I used it to get the voltages where I needed them to be. If I make it a choke input filter, the voltages go down quite a bit. Or are you saying that I should get a higher voltage power transformer, and start from there?
I've seen this advised in a few places though, to use a small first cap to mimic some of the behavior of a choke input filter, even though it's really a capacitor input filter.
Would you mind explaining that?
Correction
You are right. I was considering the ourput voltage difference between CLC and LC, and getting mixed up with "swinging" chokes.
The buzz, is of course due to the considerably greater AC component.
Premium manufacturers such as Sowter, make special chokes to cope with this.
Cheers,
Dave,dave slagle said:
You are right. I was considering the ourput voltage difference between CLC and LC, and getting mixed up with "swinging" chokes.
The buzz, is of course due to the considerably greater AC component.
Premium manufacturers such as Sowter, make special chokes to cope with this.
Cheers,
Saurav said:
Actually it's halfway between.
Think of a cap input filter: with no load, it charges to peak voltage. As it's loaded down, the cap discharges more and more between charging peaks. At worst case, the cap does nothing and the output ripple form resembles a continuous train of positive sine wave peaks, this comes at the point where the rate of discharge is greater than the rate at which the sine wave falls to the valley between peaks. Anyway, the voltage output in this condition is the average, .9*RMS for a sine wave.
Now, add a choke on after that, and you remove the AC component, resulting in the average voltage being seen. Thus at no load, the first cap is charged to peak voltage, and has no ripple on it since remember we have no load. Likewise the next cap will have the same voltage on it... Now under load, that first cap nearly disappears as described above, and the choke does its job essentially as a choke input supply. The first cap is just extra luggage...
Now of course this is all fine for a class A design but even so a PSU should have good regulation. It's good design practice.
Tim
I see what you're saying, but if I remove the first cap and model it in PSUD, the output voltages drop by about 70V. I bumped the 100uF capacitors up to about 470uF, and that didn't make a difference. So, short of replacing my 300V transformer with a 350V transformer, I think I'll have to keep a cap at the input to get the voltages I need. Or can you suggest some other way around it?
Thanks for the explanation.
Thanks for the explanation.
You can:
- Leave it as-is (and stick with the bad regulation)
- Use full cap-input (which will net better regulation than this) and drop the remaining voltage with a regulator or resistor (er, so much for regulation)
- Drop the primary's AC voltage with a buck transformer, and use cap input. This is N/A if the PT has filament windings in use.
- Or, of course, get a new PT.
Anyway, what's it powering?
Tim
- Leave it as-is (and stick with the bad regulation)
- Use full cap-input (which will net better regulation than this) and drop the remaining voltage with a regulator or resistor (er, so much for regulation)
- Drop the primary's AC voltage with a buck transformer, and use cap input. This is N/A if the PT has filament windings in use.
- Or, of course, get a new PT.
Anyway, what's it powering?
Tim
That's what I thought, resistors would make the voltage sag with current, and I thought that would be worse. I hadn't thought through the effects of a small cap.
What's a buck transformer? The filament windings are in use, but that could be changed. I've been thinking of trying regulated DC heaters, which would need higher voltage transformers, I think.
A stereo SRPP 6SL7 into SE 2A3. I took the design from the JE Labs/Angela website.
http://www.angela.com/catalog/how-to/EZ.2A3.html
I used the same values as the original desgn for the audio section, but decided to try my own PSUD model for the power supply.
Pictures: http://mywebpages.comcast.net/saurav/2a3/
Nooo.... I think I saw that Loftin-White circuit when I was looking for a circuit to build (or at least, I found another schematic that said Loftin-White 6SL7 SRPP direct coupled to 2A3" or something like that). This one's cap coupled. The L-W looked a little more complicated, because the 2A3's biased up higher in order to meet the voltage on the 6SL7, and that biasing is done with a slightly more complex resistor arrangement. It also needed a higher B+, which meant a more expensive transformer, and.... well, I ended up picking the other one. I also read about risks with a direct coupled design if someone turns the amp on with some tubes not inserted, and things like that (don't remember the details any more). And more solder joints = more mistakes I could make. My amp is laid out pretty much symmetrically, and I'd swapped grid and plate connections on one 2A3 socket because I was automatically mirorring the other side. Luckily I spotted it before I powered the thing up.
I compared the SRPP stages of the two, and the L-W amp uses different resistor values. They both have equal resistors for the cathode bias of the lower tube and the plate-cathode between tubes, but the resistor values are different, IIRC.
I compared the SRPP stages of the two, and the L-W amp uses different resistor values. They both have equal resistors for the cathode bias of the lower tube and the plate-cathode between tubes, but the resistor values are different, IIRC.
L-W
Hi,
Don't be afraid of direct coupling...it looks a bit more complicated but is quite simple to implement in SE designs.
It only gets complicated when you have to DC couple various stages in a row.
To give you a modest example: DC coupled input voltage amp DC coupled into phase splitter, DC coupled into a pair of CF buffers cap coupled to powertubes...Now that's no bed of roses.
That's quite normal, the regular SRPP circuit will always have equal cathode Rs for both triodes. They only are oprating at different operating points in the two circuits.
Here's the Loftin-White SE 2A3 amp I refered to a Japanese design from looking at the way it's drawn.)
Cheers,
Hi,
Don't be afraid of direct coupling...it looks a bit more complicated but is quite simple to implement in SE designs.
It only gets complicated when you have to DC couple various stages in a row.
To give you a modest example: DC coupled input voltage amp DC coupled into phase splitter, DC coupled into a pair of CF buffers cap coupled to powertubes...Now that's no bed of roses.
That's quite normal, the regular SRPP circuit will always have equal cathode Rs for both triodes. They only are oprating at different operating points in the two circuits.
Here's the Loftin-White SE 2A3 amp I refered to
a Japanese design from looking at the way it's drawn.)Cheers,
Attachments
That's the schematic I'd seen. I considered that for a while, once I realized how similar it was to the Angela one.
I understood that, I didn't know what sonic difference the changed operating point would make on the sound. Though it's probably the direct coupling that would have a much greater impact.
And like you said, I'd have to tweak the voltages more precisely. Right now my driver B+ is a little under 10% over the schematic, but I decided to leave it as it is (the heater-cathode for one triode half is a little close to the limit, so I'll change that... soon). I don't think I could do that with a direct coupled amp quite as easily.
I understood that, I didn't know what sonic difference the changed operating point would make on the sound. Though it's probably the direct coupling that would have a much greater impact.
And like you said, I'd have to tweak the voltages more precisely. Right now my driver B+ is a little under 10% over the schematic, but I decided to leave it as it is (the heater-cathode for one triode half is a little close to the limit, so I'll change that... soon
). I don't think I could do that with a direct coupled amp quite as easily.
Hi,
Sure, don't see why not...all you do is put the heater to a higher potential with respect to heater.
Direct coupled or not, this a typical SRPP and derivatives thing you have to cater for....if you don't you'll hear it loud and clear on most tubes potentially taking out a nice set of tweeties in the process.
While coupling caps are becoming very good, eliminating them is still very worthwile from a phase behaviour, time constant POV, IMHO.
Add a really stiff PSU and all of a sudden you have yourself a killer amp...
Cheers,
Sure, don't see why not...all you do is put the heater to a higher potential with respect to heater.
Direct coupled or not, this a typical SRPP and derivatives thing you have to cater for....if you don't you'll hear it loud and clear on most tubes potentially taking out a nice set of tweeties in the process.
While coupling caps are becoming very good, eliminating them is still very worthwile from a phase behaviour, time constant POV, IMHO.
Add a really stiff PSU and all of a sudden you have yourself a killer amp...
Cheers,
I was talking about not caring that the B+ on the driver stage was off by 10%. That would mean that the cathode of the upper tube (which is the grid of the 2A3) is off by 10% too, so the 2A3 isn't biased correctly, right? You're right, of course, the heater issue would be the same, direct coupled or not.
Would you mind elaborating? I haven't tested for oscillations or anything like that. In fact, I don't even know how to test for them - anything I can do with only a scope (i.e. no function generator)?
Would you mind elaborating? I haven't tested for oscillations or anything like that. In fact, I don't even know how to test for them - anything I can do with only a scope (i.e. no function generator)?
Hi,
Since in most cases, yours included, the B+ is in common to all stages it would still work...while you may deviate from the ideal operating point it won't mean sonic catastrophy as such.
Tubes are pretty tolerant, they usually always work...having them work optimally is crucial to me though...not everyone is as critical as me.
No need for a scope, you here them whistle and squeal like crazy.
When looking at tube datasheets you'll notice that most of them stipulate a max. heater to cathode insulation voltage.
With stacked triode configurations such as the SRPP the top triode kinda replaces the function of the anode load resistor in an active way...as the name SRPP suggest it's mimicking a Shunt Regulated Push-Pull.
Well, that was a big misnomer from day one but the heater problem is there because the top tube sees the entire B+ wich is often twice as much ( roughly) as the operating voltage a single anode resistor loaded stage would see.
Remember, we use a tube as an anode R and we assume that both triodes share the same envelope, meaning that the heater is common to both sections' cathode.
Naturally the cathode is now operating at a much higher potential as well...and there's the rub: it's insulation between itself and the cathode is exceeded and if care is not taken the heater will ultimately stick to the cathode leaving the tube in a useless state.
So, what to do?
One way is to use separate tubes.
Another is to lift the potential of the cathode of the top tube further away from the heater by injecting a portion of the B+ into the heater supply by the use of a voltage divider.
I'll post a pic with a generic solution asap if needed.
Cheers,
Since in most cases, yours included, the B+ is in common to all stages it would still work...while you may deviate from the ideal operating point it won't mean sonic catastrophy as such.
Tubes are pretty tolerant, they usually always work...having them work optimally is crucial to me though...not everyone is as critical as me.
No need for a scope, you here them whistle and squeal like crazy.
When looking at tube datasheets you'll notice that most of them stipulate a max. heater to cathode insulation voltage.
With stacked triode configurations such as the SRPP the top triode kinda replaces the function of the anode load resistor in an active way...as the name SRPP suggest it's mimicking a Shunt Regulated Push-Pull.
Well, that was a big misnomer from day one but the heater problem is there because the top tube sees the entire B+ wich is often twice as much ( roughly) as the operating voltage a single anode resistor loaded stage would see.
Remember, we use a tube as an anode R and we assume that both triodes share the same envelope, meaning that the heater is common to both sections' cathode.
Naturally the cathode is now operating at a much higher potential as well...and there's the rub: it's insulation between itself and the cathode is exceeded and if care is not taken the heater will ultimately stick to the cathode leaving the tube in a useless state.
So, what to do?
One way is to use separate tubes.
Another is to lift the potential of the cathode of the top tube further away from the heater by injecting a portion of the B+ into the heater supply by the use of a voltage divider.
I'll post a pic with a generic solution asap if needed.
Cheers,
Oh, I've done that, and I understand why it's needed. In my case, the two cathodes are at around 1V (for the lower one) and ~140V (for the upper one). Ideally I'd like my heater to be at 70V or so, but it's either at 60 or at 80, I don't remember which one. I used whatever resistors I had on hand for the voltage divider, and then my B+ ended up higher than expected, so the heater isn't exactly where I'd like it to be. Since the maximum heather-cathode voltage for the 6SL7 is 90V or so, 80 is a little close. I'd be happier if both cathodes were 70V from the heater, instead of one being 60V higher and the other being 80V lower (or vice versa).
Or I could get a second 6.3V transformer, change my wiring so one 6SL7 handles the lower triode of both channels and the other handles the upper, and then both tubes can be made to have heater voltages close to their cathode potentials.
You mean, lift the potential of the heater so it's closer to the upper cathode, right? If not, I don't think I understand this at all.
OK, I don't have that. Just a hum that's audible from the listening position really late at night, and a buzz that's audible 6" from the speaker. This is on 96dB speakers. That's good enough for me, but I'm curious about what dhaen's been saying about regulated DC on DHTs. If I can get them quieter without sacrificing anything, I'd like to try it.
Or I could get a second 6.3V transformer, change my wiring so one 6SL7 handles the lower triode of both channels and the other handles the upper, and then both tubes can be made to have heater voltages close to their cathode potentials.
You mean, lift the potential of the heater so it's closer to the upper cathode, right? If not, I don't think I understand this at all.
OK, I don't have that. Just a hum that's audible from the listening position really late at night, and a buzz that's audible 6" from the speaker. This is on 96dB speakers. That's good enough for me, but I'm curious about what dhaen's been saying about regulated DC on DHTs. If I can get them quieter without sacrificing anything, I'd like to try it.
Both times I've used this for DHT's, it was added into an existing passively filrered PSU, using the parts I had on hand, so you'll need to adjust voltages to what you need.Saurav said:
But simply use something like,
transformer -> bridge or FW (pref Schottkys) -> 10000u -> R -> 10000u -> LT108x, voltage reg -> LT108x, current reg -> common mode choke -> 10n across filament terminals -> filament.
For a 2A3 use LT1085's, allowing about 1.4V across each reg, plus the voltage across the current-set R.
The R in the pi I use for some extra filtering and to adjust the final voltages as neccessary. I'd rather a power R was taking the excess than the regulators (I had several failures at 20V/6A). Depending on your transformers, you may not need it, or may need a big-un.
I have some big power resistors that I use to set the regulators up before I plug tubes in, and a 2A3 is easy, 1 ohm. Then I tweak to ensure the correct voltage at the tube socket terminals and current to be close to spec; it will vary a bit. Youll ideally need 2 meters to do it.
All the rest of the implementations for the regulators is straight off the datasheets. Don't forget to heatsink them well.
Thanks, that helps a lot. Looks like I'll need to start with new filament transformers, there's no way I'm getting regulated 2.5V DC from a 2.5V transformer. And if I understand you right, I should aim to drop any excess voltage in the PS resistor, and have just the 1.4V across the regulators. Do you use pots for fine tuning values, or do you just have lots of resistors and add as many as you need to get to the value? A regulator's output can only be tweaked by changing the value of the sense resistor, right?
Edit: And of course, there are a dozen different kinds of LT1085s on Digi-Key. Let's see... there are only 2 that are both adjustable and in stock, so that narrows it down. Should I go for a 3-DD package or a TO-220 package? Thankfully, it looks like that's all my decision has been reduced to. I guess TO-220, since I've heard of that one before.
I think I should be able to handle the rest of the parts. The regulators are the only thing I've never used/bought before.
Thanks once again.
Saurav
Edit: And of course, there are a dozen different kinds of LT1085s on Digi-Key. Let's see... there are only 2 that are both adjustable and in stock, so that narrows it down. Should I go for a 3-DD package or a TO-220 package? Thankfully, it looks like that's all my decision has been reduced to. I guess TO-220, since I've heard of that one before.
I think I should be able to handle the rest of the parts. The regulators are the only thing I've never used/bought before.
Thanks once again.
Saurav
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