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Just bought this very cool and retro looking el84 amp on eBane. Should have taken my medication.
So, what does anyone know about this amp?
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I went thru my Heathkit set (AA-121, AA-151, AA-201) a few months ago, and of the 3 the 151 showed the most improvement - the 121 was already verrry nice, I did the recap and bias rebuild to accommodate a rebuilt power transformer - the 201 is an el cheapo SE el84 amp with a nice voice, otherwise not a lot of audiophile refinement or sophistication, but the 151 ... wow. It was already one of my mostest favorite amps, but with a FULL recap, every one single one replaced with at least good-quality, if not top-notch caps and some resistors at the filter caps, it is really a very fine amp: refined, loads of bass punch, supersweet mids and, yes, airy highs. I use an Empire or GE VR cartidge, so the phono section gets enough swing to drive the phono section, even 6eu7s, and anyway, I don't ever recall being particularly upset about the legendary AC on the heaters, anyway ;)
OK, I admit that I have all the original Mullard tubes, and the "good" transformers like Ron mentioned.
I use a set of sorta-contemporary Heathkit speakers which are really JBL Lancers, OR a pair of DiY Buschorns with FE-108EZ drivers, and the only reason it's not my main amp is because I have the 121, a late-70's HK amp, and a big-ass Philips Lab Series receiver to make it tough competition. I'm not so far gone that I would spend multi-thousands to beat it, but from what I hear from listening to really top-notch and top-dollar gear at various salons around town, I would have to to even get close to excelling it, so ... that's my story.
Go thru it, thoroughly, with the attitude that you have a time machine that has delivered you 2012-quality caps to 1962, check the resistors (esp in the phono section, if you are a vinyl guy), keep it stock and use a suitable cart and easy speakers, and you will have a very desirable, very pleasant and rewarding amp to use.
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.
Best,
Ross
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.
Eli D.
"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
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.
Randy
Edits: 04/02/12 04/02/12
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.
Best,
Ross
oops- AA151!
A couple hundred returns awaits an asylum-wide forum search for aa 151. A copy of the assembly & operation manual is useful for restoration plus ensures correct assembly or modification-reversal(these were user-assembled from supplied parts & instructions); original manuals may be sourced from ebay or copys from specialty manual vendors. I have one plus its' matching AM/FM tuner. Mine has the desirable Stancor outout transformers with cloth-covered leads; a Google search for EIA Codes will return the decipher. A Google search for Heathkit returns a link to the Heathkit Virtual Museum with some info & specs for the AA-151.
A link to the left side of the AA151 schematic follows below. Change aa151l into aa151r, to view the right side.Heath used RCA's phono preamp (graphic uploaded). The triodes in a 6EU7 are identical to those in a 12AX7/7025. AC heating is employed in the phono section, which is barely tolerable only because the 6EU7 contains a spiral wound, hum bucking, heater. Given sourcing realities, rewire the phono section sockets for 12AX7s and install Sovtek 12AX7LPSes. The 'LPS is a genuine 7025 equivalent and contains the crucial spiral wound heater. :> )
Eli D.
Edits: 04/02/12
You must recap it as you risk the transformers. Check resistors too. There is also info on the diytube.com site by some that used different driver tube in amp part. On diode mode UF4007 are newer, faster and quieter for a few cents more. Will help rectifier tube last with no loss in sonics.http://www.audiokarma.org/forums/showthread.php?t=432956
http://www.audiokarma.org/forums/showthread.php?t=331683
Join Eico and Heathkit yahoo.com sites for manual and info. Also check AK (audiokarma) database.
Randy
Edits: 04/02/12 04/02/12 04/02/12 04/02/12
I'd be careful in putting in SS diodes - this guy runs the EL84's at pretty high voltage to start with, the SS diodes will make that worse. FWIW, I've restored a couple of these. The AC heating in the phono section can result in a bit of hum! Also, note how much each of the other inputs is "padded down" to match the phono level going into the 2nd 6EU7. If you get rid of that you can over load the input tube and/or have a hair trigger on the VC. This can be a very nice sounding amp, but as noted, needs a bit of work (like recap, etc.).
The diode trick shown in the pictorial does not raise the B+ rail voltage, whatsoever. What it does is provide some extra PIV headroom, which can be critical, when current production, Russian, 5AR4s are employed. The Russian tubes can supply the full 250 mA., but they can be troublesome at the upper end of the documented voltage range. The SS diodes protect against start up arcing, etc.
Chinese 5AR4s can take the full documented voltage, but can't deliver the full 250 mA. All of JJ's Octal production is no better than HIGHLY suspect. All in all, Russian 5AR4s are the best of current production and, with the diode trick, acceptable for use. NOS prices are SKY HIGH, which leaves very little wiggle room. :> (
Eli D.
I believe the Russian 5AR4 will survive nicely in an AA151 w/o diode mods. I've got one in my 151 and it's been there for nearly a decade w/o a trace of misbehavior. It would probably be stressed in something like a MK3 or an ST70 but it seems quite at home in the 151. I've also had a quad of 6P14P-ERs in it for about the same period and they too have worked out very nicely. They show no evidence of wear after probably 3-4K hrs of use. I even like the sound although this is a personal thing. Highly recommended for this amp.
Edits: 04/02/12
Goes to show you, I should have looked more closely at what was being done with the SS diodes!
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