sheldon]

Originally posted by oldeurope
Don't forget this cap: post 13

My German is not too good, but I interpreted your comments to suggest that it might not be technically required. I figured to try it without first, then try it with the extra cap, after I had a chance to wire thing up neatly and listen in stereo. Have to wind up a couple of cores first.

Originally posted by oldeurope
I recommend to make the 6SN7 AC heater voltage symmetrical.
(Two resistors to 26VDC rail.)

I have that, just not shown on the schematic that I quickly threw together. My haste explains the errors noted by Jose. It is SN7 and the resistor is 1.8k. I'll correct the schematic.

Originally posted by korneluk
The schematic shows a 6SL7, which has a u=70, not 20, and the cathode resistor is 1.8k not 18 like in your description.

Sheldon

[Sheldon]

Here's what it actually looks like now, with errors fixed. One channel with transformer, one channel just wired to bias up the tubes. I did this to load the power transformer so I could check voltages with the stereo load.

Not noted on the schema is that each 801 has it's own filament supply.

Sheldon

[Sheldon]

Finished up a second transformer and did some testing. The final cap to ground after the PS choke, as suggested by Darius, does make a significant difference in residual hum. Without the cap, I hear hum (maybe 3mV or so), with the cap it's less than 0.5mV. Not shown on the schematic are fuses in the B+ line, in case of input tube failure.

Clip occurs at 2.6V in and, coincidentaly, 2.6V output, which calculates to about 850 blazing milliwatts of power.

The -3dB point for one transformer is 65kHz, and the other is 45kHz, which suggests that my winding technique needs a little work. Saturation at full power starts to show up at 100Hz.

At about 1V input (Im guessing here, as that is with full output on the soundcard) I see the 2nd at -60dB, 3rd at -70db, and some 4th and 5th below 100dB.

Here's the updated schematic.

[Sheldon]

Here's my winding jig. My mom's old sewing machine (she was an excellent seamstress). Note the counter, actuated by the nut that holds the sewing needle. The bobbin slides on a fixture attached to the main shaft.

[Sheldon]

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