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In Reply to: RE: Heathkit AA150 posted by fstein on April 01, 2012 at 19:49:40
AA-151 ? A great amp that may have been let down by the preamp. The Amp section is essentially a stereo version of the UA-1/2. (A stereo basic amp was sold AA-111....GREAT AMP).
I have owned several AA-151 and the earlier SA-2 integrateds over the years. And while they sound ok, I have always felt that the preamp was the weak link and give the sound an opague/veiled quality.
That said, I had a brief conversation with Peter Brueniger at the recent Axpona show. I noted in his TAS online bio that he used the AA151 so I asked him about it. He raved about the amp, but with the provision that it should be fully rebuilt before listening and preferably by Sam Kim in Toronto.
There you have it- 2 separate opinions of the same unit.
Personally, I think the truth lies somewhere in the middle. The units I owned were serviced to replace worn parts, but were not fully and carefully rebuilt. Maybe a rebuild with new higher quality parts would be enough to improve the preamp section ?
In any event the amp is inexpensive and easy to work on.
If the amp is that good, what me about running my Peachtree decco (great D/A, Good preamp, so so amp) as the preamp?
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.
"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, which Antek model would you use for a pair of 12AX7LPS.
edit: sorry didn't see it a first.
One thing that will stress the 5AR4 is the first cap after it being 60 uF. 30-40 UF would be easier on that tube as most amp makers like Dynaco & Scott
used in that position.
You could try your preamp. There is more info at diytube.com and audiokarma on changing the driver tube in the amp section and bypassing and improving preamp. Also on improving the phono preamp like Eli mentioned.
I looked at Sam Kim's web site. It seems that his AA-151 rebuilds include a full conversion of the preamp to triode operation as well as modifications to the power supply and amp section. His most extensive rebuild includes the ability to switch between triode and ultralinear operation.
Point is that to extract the best from this unit, modification of the preamp design is needed.
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