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Old 28th December 2010, 04:26 AM   #1
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Default another 12ax7 preamp

hello community

I am in the mist of building a 12ax7 preamp that i could use with my fender twin reverb. I have completed my power supply schematic and ordered all the parts. I am now building the schematic (using TINA) for the main curcuit. I am trying to keep is nice and simple. I came here looking for some feedback on the following.

First- it wouldnt hurt to get some eyes on my power supply schematic. (see attached)

Second- well It got too late to finish the mainbaord schematic tonight, but i hope to have that posted tomarrow.

The power supply is there now. I will appreciate all feedback on the design.

Thanks for your time.
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Old 28th December 2010, 11:48 AM   #2
M Gregg is offline M Gregg  United Kingdom
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Just a quick question,

Are the AC voltages peak or RMS? I think the DC values may be low for some of the caps!

Think about off load.(warm up)

How many heaters are in the circuit I guess in series?

Think about start currents! The lm7812 dissipation may be an issue.
The cold resistance of the heaters is very low even in series.
Just for interest measure them cold DC resistance and work out the current.
Work out the Vd across the regulator and wattage.

Regards
M. Gregg
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Old 28th December 2010, 12:36 PM   #3
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300 V ac in -> 275 V rectified??
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Old 28th December 2010, 12:53 PM   #4
SY is offline SY  United States
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LEDs with 12V and 300V across them will make a spectacular flash.
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Old 28th December 2010, 01:12 PM   #5
Sheldon is offline Sheldon  United States
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Second Miniwatt's comment. How do you get those voltages?

Why not split your dropping resistor and make a CRCRC supply? It will reduce your ripple.

The best way to look at both of these issues, is to download PSUD and do some sims. It's accurate and dead easy to use.

SY, I wondered the same. But he does specify a series resistor in a note.

Sheldon
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Old 28th December 2010, 01:34 PM   #6
SY is offline SY  United States
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Quote:
Originally Posted by Sheldon View Post
SY, I wondered the same. But he does specify a series resistor in a note.
Yeah, but that's pretty ambiguous. And if true, he'll be burning more current running the LED in the HV leg than used by the preamp circuit! Perhaps the LED would do better in the cathode of one of the 12AX7s?

Also, depending on the preamp circuit, it may or may not be a good idea to have the heater supply referenced to ground. It would also be a good idea to set it up so that common mode noise is not inadvertently injected through the heaters.
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Old 28th December 2010, 02:31 PM   #7
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M greg- thanks for catching the cap v values error- that was supposed to be listed as 350v rating, not at 250v.

There will be 2 heaters in the circuit. I will have to give some thought on how to run them- I will be working on the schematic today/tomarrow.

The lm7812 issue and the cold heater resistance- Ill get back to that in a few days when parts arrive- Ill give it some thought.

Miniwatt- I am not sure about these values either- I have the transformer and when i tested it, it had an output of 300vac. But that was just the transformer, I havnt been able to measure these values activly. the 275vdc rectified- I had an extra bridge rectifier and wired it to the transformer and measured 275vdc from it. It is not the rectifier specified in the circuit. I understand your skeptisism of those listed values. I prob should have listed them in a note instead of lying them right in the schem.

SY- I hadnt given much thought to the LEDs. as soon as i read your comments i realized there has to be a better way to incorperate them- Ill work on them.
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Old 28th December 2010, 04:50 PM   #8
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Quote:
I had an extra bridge rectifier and wired it to the transformer and measured 275vdc from it. It is not the rectifier specified in the circuit.
Your measurement gave an erraneous result since there was no capacitor.
As soon as you add a filtering capacitor(s) after the rectifier the DC voltage will be some 350 to 400 volts. So you should use capacitors with 450 V rating.

Forget the LED at high voltage line. Instead use some other indicator having higher voltage rating and use proper series resistance.

In case of LED, some 99,5 % of the used power will dissipated in series resistor.
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Old 28th December 2010, 04:50 PM   #9
M Gregg is offline M Gregg  United Kingdom
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Quote:
I prob should have listed them in a note instead of lying them right in the schem.

Please put the values on the schematic notes are OK, however it helps so people do not over look information!

I think you will find that a capacitor on your DC after the rectifier will give you a different voltage. Probably higher than you have allowed for.


You may find that a soft start for the heaters will help! Lots of ways to do it , resistor with timed shorting contact, soft start regulators etc.

Hi Artosalo Same thoughts bet me to it!lol

Regards
M. Gregg
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Old 29th December 2010, 02:12 PM   #10
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by SY View Post
Yeah, but that's pretty ambiguous. And if true, he'll be burning more current running the LED in the HV leg than used by the preamp circuit! Perhaps the LED would do better in the cathode of one of the 12AX7s?
True enough. If not it the cathode, then just in series with the anode.

Sheldon
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