Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on July 15, 2022, 05:09:29 pm
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Tube data charts shows 20uf filter cap for 5Y3GT. But l noticed the 5F2A Princeton’s schematic has two 16uf filter in parallel for 32uf…?
Just curious why Leo used two 16uf’s, knowing he liked to keep cost down?
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The 5Y3 can take a lot more than 20uFd; that was just a suggested application.
The amp HAS to not hum/buzz bad on the showroom floor.
And every year the customers get a little fussier.
And Leo may have over-bought 16uFd caps as they went out of fashion.
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The 5Y3 can take a lot more than 20uFd; that was just a suggested application.
Thank you PRR, l had a 33uf on 5Y3 for some time with no issues, was wondering if the tube was ailing.
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... l noticed the 5F2A Princeton’s schematic has two 16uf filter in parallel for 32uf…?
Just curious why Leo used two 16uf’s ...
Look at every other Princeton before the 5F2-A (5F2 & earlier). They all had a better system of filtering, with an extra C-R or C-L section.
Fender eliminated the extra stage of filtering (which made hum worse), then doubled the filter capacitance to try to make up for it (and didn't quite get there).
It's better not to copy the very-last tweed Champ or Princeton, as their power supplies went backwards.
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^What HPB said^
A whole-of-supply CLC filter (with an HT choke as the 'L' part), is infinitely superior for filtering a SE amp. You only need a tiny-ish 50mA choke for an amp like that
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... l noticed the 5F2A Princeton’s schematic has two 16uf filter in parallel for 32uf…?
Just curious why Leo used two 16uf’s ...
It's better not to copy the very-last tweed Champ or Princeton, as their power supplies went backwards.
I am going to start building a 5F2A using the Hoffman schematic and turret board. Are you saying there is a better filtering layout for it? Can you attach an improved layout?
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....Are you saying there is a better filtering layout for it? Can you attach an improved layout?
Look at every other Princeton before the 5F2-A (5F2 & earlier). They all had a better system of filtering, with an extra C-R or C-L section.
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^What HPB said^
A whole-of-supply CLC filter (with an HT choke as the 'L' part), is infinitely superior for filtering a SE amp. You only need a tiny-ish 50mA choke for an amp like that
No choke, just three caps, node A-33uf, B and C 22uf each. The champ is quite, not a peep but l got rid of that ground filament supply voltage in champs and did the standard wiring for heater supply in almost all Fender amp. That cut any noise. This champ is the quietest amp l have, turn it on, l wouldn’t know it if it was on till l looked at the “bright red light” :l2:
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You have me convinced. If I replace the 10k ohm resistor with a 4H choke in the 5F2A, do I need to change the 22k ohm resistor since the voltage at B+2 will be higher at about 370V instead of around 325V. If I leave the 22k ohm resistor what B+3 voltage can I expect? I couldn’t figure out the voltage drop. See the revised layout below.
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That revised layout will work but you now need to connect the OT to B+2 rather than pin 8 of the rectifier. Leave the 22K as is.
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2.pdf
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Why use a cap before the OT CT, using that 470 ohm R (that lowers the voltage about 25v's). I have build a few 5F2A circuits without the extra filtering with no issues, no excessive noise, higher voltage. I understand HBP saying the latter filtering was a step down, but is the extra filtering used because of reported noise issues? Forgive my ignorance if I'm out to lunch here? Just trying to understand the reasoning.
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Why use a cap before the OT CT
Not just a cap. Adding a cap ***AND*** a resistor before the plate node will decrease the 120Hz hum considerably as compared to the 5F2A circuit because the B+ ripple will be reduced (B+ will also be reduced). In the 5F2A circuit the ripple at the first cap (plate node) is on the order of 5 to 10Vac. Adding an extra cap and resistor drops the ripple to below 1Vac. Single ended amps are very suceptible to high ripple at the OT. High ripple equals more 120Hz hum. Fender got away with it by using a cheap 8" speaker that could not reproduce low frequencies very well. After all, the 5F2A was meant to be a cheap entry level student/practice amp.
Now, everything I just said about hum reduction when using an extra cap/resistor before the plate node applies to using an extra cap/choke before the plate node, ***EXCEPT*** the choke provides much better hum reduction than the resistor. For example, a 4H choke offers 3KΩ resistance to 120Hz, but practically no resistance to dc. You could use a 3K resistor to achieve the same ripple reduction as that 4H choke, but the resistor also reduces the dc B+. Big power waste!
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Well explained, thank you
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How do you tell which capacitors to ground with the power side and which to ground to the input/signal side? I'm using a choke and connecting the B+2 junction to the power tube. The Hoffman layout I've started with shows all 5Y3GT filter capacitors grounding to the power side. Most other grounding schemes I've seen spilt up the capacitors. Would I ground one or two of the 8uF capacitors to the input side?