27 pre PS question..

[JandG]

I have been experimenting with configurations on my PS on a 27 pre I built, PS is outboard, so tweeking it is easy. I use a pair of 6w4gt damper diodes & then CLCLC, I tried using LCLC & then LCLCLC, I could not get hum acceptable. but every time I put a cap @ the 1st choke input to ground, Hum was reduced to acceptable, almost non existant. So I now am useing a 2uf Dayton motor run oiler/by-passed with a .033uf/600v Vitamin Q. then 50uf & lastly 100uf ASC oiler also by-passed w/ Vitamin Q .033uf/600v. It sounds almost as good as choke input to me & MUCH more quiet. I also use 2- 0A3's for tube regulation. Why is it that I can not get a choke input to be quiet & is the configuration I have generaly considered a bad idea..? I am failry new to tube DIY, the 27 pre was my 1st build..I do have this thing almost dead quiet & that is with meshplate globes. Any tricks would be appreciated. I have lots of chokes & more motor run oilers on the way.

[Original Burnedfingers]

Can you post a schematic of what you have now?

[JandG]

I am useing a PS configuration similar to the 26 schematic on my 27. Except I have a 2uf oiler w/a .033uf Vitamin Q by-pass in front of the 1st choke. 2nd cap is 50uf oiler, 3rd is 100uf ASC oiler, this is the configuration I have at the moment.I have 45uf & 55 uf oilers coming & variouse vintage chokes of differant HY & such. So I can play with this to get the sonics I am looking for, I had no idea a PS had so much to do with overall sonics. Major part of the gig I would say..... I am not sure about the by passes as of yet, they do make a differance. just got to listen this all thru step by step. The by-passes seem to shift frequency range...? more top end detail, maybe to much..more listening needed. i even tried the 100uf oiler were the 2uf is now & is nice, very relaxed, but to much so...

[Gluca]

Is that hum buzzing at 50/60Hz or 100/120?

I'd use a CCS on top of the gas regulator.

Ciao
Gianluca

[EC8010]

Originally posted by JandG
Why is it that I can not get a choke input to be quiet?

Pure choke input causes 100Hz switching spikes when the rectifiers switch off. You need a snubber to prevent these; either a 100 Ohm resistor and 10nF 1kV series combination across the choke, or a pair of back to back series 220nF 1kV across the choke with the centre tap taken to ground.

[JandG]

Pure choke input causes 100Hz switching spikes when the rectifiers switch off. You need a snubber to prevent these; either a 100 Ohm resistor and 10nF 1kV series combination across the choke, or a pair of back to back series 220nF 1kV across the choke with the centre tap taken to ground

I don't mean to be slow, but the center tap taken to ground has me confused.?? Center tap from my PT is soldered to my copper ground buss. I use a all wood chassis. The above combinations are just in series/across the 1st choke & nothing to ground..? I must admit that using chike input does have a sonic quality I prefer over cap input. Can't quite explain it, but after a good listen last night, I would say this is the case. So I will try these snubber's once I am sure I got it sorted in my head correctly.

[Dave Cigna]

Originally posted by EC8010
... or a pair of back to back series 220nF 1kV across the choke with the centre tap taken to ground.

Two separate snubbers between HT and ground, one before the choke and one after? Or have I misunderstood?

-- Dave

[EC8010]

The reason I suggested it the way I did is to encourage wiring the capacitors across the choke terminals with a very small loop area. Having wired the two capacitors in series, then connect that composite capacitor across the choke terminals, take a single wire from the centre connection to PSU 0V. This method uses the internal resistance of the choke as the snubber resistance.

If you use the traditional alternative of 10nF and 100 Ohm, there is no connection to 0V.

[JandG]

At a quick look in parts closet, I don't have those snubber parts on hand to try today, but I did change the 2uf oiler 1st cap to a 39uf Solen & evrything shifted back to a more relaxed sound...?mmmm. Also you wouldn't think any of this would make much sonic differance with use of the OA3's, but unless I am delusional, it makes a great deal of differance, Actaully has way more impact than a tube roll or anything like that. I think I like a larger cap than 2uf-4uf in teh 1st spot, in fact I am sure of it, at least what I am hearing, I see most people start very small uf w/ CLCLC, maybe it is because I am useing the OA3's. Don't know. I went back & forth with 2-4uf 1st posistion to 39uf & up & the larger wins every time for more relaxed presentation. Just fun uneducated experiements I'm doing, I am watching B+., around 330v headed into OA3's..Even LCLC lights the OA3's at much less incoming B+, but way to noisy without the above outlined snubbers.

[Yugo]

http://www.diyaudio.com/forums/showt...93#post1178793

Hope this will help!

Regards,
Yugovitz