VACUUM TUBE USE IN AUDIO AMPLIFIERS :-
TUBE OPERATION 1.
Basic tube operation, cathodes, anodes and grids, diodes, triodes and
pentodes. Parameters of Ra, µ and gm.
Fig1. Schematic for a basic
triode amplifier based on 6SN7.
DC flow for quiescent conditions, dc equilibrium,
Mutual effect of anode voltage and grid voltage on Ia electron flow,
Effect of cathode bypassing biasing on Ra, and gain.
The tube modelled as a generator.
Fig 2. Schematic of basic 6SN7
generator model for illustrating ac operation of a tube.
NFB in the triode, AC signal flow, Cathode capacitor bypass impedances,
tube gain formula, gain without capacitor bypassing.
Fig 3. Electrostatic effects in
a 6SN7 triode.
NFB in the triode, how it reduces THD.
Fig 4. Electrostatic effects in
the 6AU6 pentode,
pentode and beam tetrode operation, pentode Ra, µ and gm.
6AU6 triode connected and its amount of internal NFB.
NFB in 6SN7, and why ß = 1 / µ , with more NFB and gain
equations.
The Miller effect.
TUBE
OPERATION 2.
Fig 5.
Graph of 6SN7 Ra curves with load lines for 47k and 32k.
How to find Ra for a given working point and plot loadlines in steps 1
to19.
Comment on THD and other topology outcomes.
Fig 6. Scanned
Ra curves from Samuel Seely, 1958.
Explanations about the Ra curves. About gain with CCS load and µ.
6SN7 THD with CCS load calculations from data curves.
TUBE
OPERATION 3.
How negative feedback works.
Fig 7. Schematic for Basic NFB
around an amplifier.
Explanations and formula for NFB gain reductions and effects of NFB.
Fig 8. Schematic for Turner
Audio 35 watt class AB triode amp using KT90.
General notes about this amp which has the same overall gain and NFB as
the basic example in Fig1.
Calculation methode for output resistance with NFB.
The Model of the tube gain stage as a voltage generator.
Fig 9. Schematic of a power
tube gain stage modelled as a generator with resistor to indicate Ra.
Explanations about the generator model.
Fig 10. Schematic of a tube
gain stage using 6SN7.
Fig 11. Schematic of a tube amp
drawn with each stage as a generator with loads and positions of
shunt C to analyse the HF response and graph all the attenuation
profiles.
A whole lot more about NFB, output resistance.
A simple formula for calculating output resistance of a real amp.
More on stability of amplifiers with NFB and the use of RC networks to
tailor open loop gain.
Fig 12. Graph of tube amp
frequency response without global NFB and with global NFB, with no
attempts to tailor open loop gain or phase shift.
Fig 13. Graph of tube amp
frequency response without and with NFB but with RC gain and phase
shift tailoring
networks in place.
More about stability and NFB.
Critical damping methods for tube amps with NFB.
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LOAD
MATCHING 1. SE TRIODES.
Fig1. Anode resistance curves for GE6550A in triode from the
1950s.
Explanations of how the curves were obtained and what the curves mean.
Fig 2. Schematic for testing
power triodes.
Triode voltage generator model is included in explanations.
Fig 3. GE6550 triode curves
with 2.5k RL and tangent to calculate Ra, µ and gm for any chosen
working point.
Explanation of what load lines are.
How to plot them graphically to read the graphs for gain, calculate
power output, and 2H distortion.
Fig 4. Measured power output vs
distortion for 3 load values used with EH6550 in triode.
Fig 5. Measured power output vs
distortion for 3 load values used with GE6550A in triode.
Fig 6. Measured power output vs
distortion for 3 load values used with KT88jj Tesla in triode.
Fig 7. Measured power output vs
distortion for 3 load values used with KT90EH in triode.
Fig 8. My corrected Ra curves
for EH6550 in triode with 3 values of RL plotted.
Comments on loads recommended for EH6550 in triode.
Fig 9. My anode curves for
EH6550 in triode with no load lines so you can download them for use.
Fig 10. Ra curves for 300B
measured after 1990 with 4.5k loadline details.
Comparison of THD with beam tetrodes strapped as triodes.
Fig 11. Ra curves for trioded
GE6550 measured after 1990 with 4.5k load line details.
List of conclusions about beam tetrodes used as SE triodes.
Fig
12.
Intermodulation test rig schematic for measurement.
Fig
13.
Measuring the intermodulation distortion using an oscilliscope wave
form.
Comments about THD and IMD significance.
LOAD MATCHING 2. SE BEAM
TETRODES and PENTODES.
Description of the operation of the beam tetrode with cut-away sketch by RCA.
* Fig1. Graph of anode
resistance curves for GE 6550A beam tetrode with screen supply = +350V.
* Fig 2. Graph of anode
resistance curves for GE 6550A beam tetrode with screen voltage = +350V
plus 3 load lines and
calculated results for gain, power output and second harmonic
distortion.
* Fig 3. Graph of anode
resistance curves for GE6550A beam tetrode but with screen supply = +200V.
* Fig 4. Graph of anode
resistance curves for GE6550A with one load line for 3.5k and
calculated gain, power and 2H .
Explanation of the effects of NFB application for 6550 beam tetrodes
Formulas for NFB and output resistance.
* Fig 5. Graph for power output for single 6550 beam tetrode vs
anode load value.
Choosing the OPT ratio.
* Fig 6. Graph of 6550 anode
resistance curves for 6550 in UL mode with 43% screen taps.
Calculated gain, power output and 2H for 4 different load values.
Data for Ra, µ and gm for UL and comment on the effects of NFB
and distortion outcomes.
LOAD
MATCHING 3. PP TRIODES.
A very brief history of triode use. Class A Push Pull
operation basics.
Fig 1. Schematic of basic
PP triode output stage with current waveforms to explain 2H
cancellations.
Comments on class AB1 amps, Williamson's amp, Class AB
efficiency, preferences, Class AB1 basics.
Fig 2. Graph of
EH6550 triode curves with load lines for 5k a-a.
Minimum load for PP triode AB amps, Class AB1 operation explained.
How to plot a load line to give the outcome for class AB triodes,
Anode heat dissipation, Comment on B+ regulation, Biasing the class AB
PP stage,
Distortion, Output resistance, Negative feedback.
Using a higher RL value.
Fig 3. Graph for PP 6550 triode
class AB1 power output vs RL values.
Class AB power and portion of class A power listed for 8k : 6 ohms
loading.
Fig 4. Schematic for 35 watt class AB1 PP triode
amp with KT90.
Speaker SPLs with 25 watts. Output transformer ratios. Comment on using
KT90 or multiple tubes.
LOAD MATCHING 4. PP BEAM TETRODES.
Beam tetrode background,
Fig 1. Loadline graph for PP
beam tetrode 6550 with 4k a-a load.
Plotting loadlines for PP tetrodes, 17 steps to find
maximum class AB power, class A power.
Heat dissipation considerations and measurement, 92 watt Class AB power
with Ea = 600V.
Biasing the output tubes, Distortion, Output resistance.
Using a higher RL such as 8ka-a,
Global NFB, its effect on output resistance, Calculation of amount of
applied NFB and the output resistance with applied NFB.
Fig 2. Graph of power out
vs RL .
Loading the PP beam tetrode output stage, OPT ratios.
Ultralinear and other output tube configurations, Driver amplifier
comments.
How to match loads to power tubes, load matching to 6550 & KT88 and
to some other beam tetrodes, pentodes and triodes.
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OUTPUT
TRANSFORMER THEORY.
Function of the output
transformer, ( OPT ). How the OPT works. Impedance and resistance
transformation.
Fig1. Equivalant model of
Ultralinear output stage tubes and OPT. Functions in the model and
prefered OPT chracteristics.
Test conditions for specifying OPT performance. Description of OPT No1,
wire and turns and insulation description.
Fig 2. Cross section of
bobbin winding details.
Fig 3. Schematic of OPT
No1 when used in UL with two impedance matching settings shown.
Fig 4. Schematic of OPT
No1 when used with 12.5% CFB windings and with two impedance matching
settings shown.
Notes re recommended amounts of CFB to be used. Impedance matching
notes.
Table 1. Impedance
matchings available with OPT No1.
Table 2. Recommended
output tubes for OPT No1 with winding losses. Comments about
alternative tubes used with OPT No1. Specification for OPT No1, notes
re LF behaviour, power handling ability, HF behaviour, HF resonances,
and distortion. Description
of PP OPT No2. Description of SE OPT No3. Brief note about SE OPT No4.
PUSH
PULL OUTPUT TRANSFORMER CALCULATIONS.
Brief
reference to Radiotron Designer's Handbook, 4th Ed, 1955
Fig 1. OPT No1 from page on
'Output Transformer Theory'
Design logic method, steps 1 to 45 for designing a push pull OPT using
OPT No1 as the example.
No apologies for the
complexities involved, go fishing if its all too hard!
Design method contains lots of calculations and list of Primary and
Secondary interleaving configurations likely to be used with
tube audio PP OPT.
Fig
2. OPT Secondary sub-sections for load matches with 2 and
3 secondary layers.
Fig 3. OPT Secondary
sub-sections for load matches with 4 secondary layers.
Fig 4. OPT Secondary
sub-sections for load matches with 5 secondary layers.
Fig 5. OPT Secondary
sub-sections for load matches with 6 secondary layers.
More checks of final design,
calculation of leakage inductance, acB, dcB, inductance
and final winding height.
Fig 6. OPT bobbin
winding details for OPT No1.
Calculation of shunt capacitance.
Metric
winding wire size chart for
grade 2 polyester-imide wire.
SINGLE
ENDED OUTPUT TRANSFORMER CALCULATIONS.
Brief reference to Radiotron
Designer's Handbook, 4th Ed, 1955,
Comments about SE amplifier OPTs and Ongaku.
Fig 1. Schematic of OPT
No3 for 25 watt SEUL amp for two 6550/KT88/KT90.
Design logic method, steps 1 to 51 for designing an SE OPT using OPT
No3 as the example.
( Still no apologies for the
complexities involved, go fishing again if its all too hard!)
Design method contains lots of calculations and list of Primary and
Secondary interleaving configurations likely to be used with
tube audio PP OPT.
Fig 2. OPT Secondary
sub-sections for load matches with 2 and 3 secondary layers.
Fig 3. OPT Secondary
sub-sections for load matches with 4 secondary layers.
Fig 4. OPT Secondary
sub-sections for load matches with 5 secondary layers.
Fig 5. OPT Secondary
sub-sections for load matches with 6 secondary layers.
Fig
6. OPT bobbin winding details for OPT No1 if used for an
SE amp.
Calculations for shunt capacitance at the input of the SE OPT.
More checks of final design, calculation of leakage inductance, acB,
dcB, air gap, primary inductance
and final winding height.
PRACTICAL TESTING OF SE AMPLIFIERS AND
OPT.
Adjusting the air gap and
practical checking of the gap and primary inductance.
Metric winding wire size chart
for grade 2 polyester-imide wire.
OUTPUT TRANSFORMERS FOR 1 x 13E1 OR
PARALLEL OCTAL OUTPUT TUBES.
Design flow for a couple of 25W
SE OPT to suit 1 x 13E1 or 3 or 4 KT88/6550/KT90/300B
or 4 x EL34, 5881, 6L6GC, 807.
Fig 1. OPT5 for SE 1.8k : 5
ohms with 44T x 50S core and details of bobbin layers, insulation, wire
etc.
Fig 2. OPT6 for SE 1.8k : 2.2,
3.5 6.2, 13.9 ohms with 51T x 51S core and details of bobbin layers,
insulation, wire etc.
Metric winding wire size chart
for grade 2 polyester-imide wire.
WINDING
OUTPUT TRANSFORMERS.
Practical winding methods and description.
What you need to consider if wanting to wind an OPT for yourself.
Image 1.
Bobbin winding details for OPT No1 mentioned in OPT transformer
theory.
Image 2. Four
transformers on a work bench.
Image 3. Two 300w OPT
on bench.
Image 4. 500w OPT
on table.
Image 5. Winding
lathe with bobbin being wound.
Image 6. Wound
bobbin close up.
Image 7. Close
up of 300w OPT handmade bobbins on bench.
Winding procedure, varnishing, alternative to varnish is applying
Estapol 7008.
Metric winding wire size chart
for grade 2 polyester-imide wire.
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TUBE AMP POWER SUPPLIES.
Definition of linear power supplies.
Fig 1. Basic wave forms in
rectifier circuits.
Single phase house wiring, ac waveform basics, filling the bath with
water, diode resistance, ripple voltagevs C vs Idc.
Cap ripple I and V ratings, ac to dc conversion ratios, doubler
rectifiers.
Fig 2. Schematic of 8585 amp
power supply used as example for PS calculations.
Minimum C value for resevoir C1 input cap, C reactance, Ripple voltage
calcs, peak charge currents, charge current limiting,
CRC and CLC filters, R and
choke values, LC resonance, choke reactance, LC damping resistance, CT
cap values,
need for chokes. DC heater supplies, B+ regulators.
CRCRC and CLCLC filters.
Fig 3. Schematic for solid
state regulator for screen supplies in 300 watt amp.
Send me your SMPS schematics for B+ supplies.
POWER
TRANSFORMERS AND CHOKES.
Due to huge demand for information about power transformer and choke
design,
I have refurbished the 2006 page and created a sub page solely for
chokes.
For choke info go to Choke Design for
Audio Amplifiers.
This page concerns power transformer design only.
(1) Fig1 Tube
amp PSU schematic for 5050 amplifier from 2006, and notes,
Fig2 480VA power supply schematic simplified for 2007
and suitable for a range of amplifiers.
(2) Define the power requirements for the amplifier, wanted
secondary voltages and currents, and transformer VA rating.
(2a) AC Heater Supply.
(2b) B+ Anode Supply for range of class A and AB conditions.
(2c) Negative Bias Supply.
(2d) DC Heater Supply.
(2e) Summary of transformer secondary windings needed.
(2f) Primaries.
(3) Selecting the core type and size for the transformer.
(4) Calculating turns per volt, TPV.
(5) Check for iron heat losses in the core.
(6) Magnetizing current, iron µ check, measuring inductance
and µ with a variac.
(7) Working out the winding layers and turns.
(7a) Mains Primaries, 480VA transformer.
(7b) AC heater windings.
(7c) HT for B+ anode supplies.
(7d) Negative Bias supply.
(7e) AC heater windings for DC heaters.
(8) Check that proposed windings will fit onto bobbin.
Fig3 480VA transformer bobbin winding details.
(9) Transformer Assembly,
Varnishing, Potting.
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TURNER AM-FM TUNER.
A totally tubed radio tuner including tubed multiplex decoder.
Schematics, notes, and an image.
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MISCELLANEOUS SCHEMATICS 1.
1010w Integrated amp
One channel shown with 2 x 6GW8, UL, classAB
Two triode phono stage NFB eq
1 x 12AX7, feedback RIAA eq
Three triode phono stage NFB eq
1 x 12AX7, feedback RIAA eq, buffer
Three triode phono stage Passive eq
1.5 x 12AX7, passive RIAA, buffer
Phono Amp PSU Schematic
LowPower supply for 2 and 3 tube preamps
10 tube Preamp April 2000
5 x twin triodes per channel
1 x j-fet 2SK369 simple
pre-preamp
A test circuit showing THD for a single 2SK369 j-fet
Test filter, Reverse RIAA eq
A simple test filter with discrete RC components
MISCELLANEOUS SCHEMATICS 2.
Basic Balanced Shunt FB
2 x 6L6GC
Balanced
Shunt FB 100W
4 x 6550 per channel
Automatic servo bias
control
1 x KT88 etc.
Error correction standard UL amp
2 x KT88
etc.
Error correction fully balanced
2 x KT88 etc.
Simple line preamps 1 and
2.
2 triodes.
Simple line preamps 3, 4 and 5.
2 and 3 triodes.
Line preamp with switched gain and
CCS 2 triodes.