Soldano SLO-110 reservoir cap and B+ caps

[RoadShow]

I’ve been looking over schematics for the Soldano SLO and have a question regarding the caps for the power distribution system.  The schematics are Rob Robinette’s and one that I think is a Soldano for the SLO-100.

The RR and Soldano reservoir caps are a series parallel arrangement of (4) 200uF 300V caps equaling 200uF @ 600V.  I get that.

In Rob R’s simplified 6V6 the reservoir caps are a parallel arrangement of (2) 80uF 500V caps equaling 160uF @ 500V.  I get that.

When I look at the

• 2203 with its reservoir cap parallel arrangement of (2) 50uF 500V caps equaling 100uF @ 500V,
• JTM45 with its 32uF 500V reservoir cap
• Fender TR with its series 70uF 350V caps equaling 35uF @ 700V
• Fender TRRI with its series 220uF 285V caps equaling 110uF @ 570V

How are the reservoir cap values determined? 
I’ve looked through my Blencowe Power supply book, Kuehnel Soldano book, and others and while they mention reservoir caps (as well as other B supply caps down the line) they don’t discuss how the values are chosen.  I understand the voltage requirement based on rectification of the transformer, but it seems like a wide range of capacitance, 32uF – 200uF.

So, can someone either point me to an explanation or give me one?
Or, targeting the simplified 6V6/6L6 versions can I just reuse one, say the 2203 arrangement, or JTM45 arrangement with the SLO voltage drop resistor.

Thanks.

[NewYorker]

Others here can explain in more detail, but, in general semiconductor diode rectifiers can tolerate higher filter cap values than tube rectifiers can.

[Latole]

https://el34world.com/charts/Schematics/files/Soldano/Soldano_slo100.pdf

[j-b-c]

Generally speaking in that type of amp you want the power supply to be as stiff as practical.

The capacitance of the first filter stage will be constrained by the peak forward current of the rectifier, that will be specified on the devices datasheet.

Duncan's PSU designer software is simple and good for showing whats up.  It runs good under WINE emulation on Linux.

https://www.duncanamps.com/psud2/index.html

On further thing to be aware of.  The peak forward current is reflected back from the rectifier into the transformer core.  Diodes switch harder than tubes making the RFI ringing more severe.  That ringing is one of the major sources of hum that is amplified by single coil pickups.

Power supplies really should be snubbered.  A proper snubber is just 3 inexpensive parts.  It is a little tricky however to tune them to the transformer.

https://www.diyaudio.com/community/threads/simple-no-math-transformer-snubber-using-quasimodo-test-jig.243100/

[RoadShow]

https://el34world.com/charts/Schematics/files/Soldano/Soldano_slo100.pdf

Yes, that's the official Soldano schematic I have and mentioned, along with the RR versions.

[acheld]

How are the reservoir cap values determined?

I've had the same question, and looked in the same books.  My conclusion is that designers have chosen the values to be as large as reasonable, primarily to provide as "stiff" a power supply as they can.   

In other words, when you know a .22 is not enough, and a .50 is too big to carry around, you might then choose a .45 -- a .30 or .38 might work fine, but you have a .45, and into the amp it goes. 

So yeah, I think the power supply design is a bit arbitrary.  But it works . . .

I do wish Merlin would rewrite his Power Supply book.

PSUD2 does work well IF you are able to give it the right parameters. 

[RoadShow]

Generally speaking in that type of amp you want the power supply to be as stiff as practical.

The capacitance of the first filter stage will be constrained by the peak forward current of the rectifier, that will be specified on the devices datasheet.

Duncan's PSU designer software is simple and good for showing whats up.  It runs good under WINE emulation on Linux.

https://www.duncanamps.com/psud2/index.html

On further thing to be aware of.  The peak forward current is reflected back from the rectifier into the transformer core.  Diodes switch harder than tubes making the RFI ringing more severe.  That ringing is one of the major sources of hum that is amplified by single coil pickups.

Power supplies really should be snubbered.  A proper snubber is just 3 inexpensive parts.  It is a little tricky however to tune them to the transformer.

https://www.diyaudio.com/community/threads/simple-no-math-transformer-snubber-using-quasimodo-test-jig.243100/

Thanks for the guidance.  I modeled several configurations as spec'd by the schematics, some simulations attached below.  I was surprised at the one configuration I came up with while changing values showing minimal overshoot & ringing.

I'll look into snubbing as well. I looked at the first and last page of the subject and will probably sub in a PIC for the 555 using a pot to adjust and play with the square wave frequency.

[j-b-c]

I'll look into snubbing as well.

The design files for the Quasimodo jig are open source.  Any PCB manufacture will make them for very little money.

https://github.com/analoghifi/Transformer-Snubber

Maybe this is something the proprietor of this forum could organize on a group buy and/or add to his catalog.