I'm seeking to understand the filtering characteristics of chokes but haven't had success in finding a document which addresses them specifically.
I know that a choke has a filter rating (henry), current rating (mA) and resistance rating (ohm). Current and ohms ok but henries?
A 4 henry rating for Fenders seems fairly common, and then I read an the AC30 came stock with a 30 henry choke - which seems to be a significant variance.
How is the filtering requirement (henries) determined for a circuit?
Thanks in advance - Greg
Choke primer
Moderators: pompeiisneaks, Colossal
Re: Choke primer
Do you understand how to design an R-C ripple filter?
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gregarious
- Posts: 49
- Joined: Mon May 08, 2006 11:21 am
Re: Choke primer
No, but thanks for the pointer... 
Re: Choke primer
Try for a start http://www.aikenamps.com/gregarious wrote:I'm seeking to understand the filtering characteristics of chokes but haven't had success in finding a document which addresses them specifically.
section: Advanced Tech: Chokes Explained
Work in rejection (attenuation) ratios. Assuming you are familiar with a CRC pi filter, then the CLC works similarly but with lower losses where it matters (at DC). Higher freq = higher impedance due to action of inductance = higher ripple rejection.
In a typical pi filter (C1 L C2), where C1=C2 = 20uF, and X=impedance
XL = 2pi f L
at 120Hz and 5H = 3k7 ohm
at 120Hz and 10H = 7k5 ohm
XC = 1/2pi f C
120Hz = 66ohm
Think of the pi filter (XC1, XL XC2) as the input cap followed by a divider XL and XC2.
Then in terms of ripple voltage reduction,
Ripple In/Ripple out = XC2/(XC2 + XL)
120 Hz 5H = 0.0175 = -40.5dB (dB = 10 ln(XC/XC+XL)
120Hz 10H = 0.0087 = -47.4dB
Re: Choke primer
From practical observation I wouldn't get too worried about choke values. It seems that most values from 2 or 3H to 10 or 12H work fine in instrument amps.
I'm surprised at the Vox value, perhaps someone might shed some light on it.
I'm surprised at the Vox value, perhaps someone might shed some light on it.
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gregarious
- Posts: 49
- Joined: Mon May 08, 2006 11:21 am
Re: Choke primer
Thanks Richard for all the info and for lighting the path...
Randall, of course, leads to Duncan.
Cheers - Greg
Randall, of course, leads to Duncan.
Cheers - Greg
Re: Choke primer
With an R-C filter, you must compromise between filtering and voltage loss.
With an L-C filter, the L (choke) has low DC resistance and high AC (ripple) impedance. It is almost no compromise at all (until you look at cost and weight).
Say you design an R-C filter, and decide you need 3K to get good filtering, but need less than 300 ohms to get low voltage drop. A 4H choke will give near 3K impedance at 100Hz/120Hz, and one can probably be found with low DC resistance.
When looking at existing amplifiers, note if the choke filters the whole amp, or just part of it. Fender tended to let the power plates eat raw DC, used a choke to filter the much smaller current to screens and preamp. The smaller current allows a smaller cheaper lump of iron. Really good filtering of the entire amplifier's current tends to need a choke half the size of the PT, and has rarely been done in domestic audio since electrolytics came along.
With an L-C filter, the L (choke) has low DC resistance and high AC (ripple) impedance. It is almost no compromise at all (until you look at cost and weight).
Say you design an R-C filter, and decide you need 3K to get good filtering, but need less than 300 ohms to get low voltage drop. A 4H choke will give near 3K impedance at 100Hz/120Hz, and one can probably be found with low DC resistance.
When looking at existing amplifiers, note if the choke filters the whole amp, or just part of it. Fender tended to let the power plates eat raw DC, used a choke to filter the much smaller current to screens and preamp. The smaller current allows a smaller cheaper lump of iron. Really good filtering of the entire amplifier's current tends to need a choke half the size of the PT, and has rarely been done in domestic audio since electrolytics came along.
Re: Choke primer
P-P obviously rejects alot of the noise when B+ taken prior to the choke. PRR, what is your opinion/observation of using the same "raw DC" for the plate in a Single Ended output. I seem recall that the Champ did that (haven't checked - I could be wrong). Is it worth taking the plate supply after a, say 150mA, choke to supply a 6V6 SE ? I ran a simulation on Duncan Munroe's PSU Designer II which seemed to work OK.PRR wrote: Fender tended to let the power plates eat raw DC, used a choke to filter the much smaller current to screens and preamp. The smaller current allows a smaller cheaper lump of iron. Really good filtering of the entire amplifier's current tends to need a choke half the size of the PT, and has rarely been done in domestic audio since electrolytics came along.
Re: Choke primer
> P-P obviously rejects alot of the noise ...opinion/observation of using the same "raw DC" for the plate in a Single Ended output.
SE Pentode has rejection too.
Book-data for 6V6 shows plate resistance about 10 times best-power load resistance for SE conditions. So 10V of ripple divides about 1V across a 8.5K load and 9V across the 77K 6V6 plate resistance. That's about 0.1V at an 8 ohm transformer tap, or 30dB below 1 Watt. Assuming 90dB SPL/W speaker, that is 60dB SPL, awful high. However Champs tend to use lesser speakers, the "91dB" is measured 400-4,000Hz and will be less at 120Hz. Also the speaker is often mounted open-back in a box small enough to be dropping at 250Hz.
Skip the math. Build the Champ. Does it hum? Verify that it hums with 6V6 grid shorted to cathode return. Clean your screen supply more. Try a 6V lantern battery to heat the 6V6. Clean-up your power supply loop (PT, rectifier, first cap must be a tight loop). If it still hums, try a bigger main cap. If that helps but not enough, and a way-big cap costs more than a choke, add a choke.
Depending on your religion, you should try NFB from the speaker winding (not the plate winding as seen on some very old amps). Cut down gain 4:1, your THD number goes down 4:1, and so does your hum.
There's "no" hum in a champy-thing I recently built. I see a bit when the preamp is "dimed" but that's open-chassis on a nasty bench, not coming in the 6550 plate. The power supply is (simplified) 165uFd, 100 ohms, 165uFd. Yes, that's far beyond what Leo put in Champs. But huge-uFd snapin caps are small and cheap, and I didn't want any question of hum.
SE Triodes are much more susceptable to PS garbage. Instead of 10:1 rejection is is more like 3:2.... most of the ripple appears on the load. Since for the same output a triode needs nearly twice the supply power, this makes pentode power much cheaper for a given power and humlevel.
SE Pentode has rejection too.
Book-data for 6V6 shows plate resistance about 10 times best-power load resistance for SE conditions. So 10V of ripple divides about 1V across a 8.5K load and 9V across the 77K 6V6 plate resistance. That's about 0.1V at an 8 ohm transformer tap, or 30dB below 1 Watt. Assuming 90dB SPL/W speaker, that is 60dB SPL, awful high. However Champs tend to use lesser speakers, the "91dB" is measured 400-4,000Hz and will be less at 120Hz. Also the speaker is often mounted open-back in a box small enough to be dropping at 250Hz.
Skip the math. Build the Champ. Does it hum? Verify that it hums with 6V6 grid shorted to cathode return. Clean your screen supply more. Try a 6V lantern battery to heat the 6V6. Clean-up your power supply loop (PT, rectifier, first cap must be a tight loop). If it still hums, try a bigger main cap. If that helps but not enough, and a way-big cap costs more than a choke, add a choke.
Depending on your religion, you should try NFB from the speaker winding (not the plate winding as seen on some very old amps). Cut down gain 4:1, your THD number goes down 4:1, and so does your hum.
There's "no" hum in a champy-thing I recently built. I see a bit when the preamp is "dimed" but that's open-chassis on a nasty bench, not coming in the 6550 plate. The power supply is (simplified) 165uFd, 100 ohms, 165uFd. Yes, that's far beyond what Leo put in Champs. But huge-uFd snapin caps are small and cheap, and I didn't want any question of hum.
SE Triodes are much more susceptable to PS garbage. Instead of 10:1 rejection is is more like 3:2.... most of the ripple appears on the load. Since for the same output a triode needs nearly twice the supply power, this makes pentode power much cheaper for a given power and humlevel.
Re: Choke primer
Thank you PRR, (hoping may we yet get a friendlier moniker by which to address you? <smile>
I enjoy and appreciate all your replies. Please don't hold back on the math, such ballpark calculations are hugely educational.
I used a poor comparison when talking about a champ (they seem to use RC filters in any case). What I'm really after is experienced judgment on whether in terms of hum it is ever worth while taking the plate supply post choke, not before the choke. Early Bassman did so (5D6, 5E6 according to Weber), they later changed the orientation. Given the Bassman is PP, and such smoothing of plate supply isn't called for, I wondered if the earlier, perhaps more conservatively designed, set up of the 5D6 was worth applying to SE ccts.
I have a suitable choke at hand but rather limited ability to breadboard, so perhaps unrealistically, I hope to get it 90% right first time. The cct I'm envisioning is happy to induce sag and powersupply compression, so that isn't an issue. I just hoped I hadn't missed anything obvious such as supply ringing due to transient response or inductor flyback issues.
I enjoy and appreciate all your replies. Please don't hold back on the math, such ballpark calculations are hugely educational.
I used a poor comparison when talking about a champ (they seem to use RC filters in any case). What I'm really after is experienced judgment on whether in terms of hum it is ever worth while taking the plate supply post choke, not before the choke. Early Bassman did so (5D6, 5E6 according to Weber), they later changed the orientation. Given the Bassman is PP, and such smoothing of plate supply isn't called for, I wondered if the earlier, perhaps more conservatively designed, set up of the 5D6 was worth applying to SE ccts.
I have a suitable choke at hand but rather limited ability to breadboard, so perhaps unrealistically, I hope to get it 90% right first time. The cct I'm envisioning is happy to induce sag and powersupply compression, so that isn't an issue. I just hoped I hadn't missed anything obvious such as supply ringing due to transient response or inductor flyback issues.
Re: Choke primer
I've just had a sudden "Doh" moment.
Does the inductance of the primary of OPT bear upon the common practise of not bothering to take the plate supply post filter?
Does the inductance of the primary of OPT bear upon the common practise of not bothering to take the plate supply post filter?
Re: Choke primer
The OPT inductance should be negligible -- all but the cheapest OTs pass frequencies down to 60hz. Most of the impedance the output tubes see is being reflected from the load connected to the speaker side of the OT.
--mark
Most of the impedance the output tubes see is being reflected from the load connected to the speaker side of the OT.--mark
Re: Choke primer
Thanks Mark, I wondered if the question was dumb after posting it! So back to my main concern, is the practise of not smoothing the plate supply with a choke purely down to savings on cost of iron for the greater current rating and nothing to do with some hidden phenomena? Is there any reason apart from expense NOT to smooth plate supply of output tubes especially in SE config?
Re: Choke primer
The main reasons for not using a choke (from a manufacturer's point of view) are expense and weight, not any technical reason.
It is instructive to watch B+ of both types under load with a 'scope. The RC type have very regular (and fairly large) 100 or 120hz ripple, while the LC type look smooth on first glance, but are actually sloshing up and down at a very low and irregular frequency.
This latter is way too low to be heard (or passed through an OT), so the audible difference is less hum coming out of the speaker when using a choke, particularly with SE amps.
--mark
It is instructive to watch B+ of both types under load with a 'scope. The RC type have very regular (and fairly large) 100 or 120hz ripple, while the LC type look smooth on first glance, but are actually sloshing up and down at a very low and irregular frequency.
This latter is way too low to be heard (or passed through an OT), so the audible difference is less hum coming out of the speaker when using a choke, particularly with SE amps.
--mark
Re: Choke primer
Thanks for that. But what exactly is the merit behind taking the plate supply from the 'front' of filter and taking the screen post filter? Why not both after the filter - i.e. from where the screen supply is commonly taken? Especially in low power SE jobs where DC idle currents won't demand chokes with windings that are hugely rated for current. Surely, the smoother the plate supply the better.