Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on May 29, 2011, 02:54:15 pm
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Hello,
i bought an old RCA LMI- 32215 A tube preamp.
Searching for a power supply for make it work i found the original power amp schematic:
http://www.novacon.com.br/audiorcbri.htm (http://www.novacon.com.br/audiorcbri.htm)
(Another one with voltages, attached)
I a just whant to make a power supply for the preamp.
I suppose:
Trafo (350v outs) -----Rectifier (solid state, maybe)-----16uf cap-----Choke------16uf cap-
I´m right or can i just go for a 400v after filtered supply and a single 16uf cap?
thank in advance.
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350V, you mean, 350-0-350? That will be too much with solid state rectification.... it will give you about 494VDC, even
after a choke. If you get a PT that supplies the correct amount of current (one that supplies more will put out more voltage
after rectification) then you can just use straight math to calculate the PT secondary...
In your case you want about 420V (I would shoot for less, a small properly sized choke will not bring it down much), so say
405 or 410. That would indicate that you want a PT with 290-0-290 secondaries. 300-0-300 will give you about 424V, so
if you go that route (since a 300-0-300 would be and "off the shelf" value) get a choke with more DC resistance...
OTOH, if you go tube rec, you will want more from your PT, check the tube curves (with your current demands) for proper
PT secondary output.
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The high voltage, big rectifier, and choke are for the POWER amp.
In a preamp, R-C filtering works fine at much lower cost.
> a single 16uf cap
It needs more filtering than that. Filtering small power is cheap.
To power just the preamp, try 300V-350V over 20mA AC, rectifier, 40uFd, 4K7 2W, 40uFd, 1K 2W, to the preamp power feed.
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i found this one.
http://cgi.ebay.de/310-280V-Audio-Power-Transformer-for-DIY-Tube-Pre-amp-/300561635192?pt=Vintage_Electronics_R2&hash=item45fade9778 (http://cgi.ebay.de/310-280V-Audio-Power-Transformer-for-DIY-Tube-Pre-amp-/300561635192?pt=Vintage_Electronics_R2&hash=item45fade9778)
Is just an example, but,
Could this one do the work?
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updated
i found this. Help me to choose one, please:
http://shop.ebay.es/dieroehre1/m.html?_nkw=Netztrafo&_sacat=0&_odkw=&_osacat=0&_trksid=p3911.c0.m270.l1313 (http://shop.ebay.es/dieroehre1/m.html?_nkw=Netztrafo&_sacat=0&_odkw=&_osacat=0&_trksid=p3911.c0.m270.l1313)
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solid state recto full wave CT w/ two strings of 2 x 1n4007.
220uF filters
47K 5W bleeder - burns off ~7.5mA
hammond 269JX - 500VCT @ 70mA - or yank transfo out of 6BQ5 amp - should get you 350V +/- DC w/ preamp load.
hammond 157G choke - 30 H @ 40mA DCR 600ohm.
~~350VDC B+ tapped @ after LC network.
$22 filters & resistors.
$47 PT
$25 choke
--DL
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Cool! thanks!
now i have to found the EU equivalent. This hammond dont have 220v primary.
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thanks everybody.
i´m gonna try the PRR option. Few parts involved. Anyway, thanks dummy!
I think thist trafo is perfect, and the 3 HT secondaries are cool.
http://cgi.ebay.es/Rohren-Netztrafo-160VA-280-320-350V-0-35A-50V-6-3V-/120627526322?pt=LH_DefaultDomain_77&hash=item1c15f5fab2 (http://cgi.ebay.es/Rohren-Netztrafo-160VA-280-320-350V-0-35A-50V-6-3V-/120627526322?pt=LH_DefaultDomain_77&hash=item1c15f5fab2)
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forgot you are in EU. 369JX - has universal primary with same secondaries. slightly larger because of the 50Hz rating.
--DL
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Thanks Dummy!
As i said, i going to try the PRR idea. i made a fast schematic.
Please, check for errors.
And first question:
Do i need a full wave rect. bridge or can it done like my draws?
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And first question:
Do i need a full wave rect. bridge or can it done like my draws?
You can do it that way, but that's a half wave rect. and will not be as clean as a FWB rect.
I would use the FWB, and I would put the SB switch after the 1'st 40uF filter cap, it's a little easier on the switch that way. Just my .02's.
Brad :icon_biggrin:
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If you haven't bought it yet. Also see triode transformers PA522 trans. Has 240v
And 120v mains. 330v and 12.6v secs, no center tap on either, so you get
360 or so after a bridge rectifier. I've used them several times.
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If you haven't bought it yet. Also see triode transformers PA522 trans. Has 240v
And 120v mains. 330v and 12.6v secs, no center tap on either, so you get
360 or so after a bridge rectifier. I've used them several times.
too late.... :embarrassed:
anyway, i had some problems in the past with 240v primary transformers. in my area we use to have 215-225vAC and that means lower voltages in the secondaryes. Thanks anyway!
updated schemo with Willabe corrections.
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> Röhren Netztrafo 160VA
You only need 10VA. 20VA would be generous. 160VA is WAY more than you need, and 60 EUR is not cheap.
Find a euro forum for hifi preamps. Ask what they use. Need 350V-450V DC at 10mA-20mA (0.020A) plus 6.3V at 1A.
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> Röhren Netztrafo 160VA
You only need 10VA. 20VA would be generous. 160VA is WAY more than you need, and 60 EUR is not cheap.
Find a euro forum for hifi preamps. Ask what they use. Need 350V-450V DC at 10mA-20mA (0.020A) plus 6.3V at 1A.
I knowed it´s oversized, but sometimes i´m really impatient.... :BangHead:
Any way, i can use it for another proyect. :icon_biggrin:
thanks!
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ok.
i found this one for a more affordable proce:
http://cgi.ebay.de/Trafo-Netztrafo-f-Rohrenverstarker-220-V-315Vu-6-3V-/230623845696?pt=DE_TV_Video_Elektronik_Funkger%C3%A4te&hash=item35b2409d40 (http://cgi.ebay.de/Trafo-Netztrafo-f-Rohrenverstarker-220-V-315Vu-6-3V-/230623845696?pt=DE_TV_Video_Elektronik_Funkger%C3%A4te&hash=item35b2409d40)
no information about the power capability, but i suposse is a ball park.
I mailed the seller asking for some more information.
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too late.... :embarrassed:
anyway, i had some problems in the past with 240v primary transformers. in my area we use to have 215-225vAC and that means lower voltages in the secondaryes. Thanks anyway!
at 215v, the PA522 would run 300v and 10v. 300v is still good for the B+, and you could rectify and use a 7806 6v regulator to get 6v for the filaments (min. 9v, so 10v would be fine..) plus its small (compared a 160VA PT must look like).
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To power just the preamp, try 300V-350V over 20mA AC, rectifier, 40uFd, 4K7 2W, 40uFd, 1K 2W, to the preamp power feed.
Ok. One more question.
If you look at the preamp schematic, there are a 15k resistor in the H.T. rail, just after the link octal shocket
I suposse that i put a 1k one after the last 40uf node, that resistor is redundant.
Isn´t?
thanks in advance!
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I like a resistor (if possible) where power leaves or enters a box.
I suggest a 1K between the main DC power and the DC power socket/cable, so that an accidental short on the cable does not suck "infinite" current from the main filter cap.
With no resistor, you could realistically get 50-100 Amps. Only for a fraction of a millisecond, but still a huge current.
Such surges burn the corner off of small screwdrivers. Can do real damage to connector or a poor connection.
With 1K you can't have over a half Amp. Much less damage.
The same applies for the cap inside the preamp. If it were directly connected to cable, you broke it and shorted it, you could have a spike of "infinite" current while that cap discharged.
You may say "I don't pinch, yank, or break connectors and cables!" I try not to, but last night a picture fell off the wall and sheared a lamp-cord plug in half. Bits of plastic and brass prongs. Somehow, nothing shorted, and there was no plug-prong stub stuck in the wall outlet to shock the dog's nose. But if it had fallen a little differently....
Yes, since you know there is 15K in the preamp, you "could" re-design with maybe 5K in the power supply and 10K in the preamp. Same number of resistors.
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OK.
I understand. Thanks!!!
I,ll recive the preamp in a few days.
I supoose, i´ll back with some more questions!
:icon_biggrin:
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Preamp arrived.
Thats a beatifull piece of art!
And the tubes! 1 EF86 and 2 ECC81, Mullard!
I mounted the power supply in a test board and get 365vDC after rectifier.
There are so few componets in the power sup that i gonna use a this stuff (i don,t know the name in english)
(http://imagenes.solostocks.com/z1_4010881/regletas-de-conexion-electricidad.jpg)
Tomorrow, burn test. :icon_biggrin: