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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6sn7 Preamp power supply

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I found this design from a google search that got some good feedback, and I want to build it because I have been looking for a good 6sn7 preamp. I also found a power supply, but it seems like the transformer has an odd VAC, and the regulator s a bit complex and out of my tight, tight budget. Is there a good 250v B+ design that uses either SS or tube rec. and uses a common transformer voltage? Any help would be greatly appreciated.

Thanks, ((AE))
 

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ambience exists said:
That looks nice, but pardon my noobness: what is the symbol at the end of the circuit?

Thanks for the reply.

Also, does anyone have a SS suggestion?

The symbol at the end is my best guess at the current draw of the preamp.

About your transformer, I'm sure there is a propper way other then just plugging it in and measuring the leads with a meter (which is likely not a wise thing to do) so I'll let someone else field that question.
 
Actually, that's a fine way of doing it; it will give you the off-load voltage, and by plugging that in to their calculator along with primary and secondary resistances, you can get a decent representation of your transformer.

But there's danger involved, so watch the voltages.

There's a very nice solid-state regulator kit that should work well with this preamp circuit:

http://tech-diy.com/DIY_PWR_LastPass.htm
 
So.. that kit would be the whole supply? hook it right up to the xformer? Looks very cool and noob friendly. I like PCB's!

When I am figuring out this power transformer, what else should I measure besides the volts coming out of the other end?

How do I get the primary and secondary resistances?

Once I get the volts, I'll post the results so maybe someone will know what it is comparable to what I could get new.

My real question: What if it ends up too large? Could I use a resistor to drop the voltage before it goes into the regulator(I have decided I like that kit)? I guess I am just confused about how much of the power supply that sweet PCB kit includes.


Any Ideas?
 
Contact Jack directly about what is/isn't included, though I'm sure the transformer wasn't part of the deal.

The primary and secondary resistances are easily measured with a multimeter. But again, if you start measuring voltages, there are definite safety hazards for a noobie. We don't want to lose you! Check the Safety and High Voltage Practices sticky thread. And I would HIGHLY recommend you invest in Morgan Jones's two books on tubes and tube amps. That will help keep you safe and give you a lot more understanding of what you're doing.
 
Sorry about not being clearer, I knew there was no xformer in the deal!

I have looked closer at this kit, and it seems like you just hook it up and it is the complete power supply. Pretty cool.

But I guess if you needed to drop the B+ after it is spit out of the PCB you could use a high wattage resistor eh?
 
Nice simple design on the preamp... note that it has an inverted output, so somewhere in your signal chain you need to invert polarity again. Most likely at the speaker side if your amplifier is non-inverting.

On the tube regulated supply... also nice and simple, but two notes:

1- the 5651 is a voltage reference tube, not a shunt regulator. Maximum current is recommended at 3.5ma and operating design point should be around 2.5ma. The design runs it beyond 3.5ma based on the 47K dropping resistor plus the current flowing thru the triode section... probably closer to 4ma total which will reduce tube life. A 68K would be better.

2- pretty much all gas regulator/reference tubes can oscillate so putting a small value cap in parallel with it usually alleviates the problem. Sylvania recommend a 0.02uF shunt capacitor, but a 0.01uF should be fine.

Happy building... post some pictures when completed.

Regards, KM
 
SY said:
No need for something that inelegant, you can use the Zener reference string to exactly set the output voltage.

Yeah, but how do you explain that to someone who is not an engineer? I'm not sure of what you mean.

I did measure the xformer, and I think the main winding is 430 volts. There are red wires. The other windings which I assumed were the filament windings and are green/brown/yellow/etc... are giving me strange, un-filament numbers, but I may not be measuring them correctly.

Really what I want to know is if I can use this xformer with that PCB kit to power franks preamp?

Thanks for the help!
 
Standard plate/filament wire color codes are usually:

Black - Black = primary, 120 volts (for US)

Red - Red = Plate supply winding - Red-Yellow (trace) is the center-tap

Green - Green = 6.3VAC filament winding - Green-Yellow (trace) is the center-tap

Yellow - Yellow = 5.0VAC rectifier filament winding (usually no center-tap)

Any other colors could be additional filament windings or primary tap options. You should measure continuity/resistance of each winding and make yourself a schematic drawing. Some power transformers have a static shield which can be on a separate wire.

Also note that an unloaded power transformer will read high on voltages. You can safely assume the yellow pair is good for a minimum of 2 amperes as most rectifiers require 2 amperes... some big ones are 3 amperes. The 6.3VAC winding can vary anywhere from 2.0 amperes for something small to 4.0 or more for large iron. Plate windings vary greatly on voltage and current based on intended use.

Zener diodes are basically shunt regulators, ie, they have a specific junction voltage and maintain that over a current range. You could take five 47-volt zeners in series and be close to 250 volts with simple shunt regulation.

Regards, KM
 
To follow on to KM's excellent post, the Zener string as shown has two 100V Zeners in series with an 80V Zener. If you use two 80s and a 100, you'll be at 260V (actually, 261.25V because of the regulator chip's 1.25V reference). That's close enough for this circuit without doing anything more.

Don't worry about not being an engineer. I'm not one, never even had an engineering course, but I still manage to design some reasonably good equipment. Read and learn, then experiment, it's terrific fun. You'll smoke some parts along the way and probably hit some frustrating walls. That's all part of it. Just be careful around these voltages!
 
Regarding design and engineering, most basic concept is "Smoke Theory"

- All electronic components run on smoke. When the smoke leaks out, they stop working.

On a more serious note.... here's a link that should be useful for designing a zener-based shunt regulator:

http://www.ibiblio.org/kuphaldt/electricCircuits/Semi/SEMI_3.html#xtocid1094012

For vacuum-tube and basic electronic theory you really can't go wrong (or do much better) than getting an old ARRL Handbook... from the 1950's... mine is a 1958 (and I have others too). You can find them on ebay reasonably priced.... gee, look here:
http://cgi.ebay.com/Radio-Amateurs-...ryZ15050QQssPageNameZWDVWQQrdZ1QQcmdZViewItem


Regards, KM
 
Cool, thanks.

So, I really don't know how many volts my xformer is because I was reading it unloaded. How do I find the REAL voltage?

Thanks a lot for the links, there is so much info on the web obviously and I never know where to start.

Oh yeah, one of the filament windings has green wires like the others but has three and the third has some yellow striping on it. Is that the center tap?

Also, if I am not using all of the filament windings, how do I "not use them"? Just tape them off??

Maybe Ill post a pic of this transformer...
 
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