Power supply variation

[nsubulysses]

I have always been curious why old Sunns typically do the power supply in a way where there is a reservoir capacitor after the rectifier and this node is not used to power anything. Then they have a choke, then all power supply nodes come after the choke. So the choke has to handle all current that is drawn for the whole HV winding, BUT it is still not considered a "choke input filter, right? Because a capacitor still comes before the choke?

Most other amps you see are similar to the other image i attached. There is the rectifier, then reservoir cap which is the B+ node, then a choke, then screen supply node, etc.

So in one way it makes me wonder if the sunn style has a stiffer power supply because it has the extra power supply node, and in another way I wonder if the whole power supply sags more becasue of the DC resistance of the choke in series with everything. Which is it, or is there another reason entirely to do it this way? Does it possibly have something to do with a lot of Sunns having an ultralinear power amp style while most other amps are not ultralinear?

Thanks for any comments

[Dave H]

I have always been curious why old Sunns typically do the power supply in a way where there is a reservoir capacitor after the rectifier and this node is not used to power anything. Then they have a choke, then all power supply nodes come after the choke. So the choke has to handle all current that is drawn for the whole HV winding, BUT it is still not considered a "choke input filter, right? Because a capacitor still comes before the choke?

I think the choke has to come first (no capacitor) for it to be considered a choke input filter. If a capacitor comes first (even if that node isn't used) it's a capacitor input filter.

[pdf64]

The way I see it, in regard of power tube characteristics, the screen grid IS the plate. For regular amps, the screen grid HT node is as stiff and ripple free as feasible, by use of a pi (ie CLC) filter. Any significant degree of ripple there modulates the signal horribly.
UL/DL amps (including the Sunn amps in question?) use the same HT supply for screen grid and plate, but to avoid the modulation, they still have to conform and use a pi filter on the HT to the screen grids. It’s just that with UL, the plate supply also has to be taken from there too.
Yes it’s saggier than no choke, but that’s better than ripple modulation.
Of course with power triodes (and triode connected tetrodes/pentodes) the screen grid really is the plate, so pi filtered HTs are a good idea.
That’s why ripple modulation at high power levels tends to be an issue when triode/pentode switches are implemented without a pi filter feeding the OT.

In regard of rectifier smoothing / filtration classification, where the load is taken from is immaterial. The presence/absence of a reservoir cap at the rectifier output fundamentally changes the ac - dc relationship, irrespective of what follows.

[J M Fahey]

I have always been curious why old Sunns typically do the power supply in a way where there is a reservoir capacitor after the rectifier and this node is not used to power anything. Then they have a choke, then all power supply nodes come after the choke. So the choke has to handle all current that is drawn for the whole HV winding, BUT it is still not considered a "choke input filter, right? Because a capacitor still comes before the choke?

Most other amps you see are similar to the other image i attached. There is the rectifier, then reservoir cap which is the B+ node, then a choke, then screen supply node, etc.

The detail is in the capacitors used.

The Old Sunn has TINY 20uF caps , so a ton of ripple.
They tame it by using a CLC filter,so capacitor input , smoothed by an LC filter.
Heritance from old times where capacitors were more expensive than chokes.

Later on, capacitors became cheap so they use huge 200uF ones, good enough to straight feed plates from them, and they keep a small choke for screens.

Even later, screen supply capacitors were increased more so chokes were fully disposed of.

[nsubulysses]

Great, I think this is what I needed to know. This is not some superior, ideal power supply arrangement (in the case of the sunn with unused reservoir cap), it is just a historical throwback because power supply caps back then were expensive and there were no high values.

Thanks

[J M Fahey]

Well, if cost is no concern, you can have BIG input caps, FULL current chokes and BIG secondary caps feeding plates

[loudthud]

It''s worth mentioning that the chokes are different. The Sunn chokes are 1.5 Henry, DC resistance about 50 Ohms. Current rating is something like 300mA. The Screen only chokes are usually between 5 and 20 Henry and several hundred Ohms. Current rating is under 100mA in most cases.

[trobbins]

Sunn started in 1965 and used the Dynaco Mk3 amp block and PAM-1 for their 200s - supposedly for its relatively better bass performance (being a hi-fi amp), but also possibly it was an available kit that they could re-purpose. It's filter was 20uF/1.5H/30uF, with the choke small enough to fit under the chassis. The Dynaco Mk3 kit amp was well known given the Mk2 was from 1955 and the Mk3 from 1957. I doubt there was any incentive to modify the amp, as that was it's edge with musicians, and Sunn was about adding a cab - so it stayed as is - a UL amp that was the norm for high power hi-fi in the mid to late-1950's. Interestingly Sunn then marketed the 2000S, a quad with twice the output power rating, but used the same CLC filter !

I have a mid-1964 guitar amp using that circuit - it was a prototype for a local maker here who wanted to get in to guitar amps - he subsequently made an amp line that didn't use hi-fi rated output transformer.

[Leo_Gnardo]

I have always been curious why old Sunns typically do the power supply in a way where there is a reservoir capacitor after the rectifier and this node is not used to power anything.

FWIW I have experimented with altering the filter/amp scheme so that the first filter node powers the MkIII type amp. Gained a whopping watt or two, but ended up with a little instability. A small pip of ultrasonic hash that rode the test sine wave signal, and I couldn't get rid of in any conventional way. So... I went back to the stock circuit. End of story.

[g1]

It''s worth mentioning that the chokes are different. The Sunn chokes are 1.5 Henry, DC resistance about 50 Ohms. Current rating is something like 300mA. The Screen only chokes are usually between 5 and 20 Henry and several hundred Ohms. Current rating is under 100mA in most cases.

On top of all that, the schem. in post #1 shows two chokes in parallel.

[nevetslab]

I just looked thru my notes on the restoration of a Sunn 1200S that I finished I finished in 2016, to see if I had measured the two parallel chokes it also had between the first 20uF cap and the next filter stage following the chokes, which fed the OT C/T. Turns out I never did removed them from the chassis to get those measurements. I had wondered if there was any phasing issues with them, though I never swapped leads on one to see if anything changed.

[nsubulysses]

A couple months ago a friend asked me if I could build him a "Sunn 200S but louder" so of course I started looking at Sunn 1200S and 2000S schematics. in my search I came across the Nevetslab restoration and an old thread where LT recommended using Hammond 159V choke to get what you need from just a single choke. All that old info really helped, thanks. THe choke was very large.

https://www.cedist.com/products/filt...et-15-h-500-ma

Not sure if it matters much, but now in retrospect I guess it should have been 3 henries instead of 1.5? Do Henries add in parallel?

[Helmholtz]

I had wondered if there was any phasing issues with them

What do you mean with "phasing issues"?

[nevetslab]

What do you mean with "phasing issues"?

Would there be any difference if I switched the wires of one of the two parallel chokes? I didn't think so, but..........

[Helmholtz]

Would there be any difference if I switched the wires of one of the two parallel chokes?

No, there isn't enough coupling flux outside the cores to noticeably change resultant inductance.