Which Choke?

[passaloutre]

I'm planning my next amp around a Gibson GA-40 (schematic below), excluding the tremolo and second channel: so a single 5879 into 12AX7 paraphase inverter, into push-pull, cathode biased 6V6s. With my transformer set (borrowed from a junked RCA Victor console), I'm expecting just over 300VDC out of my 5Y3 rectifier. By my calculations, the amp should idle at something like 94mA from the high voltage power supply.

This is my first time using a choke upstream of the output transformer, and I have two chokes on hand: one is 1Hy/160mA, the other 3Hy/100mA.

The 3Hy choke matches the spec for the original Gibson, but am I playing too close to the current rating of this part? The 1Hy choke offers less filtering, but gives me a bit more wiggle room for current.

https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga40.pdf

[shooter]

i'd agree you are "close" at 100mA since it feeds everyone, the 1H will still give you better "results" than a resistor

[bmccowan]

I'm guessin that you might be closer to 80mA. Shooter, if that's the case would you consider the 3H choke to be risky?

[DummyLoad]

Hammond 156M is 3H @100mA - That should work well for your GA-40 clone. If the marginal current rating (actually slightly higher than you need), then the Hammond 157Q 3.5H @150mA would also work well.


--Pete

[DummyLoad]

I'm guessin that you might be closer to 80mA. Shooter, if that's the case would you consider the 3H choke to be risky?

Looking at the schematic, 18.5V on the 6V6 cathodes is ≈93mA across 200Ω Rk for output pair at idle at the B+ indicated - another 3-4mA for the rest of the preamp ckts - call it 100mA total, will peak slightly more, but that output stage is mostly running class A, so maybe 15-20% rise in plate+g2 current under heavier load.

Too large of a choke (higher inductance) will be too stiff, P-P guitar amps sound better with a just enough filtering PS plan, of course the exception being P-P UL output stages. If you're going to clone, get as close as possible to the original parts values/type, no?   


--Pete

[passaloutre]

I’m not going for a super accurate clone, just something of that flavor with parts from the spare parts drawer. I’ll probably go with the higher current choke and sacrifice a little bit of filtering.

[shooter]

probably go with the higher current choke and sacrifice a little bit of filtering.

not going for a super accurate clone

I would up the filter caps to all 40uF then, especially for the preamp section.

[ac427v]

I like the squishiness of small filter caps in old amps. If it were my project, I would build as designed and try it before changing to big caps. The capacitor-inductor-capacitor first stage of filtering should flatten much of the power supply ripple. It may be enough to keep the original values and sound.

[tubeswell]

Agree with Shooter -  increase the filter capacitance around the 1H choke until you can't hear any ripple hum. Won't do any harm if the rectifier can handle it

[DummyLoad]

Use your 3H 100mA choke - it will work fine.


--Pete