LAST update June 2013.

Click the pages you think might be interesting :-


Listening pleasure, class A, log sawing, class B, class AB, solid state history,

measurements, enough is enough,
NFB, triodes and NFB,

NFB and sales, circuit development, in house transformers,
future amps, preamps, SS amps,
NFB, tubes not bad, bandwidth, parts quality, tube choice, capacitors,

NOS tubes, testing tubes, add-ons, news groups.


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 modeled 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.

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.

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 method 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 modeled 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 analyze 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.

About NFB within triodes,
Testing a 6550 to measure Ra, Gm and µ.
Fig 1. Schematic A and B for testing 6550.
Schematics A and B explained,
How to determine Beam Tetrode and Triode Ra,
µ, and Gm.
Simple method to find Beam Tetrode and Triode Ra,
µ, and Gm.
Equivalent Model, Beam Tetrode, UL, Triode, 6550
Fig 2. Equivalent Model for calculation gain of UL connected tubes.
Fig 3. Graph of Ra vs UL tap % for SE 6550.
SEUL 6550 and SEUL 6550 with CFB.
Fig 4. Schematics for SEUL and SEUL+CFB for 6550.
CFB with UL taps and without UL taps explained with maths.

About Input and Driver stages for PP tube amps.
Fig1. Schematic for basic input LTP.
Fig 2. Schematic for Driver LTP with balanced or un-balanced inputs.
Fig 3. Schematic for Optimal LTP input and driver LTP for high level operation.
All explained with many calculations and considerations.

Content of this page is based around schematics. 
Fig 1. Three most used basic SE amp stages, 13W to 10W, SE pure tetrode,
SE Ultralinear, SE triode, 1 x KT88/6550.

Fig 2. 20W+ amp with SEUL with 2 x EL34, KT66, KT88, KT90, KT120, etc.
Fig 3. The Equivalent Model of KT88 with g1 and g2 inputs treated as current

This allows understanding of operating properties of a KT88, and its Ra, gm g1,
gm g2, and to analyze all voltages
and currents in all electrodes to determine
voltage gain, and effect of local NFB.
The theory may be applied to all power tubes
including pure triodes which do not have a screen, such as 300B and 2A3.

Fig 4. 20W+ amp with CFB, with 2 x EL34, KT66, KT88, KT90, KT120, etc, using
same SEUL OPT in Fig 2.

Fig 5. 36W SE amp with CFB with 3 x KT88 etc, using SEUL OPT, 25% UL tap,
1k3 : 5r6 Z ratio.

This was designed to include "choke sink" for cathode current, and choke in anode
feed for driver tube.
Probably nobody
has ever built an amp like this because they need to source
good quality chokes.
The THD and Rout is much lower than
conventional SEUL amps.
Fig 6. 25W PSEUL + CFB amp designed around the Hammond 1640SEA output
transformer with mosfet CCS at KT88 cathodes,
Fig 7.
25W PSEUL + CFB amp designed around the Hammond 1640SEA output
transformer with choke at KT88 cathodes,

Fig 8. SE CFB output stage and SEUL output stages with OPT with 3 windings.
Fig 9. Choke Feed SEUL and SET output stages.
Fig 10. Choke Feed SET amp with 845.
Fig 11. Choke Feed SEUL with floating B+ supply.
Fig 12. Choke Feed SET with floating B+ supply.


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 LL graphically and read for gain, calculate power output, and 2H distortion.
Fig 4. Measured Po vs distortion for 3 load values used with EH6550 in triode.
Fig 5. Measured Po vs distortion for 3 load values used with GE6550A in triode.
Fig 6. Measured Po vs distortion for 3 load values used with KT88 JJ-Tesla in triode.
Fig 7. Measured Po 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 curves for EH6550 in triode without LL 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 GE6550 triode 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 oscilloscope wave form.
Comments about THD and IMD significance.

This page has the following content :-
Operation of the beam tetrode with cut-away sketch by RCA.
Fig1. Graph of Ra curves for GE 6550A beam tetrode with screen supply = +350V.
Fig 2. Graph of Ra curves for GE 6550A beam tetrode with screen voltage = +350V
plus 3 load lines and calculated results for gain, power output and second harmonic
Fig 3. Graph of Ra curves for GE6550A beam tetrode but with screen supply = +200V.
Fig 4. Graph of Ra 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 Ra 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.

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.

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.

LOAD MATCHING 5.  About KT120, KT90, KT88, 6550.


This page is about :-
OTL amps, strengths and weaknesses, why OTL is so flawed.
Improving OTL performance, 1 to 6.
Table 1, Helps to explain use of mains toroidal PT as a speaker matching transformer.
Notes about Toroidal PT use.
Fig 1. Using a pair of 6AS7, loadlines for 4r, 8r, 16r, 32r, 64r - and what happens.
Calculating Pda for any level of operation.
Fig 2. Graph for Pda when using 4r to 64r loads.
Fig 3. Loadlines for class A1 with 6AS7.
Notes on class A1 use of 6AS7 - with OPT.
Table 2. Loading for class A1 using various numbers of pairs of output tubes.
Fig 4. Use of Class AB1 operation with 6AS7
Table 3. Use of 6AS7 with a "normal" PP OPT for class AB1 operation.
Table 4. Use of 6AS7 in Series Connection and with OPT for PP AB1 operation.
Use of 6C33c, notes.
Fig 5. Loadlines for 4r using 6C33c, Class B1, with Ea = +80V and +150V.
Discussions on peak tube current ability of 6C33c.
Fig 6. Class A1 operation and loadlines for 6C33c.
Table 5. Class A1 loading for various Ea and Ia and for Pda at idle = 40Watts.
Table 6. Configurations and output loads for a pair of 6C33c.
Fig 7. Understanding Class B1 loadlines for 6C33c.
Fig 8. Understanding Class AB1 operation of 6C33c.
Fig 9. Three basic ways for PP output tubes to be configured.
Fig 10. Schematic 30W class AB1 amp, 6CG7, EL34, IST, 6C33c Series Connected, + OPT.
Fig 11.
Schematic 28W class AB1 amp, 2 x 6CG7, EL34, 6C33c Series Connected, + OPT.
Fig 12.
Schematic 28W class AB1 amp, 6CG7, 2 x EL84, Technics FB circuit, 6C33c Series Connected, + OPT.
Fig 13. Schematic 30W class AB1 amp, 6DJ8, 2 x EL84, 6C33c, Normal PP OPT.
Fig 14. Schematic 30W class AB1 amp, 6DJ8, 2 x EL84, 6C33c, Circlotron PP OPT.
Class A1 SET with 6C33c.
Fig 15. Loadlines for class A1 with 6C33c SET.
Fig 16. Schematic 29W Class A1 SET amp, 6DJ8, EL34, 2 x 6C33c parallel + air gapped OPT.
Fig 17. Schematic 29W Class A1 SET amp, 6CG7, EL34 with g2 FB, 2 x 6C33c parallel
+ air gapped OPT with 50% CFB.


Function of the output transformer, ( OPT ). How the OPT works.
Impedance and resistance transformation.
Fig1.  Equivalent model of Ultralinear output stage tubes and OPT.
Functions in the model and preferred OPT characteristics.
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 matching 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 behavior, power handling ability,
HF behavior, HF resonances, and distortion. Description of PP OPT No2.
Description of SE OPT No3. Brief note about SE OPT No4.
There are 5 pages covering this subject.

PP OPT Calc Page 1 :-
Contents,  Brief reference to Radiotron Designer's Handbook,  4th Ed, 1955.
Some general notes.
Design steps 1 to 34 for 2 x 6550/KT88/KT90, for TETRODES and PENTODES.
Design example is for OPT-1A for up to 75Watts of audio power between 14Hz and 65kHz.
Loadline analysis, waveforms, metric wire size table, bobbin winding diagrams, blank
sheets you may use.
PP OPT Calc Page 2 :-
Design of OPT-1A continued....

Design steps 14 to 29 for 2 x 6550/KT88/KT90, for TETRODES and PENTODES.

Many drawings, explanations, calculations etc.
PP OPT Calc Page 3:-

Design of OPT-1A continued...

Design steps 30 to 46 for 2 x 6550/KT88/KT90, for TETRODES and PENTODES.

Many drawings, explanations, calculations etc. 


PP OPT Calc Page 4 :-


Design steps 47 to 53 for 2 x 6550/KT88/KT90, for TETRODES and PENTODES.

Many drawings, explanations, calculations etc.

PP OPT Calc Page 5 :-

Design steps 55 to 63, then 14T to 29T for 2 x 6550/KT88/KT90, TRIODE CONNECTED.
This page is about speaker matching transformers.
Fig 1 to 13 show details of transformers and all is explained.  

This page still being written 2014...
Calculations of anode heat dissipation for class AB and B amps with OPTs.
Many graphs and explanations and formulas etc.
This page still being written 2014...
Calculations of tube or solid-state device heat dissipation for low bias Class AB
with 6C33C or 6AS7G.
Graphs and calculation examples, etc.

There are now 3 pages for this subject.

SE OPT Calculations Page 1.
Notes on RDH4, SE amp history, trends and preferences.

For operating conditions of common tubes.
Table 1.  SE Pentodes, Beam Tetrodes, CFB, UL,

Table 2.  SE Triodes.

Interpreting Table 1 and Table 2.

Design example for SE OPT4 with EL34.
Steps 1 to 30 with many diagrams, tables, and calculations.
Blank sheets for drawing response curves.
SE OPT Calculations Page 2.

SE OPT4 design continued,

Steps 31 to 44 with many diagrams, tables, and calculations.

SE OPT Calculations Page 3.

Practical Testing of 8 Watt SEUL OPT for 1 x EL34 for old AM radio.

Fig 19. Schematic for rather good 8 Watt SE amp, notes, old radio picture
Oscilloscope pictures of waveforms produced with 8 Watt SEUL OPT.

SE OPT Easy Method for calculating any SE OPT, f
or Basic Parameters only.
Steps 1E to 10E.

High Voltage testing of transformers, test schematic, OPT3 details.

SE OPT for 1 x 13E1
, 25W, or for multiple parallel octal 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, all details.
Fig 2. OPT6 for SE 1.8k : 2 to 14 ohms with 51T x 51S core, all details.
Metric winding wire size chart for grade 2 polyester-imide wire.

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 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.


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 voltage vs C vs Idc.
Cap ripple current and voltage 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 reservoir 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.
About power transformer design and choke design,
I have refurbished the 2006 page and created a sub page solely for chokes.
Due to demand for information about power transformer and choke design,
there are 3 pages about chokes :-
Chokes 1 about basic chokes, testing chokes and for CLC PSU filters.
Chokes 2 about
Filter chokes for "choke input" or LC filters in power supplies.
Chokes 3 about Chokes for DC anode feed.
Here is a rugged PSU design with HV rated bjts for adjustable regulated B+ voltages
from +126Vdc to +585Vdc for up to 500mAdc output, and short circuit proofed.
Several examples of radical re makes of old radios with serious quality problems.
Many pictures, some schematics. 
A totally tubed radio tuner including tubed multiplex decoder.
Schematics, notes, and an image.

Sheet 1, Block diagram of THD measurement of amplifier,
Sheet 2, Wien Bridge oscillator, 1kHz, 0.004% thd,
Sheet 3, Attenuator and buffer and filter after 1kHz oscillator,
Sheet 4, L&C Bridged T notch filter for nulling 1kHz, THD amp and filters.
Sheet 5, Hi Zin buffer for use with hi Z source to be tested.
Sheet 6, Front panel for THD testing unit.


Listed test gear needed for testing amps,
Graph 1, blank sheet for use to plot response graphs.
Fig 1, Wien bridge sine wave oscillator with op-amp and switched F.
Fig 2. Square wave oscillator with switched F and bjt amp.
Fig 3, Wien bridge sine wave oscillator with bjts amp.


6 decade frequency ranges, 1Hz to 1MHz, with square waves.
There are 3 schematics :-
Fig 1 shows the switched R & C positive FB network and the simplified amp schematic with output attenuator
and the negative FB network with j-fet to vary the amount of NFB.
Fig 2 shows the full details of the amp using discrete j-fets and bjts which gave wider BW than most common op-amps.
Fig 3 Gives basic properties of the Wien Bridge network. 
Fig 4 shows the Schmitt Trigger square wave schematic.
Full descriptions of schematics and expected problems DIYers and dummies will face


This page is about Wien bridge oscillators using vacuum tubes and some solid state for square waves.
Fig1, Wien bridge oscillator, tubed, from 2005.
1Hz to 220kHz
Fig 2, Wien bridge oscillator, tubed, SHEET 1, amp, 2013. 1Hz to 2MHz
Fig 3,
Wien bridge oscillator, tubed, SHEET 2, buffer, output.
Fig 4,
Wien bridge oscillator, tubed, SHEET 3, Schmitt trigger and amp.
Graph 1, Square wave harmonic content.
Fig 5,
Wien bridge oscillator, tubed, SHEET 4, Power supply.
Fig 6, Small signal bjt discrete bjt op-amp.


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                      Low-power 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

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.

Where appropriate I have included explanations.

Should you have any indispensable wisdom you'd like to have added then email me.

 To Index Page