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 Post subject: Re: 12B4 Preamp
PostPosted: March 18th, 2021, 8:44 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Update on the 12B4A preamp. After an extensive burn-in streaming non-stop since Sunday evening from the Naim Classical Radio channel (CD resolution FLAC), I brought the preamp up to my main system. Outside of it having more gain than I need, it is pretty transparent sounding. I have been using a P&G passive attenuator, and it is almost that transparent. Tonally well balanced, I think it could have a little more low frequency, but it seems clean and well defined. Open soundstage. Based on what I am hearing, I would place it ahead of my TVA using Silk transformers, and second to my P&G passive. I know the output caps I am using could need a bit more body, so maybe that is something to work on. One thing for me to experiment with is a slight rearrangement and addition of small input caps, and I can run this as a current sink loaded buffer and work with my system gain structure. Fun!

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 20th, 2021, 10:20 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Well, the 12B4 preamp is sidelined. Since the beginning of the week we had been trying to track down a odor in the house, a strong electrical or electronics smell. I knew my preamp was running warm with all the heatsinked mosfets in the current source. I also knew the zener diode string I built on an FR4 perf board also was running hot, but after several days of continuous running, it seemed to reach some thermal stability, warm, but not hot. I did have a bit of a smell at first, but it seemed to not as bad over time. I had it off most of the day today, and the smell in the house dissipated. When I turned it back on for about an hour or so, my wife complained the smell returned. She told me to turn it off. Good that she did, I inspected the zener regulator board, and the board was very discolored and the back side of the board with the solder connections showed the solder had turned from a shiny to pasty surface, like some melting was occuring. I now understand why Morgan Jones wires the zeners on a tag board keeping full airflow and lead length to dissipate the heat. Sinking 20mA at 220V is dissipating 4.4 Watts, and I had two regulators stacked one over top of each other. So don't do this! I have to figure out a way to recover, since I don't have a lot of space to work with. May have to look into a simple tube or mosfet regulator to either get the heat out of the chassis or sink it to the already warm chassis.

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 7:53 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Well, I figured out a way of building the two diode strings for the statistical regulator in a compact form that should solve the thermal issues. Zener diodes use the leads for heat dissipation, so I soldered up an array on spaced perfboards that provide for air circulation around the leads, rather than soldering the leads short on a circuit board. I have two 207V regulators (string of 37-5.6V zeners) on this little module. I will cut back on the current through them to around 10-15mA (I was running 20mA). A picture of the original regulator boards and the new assembly is attached. This fits in the same space in the preamp chassis as my original stacked boards. I am adding (drilling) ventilation holes on the bottom of the chassis directly below the regulators (I only had ventilation holes on the top originally).

You can see the overheating on the old boards. Basically a fire waiting to happen. The back side (not shown) is even worse.

David


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old and new regulator.jpg
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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 9:16 am 
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Joined: February 28th, 2013, 3:31 pm
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Did you derate the zeners? I never use them at more than half their rated dissipation. I also rely on the connections at the end of the leads rather than the leads themselves to act as a sink. The only reasons I can think of that your original boards got so hot was that either each diode was dissipating a lot of heat or there were so many in close proximity that the total power on the area of the pads was excessive. What was the total power calculated for the string?


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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 9:32 am 
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Joined: July 24th, 2015, 4:17 pm
Posts: 1374
Location: Parkville, Maryland
David McGown wrote:
Well, I figured out a way of building the two diode strings for the statistical regulator in a compact form that should solve the thermal issues. Zener diodes use the leads for heat dissipation, so I soldered up an array on spaced perfboards that provide for air circulation around the leads, rather than soldering the leads short on a circuit board. I have two 207V regulators (string of 37-5.6V zeners) on this little module. I will cut back on the current through them to around 10-15mA (I was running 20mA). A picture of the original regulator boards and the new assembly is attached. This fits in the same space in the preamp chassis as my original stacked boards. I am adding (drilling) ventilation holes on the bottom of the chassis directly below the regulators (I only had ventilation holes on the top originally).

You can see the overheating on the old boards. Basically a fire waiting to happen. The back side (not shown) is even worse.

David

You did a remarkable job of "thinking outside the box" with a goal to improve reliability, but I have to ask -- doesn't anyone manufacture high-current Zener diodes? I would think the industrial market would have something on offer.

And what about voltage regulation using cold-cathode voltage regulation tubes? Just saying -- building a tube pre-amp (old tech) could be regulated with old tech.

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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 9:43 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Tom,

I had 20mA through each 5.6V zener. This is 0.11W per zener, for a 0.5W rated part. Based on the rating, the diodes should be able to handle a maximum of 89mA. But with the zener diodes flush against the board, and the short lead lengths, there was no way for the heat to dissipate, so all that heat was being sinked into the circuit board material itself, causing it to overheat, discolor, and smell. Having the leads completely free and able to dissipate heat to the surrounding air, and backing off slightly on the current through the regulator, should help alot with the thermal issue, and the boards hopefully should not overheat. If not, then back to square one.

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 9:57 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
SoundMods wrote:
You did a remarkable job of "thinking outside the box" with a goal to improve reliability, but I have to ask -- doesn't anyone manufacture high-current Zener diodes? I would think the industrial market would have something on offer.

And what about voltage regulation using cold-cathode voltage regulation tubes? Just saying -- building a tube pre-amp (old tech) could be regulated with old tech.


Walt,

Thanks. As pointed out, the problem was not device current, but heat dissipation of the concentrated assembly. My first choice would have to use the cold cathode gas regulator tubes, but the use of a chassis with a removal top plate and the internal arrangement made that challenging if not impossible to do. If I was starting from scratch, I would consider that, although Morgan Jones felt that the "Statistical Regulator" was a lower noise approach than using gas regulators. So it was a matter of figuring out how to get it done in the tight space I had. I will have to see how it works out. I just hope that I haven't made a mistake in the direction of one of the diodes in the string of 37 per regulator.

David


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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 10:33 am 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
David McGown wrote:
SoundMods wrote:
You did a remarkable job of "thinking outside the box" with a goal to improve reliability, but I have to ask -- doesn't anyone manufacture high-current Zener diodes? I would think the industrial market would have something on offer.

And what about voltage regulation using cold-cathode voltage regulation tubes? Just saying -- building a tube pre-amp (old tech) could be regulated with old tech.


Walt,

Thanks. As pointed out, the problem was not device current, but heat dissipation of the concentrated assembly. My first choice would have to use the cold cathode gas regulator tubes, but the use of a chassis with a removal top plate and the internal arrangement made that challenging if not impossible to do. If I was starting from scratch, I would consider that, although Morgan Jones felt that the "Statistical Regulator" was a lower noise approach than using gas regulators. So it was a matter of figuring out how to get it done in the tight space I had. I will have to see how it works out. I just hope that I haven't made a mistake in the direction of one of the diodes in the string of 37 per regulator.

David

OOPS! It only takes one in the wrong direction. Kind of like one bad apple in a barrel of apples. Anyhow -- you had to choose from different choices -- welcome to that club.

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Walt


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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 12:57 pm 
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Joined: March 2nd, 2013, 2:43 pm
Posts: 189
Location: Potomac, MD
Incidentally, There are two different principals for so-called Zener diodes. A true zener has a soft characteristic, meaning the breakdown curve is very gentle and the voltage is strongly current dependent. The other type which should not really be called a zener but almost always is, is really an avalanche device. It has a sharp breakdown characteristic and the voltage is largely independent of current. True zeners are usually low voltage, perhaps under 6 volts and the avalanche diodes are usually over 12 volts. for devices between these voltages I have seen both types.

Anyway, some pointers for audio applications: Zener diodes are quiet; avalanche diodes are noisy. So are gas tubes. Bypassing a avalanche diode with a capacitor is useless without adding a resistor to increase the impedance so the capacitor has something to work with. Bypassing a gas tube with a capacitor usually makes negative-resistance oscillator. True zeners can be bypassed, but probably unnecessary because they are quiet.

These devices were traditionally available in stud-mount packages for higher power. I have not checked, but they may be available in more modern plastic packages as well.

Incidentally, your common integrated regulators do not use zener or avalanche technology, but rather bandgap references, a superior method.


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 Post subject: Re: 12B4 Preamp
PostPosted: March 24th, 2021, 2:00 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 599
Dave,

The 5.6V zeners are the true zener type, which is why Morgan Jones used them in his "Statistical Regulator" Basically, it is current source fed shunt regulator bypassed by a 10uF PP capacitor (he recommends one with a Kelvin connection). Simple, but apparently very effective at -140dB noise reduction (his observation, not mine).

David


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