Electronics > Projects, Designs, and Technical Stuff

Replacement of tube amplifier kit filter choke

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TimFox:
Besides the cost of capacitors (and the required high-voltage rating), if a 10 H inductor has a reactance of 3 k\$\Omega\$, but a DC resistance of only 100\$\Omega\$, then it wastes far less DC voltage than a 3 k\$\Omega\$ series resistor for similar ripple reduction.  Note that a push-pull output stage is relatively insensitive to ripple on Vbb, but the input voltage amplifiers are more sensitive to it.

floobydust:
Haven't seen the 430VDC power supply schematic, but assuming it's using solid-state rectifiers - so B+ surges to a high voltage when the tubes are cold, there is no load on the power supply until the filaments warm up. The voltage rise is much higher for choke-input LC filtering on B+, rarely used though. I would expect B+ to go over 500V and this is the reason the designer uses two 450V caps in series with balancing resistors.

For sizing the choke, Radiotron Handbook advises to avoid resonance at the amplifier's lowest output frequency. That pipe organ music at 25Hz... not only mains ripple coming in to it.
The choke should be as large as possible, what you can afford and fit for best filtering.
The choke should be as small as possible, to save room and money, and have low losses.

TimFox:
10 H and 50 uF resonate at 7 Hz for this example.

floobydust:
I also found these chokes used in guitar amps:
EDCOR USA choke offerings are 4H 50mA 170Ω 0.4lbs used as Fender 125C3A, 15H 75mA 347Ω 1.4lbs.
Hammond 194 is another series, shielded.
Sowter UK also has excellent offerings.

luiHS:

--- Quote from: floobydust on June 05, 2023, 07:55:27 pm ---Haven't seen the 430VDC power supply schematic, but assuming it's using solid-state rectifiers

--- End quote ---

This is the schematic of the power supply, yes it is solid state. On the power supply there are four trimmers to adjust the BIAS of the KT88.

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