Newbie needs help. Basic PSU design and choke question.

[RockysDad]

I am planning on building a DC coupled Double Darling sort of like the one below but it uses two 1626 output tubes.

The PT I was thinking of using was the Hammond 272FX which is 600VCT at 150ma and a choke of 10H and 155 ohms. When I simulated this in PSU2, I get a huge spike over 500V at turn on. I'm figuring that total current draw will be about 110ma. Is this PT too small?

Also, a couple of question here:

1) will this spike kill the PS caps rated at only 500V?
2) Some chokes are rated at 400V max. Will I also kill this over time if I used a 400V rated choke?
3) Who makes electrolytic caps rated for 500V?
4) Can I use 1kV ceramic caps in these locations?
5) I noticed that if I changed the value of the second cap following the choke to around 220uf that the turn on spike goes way down. Problem is finding a 220uf/500V electrolytic cap.

Having B+ at 400VDC is tough. Any ideas on what to use here in my PSU? Thanks.

This is my starting point:
http://www.geocities.com/bobdanielak/darling.html

[ray_moth]

"The PT I was thinking of using was the Hammond 272FX which is 600VCT at 150ma and a choke of 10H and 155 ohms. When I simulated this in PSU2, I get a huge spike over 500V at turn on. I'm figuring that total current draw will be about 110ma. Is this PT too small? "

I think it is rather small. Have you looked at Amplimo's offerings? They have a number of toroid power transformers with high VA and small size and weight, which don't look too expensive. Most of them don't have a CT so you'd have to use a bridge rectifier. I prefer a secondary rating that's at least twice the current I expect to need. If you reckon you'll need 110mA, you might be well advised to use a transformer rated at 250mA or more.

"1) will this spike kill the PS caps rated at only 500V? "

Yes, it might. It's better to aim to subject the caps to no more than 2/3 of their rated voltage.

"2) Some chokes are rated at 400V max. Will I also kill this over time if I used a 400V rated choke?"

The risk here is insulation breakdown. One way of avoiding it is to put the choke in the negative (grounded) line instead of the positive line. The only disadvantage (if you can call it that) is that the CT or bridge negative pole will not be grounded except through the choke - not a big deal in my opinion.

"3) Who makes electrolytic caps rated for 500V?"

I don't know, but I would use a pair of 350v caps in series, with a pair of 100K 2 watt resistors forming a voltage divider between them, to ensure that each cap sees half of the total HT voltage. these are easy to find. Of course, you'd need double the required capacitance in each cap, e.g. two 470uF 350v caps in series will give 235uF at 700v max.

"4) Can I use 1kV ceramic caps in these locations? "

Can't comment.

"5) I noticed that if I changed the value of the second cap following the choke to around 220uf that the turn on spike goes way down. Problem is finding a 220uf/500V electrolytic cap."

See above.

"Having B+ at 400VDC is tough. Any ideas on what to use here in my PSU? Thanks."

Not sure what you mean. Again, you might look at Amplimo's offerings (see http://www.amplimo.nl/amplimo.html ) for th etransformer. As to the rectifier, if you're forced to use a bridge, you could consider TV damper diode tubes for the positive-going half of the bridge and SS diodes for the negative half. This would give you a soft start (about 30 seconds) and help protect your amp tubes from cathode stripping due to having HT delivered to them before they've warmed up. With this hybrid bridge arrangement, you should get about 400v from a 350v winding and NO start-up spike!

Hope that helps.

Regards,

Ray

[arnoldc]

Originally posted by RockysDad
3) Who makes electrolytic caps rated for 500V?

Panasonic does, and so do Audio Professor, Elna and their Cerafine series.

[SHiFTY]

Use a slow warm up rectifier tube like a GZ32/5V4 instead of diodes and your B+ will never go above working voltage. You will need a 5V winding on the power tranny, and about a 350-0-350V transformer. If you want to use SS diodes, include a B+ switch, usually on the power transformer centre tap to ground.

However, I find directly heated tubes draw current almost immediatley, so the voltage should be ok. Please describe your power supply topology.

If your choke is rated to 400V should be ok. They ususally rate them conservatively.

Buy an overrated power tranny, with at least 50% over capacity on all windings. If this gets too hot during operation, try seperate filament transformers. You might need to go DC on the DHT filaments to get the hum down.

[RockysDad]

I know the term, tube rectification, and would like to know more on how to use them in an audio amplifier. Does anyone know of any good links that talks about how to go about choosing a tube to best suit my purpose? I would like to learn how to design a simple PSU then simulate it using the PSU2. I don't think I will learn anything if I just throw parts at the simuilation hoping to find something that would work.

Thanks for your responses...

[Valverine]

Have a look at Joseph Marshall articles at Audio Classroom Series.

Being a beginner myself I find them very informative.

[RockysDad]

Which would give me the actual VDC from my power supply measured with a DVM? RMS or the one from the graph?

[SHiFTY]

Meh? Are you asking about the input voltage? (Hint: 300-0-300 offload will give roughly 300V RMS)

There is no RMS with DC, its an AC thing!

[Layberinthius]

Ill be a using a full paper-in-oil power supply

Thanks to those guy's in another thread my hopes of building a very cool amp are now again sky high

I've found with center tapped transformers and PSU2 i've always seen some really weird results, like with a full-wave rectifier I see 200volts out of a "400v-0-400v" tranny... reguardless of capacitor size/value or for that matter resistance/constant current etc...