6m amplifier with 2x Gi7b
This is my contribution to cheap tube amplifier gallery. Idea was to build stable and small power amplifier, suitable for portable and domestic use.
Although measures are small, this PA delivers 900W with used pair of tubes primary installed into amplifier. Toroidal transformers is used
in mains to reduce amplifier size. Step start is recommended due to excessive start current. Zener diode chain start conduct when AC at T2 primary reach
180V AC. Optocoupler activates Q2 and relay K1 which bridges R1 and apply full mains on T3.
Like in HF amplifier, in this PA genuine tube heath sinks are also replaced by heath sinks suitable for axial blowers. Many builders asked me for source of
sinks which are used in my amps. They are from my junk box and were used as heat sinks for strong rectifier diodes and I'm afraid that's small chance to
find this sinks on market (manufacturer was "Iskra" from S5).
But, there is no reason to giving up. Check your junk box, maybe you could find suitable sinks, for this project one massive cooler could be used for both
tubes lake those used in mighty audio amplifies. You should provide good thermal contact and sink with same or even bigger capacity than original one.
Output network is PI-L type which gives better suppression of unwanted emission than regular PI network.
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