I happened upon a thread somewhere on choke input and i want to try it. I think i may have in the past but not sure so i'd like to give it a shot. Just need to know how to go about it. Right now it's B+>50uf>choke>50uf>screens. Can i assume what i would need to do is just remove the choke wire at the second cap and remove the B+ from the first cap and connect them, then put about a 100R across the 2 caps? This would be B+>choke>50uf>100R>50uf>screens. Is that right?
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What is the current rating of the choke? If it can't stand the current, it will quite possible burn up. It takes a mighty big choke, something that weights 5 or 10 pounds.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school !
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Daz,
Anytime you want to try power supply mods then you can simulate it AS IS first and then as proposed to see what happens.
This is what everyone (well most nearly everyone) uses for power supply simulations.
PSUD2
Its free and I've found it to be very accurate.
Trick for using it for Class AB push pull tube amp supplies is to use the stepped current load model, set a load current equal to the idle current and then step it up by 40% after say 2 seconds. This will make sense once you have PSUD2 installed an running.
Cheers,
Ian
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It's a 20 henry, 500vdc, 100ma, 181 dcr. And as tom mentioned, i'd lose a lot of voltage which is why i wanna try it. I hope to change the PSU to get max voltage (up to the point it's at now at most) for the preamp and PI so as not to change that any more than it has to, but to run the output tubes at a much lower voltage. But i guess at 100ma i can't use it this way eh? 2xel34, 3x12ax7, 250ms PT. What if i use 6v6?
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Indeed. It's a purpose designed choke. Higher in H's and dcr and lower in overall current capacity than a choke designed for the entire power draw of the amp would be.
While an enticing experiment from a purely curious perspective, it's not really a compatible set of circumstances. Just as you wouldn't bother putting a tow hitch on a VW beetle or use a tractor as a commuter vehicle. Your amp is designed a certain way and overall design and function for both the amp and the purpose of a choke input is sacrificed by trying to amalgamate them. To do it properly should include a lot of alteration to the HV rail. Considerations of filter values and time constants, number of nodes, perhaps node grounding, etc. Far from as simple as just removing the existing choke from the screen supply and putting a bigger choke at the front end. Failing to redesign with these considerations is almost certain to yield unsatisfactory results that, for lack of any other perspective, should likely be perceived as a blanket effect of the use of choke input filtering to a novice. And incorrectly so.
I might suggest simulating rectifier tube resistance with a big resistor or even a power scaling circuit instead.
JM2C"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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I really just wanted to see what low voltage el34's sound like. I've tried sag resistors of many small to large values and i don't like that feel. It's TOO saggy. Probably because my pre already has a lot of sag in it's feel.
No big deal....just thought maybe i could try it.
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Selmer T'n'B amps used a semi-choke input with EL34 but they're anything but saggy. I say semi-choke input because the rectified DC hit a cap first, then the choke, then another cap. However, all of the amp B+ was drawn after the choke/second cap. These chokes are approximately the size of a tweed deluxe OT, they are low inductance (all I've measured are in the 2.5 to 3 H range) and very low resistance, @ 25 to 35 ohms but can handle a lot of current. They get kind of warm when the amp is really driven but not too bad, at least none that I've had. If I was going to mess around with this topology, these chokes are where I'd start.
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Originally posted by EFK View PostSelmer T'n'B amps used a semi-choke input with EL34 but they're anything but saggy. I say semi-choke input because the rectified DC hit a cap first, then the choke, then another cap. However, all of the amp B+ was drawn after the choke/second cap. These chokes are approximately the size of a tweed deluxe OT, they are low inductance (all I've measured are in the 2.5 to 3 H range) and very low resistance, @ 25 to 35 ohms but can handle a lot of current. They get kind of warm when the amp is really driven but not too bad, at least none that I've had. If I was going to mess around with this topology, these chokes are where I'd start.
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It's not feeding the WHOLE amp. Your existing choke WOULD sag. ALOT! It also might blow if you ever pushed the amp anywhere near it's rated power.
You could also try a big zener or a string of zeners in the power supply to reduce voltage without the sag affect of a resistor. Better would one of the power scaling circuits that can be bought as a kit and installed with provided instructions."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by daz View PostI happened upon a thread somewhere on choke input and i want to try it. I think i may have in the past but not sure so i'd like to give it a shot. Just need to know how to go about it. Right now it's B+>50uf>choke>50uf>screens. Can i assume what i would need to do is just remove the choke wire at the second cap and remove the B+ from the first cap and connect them, then put about a 100R across the 2 caps? This would be B+>choke>50uf>100R>50uf>screens. Is that right?
Choke input filters do funny things if they are just used willy-nilly, as you might expect from them being fundamentally current-operational things instead of the voltage-oriented stuff that capacitor input filters do.
A choke-input filter with very light load is essentially a capacitor-input filter, and the voltage at the output is 1.414 times the RMS voltage fed to the rectifiers (ignoring rectifier voltage losses). This is because the current pulses feeding the filter cap load up the inductor with current, but the inductor runs out of current before the next half cycle starts.
As the load current increases, the time the inductors conduct per half-cycle rises; this means more voltage is absorbed in the inductor and the output voltage on the cap falls. When the load current is big enough to keep the inductor conducting on both half-cycles, the voltage falls to 0.636 times the capacitor input voltage. So if you have a power supply putting out 400Vdc into a capacitor input filter, with an inductor input filter, it starts at 400V at nearly zero load and falls to 254V when enough current flows to keep the inductor current flowing through both AC half cycles. It then **stays** there as current rises until something smokes. Inductor input filters are great for voltage regulation, but suck at how much voltage you get.
If you're after some sag, you are likely to get the worst of both worlds. Inductor input filters sag massively as loads go up from no load to light-moderate loads, then do not sag essentially at all up til things burn out. You'll get too much sag at light loads and no sag at heavy ones.
Yes, you can design the inductor and capacitor values to hold it at the Goldilocks sag value - if you're very good at inductor design and can make your own custom stuff. This is because the exact amounts and places where the load is "light" "moderate" and "heavy" depend on the inductor and capacitor values. So a custom inductor is almost certain to be needed if you have specific needs in mind.
You can indeed convert your supply to inductor input, what I think you meant in the post, moving the inductor to between the rectifiers and first filter cap, but you are very much taking pot luck on what that value of inductance does to you. And the voltage will drop to 64% of the cap-input filter if you load it "right" and will have dramatic sag between your existing cap input filter now and that 64%.
You're also taking pot luck on the inductor's design for inductance and current rating. High inductances hit full conduction at lower currents than lower inductances, so the inductance value is selected for the range of currents you need. High inductances also saturate sooner in most cases for the same core size, so you *have* to go to lower inductances for higher currents, as well as designing for the min to max currents your application has. Few of us know the actual currents our amps pull, I suspect.
This is one of those things where it can be fun to play with (assuming you're careful with high voltage and have a healthy respect for not dying of electrocution, as always) but taking pot luck on the inductor you already have may be very disappointing. But you could also get lucky, too. Just don't bet the farm on it. And if someone says "swinging chokes" RUN!!Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.
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Daz and Chuck: the Selmers are different, because the choke **is** feeding the whole amp. The power supply runs GZ34/SS (depending upon whether mkII or mkIII model), filter cap, choke, filter cap and THEN it feeds the OT center tap/plates, and then a shared 1.5K screen R/cap etc. Take a look at one of the schematics - all of the amp B+ is drawn through the choke. The earlier models had @ 490V on the EL34 plates (and this was after the choke!) while the mkIII models dropped it back slightly to around 460 and increased the filters from 32 uF to 50 uF. As I see it, the only difference between the Selmer T'n'B amps and a more "classic" choke input is that the Selmers add an extra cap before the choke, so it's not a true choke input in the sense that the power supply is feeding the choke directly/immediately; however, as I've noted, nothing is drawn off the PS until after the choke.
Edit: after re-reading a couple of the posts, Chuck I can't tell whether you're indicating agreement re: the Selmers, or agreement with the comment Daz made. Sorry!
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Originally posted by EFK View PostEdit: after re-reading a couple of the posts, Chuck I can't tell whether you're indicating agreement re: the Selmers, or agreement with the comment Daz made. Sorry!"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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