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Old 8th August 2008, 06:10 AM   #1
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Default Electraprint EL84 PP with PSA-2N

Total tube noob, here- first post and all. I'll try not to plague the forum with my remedial project, but if anyone would be so kind to help me out it would be greatly appreciated.

..looking at building my first amp. I have selected a super easy (looking) one Jack Eliano has up at the Electraprint site. It's an EL84 Push Pull using hexfred rectification and one of his PSA phase splitters, and I have a few questions before tackling this little monster.

1. Has anyone actually built this thing?
2. Should I use some sort of B+ time delay (or toggle) for standby?
3. For the power supply, are 400V rated poly caps (such as Solen) beefy enough?

Here's the link: http://www.electra-print.com/el84psapp.php
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Old 8th August 2008, 06:59 AM   #2
kmaier is offline kmaier  United States
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Blackeye,

Hard to read the smallish print but it appears that the power transformer has a 300-0-300 volt secondary winding for B+. With SS rectifiers you would multiply by 1.4 resulting in a peak voltage of 420V... maybe a bit higher, considering no load until the tubes warm up and the possibility of your local power company line voltage being higher than normal.

As a result, the 400V Solens might be at risk. For power supply caps I personally prefer the (now no longer made) Axon HighVolt caps which are rated at 630V and fairly large... and expensive. As a first project I would recommend something reasonably priced like Sprague Atoms. Steve at Angela stocks these at reasonable prices.

As for a time delay, the simplest approach would be to use a vacuum tube rectifier like a 5V4G... and yes, you would have slightly less output voltage but in this design it probably wouldn't result in much of a change beyond maximum output power. Of course you would need a 5-volt winding... meaning a different power transformer or adding a separate filament transformer. Good luck and happy building.

Regards, KM
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Old 8th August 2008, 02:11 PM   #3
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Blackeye:

If you haven't already, you may want to download the Duncan Amps power supply software. It's free and is easy to use. It will allow you to model your transformer with SS or tube rectifiers and check various voltages and currents, etc.

http://www.duncanamps.com/psud2/index.html
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Old 8th August 2008, 02:46 PM   #4
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You could try a time delayed contact in series with the transformer CT. No need for a tube rectifier, extra hole in chassis, and additional 5V winding. Just use the existing 6.3V.

B+ time delay circuit - your comments

Works great for this type of application.

Edit: Solens are available in 630V ratings from Parts Connexion. Not too expensive. Then there's always motor runs........
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Old 8th August 2008, 04:15 PM   #5
kevinkr is offline kevinkr  United States
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I have a friend who has built a couple of variants of this design. Sounds quite nice.

I would use the 630V Solen caps recommended elsewhere and just build it as designed. Works fine.
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Old 8th August 2008, 08:05 PM   #6
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Default thanks.....

...for the good advice- all of it.


kmaier:
Jack was kind enough to send me a high resolution copy of the schematic (see attached). Regarding the voltage- that's what I needed to hear. I'll scrap the idea of using poly caps because they get damn expensive when you need capacitance and working voltages this high. Is there really that much to gain in a simple push pull circuit like this anyways? I know the concensus in SET amps seems to be that the type and quality of the power supply caps have a significant impact on the overall sound, but in a push pull(?). ...maybe I'm getting too ahead of myself by shooting the moon on my first amp. I'll be lucky if it plays! 'Lytics it is.

boywonder:
Excellent link; that went straight to my favorites list. Thank you.

zigzagflux:
I like the way you think. I saw a prebuilt time delay on ebay that looked interesting. But couldn't I also just add a toggle for the B+ and manually flick it on after a few minutes of filament warm up? oh, and motor runs are hard to find. I checked out ebay, mouser, allied, etc- and when I found a 100uF, it was 440V. They're expensive, too. The 630V Solens are aluminum when they reach 100uF. I'm ordering some Panasonic (105C) today. They're cheap and the opinions on these seem to be that they are quite good.
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Old 8th August 2008, 08:20 PM   #7
kmaier is offline kmaier  United States
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Blackeye,

Yes, I do feel that the film caps offer advantages over electrolytics. I'm using all Axon caps in my 45 SET amps... they are big and quite a bit more expensive than equivalent electrolytics. IIRC the Axon caps are actually made by Solen for Axon.

Some advantages, beyond sonics, should be a near infinite lifespan as there is no chemical activity and nothing to dry out, etc. and I've noticed that with a good scope you can see some little blips occur on power supply lines and even the signal lines with electrolytics which are random. Dump the electrolytics and replace with good quality film caps and the lines go dead... so to speak.

But I would tend to recommend less expensive components for a first project... I do like Dale/Vishay metal films... and you can buy new from Mouser at reasonable prices, but I won't use anything smaller than the RN65 for anything.

To give you an idea on the size of a 40uF/630V Axon cap, here's a pic of the underside of my 45 amp. The cap diameter is just slightly less than 2". The 100uF/630V Axon caps are huge but are axial same as the 40uF. Either way, they take a lot of space!

Click the image to open in full size.

Best of luck with your amplifier project...post some pics when you're done.

Regards, KM
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Old 8th August 2008, 08:53 PM   #8
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Nice work, KM. All Hashimoto? Even the busses look nice- great attention to detail. Thanks for the advice!!
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Old 8th August 2008, 09:37 PM   #9
kmaier is offline kmaier  United States
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Yes, all Hashimoto iron... my overall favorite. Also used Caddock resistors in the entire audio path, Mills resistors in the power supply and pure silver wire (Kimber AGSS) for the entire amplifier circuit... the single ground buss is 12ga solid silver. The red wire is SPC in telfon and used for non-critical wiring (filaments on the 5V4G and 12AU7) and AC input wiring. The only down side to these amps were the cost... around $1200 per chassis sans vacuum tubes... and I had to build two for stereo

Thanks for the kudos... used CAD for the chassis layout, makes repeatability simple.

Regards, KM
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