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Old 13th March 2011, 12:05 AM   #1
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Default Requesting input on preamp schematic

This is a schematic (power supply slightly modified) of the HAP-03 that Radii put out a long time ago. I've some questions about it and would love input regarding some of the passive component value choices.

Click the image to open in full size.

A few things that stand out to me right away.
powersupply
1. My cap voltage values for the 120v supply are only 200v. I have some 400v caps as well but I'd rather use the 200v caps if I can get away with it, I'm only really concerned about inrush.
2. The Second pi circuit in the 120v supply using a CRC setup is likely totally unnecessary but I'd like to play with it so that resistor will be easily jumperable.
3. Do I even need a pi circuit in the heater supply?
4. I could add a resistor in the heater supply prior to the full wave bridge to dial the heater supply to exactly 12.6v, I'm pretty sure that's a good idea.
amp
1. The cathode bypass caps seem a little on the low side.
2. I have heard (on wikipedia) that a 12au7 wants to see around 100k on it's plate but unless the following 6dj8's grid introduces resistance the 12au7 is only seeing 56k.
3. Is it worth bypassing whatever electrolytic I end up using in the cathode bypass with a .1uf polypro or something similar?


I'm pretty new at this, I've been around for a while but I've only just started actually doing the math (or trying) in order to figure out how this is all working out. I appreciate the patience and help.

Milo
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Old 13th March 2011, 12:19 AM   #2
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Hi Milo,

The cathode bypass caps are OK for value; low cut off would be some 3 Hz.

However the main weak point of the schematic is the 120V supply.
B+ should be at least 250V to move the input tube into a linear operating point.
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Old 13th March 2011, 04:18 AM   #3
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pieter, thanks for the response. I actually have a 250V CT transformer lying around. a 261C6 to be exact, which has a 6.3V filament winding but I could just run the 12au7 in parallel rather than in series. However I have heard this amp and I have liked it, I'm not saying that it is designed correctly but it's interesting that it's not running as hot as it should (a friend at work pointed out the same thing).

Ok so now my "I'm really new at this" comes out. I'd have to recalculate the resistor and cap values for running off a 250V right? Mechanics make sense to me, levers gears and whatnot, electronics are always a struggle a real

Literally I finally got that you design an amp with a specific supply (line level for example) and a specific load (headphones, etc) and that's how you start your math, I was always just staring at schematics wondering how people got from a white sheet of paper to actual passive component values.

Thanks for the help.
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Old 13th March 2011, 07:19 AM   #4
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by A3rd.Zero View Post
Ok so now my "I'm really new at this" comes out. I'd have to recalculate the resistor and cap values for running off a 250V right? Mechanics make sense to me, levers gears and whatnot, electronics are always a struggle a real
The quick answer is that you draw a load line. Then if you don't like t you draw another. One step for me is a Spice simulation. Then a bread board and measure the amp's performance. Each step either refines the design or shows you it can't work and that you have to go back a step or two. I don't think anyone gets it right the first time it is drawn on paper

But the first step to designing any tube amp is to draw a load line. This explains the process in very simple terms.
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf
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Old 13th March 2011, 08:08 AM   #5
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ChrisA, perfection. This article is brilliant, exactly what I was looking for. Many thanks.
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Old 13th March 2011, 10:49 AM   #6
mjf is online now mjf  Austria
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hello.
have a look at the rectifier(diodes) in the psu, i think they are built in wrong.
greetings
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Old 13th March 2011, 11:50 AM   #7
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Originally Posted by mjf View Post
hello.
have a look at the rectifier(diodes) in the psu, i think they are built in wrong.
greetings
Rectifier diodes are drawn correctly.
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Old 13th March 2011, 02:38 PM   #8
mjf is online now mjf  Austria
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magnified the picture.......rectifier is ok.
sorry - i think i need new glasses.
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Old 13th March 2011, 06:17 PM   #9
DF96 is offline DF96  England
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56K is fine for a 12AU7 anode load, provided the supply voltage is high enough - yours might not be.

A White cathode follower might be overkill for a line stage, unless you have very long cables. To reduce the risk of instability from a capacitive load, I would add a resistor between the output anode/cathode junction and the 47uF cap - maybe 220-470R? Depending on what you are driving, the 47uF electrolytic could perhaps be much smaller and non-electrolytic?

The second cathode bypass could be a bit bigger. The first one, on 12AU7, is fine.
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Old 13th March 2011, 07:50 PM   #10
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Quote:
56K is fine for a 12AU7 anode load, provided the supply voltage is high enough - yours might not be.

A White cathode follower might be overkill for a line stage, unless you have very long cables. To reduce the risk of instability from a capacitive load, I would add a resistor between the output anode/cathode junction and the 47uF cap - maybe 220-470R? Depending on what you are driving, the 47uF electrolytic could perhaps be much smaller and non-electrolytic?

The second cathode bypass could be a bit bigger. The first one, on 12AU7, is fine.
Plan is to drive headphones most of the time and a chip amp or a discrete Halfler the rest of the time.

The 47uf Cap on the output I just drew as an electrolytic to remind myself of the polarity in the case that I'd use one. I have a Solen Fastcap for that spot. What's the math to calculate a capacitance match for the driven output. I'm assuming I need level and input impedance of whatever I'm driving and I'm sure there is a formula (Still reading through some of the docs so if it's in there I'm sorry).

Two additions:
1. Headphones I'll drive will always be higher than 220ohm, I only own high-Z cans (this was an accident actually)
2. ChrisA, again that article is just money. It's exactly the noobish structure that I need, really well put and clarifying a bunch of things that always confused me. Thanks again!
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Last edited by A3rd.Zero; 13th March 2011 at 08:13 PM.
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