Class A simple valve amplifier

Single Ended, minimal basic.

general-purpose (flexible evolutive build)

(version franēaise cliquer ici)

foreword towards our English-American-speaking friends.

Translation is made by a software.

Please excuse errors.

Only the French text is worth reference.

This device can evolve towards superior versions. All possible upgrades.

valve KT88.

I wanted to produce a simple basic valve amplifier in class A. stereo.

Valve KT88.

It was there is more than 20 years.

I had two output transformer for EL 34 (given by Didier De Bruynes)

My only problem was the power output valves' choice.

I tested the R120 triodes (not convincing, due to resonance at a certain frequency, and not enough power).

I placed two EL 504 (not convincing)

Then KT 88 (for the moment it is the tube which I prefer).

Here with EL 504 (abandoned).

In limps out of wood (with ventilation wire netting).

The lower part (It is badly made. I was beginner... excuse me!).

This build was useful for me to learn processes and craftiness of self build (DIY).

The sound is very good, but the low ones are a little weak, I prefer the EL84 push pull sound. But it is true for the EL84 amp  I have quality output transformer.

Tube KT88, a success.

Result don't had waited value.

Vhy ? The cause is simple: my two output transformers were of insufficient quality , They were built for guitar cheap amplifiers, for small budgets.

I did not want to invest in two transformer of quality, of which I know the price: more than 152,45 euros or $ each part.



Take your precautions with assemblies in 220 Volts AC or DC.

Real mortal danger.


I wanted a food carried out with transformers and components of recovery.

  • For the high voltage produce transformers; I had a transformer 220/220, but as 220 is a too low tension, I added in series with this first secondary a second transformer with secondary 2 * 30 V, this giving me (when the connection series is in phase) 280 volts. The setting in phase takes place in experiments, either you obtain 220 - 60 = 160 volts, or 220 + 60 = 280 volts which will give in the 300 volts after rectification. The intensities of the secondaries must be compatible and the total power of each compatible transformer also.
  • For the chemical condensers, 2 * 100µF are enough. But the condensers must be able to hold 500 volts. You can putting series condensers with resistances to balance tensions, 200K to 5.5 Mohms, but you must double the capacity value. Exemple: to obtain 100 µF with two series condensers it is necessary to have 200 + 200 µF.
  • The filter coil was found in old lamp television set of the years 1958-1960.
  • The BIAS circuit uses a 80V transformer. The resistance of filtering is of high value (to be calculated in experiments). The led can control a MOC, thus in the event of breakdown of polarization, which would be fatal for the tubes, the transformer of high voltage would see its cut off supply (safety).
  • The filaments heating uses alternate 6V transformer, but able to deliver 10 amps.
  • To envisage fuses (but not fuse on the secondary of polarization).
  • Any chemical condenser is doubled in parallel by a condenser with plastic film passing the high frequencies.


  • You will notice that the rate of negative feedback is linear and very weak (it is desired).
  • Polarization is adjustable and the subsequent current in the output tube is evaluated with an electronic or digital voltemeter between the points T1 and T2.
  • The triode is a half ECC808 or any other kind ECC83 with weak slope.
  • The condensers of connection are 1600V insulated with (minimum 630V).

Examples of Upgrade (Click here).

KT88 = 6550.

This powerful tube, modern, manufactured by many marks, is a tetrode (like famous the 6L6) specialized audio.

Used in Single Ended it can deliver an audio power of 19 Watts.

In push-pull assembly the power can do 90 Watts.



High Performance Audio

Beam Power Tetrode

he Svetlana™ KT88 is a glass envelope beam power tetrode. It is intended for audio frequency power amplification service. High plate dissipation rating, close manufacturing specification tolerances and thorough processing provides improved reliability and superior sonic performance. The Svetlana KT88 is manufactured in the Svetlana factory in St. Petersburg, Russia, and is designed to be a direct replacement for any KT88, KT90, KT99 or 6550.

The new KT88 features greatly enhanced sonic performance: Increased peak cathode emission from new cathode materials; Stable operation from extended processing and aging; Gold-plated grid; New tri-plate anode; Single-piece beam forming electrode; Precise grid/screen alignment; Improved vacuum processing; comprehensive static and audio amplifier testing before and after aging.


Heater: Min. Nom. Max. Voltage (AC or DC) 5.7 6.3 6.9 V Current 1.6 A

Cathode: Oxide-coated, unipotential Cathode-to-heater potential, max. -250*/250** V

Mechanical: Operating Position Any Base Large wafer octal 8-pin with metal sleeve

AF Power Amplifier

Maximum ratings: DC plate voltage 800 V Grid no.2 DC (screen) voltage 600 V Grid no.1 (control) voltage -300 V DC cathode current 230 mA Plate dissipation 42 W Grid no.2 (screen) dissipation 8 W

Retour Home Page