Is this thing gonna blow up?

[Meat&Beer]

'Cause I kinda just wung it...

It's a power supply for a preamp (thing) I'm making that wants 3 250v supplies. It'll have 2 12AX7's, and either a 6SC7, 6SL7, or maybe even just another 12ax7.

The schematic was borrowed and modified from what was given to me to build guitar preamps in a breadboard. I'd like the leave the xformer in there, as I plan to build and develop more amps. For now, I'm making this.

So, I'll be wanting to order another xformer, preferably one that just gives up ~250v from the getgo, not 380, like this one does at the top of C3.

Warning; foolish newb alert.

I don't know what the hell I'm talking about. Moving forward, the tranny is a Hammond 369JX, 500v with a center tap. Do I want a 250-0-250, or a 125-0-125... See what I mean? I'm a doofus.

Anyway, here is my stab at getting the supply right with the tranny on hand. I was tinkering with 2 watts trying to limit the joltage, but a couple of em started smoking after a minute, so I got some big fatties.

Also, I'm working with parts I have lying around, certainly not deriving specific values with formulas.

R1 is my bleeder, right on the cap. R3 is my dropping resistor (?) that takes it down to 250v. Am i at least correct with the terminology so far?

So how does it look? Is it gonna spit capacitor acid in my face? It does work, and nicely as far as I can tell. I'm getting a nice 250-253 volts on all the supplies. R2 gets very slightly warm to the touch after about 5 minutes.

I'll stop blabbing. No one wants to read all these darn words, sorry.

What do we think? And you don't worry about hurting my feelings. All advice and help is greatly appreciated, Thank you!

[Bob S]

Safety First - read all you can on safe practices.
This shit is Dangerous!
OK - then search Zener Diode on here.
The right transformer is the best solution. But ...
You can drop voltage with a high power zener on the high voltage center tap to ground.
You'll need to move the standby switch to right after R1 - I would do that anyway.
R2 can be changed for a choke or a smaller value resistor if you want some power supply sag (1k ala express).
Bump R3 to 220k if you want a bleeder after the standby.
Not sure why you need 3 x 250 vdc.
The 2 x 10k's in the dropping string will mess that up.
Smaller values for less change or diodes maybe.
Have fun and BE SAFE.
Please.
Bob

[bb5000]

If the first dropping resistor drops the voltage to 250V, the following dropping resistors will drop the voltage even more (according to your schematic)...if you need 250V DC for all preamp tubes, you only need the first dropping resistor and feed all tubes from that point.

check this: http://www.hammondmfg.com/pdf/5c007.pdf

A 180-0-180 with Full Wave rectifier, Capacitor input load should give you ~255 VDC without the need for dropping resistors.

[Meat&Beer]

Safety First - read all you can on safe practices.
This shit is Dangerous!

Thanks Bob!! I've been working on the same amp for about 2 years now, ironing out the kinks these days but wanted to jump on this little project also. I'm well aware of the dangers already, but warnings are always nice reminders.

You'll need to move the standby switch to right after R1 - I would do that anyway.

Is this to get the first cap charged before slamming the entire thing with the switch? I ask because this is also the location of the stby in the actual amplifier I'm additionally working on. I'm always interested in alternative methods for a desired result and pro's/cons...

R2 can be changed for a choke or a smaller value resistor if you want some power supply sag (1k ala express).

This is actually a PSU for a phono preamp, then a mixer tube to sum stereo to mono. I have a killer Telefunken Opus Royal console that someone changed out the turntable with a nice Garrard. Problem is its a RatShack SS pre with a Y cable to the mono TFK. It loses the left channel, and I hate the fact of a SS RatShack POS existing inside of such a nice unit. Pretty sure I don't want any sag in here, but thanks for the tip!

If the first dropping resistor drops the voltage to 250V, the following dropping resistors will drop the voltage even more (according to your schematic)...if you need 250V DC for all preamp tubes, you only need the first dropping resistor and feed all tubes from that point.

Hi and thanks bb500! That's what it looks like, right?? I'm only getting about a 2-3 volt fluctuation between the supply voltages though. Beats me! But hey, you answered one of my questions that I didn't even ask because I tried not to be too long winded. I only need one supply for the three! I had a feeling, and thank you.

You guys are great, I knew it wasn't a difficult question for you guys! I'll be changing some stuff with it, and researching the links you've provided me with. (Very busy day today unfortunately...) Thanks again so much, this place is incredible.

[Firestorm]

Keep your 250V nodes decoupled from each other (resistor and cap on each) but don't run the supply in series, parallel them.

[Meat&Beer]

I'm thinking this is what I need for a ~250v tranny...

http://edcorusa.com/p/702/xpwr105_120

Yes?

Firestorm, I'm not sure if I follow you exactly...

Got a pile of Mouser in the mail yesterday. Drawing up the layout for the breadboard shortly.

I'm guessing it's okay that I'm here with this query? You can't plug a guitar into it, but it is a tube amp nonetheless.