A 6BQ5 Pushpull Amplifier - Motorola HS-696C
Posted: Mon Apr 15, 2013 5:03 pm
I came into a possession of a stereo amplifier and its front panel that came from a 60s vintage stereo console.
The amplifier chassis appears to be made of zinc-coated steel approximately 12" x 8" x 1/2" in dimension.
It's in a good condition albeit with a layer of rust and dust.
Marked as HS-696C.
It has one 5U4GB (GE), two 12AX7s (RCA/Motorola), and four 6BQ5s (GE).
The multi-section cap contains four caps: 40uF/400V, 40uF/400V, 30uF/400V, and 200uF/25V.
A preliminary inspection shows that the quad of 6BQ5s share a common 82-Ohm 5W cathode resistor and the 200uF cap in the CanCap.
Also, the screens of the 6BQ5s are all tied together suggesting that they are pentode connected.
(Click the photo for the higher resolution version.)
All the controls are on the "front panel" assembly which is connected to the amplifier chassis via the
umbilical and two RCA audio cables.
The front panel assembly has two 12AX7s on board:
All the pin and socket connections were cleaned with contact cleaners, the front panel assembly
was connected, and dummy loads and shorting RCA phono plugs were attached.
The amplifier was then gradually powered up through a variac without any magic smoke escaping.
With the signal source and the speakers connected, the amplifier sounded really well, almost as good as
a Dynaco Stereo 35 clone.
Reverse engineered schematic for Motorola HS-696C with actual measured voltages:
(Added schematic v0.1 on 4/17/2013)
(Updated Schematic on 4/23/2013)
(Updated Schematic on 5/06/2013)
(Updaged Schematic on 4/25/2015)
The amplifier chassis appears to be made of zinc-coated steel approximately 12" x 8" x 1/2" in dimension.
It's in a good condition albeit with a layer of rust and dust.
Marked as HS-696C.
It has one 5U4GB (GE), two 12AX7s (RCA/Motorola), and four 6BQ5s (GE).
The multi-section cap contains four caps: 40uF/400V, 40uF/400V, 30uF/400V, and 200uF/25V.
A preliminary inspection shows that the quad of 6BQ5s share a common 82-Ohm 5W cathode resistor and the 200uF cap in the CanCap.
Also, the screens of the 6BQ5s are all tied together suggesting that they are pentode connected.
(Click the photo for the higher resolution version.)
All the controls are on the "front panel" assembly which is connected to the amplifier chassis via the
umbilical and two RCA audio cables.
The front panel assembly has two 12AX7s on board:
All the pin and socket connections were cleaned with contact cleaners, the front panel assembly
was connected, and dummy loads and shorting RCA phono plugs were attached.
The amplifier was then gradually powered up through a variac without any magic smoke escaping.
With the signal source and the speakers connected, the amplifier sounded really well, almost as good as
a Dynaco Stereo 35 clone.
Reverse engineered schematic for Motorola HS-696C with actual measured voltages:
(Added schematic v0.1 on 4/17/2013)
(Updated Schematic on 4/23/2013)
(Updated Schematic on 5/06/2013)
(Updaged Schematic on 4/25/2015)