Download A Single Ended 12B4A

A Single Ended 12B4A
Integrated Amplifier.
1.5 Watts Of Pure
Single-Ended Triode Power!!!
Part 1
by Gaby Levinson
Motives
I know what you are thinking. What the hell am I going to use a 1.5 watt amplifier for? Well, as
it turns out 1.5 watts is probably more than enough for an office system. As there were a couple
of 12B4As in my tube drawer, so it made sense to put them to use. Since this project is an
experiment, inexpensive parts will be used.
A trip to Duncan's Amp Pages told me the 12B4A is a small indirectly heated power triode in a
9-pin envelope. Its intended use is in the vertical deflection amplifiers in old televisions.
Maximum plate dissipation is about 5.5 watts. Since a single-ended amplifier usually can get
around 20% of the max plate dissipation to go to the loudspeakers, I figured one could achieve
approximately one watt from these little guys.
Circuit Design
The easiest way to design is from output to input. With that in mind, the first thing to do when
designing an amp like this is to choose an operating point for the output tube (technically you
start with the speaker, but I am assuming a standard 8-ohm speaker). The tube data for the
12B4A from Duncan Amps suggests an operating point of 150V on the plate and 35mA with
bias set at -17.5V. The plate impedance at this point is about 1.3k ohms. There is a rule of thumb
that says you should pick a load impedance that is approximately 2.5-3.5 times the plate
impedance at idle. Since the plate impedance varies wildly with the AC swing of music (this is
just a suggestion).
After looking at the tube curves, I decided a slightly better operating point would be at 180V on
the plate, 27mA and a bias of -23V. This would produce about 1.56 watts when using a 5K ohm
load, with just about the same distortion level that the suggested operating point gives. The
easiest way to set this up is with cathode (auto) bias. Ohm's Law says that to get -23V drop with
27mA, therefore I need an 850 ohm cathode resistor. This resistor will dissipate about 0.6 watts.
It is always best to underrate your components, so I decided on a 2 watt part. I had some 820
ohm, 3 watt resistors on hand - which is close enough to 850 ohms. In fact, that only moved the
bias to about -22.5V, so that is not a problem. This resistor needs to be bypassed with a capacitor
to maintain the correct gain and output power. This cap works out to around 50uF, and should be
rated at least 50V. Any calculations that I do not mention and you are curious about, please email me about them and ask.
Now, on the other side of the tube, we need a plate load. Since I decided on a 5K load, we
needed an output transformer for that. The Hammond 125BSE seemed like a good choice. It can
handle up to 45mA of bias current - and I am using approximately 27mA. So this output
transformer will work. I bought mine from Angela Instruments. The last thing we need for the
output stage now is a grid leak resistor and DC blocking cap. This resistor is generally best made
as large as possible. Max rating for the 12B4A is 1Mohm. I will use a 470k 1/2 watt metal film
resistor cause I have quite a few of those. The cap is unnecessary is you are going with a direct
coupled design. However that would require some changes to the voltage points. The cap and the
grid leak resistor form an RC high pass filter, so we need to be careful what size cap we use. It is
generally a good idea to pick a nice low frequency cutoff for the high pass filter. I used 5Hz and the cap works out to around .047uF. For this I will use an inexpensive Sprauge Orange Drop,
which can usually be had for around $1. (My favorite coupling cap these days is the AuriCap,
and in this value would be around $6 each - I may switch some in later on.)
Now we need a driver stage. The driver needs to do two things: First, it needs to provide enough
voltage swing to drive the output stage to full output. Second, it needs to provide enough current
to overcome the interelectrode capacitances of the power tube. The first requirement is easy to
figure out. If we want to drive the output tube to full power, we need a AC voltage swing that is
double the bias voltage. In this case, the 12B4A's grid is at -23V, thus we need the driver stage to
swing 46 volts.
Now to determine how much current we need, calculate the slew rate the output tube needs to
overcome its capacitances. Roughly speaking, this works out to about 2mA for the 12B4A. Any
driver with at least this much bias current will do. Now to pick a tube that can do this. I have a
bunch of signal tubes that could be used on hand, it is just a matter of deciding which one.
The 12AX7 is ruled out. It will swing 100 volts (that is way too much), at a max of 2 mA. We
need some more leeway than that. A standard twin-triode as found in many pre-amplifiers will
do, such as the 12AU7 or 6DJ8. However, since I am using such an unusual output tube, why not
use an unusual driver tube? Lets go with the 6AU6. The 6AU6 is a small RF pentode in a 7-pin
miniature envelope. Its intended use is in tuner sections. It is entirely unsuitable as a driver when
used as a pentode - however when wired as a triode (screen and suppressor both tied to the plate)
it makes a fantastic voltage amplifier.
I am running the 6AU6 at around 220V on the plate, 12mA plate current, and -4V bias. The bias
is set to -4V so that the 2V output from a CD player wont clip the input. The 6AU6 is set up with
a 10K load resistor and a 330 ohm cathode resistor. The load resistor will dissipate about 1.5
watts, so I have a 5 watt resistor in place. At this operating point, the 6AU6 is providing about 50
volts of swing, which is just over the 46 volts we need for the 12B4A's full output. This means
the driver wont clip before the output stage clips - a good thing. Stick a 100k ohm log taper pot
on the grid of the 6AU6 for a volume control and that part is done. All this can be seen in the
below schematic.
Power Supply
The next thing that needs to be designed is the power supply. Since this amplifier is rather simple
to begin with, a simple power supply is in order. The maximum voltage needed is around 350,
used as the B+ for the driver stage. This works out to a transformer voltage of around 275. I
selected the Hammond 270DX, which will give 275V at 90mA, center tapped, and has a 6.3V,
3A winding for the heaters. I had a 6BW4 (9-pin) full wave rectifier in my tube stash, so I will
use that one. The output of the 6BW4 is sent into a CLCRCRCRC filter using a 10uF capacitor,
a Hammond 156M choke, and then a JJ 20/20/20/40 can-type capacitor. The B+ for the driver is
taken from the first 20uF cap. The B+ for the 12B4A is taken off the 40uF cap. The schematic
can be seen in the below schematic.
Experiment Time!
It is now time to put this brainstorm to the test. Essentially, I need to make sure everything that
was calculated would happen in real life. First, I needed to make sure my power supply design
actually gives me the right voltages. So I wired it up using a few tack-solders and clip leads. Sure
enough, there are the right voltages so that solves hurdle number one. Hurdle two involves
checking to see if the driver stage indeed gives me 50V as calculated. I wired the output stage
using tack-solders and clip leads as well, this time adding the DC blocking cap and 470k grid
leak resistor (you need to have something for the driver to drive, even if it is just a resistor). Sure
enough - I was getting about 57V from the driver tube using a 2V input from my signal generator.
OK so it is a bit high, but that is of little concern. It will still work fine.
Now for the real big money - the power stage. I am starting to run out of clip leads at this point,
but we will forge on ahead. With the power tube wired in my oscilloscope is telling me we are
getting a whopping 1.56 watts at full output power before distortion takes over. At this point, this
whole mess looked like the photo above. I think its time to listen to it. Sure only one channel is
done (if you can call that mess 'done'), but that never stopped me!
The Real Test
I was fully expecting this thing to sound like absolute junk. After all, its clip leaded together, and
it was a circuit I just thought up in my head. But boy was I wrong! This 1.5 watt flea powered
amplifier sings! Its missing a touch of refinement that my 300B monoblock amplifiers have, but
it certainly sounds like a single-ended triode amplifier. And yea, its only 1.5 watts... though my
loudspeakers are efficient enough. I found myself groovin' at quite a decent volume with my
97dB horns. It had all those nice qualities like pacing and timing. Qualities I find lacking in some
commercial tube amplifiers (perhaps the manufactures want to over-emphasize the 'tube sound'
and end up making it sound ooey-gooey?). In fact, this amplifier sounds downright wonderful.
This mess of clip leads sounded nice and fast, yet smooth. It will not play extreme loud, but
within its useable volume range with is quite nice. Now I just need to find a tube that sounds like
this wonderful indirectly heated triode that can give me a few more watts. This is one fun little
tube!
Now all that's left is to build a chassis for it and do the final construction, and the all important
critical listening test. Tune in next month for part 2 to see how this project comes out. Need
some "office" speakers to go with this little "office" amplifier? Look for an upcoming article on a
matching speaker...
12B4 Pinout
12B4 Substitutes
Different rating or performance 12B4A
Warning: Substitutes are given as a guide only - please refer to original manufacturers data
sheets to ensure that a substitute is safe and appropriate for your application.
12B4 Ratings
Vh
Ih VaPeak VaMax PaMax IkMax Cgk Cak Cga
12.6 0.3 1,000
550
5.5
30
5.0 1.5 4.8
6.3 0.6
12B4 Application Data
Class
Va Vg1
Ia
Ra
S
Frame output 150 -17.5 34.0 1,030 6.3
12B4 Data sheet links
No information available
Hammond 125BSE Transformer
Key Features:
Designed for general purpose or replacement use (not Hi-Fi), in single ended, tube output
circuits.
Frequency response: 100 Hz. - 15 Khz (+/- 1db max. - ref. 1 Khz).
For full frequency response (20 Hz. to 20 Khz.) - see our 1627-1642 series.
For push-pull output use, see our 125 series - optimized for push-pull tube output circuits.
Open style with minimum 12" long primary & secondary leads leads.
All sizes use butt stacked cores (using 29M6 steel) with an air gap, to reduce D.C. core
saturation.
Primary impedance range from 2,500 to 10,000 Ohms.
Secondary impedance range from 4 to 32 Ohms.
Max. D.C. Bias
Cat. No. Audio Watts
Wt. Lbs.
(ma)
45
1.1
125BSE 5
Mechanical Data:
Dimensions (Inches)
G
A B
C D Mtg.
Hole
125BSE 3.25 1.63 2.00 2.81 0.187
Cat. No.
Schematic & Hook Up Data: