MikeS
03-20-2006, 01:38 PM
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Curious to hear the rave about the 12B4A tube, I found one around here that tested fairly strong.
Couldn't find two 12B4A's, so it's a mono linestage.
I used the control unit chassis for a reverb box from a old Admiral 6BQ5 push-pull console stereo. The main 6BQ5 PP amplifier chassis had a 117 VAC power take-off point to the reverb control unit chassis.
The control unit has it's own little PS transformer that powered a 6X4 full-wave rectifier, a 12AU7, and a 12AX7.
The heater current for 12AU7/12AX7 at 12.6 volts is around 300 milliamps, 600 milliamps for both, sumthin' like that...
One 12B4A's heater current is at 600 milliamps, so I knew the little PS xformer filament winding would swing one 12B4A, but probably not two.
(Another reason why it's mono...)
I messed the Duncan PS program, and came up with a rather primative supply, CRCRC, with a 3K plate load resistor. (It still needs a bit more refinement, there is still some noise.)
The tube rating for 12B4A is 150 VDC on the plate, at around 34 milliamps plate current. With -17 volts on the grid.
Amp factor of 6.5.
It's a mini nine pin tube, with a single triode in the bottle. It has a lowish plate resistance of 1030 ohms, and a transconductance of 6300.
I'm running the 12B4A's plate at 100 volts, with -10.5 volts on the grid, and 10.5, 11 volts on the cathode. At around 25-28 milliamps plate current.
100K volume pot, with a 220 ohm suppressor resistor, with 470K to ground. (There is a teeny bit of hash, but that's through JBL 075 tweeters.)
A green russian 0.47 uF 600 volt cap for the output coupling cap, with a 470K to ground.
It didn't seem to have much output running a portable CD player to the input, with a 8 ohm sansui midrange clip leaded to the output.
It seemed like it wasn't working...... so whatever...I went ahead and tried the 12B4A linestage on one side of a Magnavox SE 6BQ5 amp, and the other side hooked up to 100K stepped attenuator.
It sounds pretty good, in spite of the little bit of PS hum.
The stepped attenuator sounds kinda dead compared to the linestage.
With plenty of gain, it has a open sound with a little bass grunt to it.
Not overblown, just a little bit of punch.
It gives the SE 6BQ5 amp some bass punch and opens it up compared to the 100K stepped attenuator.
It sounds a bit better than I thought it would.......it seems to be a decent tube to try for a stereo linestage.
Only the 12B4A can be microphonic, this RCA sample is anyway.
It pings a bit when you tap on it. Just like the 6SN7's.
So one would have to check many samples in circuit to come up with the least microphonic ones. They are cheap, however......
It's a butt-ugly fucker......but it is a glorified breadboard/experiment....
Curious to hear the rave about the 12B4A tube, I found one around here that tested fairly strong.
Couldn't find two 12B4A's, so it's a mono linestage.
I used the control unit chassis for a reverb box from a old Admiral 6BQ5 push-pull console stereo. The main 6BQ5 PP amplifier chassis had a 117 VAC power take-off point to the reverb control unit chassis.
The control unit has it's own little PS transformer that powered a 6X4 full-wave rectifier, a 12AU7, and a 12AX7.
The heater current for 12AU7/12AX7 at 12.6 volts is around 300 milliamps, 600 milliamps for both, sumthin' like that...
One 12B4A's heater current is at 600 milliamps, so I knew the little PS xformer filament winding would swing one 12B4A, but probably not two.
(Another reason why it's mono...)
I messed the Duncan PS program, and came up with a rather primative supply, CRCRC, with a 3K plate load resistor. (It still needs a bit more refinement, there is still some noise.)
The tube rating for 12B4A is 150 VDC on the plate, at around 34 milliamps plate current. With -17 volts on the grid.
Amp factor of 6.5.
It's a mini nine pin tube, with a single triode in the bottle. It has a lowish plate resistance of 1030 ohms, and a transconductance of 6300.
I'm running the 12B4A's plate at 100 volts, with -10.5 volts on the grid, and 10.5, 11 volts on the cathode. At around 25-28 milliamps plate current.
100K volume pot, with a 220 ohm suppressor resistor, with 470K to ground. (There is a teeny bit of hash, but that's through JBL 075 tweeters.)
A green russian 0.47 uF 600 volt cap for the output coupling cap, with a 470K to ground.
It didn't seem to have much output running a portable CD player to the input, with a 8 ohm sansui midrange clip leaded to the output.
It seemed like it wasn't working...... so whatever...I went ahead and tried the 12B4A linestage on one side of a Magnavox SE 6BQ5 amp, and the other side hooked up to 100K stepped attenuator.
It sounds pretty good, in spite of the little bit of PS hum.
The stepped attenuator sounds kinda dead compared to the linestage.
With plenty of gain, it has a open sound with a little bass grunt to it.
Not overblown, just a little bit of punch.
It gives the SE 6BQ5 amp some bass punch and opens it up compared to the 100K stepped attenuator.
It sounds a bit better than I thought it would.......it seems to be a decent tube to try for a stereo linestage.
Only the 12B4A can be microphonic, this RCA sample is anyway.
It pings a bit when you tap on it. Just like the 6SN7's.
So one would have to check many samples in circuit to come up with the least microphonic ones. They are cheap, however......
It's a butt-ugly fucker......but it is a glorified breadboard/experiment....