[Engineer]

Hi, Vacuumlanders,
Can anyone point me towards the schematic for a Heathkit A8A
integrated mono tube amplifier? (2 x 6SJ7, 6SN7, 2 x 6L6 and 5U4 - as
found.) I'm refurbishing one and it's nearly done save for the mic/
phono pre-amp.
The circuit up to the "aux" (called "xtal") input was easy to reverse
engineer and fix (someone had used a too high B+ filter resistor
feeding the 6SN7 and the two 6SJ7's.) However, the mic/phono stage,
1st 6SJ7, looks "odd" - can't quite put my finger on it. The gain is
low and the tone controls are strangely marked (by Heathkit) - need
original schematic!
Many thanks for all replies.
Cheers,
Roger

[TerryJ]

Available here.

http://www.historicphotoarchive.com/...athkit_a8a.htm


"Engineer" wrote in message
ups.com...
Hi, Vacuumlanders,
Can anyone point me towards the schematic for a Heathkit A8A
integrated mono tube amplifier? (2 x 6SJ7, 6SN7, 2 x 6L6 and 5U4 - as
found.) I'm refurbishing one and it's nearly done save for the mic/
phono pre-amp.
The circuit up to the "aux" (called "xtal") input was easy to reverse
engineer and fix (someone had used a too high B+ filter resistor
feeding the 6SN7 and the two 6SJ7's.) However, the mic/phono stage,
1st 6SJ7, looks "odd" - can't quite put my finger on it. The gain is
low and the tone controls are strangely marked (by Heathkit) - need
original schematic!
Many thanks for all replies.
Cheers,
Roger

[Engineer]

On Mar 10, 10:54 am, "TerryJ" suptjudatcomcastdotnet wrote:
Available here.

http://www.historicphotoarchive.com/...athkit_a8a.htm

"Engineer" wrote in message

ups.com...



Hi, Vacuumlanders,
Can anyone point me towards the schematic for a Heathkit A8A
integrated mono tube amplifier? (2 x 6SJ7, 6SN7, 2 x 6L6 and 5U4 - as
found.) I'm refurbishing one and it's nearly done save for the mic/
phono preamp.
The circuit up to the "aux" (called "xtal") input was easy to reverse
engineer and fix (someone had used a too high B+ filter resistor
feeding the 6SN7 and the two 6SJ7's.) However, the mic/phono stage,
1st 6SJ7, looks "odd" - can't quite put my finger on it. The gain is
low and the tone controls are strangely marked (by Heathkit) - need
original schematic!
Many thanks for all replies.
Cheers,
Roger- Hide quoted text -

- Show quoted text -

Thanks, Terry. Actually, I found this one earlier from a Google
search. I only want to confirm a few resistor values so don't need
the whole manual but I might break down and buy it, anyway, as an
accessory to this only semi-collectible amplifier (the A8A is a bit
primitive all round, e.g. shared 6L6 cathode bias resistor - not a
good idea!)
The fuzzy picture confirms the primitive tone controls and phono
equalization (can't see the values but I might presume the original
constructor used the correct ones so I can fill them in on a sketch),
so that's two mysteries solved. Also the first 6SJ7 is a straight
gain mic. amplifier with no equalization - no sweat to redesign if I
have to. Actually, I'm thinking of redesigning the whole front end
(plus the poor O/P stage bias) to more "modern" specifications, i.e.
proper phono EQ + tone controls, a legitimate upgrade IMHO.
Regardless, any original schematic still gratefully received...
Cheers,
Roger

[John Byrns]

In article . com,
"Engineer" wrote:

On Mar 10, 10:54 am, "TerryJ" suptjudatcomcastdotnet wrote:
Available here.

http://www.historicphotoarchive.com/...athkit_a8a.htm

"Engineer" wrote in message

ups.com...



Hi, Vacuumlanders,
Can anyone point me towards the schematic for a Heathkit A8A
integrated mono tube amplifier? (2 x 6SJ7, 6SN7, 2 x 6L6 and 5U4 - as
found.) I'm refurbishing one and it's nearly done save for the mic/
phono preamp.
The circuit up to the "aux" (called "xtal") input was easy to reverse
engineer and fix (someone had used a too high B+ filter resistor
feeding the 6SN7 and the two 6SJ7's.) However, the mic/phono stage,
1st 6SJ7, looks "odd" - can't quite put my finger on it. The gain is
low and the tone controls are strangely marked (by Heathkit) - need
original schematic!
Many thanks for all replies.
Cheers,
Roger- Hide quoted text -

- Show quoted text -

Thanks, Terry. Actually, I found this one earlier from a Google
search. I only want to confirm a few resistor values so don't need
the whole manual but I might break down and buy it, anyway, as an
accessory to this only semi-collectible amplifier (the A8A is a bit
primitive all round, e.g. shared 6L6 cathode bias resistor - not a
good idea!)
The fuzzy picture confirms the primitive tone controls and phono
equalization (can't see the values but I might presume the original
constructor used the correct ones so I can fill them in on a sketch),
so that's two mysteries solved. Also the first 6SJ7 is a straight
gain mic. amplifier with no equalization - no sweat to redesign if I
have to. Actually, I'm thinking of redesigning the whole front end
(plus the poor O/P stage bias) to more "modern" specifications, i.e.
proper phono EQ + tone controls, a legitimate upgrade IMHO.
Regardless, any original schematic still gratefully received...

Roger, are you sure the first 6SJ7 is a straight gain stage? The high
frequency equalization was usually done by selecting the value of the
input resistor to ground so that it provided the required roll in
conjunction with the inductance of the pickup cartridge. It is hard to
tell from the blurry photo but it appears that there is an RC network
connected to the selector switch which may be switched in to furnish the
low frequency equalization.

The tone controls don't seem seem all that "primitive" and are right in
tune with the amplifiers times, providing variable treble roll off for
getting rid of annoying clicks, pops and static, while the variable bass
boost control provides the bass heavy sound that was favored by many.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Engineer]

On Mar 10, 12:59 pm, John Byrns wrote:
In article . com,





"Engineer" wrote:
On Mar 10, 10:54 am, "TerryJ" suptjudatcomcastdotnet wrote:
Available here.

http://www.historicphotoarchive.com/...athkit_a8a.htm

"Engineer" wrote in message

oups.com...

Hi, Vacuumlanders,
Can anyone point me towards the schematic for a Heathkit A8A
integrated mono tube amplifier? (2 x 6SJ7, 6SN7, 2 x 6L6 and 5U4 - as
found.) I'm refurbishing one and it's nearly done save for the mic/
phono preamp.
The circuit up to the "aux" (called "xtal") input was easy to reverse
engineer and fix (someone had used a too high B+ filter resistor
feeding the 6SN7 and the two 6SJ7's.) However, the mic/phono stage,
1st 6SJ7, looks "odd" - can't quite put my finger on it. The gain is
low and the tone controls are strangely marked (by Heathkit) - need
original schematic!
Many thanks for all replies.
Cheers,
Roger- Hide quoted text -

- Show quoted text -

Thanks, Terry. Actually, I found this one earlier from a Google
search. I only want to confirm a few resistor values so don't need
the whole manual but I might break down and buy it, anyway, as an
accessory to this only semi-collectible amplifier (the A8A is a bit
primitive all round, e.g. shared 6L6 cathode bias resistor - not a
good idea!)
The fuzzy picture confirms the primitive tone controls and phono
equalization (can't see the values but I might presume the original
constructor used the correct ones so I can fill them in on a sketch),
so that's two mysteries solved. Also the first 6SJ7 is a straight
gain mic. amplifier with no equalization - no sweat to redesign if I
have to. Actually, I'm thinking of redesigning the whole front end
(plus the poor O/P stage bias) to more "modern" specifications, i.e.
proper phono EQ + tone controls, a legitimate upgrade IMHO.
Regardless, any original schematic still gratefully received...

Roger, are you sure the first 6SJ7 is a straight gain stage? The high
frequency equalization was usually done by selecting the value of the
input resistor to ground so that it provided the required roll in
conjunction with the inductance of the pickup cartridge. It is hard to
tell from the blurry photo but it appears that there is an RC network
connected to the selector switch which may be switched in to furnish the
low frequency equalization.

The tone controls don't seem seem all that "primitive" and are right in
tune with the amplifiers times, providing variable treble roll off for
getting rid of annoying clicks, pops and static, while the variable bass
boost control provides the bass heavy sound that was favored by many.

Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/- Hide quoted text -

- Show quoted text -

Good forensics, John... I'll take another look but I'm pretty sure
it's straight mic. gain. I've saved that fuzzy picture form the adv.
and will run it through photoshop to see if I can get some values out
of it. As for the tone controls, you are right - they are crude top
cut and bass boost only. My idea is to replace then with, say, a
Baxandal (sp?) circuit from the 1950's with +/- 10 dB wrt 1KHz at both
ends.
Another idea is to junk the first 6SJ7 stage altogether and use the
socket to power a spare Heathkit WA-P2 preamp (with proper tone
controls) that I have lying about the place, but that looses the mic
stage and leaves me with just an "A8" amplifier with surplus knobs.
I'll see... might be more fun to rebuild the A8A front end
properly.
Cheers,
Roger