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In Reply to: RE: Heathkit AA150 posted by fstein on April 02, 2012 at 19:59:00
You may find that the Heath power section has too much gain. The pentode section of the 6AN8 partially makes up for losses in the tone control circuitry and also produces the drive needed by the "finals".
FWIW, I'm not at all surprised by the poor preamp remarks being made. Heath's tone controls SUCK. Really good tone controls need to be 100% out of circuit, unless the rare recording that's actually helped by them is being played. They also have to be "unity" gain, which requires a fair degree of circuit complexity.
Were it my amp, the tone controls would be eliminated and I'd retain the power section small signal topology, but switch to a 6GK5 voltage amplifier and a section of an ECC99 as the phase splitter. The 6AU6 would go, as its gain is completely unnecessary, when the tone controls are eliminated. A 6GK5 would go into the socket currently occupied by a 6AU6 and an ECC99 would go into the socket currently occupied by a 6AN8. Set the ECC99s up so that each channel uses a different section. When the ECC99s show signs of wear, flip/flop them and get double the service life.
A proper DC supply for the phono section heaters can be constructed around an inexpensive AnTek model AN-0212 toroidal trafo.
Eli D.
Follow Ups:
"A proper DC supply for the phono section heaters can be constructed around an inexpensive AnTek model AN-0212 toroidal trafo."
Eli, could use some verification on how to wire this.
Thinking wiring secondaries in series to get 24vac wired to a full wave bridge.
Off this to a CRC then power the heater in series to pin 4 out of pin 5 to other tube pin 4 then pin 5 to ground.
If that is right would the caps need to have its positive side to ground.
A 300 mA./12 VDC supply with that trafo is easy. Wire the 2 secondaries in series. Full wave center tap (FWCT) with a MBR20100CT common cathode twin Schottky diode. Filter with a 4700 μF./25 WVDC 'lytic. Follow the filter with a 7812 3 terminal regulator. I would let the heater supply "float". You can always ground the 'X7 heater via a cap. connected to pin 9, if hum or other noise raises its ugly head.
Eli D.
Thanks Eli
Eli, which Antek model would you use for a pair of 12AX7LPS.
edit: sorry didn't see it a first.
Edits: 04/04/12
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