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The "Hybrid+", an enhanced version of the Hybrid amplifier for extended output power

The Hybrid+ is a version of the Hybrid amplifier that provides a way to increase the delivered output power. Basically, the Hybrid+ maintain the same ECC88 tube based input stage but supports up to three class-A output stages in parallel. The output stage is composed by the class-A power stage (Q5) and the associated current source (Q4). In this way, the amplifier output power is increased by simply paralleling more class-A stage (Q5) and its associated current source (Q4) as showed in the schematics. Up to three output stages can be connected to the ECC88 tube driver stage. More than three output stages will increase the capacitive load to the ECC88. With three output stages in parallel, the Hybrid can deliver approximately 90-100Watt RMS of pure class-A power.

Each couple of power output stage must be effectively cooled with the appropriate heat sinks as specified for the Hybrid.

The following table resumes the basic requirements for an Hybrid+ amp supporting more than one output stage:



Power (W RMS)




Current  (A)

Suggested Output

Mosfet type

25-30 1 3 IRF9540/540
50-60 2 3 IRF9540/540
40-50 4 BUZ900DP/905DP
90-100 2 4 BUZ900DP/905DP

"Hybrid+" Power Supply requirements

The Hybrid+ amplifier requires an enhanced version of the power supply, able to deliver 12 Amp for a dual output or 18 Amp for a triple output.

In this way, T1 must be a 25+25 VAC 12Amp or 18Amp  transformer, the bridge rectifier must support at least 50 Amp 400V such as the Semikron SKB60/08 or Powersem PSB105/08. In addition, the number of the power supply capacitors must be double or triple.

The following table resumes the main power supply characteristics for the different single, dual, or triple Hybrid+ versions.


Number of Output


T1 power capacity (A)

D1 bridge capacity

1 300 Watt 25+25VAC 6 Amp 100V 26Amp
2 600 Watt 25+25VAC 12 Amp  100V 35Amp
900 Watt 25+25VAC 18 Amp 100V 50Amp


"Hybrid+" PCB & Building Kit

For those that would like to concentrate in building the hybrid+ instead of in finding parts and developing&testing PCBs, I have prepared a PCB (Printed Circuit Board) and a complete kit to build  the Hybrid+ supporting up to three output stages.


I already have some PCB available and selling it at cost for 40 US$ each. 

HYBRID+ Complete Kit

A complete Hybrid+ amp kit with two output stages costs 160 US$ for each channel. The kit includes: PCB, tube socket, mounting instructions, ECC88 quality tube, all passive components, all semiconductors, high power rated Mosfets, and shipping (excluding heat sinks).


A complete power supply kit supporting up to two Hybrid+ amplifiers (stereo version), each with two output stages costs 180 US$. The kit comprises both the 35+35 VDC power supply and the heaters 6.3 VDC regulated power supply. The power supply kits includes: PCBs, mounting instructions, all semiconductors and passive components, high quality/capacity filtering capacitors, and shipping (excluding transformers).

Main power supply PCB only costs US$27.

Regulated 6.3 V Heater power supply PCB only costs US$19.

i.e.: For a complete "HYBRID+" 50W power amplifier stereo version you need:


If you are interested in PCB or complete Kits, please send me an email to:


Generoso Cozza