ECONOMICAL VALVE BASED AUDIO POWER
Here is a design for a valve based, stereo audio-amplifier, not using
valves types which are expensive to procure because of their esteemed nature in
performance and purity of sound for music reproduction, guitar amplifiers
e.g. EL84, 6L6, KT66, EL34, 6550A, 6SN7 types etc which typically
command high prices when offered for sale.
A Stereo 17 watt per channel
Ultralinear, push-pull Amplifier using 6CM5/EL36 output valves.
design uses 6CM5/EL36 power pentodes in a push-pull Ultra-linear configuration,
with a fixed - 50 volt DC grid bias.
Anode current of each o/p valve is a
conservative 25 mA approx, and maximum undistorted output power is about 17
The input stage is based on the Mullard 5-20 but I have used a
6AU6 instead of the popular EF86 pentode. (see later)
The phase splitter is
of the well known cathode coupled configuration using a 12AX7 twin triode, being
as in the LEAK TL 12+ amplifier.
I acknowledge inspiration for my design
came heavily from the following web pages;
The 6CM5/EL36 pentode was originally intended for
use both as a “line output or horizontal deflection” valve in television
receivers, and (somewhat rarely) as a Class B audio power amplifier.
valve was once plentifully available (as a service spare for B/W TV receivers)
and because it is not a highly prized audio tube, by valve collectors and audio
buffs (like the EL84, KT66, 6SN7, EF86, 12AX7's etc) you should be able to
obtain some cheaply from valve vendors.
The Soviet 6pi31 (6P31) is a direct
equivalent, and the well regarded Sveltana branded type can be purchased cheap
and in quantity.
6π31S (a.k.a. = 6P31C, 6p31C, 6pi31C) = 6n31C (Analogue of
The input stage is based on the Mullard 5-20, however instead
of the popular EF86 I have used a 6AU6 which is cheap and easy to
An interesting comment is sited in: " VALVE AMPLIFIERS: MORGAN
JONES. NEWNES (1995) pp224 "
The EF86 pentode has a heater structure
as a helix and an electrostatic shield around the anode (mesh) both designed to
reduce hum in audio circuits. The EF86 pentode, is responsible for high
sensitivity but poor noise performance.
So the humble 6AU6 should be
The screen decoupling capacitor is connected from G2 to cathode
(rather than hard to ground) because the cathode is not at AC ground on account
of the negative feedback into the cathode. Grounding the screen decoupling cap
would produce positive feedback to G2 and hence instability.
negative feedback gives the amplifier a substantially flatter frequency
response, than otherwise would be.
Only the DC biassing cathode resistor is
decoupled for audio variations by a 22uF electrolytic cap'.
A (12AX7) cathode
coupled phase splitter is used, where V1 (first triode) is a voltage amplifier
and V2 (second triode) can be considered as a grounded grid amplifier fed from
the commoned cathodes of V1 & V2.
This circuit is well proven and is
exactly as in the LEAK TL 12+ amplifier.
Unfortunately the 12AX7 is highly
revered by audiophiles and some brand names attract unusually high prices when
sold at on-line auction.
It is still in current manufacture (Soviets and the
Chinese) so look around and you will find some budget priced units, I'm
The total valve filament current requirement is
||1.25 X 4
||0.45 X 2
||0.3 X 2
The power transformer I've used was
salvaged from a c1980's japanese Color TV as was some winding wire (from
It was more than suitable, because it was fitted with an
external copper electrostatic shield and also fully metal clad .
transformer (E & I laminations) were disassembled, the existing outer 6.3v
filament winding removed and new windings installed, to provide enough current
to supply the heater current needs of 6.5 amp. The existing mains primary and
110v secondary were retained.
I didn't have any thick enough gauge wire to
satisfy the above amperage specification, so I made 8 (hand wound) windings (
yes 8 ! ) of #24 gauge wire producing 3.5 volt each (no load).
polarity series'd and paralled (2 x 4) to give an effective 7v centre tapped
filament winding (see diagram).
The earthed center tap is good practice as it
reduces induced AC hum in an audio amplifier.
In operation with the voltage
drop caused by the 6.5 amp load, filament voltage measured was correct at 6.3
The winding wire was salved from the de-gaussing coil of the same donor
color TV and there was more than enough for the rewind.
I did consider
single-point earthing as a hum reduction measure but with tagstrip component
mountings each earth lug carries as many earthy connections as practical.
took particular care to bond the earth lugs of tag strips solidly to the metal
chassis . Good construction practice was observed, particularily with filament
wiring (see diagram) and also extra care taken with the arrangement of all
components so that the underside is aesthetically pleasing !
physical appearance of these "naked" hand crafted amplifiers, is a
consideration, as well as their electrical performance;
I have included the
OC3 Voltage Regulator tube, which glows purple when operating! The input voltage
amplifier is hence fed from 108 volt regulated HT supply.
While this is not
strictly needed, rather than have the gas-filled VR tube purely for appearance,
I used it to supply stabilized B+ volts for the 6AU6 input stage.
The output transformers are sold as 100v line
output speaker line transformers, but astute observation by experimenters has
revealed that these transformer have the centre tap and the 43% screen tap
(Ultralinear operation), and have an overall primary to secondary ratio of 8000
: 8 ohm
These transformers are sold by Altronics in Australia are very
cheap and rated at 15 watt.
They proved ideal for the application.
well written analysis is available at:
of the M1115 100V line, 15W Speaker Transformer to Valve Push-Pull Output
The on-line catalogue listing and pricing of the M1115 transformer is
transformers may be readily available worldwide , I haven't bothered to
8 Ohm to 1.25W - 15W PA Speaker Transformer
15 W 100V line speaker
transformer with multi-tapped primary and secondary to suit 8 Ohm
High quality, high performance 100V line audio transformers.
models are wound on professional grade grain orientated steel core for extended
Designed for use in high grade constant voltage 100V and
70V PA Systems.
Frequency Response: 30Hz - 20kHz
Secondary Tap: 8 Ohm
Power taps: 1.25W, 2.5W, 5W, 10W, 15W
quote from some learned text's :
In a push-pull amplifier (compared to a
parallel pair of valves): 2nd Harmonic distortion produced by either valve is
cancelled by equal and opposite distortion from the other.
Two triodes in
Push-pull therefore give greater undistorted output than if connected in
3rd harmonic distortion does not cancel in this way; pentodes whose
output is limited by 3rd harmonics therefore do not share in this particular
They can be made to do so by connecting their screen grids to
suitable tapping’s on the primary of the output transformer, in what is rather
absurdly called the "Ultra-Linear" system.
FOUNDATIONS OF WIRELESS 7TH
EDITION: SCROGGIE (1958) pp319
Ultra-Linear operation provides a
compromise between the high efficiency of tetrodes and the low distortion and
uncritical load impedance of triodes.
There is an undoubted advantage in
modulator service because the matching of the modulator to the RF stage is not
so critical and the screen (DC) supply is obtained "automatically" from the
Screen grid tappings: 43% gives minimum distortion 20% gives
RSGB handbook 4th Edition (1968) pp9.19
496,883 dated 5 June 1937 is one which is considered to be among the most
important, which later became known as the "Ultra - Linear" amplifier.
so many of Blumlein’s circuits the design is deceptively simple.
It shows a
pentode output stage of a single ended audio frequency amplifier with a tap on
the primary winding of the output transformer.
This tap provided feedback to
the second grid to improve the linearity of the amplifier.
that if the tap was placed at the Anode end of the primary winding, the valve
would then be connected as a triode, and if the tap was at the supply end, it
would be a pure pentode.
It was well known that when a pentode was connected
as a triode it would be far less efficient and therefore provide a lower power
output. If however the tap was placed at a distance 15-20% down from the supply
end of the supply end of the transformer, the valve would combine positive
features of both the triode and the pentode.
The Ultra-Linear circuit in its
push-pull form became widely used in high-fidelity valve amplifiers during the
second world war, though it was a good many years before general practice caught
up with Blumlein’s thinking.
While Blumlein did not consider the problems
associated with distortion resulting from feedback to the screen, he regarded it
mainly as a convenient alternative to the control-grid feedback for reducing the
undesirably high impedance of pentodes while retaining their efficiency.
did suggest lowering the output impedance of the valve to approximately equal
the optimum load. This would have been done in order to prevent damage to the
valve or the output transformer if, the load became disconnected.
LIFE AND WORKS OF ALAN DOWER BLUMLEIN:
ROBERT CHARLES ALEXANDER (2000)
Final stage fixed-bias allows slightly higher
anode-to-cathode voltage for the output valves than using cathode bias.
-50v bias voltage is obtained by zener regulation from a seperately rectified
output from mains transformer secondary.
A 10 Ohm resistor in each 6CM5
cathode earth return is used to monitor cathode current (by Ohms Law the easily
measured voltage across this resistor, divided by 10, yields the value of
standing cathode current through the valve, this in turn being the sum of
principally the anode current plus a lesser amount of screen current) and acts
as a fuse under overload conditions.
(Max power of 17W was measured by
applying a 1kHz sinewave to the amp and measuring the peak to peak voltage
across an 8 Ohm dummy load at the point of clipping. The RMS volts = pp volts
divided by 2.88 (twice the square root of 2). Power then = Vrms2/8 Ohm.
power supply is straightforward; voltage doubler B+ and seperate negative bias
The front panel is uncomplicated; stereo input jack, ganged dual
audio gain control, and a 3 position switch OFF, STBY (valve heaters) and RUN
In summary I can state that the amplifier gives a good account of
itself, certainly loud enough for my needs, and has the rich, distortion free
mellow audio, that (as we are told) only vacuum tubes can produce!