Hybrid Amps: Some more circuits
Using the first article to get our creative juices flowing I found some more well written well designed amplifiers using tubes and semiconductors.
The design link below shows various permutations to feed a high voltage swing with little distortion to an emitter follower output stage. This circuit, titled Hi-End Hybrid Amplifier 100W uses a high end K&K solid state current generator (source) in series with a pentode voltage amplifier. The tube in this case can be a Siemens D3A or Russian 6C45.
OK, so what is our pitch here?
High safety margin, for a 100W amplifier using 4 x MJE15032 and 4 x MJE15033 output transistors in the output, the input to the current amplification circuit is through a capacitor (DC block) from the voltage amplifying pentode and an LM308 op amp for stabilising the DC offset at the output, removing the need to apply manual adjustment.
The MJE15032/MJE15033 can be obtained at a good price from Mantech (Communica advertise only the PNP at R17 each excl).
The K&K current source can be found under the two terminal current source on the K&K website. The source is a design around the depletion mode IXYS IXTP08N100D2. As these are not locally available I’d advise the reader to first open the Two TerminalCurrent Source Kit pdf file from K&K – Microsoft Word then get your nose into Rod Elliot’s article on sinks, sources and mirrors. A few minutes read up on current sources should throw some light on the subject and have you amped for your next journey.
An auto-bias heater filament Mosfet load headphone amplifier
Quite a mouthful but nice clean design pulls all the stops in hybrid design. Design found here.
Hybrid vacuum tube/solid-state audio power amplifier
Nice design which will break the bank amplifier found here – this amplifier uses an 6AN8, a pentode-triode combo. The output devices are BUZ900 and BUZ905. The 6AN8 may not be that easily available in South Africa but the good news is that Mr. Valve lists this valve for R150.00. Based in Pretoria, excellent service.
The BUZ900 and 905 are expensive, TO3 equivalents include the Hitachi 2SK134 and 2SJ49. Available (no price given) at Mantech otherwise Yebo listed as ECF20N20 and ECF20P20 going for over R300 per pop. Ouch! Note that these are lateral type MOS devices and because of the TO3 can this amplifier is ideal for experimenters wishing to modify the output stage with cheaper alternatives. After all, we are looking at the voltage gain devices, chiefly pentodes driving Mosfets.