"sloclone" HT PS filtering mod question - diyAudio
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Old 17th February 2013, 02:48 PM   #1
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Default "sloclone" HT PS filtering mod question

Hello, I'm trying to make a hybrid using the 4 tube, slo/Weber Heather preamp feeding a SS power amp.

The question is, since the outputs D & E (A & B on Heather) are not needed, how should the circuit be modified to provide the best filtering, or how is it calculated?

I noticed these types of supplies often use series caps in the beginning, which I thought was to beat the need for large HV caps. But in my research on EMC, the additional ESR may actually be a neccessary for a quiet and safe filtering scheme, and ties into inductor selection.

Moreover, there is a huge iron core inductor measured in Henries, of which there are very many to select from.

I was also wondering if oil-filled motor run caps will work here, and if there is any advantage besides long life and low noise? 50uFs are cheap and "fairly" small.

The slo and Heather snips are provided for convenience. The slo uses a 10H 125mA 115 ohm, the Heather spec is unkown. Noteworty is Heather uses less total capcitance for essentially the same circuit. It also uses suppression caps on the rectifier diodes whereas the slo does not! Well, at least not in the published diagram.
slo ps snip.JPG
Heather ps snip.JPG

Last edited by ITPhoenix; 17th February 2013 at 02:56 PM. Reason: forgot something
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Old 28th February 2013, 04:23 AM   #2
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Join Date: Oct 2004
Location: Adelaide South Oz
Since no-one had responded I will put something here:

For a 4 tube Heather Clone preamp you will need ONLY nodes C D and E on that diagram and a total of less than 10mA current - that means the original powerr tranny which was designed to cope with the power section as well is WAY over what is required, no choke is required etc.

I would suggest a 2 transformer solution:

Buy 2 toroidal trannies with dual 115V primaries and 6 volt secondaries. As your line voltage is closer to 120V then you will get about 6.1 or 6.2 V from the first tranny, that is fine for the heaters.

You need to supply 4 of 300mA @ 6.3V for the heaters, that is 0.3 x 6.3 x 4 = 7.5VA

To get the High Voltage grab a second tranny - wire its 6.3 V secondary across the 6.3 V secondary of the first tranny. That will give you 230V AC from the old primary ( 2 x 115 V in series this time) which is now the secondary (if you follow), rectify and filter that and you will get a supply of around 270 to 280 Volts. You only need to supply 10mA from this so that tranny needs to be rated for about 3 VA and it adds 3 VA to the load of the first tranny.

So 1st tranny use say a 15VA toroid (12 VA minimum).
Second tranny use say a 5VA toroid (3 VA minimum) or just buy 2 of the first type.
Full bridge rectifier using UF4007 diodes, a 100uF 450V filter cap (add a 100nF 630V polypropylene cap across the electrolytic cap for extra spiffy supply), that gives you node C, take D and E off C as per the original diagram.

The 270 to 280 volt rail is a little lower than would probably be in the Heather preamp section but it is enough.

This wiring a second transformer in reverse across the first is a good cheap way to get a low current high voltage supply for tube preamps - NOT RECOMMENDED for high current power amps thou'.

For preamps you can play around with the plan a little too -
Example: Use 12V secondary for the first tranny (run the tube heaters at 12V) use a 15 or 18V secondary for the second reverse connected transformer then voltage double the 115 + 115 V Old primary for:
About 400V if using a 15V secondary for the second tranny
About 360V if using a 18V secondary for the second tranny:

Voltage numbers are back of the envelope calcs but should not be too far off.

You can still use a voltage divider off the High voltage output to elevate the heaters if you want.

Hope this is helpful,
Cheers,
Ian

Last edited by gingertube; 28th February 2013 at 04:36 AM.
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Old 28th February 2013, 04:30 AM   #3
jcdrisc is offline jcdrisc  Australia
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Maybe you guys would benefit from doing a bit of research on capacitance multipliers.
Best way to get very low ripple for low cost and space.
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Old 28th February 2013, 05:06 AM   #4
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I knew I had to go to South Oz for this question. Thanks for the ideas.

Interesting with the toroids; they are very flexible. However, 382V is required for the last tube in the chain. Then 300VA is required to run the power section. 25-0-25AC @ ~3A per rail. I was hoping to find all this in a stock, audio quality toroid, but no luck.

I did wind up with two trannys. One 500va line conditioner which maintains the voltage to within 4% of 110 or 220, whichever I want. But this has no real secondaries. It is essentially a toroidal autotransformer. One coil with a bunch of taps and a control card.

So I wound the secondary for the 25-0-25 which was not that difficult. I was not looking forward to 720 turns for the HT (still requiring a doubler!), plus the heaters, so I found a small audio torroid, fully shielded, with 170 plus a 160V tap, and two separate 6.3V windings.

It is yet to be found if the line conditioner will inject noise into circuit, whether from the unshielded torroid, or the switching circuit. I have not looked to see if it uses SCRs or not, which may add noise. The torroid can be shielded with mumetal or magnetic foil. It's going inside the box.

Yes, I realized the 10H choke and all that reservior capacity probably would not be needed since most of the current goes to the inverter and power section, but I was not sure.

Why the line conditioner? It was cheap, and if it doesn't work, it could be used as a crude stepup-stepdown Variac for the bench, or a choke for a spaceship.

Last edited by ITPhoenix; 28th February 2013 at 05:17 AM.
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