• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Amp - Baby Huey

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M

malcolmfraser

Member
Joined 2010
First timer, Building a Baby Huey

Hi all, well all systems go, got the iron.

An externally hosted image should be here but it was not working when we last tested it.


PSU based on gingertube's recommendations in this thread...

An externally hosted image should be here but it was not working when we last tested it.


Design goal...form follows function!! And have Morgan Jones on the bookshelf.

Looking forward learning heaps on this little project...
 
M

malcolmfraser

Member
Joined 2010
Help with BH PSU unit, as described by Ian

gingertube said:
In my on-going efforts to "convert" the youth of the world to Hollow State my apprentice at work has just finished building a fixed bias version of the Baby Huey for his home HiFi.

He wanted to use a single power supply for both channels and use readily available Hammond Trannies.

Here is what we came up with for his amp:

Power Supply (for 2 channels).
Power Transformer - Hammond 370HX (275-0-275 V @ 200mA).
Solid State Rectifier Diodes (Ultrafast Soft Recovery)
"C-L-C" arrangement power supply.
10uF/900V Polypropylene first capacitor.
Hammond 158S (1.5H 250mA 60 Ohms DCR) choke
470uF/450V final electrolytic with 5uF/900V polypropylene in parallel for the 2nd capacitor.
This gives a rock solid 330V rail - just right.

Hammond 1608 Output Trannies.

Notes:
The 900V polypropylene for the first capacitor is an overkill. You don't need a fantastic quality cap in this position.
The Ripple Current for this capacitor (according to PSUD2) is 320mA RMS
A Panasonic ED 10uF/450V has a 350mA RMS ripple current rating and could be used instead.
BUT
I would instead RECOMMEND using 2 off Panasonic ED 22uF/200V in series, with 220K voltage share resistors (1 or 2 watt) across each capacitor. These caps have 600mA ripple current rating which means a far longer lifetime.

For the cap in parallel with the 470uF/450V second capacitor, any 630V rated film cap of 2.2uF or 4.7uF would do. Polypropylene for preference but even polyester will be better than no parallel cap.

This supply would suit either the original (CCS biased) or the fixed bias versions of the Baby Huey.

Cheers,
Ian
Click to expand...

So, is this right then?

An externally hosted image should be here but it was not working when we last tested it.
 
M

malcolmfraser

Member
Joined 2010
Power Supply Loading

Hi Bill, was just following the loading on the designers thread ;-)

gingertube said:
After you run the sim look in the Results Table for I(C1). The right most column has the RMS Amps values in it - that is the Ripple Current.
Plugging in all of thh transformer, cap, inductor etc, values in my post above then I(C1) was reported as 321.33mA RMS.

I used current source load at 140mA (35mA idle current each off the 4 EL84s) with a step to 180mA at 1.5 seconds to see what will happen when you run the amp really hard.

To check I just re did the sim but looked at values for 2 seconds after a startup delay of 1 second (same load step from 140mA draw to 180mA draw) at 1.5 seconds- that gave a ripple current of 260.9mA.
That is a more realistic value, since the initial capacitor charge currents at power on are then not included in the "operating" ripple current calc.

That means that the single 10uF/450V Panasonic ED (360mA ripple current rated) would be satisfactory. No need to go for the 2 lower voltage units in series.

The ripple current calculation is not really simple - its one of those formula where you take the Root Mean Square value of the charge current and the discharge current but where there is an adjustment according to the charge time and discharge time ratio.
There is a "White Paper" on it on the EPCOS capacitor Website if you are all that interested. It also covers how to then calculate capacitor expected lifetime. Basically there is an inverse square relation ship with Ripple Current. Halve the ripple current (or use a cap with 2 x the ripple current rating) and you will get 4 times the life expectancy.

Cheers,
Ian
Click to expand...
 
boywonder

boywonder

Member
Joined 2008
malcolmfraser said:
No worries Bill, actually had it built for 0.5A!! (just in case I want to swap out the EL84s for something larger (or do a PPP EL84) down the track.
Click to expand...

Pulling 140-180ma from a 500ma rated transformer will probably result in B+ voltages that are a bit higher than you expect since it will be lightly loaded.....if you are doing a CLC PS design, you can always use a small value first cap to reduce the B+ if needed.
 
boywonder

boywonder

Member
Joined 2008
Malcolm: Your rectifiers are not wired properly. Please refer to a solid state full wave center tap schema....if you are using PSUDII, look at that schematic.

Why not just use a 10uf/400V or 450V rated cap for C1? I don't see the point in using 2 200V rated parts, as it's unnecessarily increasing ESR, space requirements, and cost. Also, for safety, add 220K/2W (or so) bleeder resistors across each PS cap so that they will drain down when the amp is powered off.

You may also want to pick up a few cap values less than 10uf for C1, say an 8uf, a 6.8uf, 4.7uf, as these will allow you to easily lower your B+ if required.
 
Last edited:
Soonerorlater

Soonerorlater

Member
Joined 2008
Charlie Slee said:
OK, reading 42mA for the bias block. Much better, but is it a little too high?

Charlie
Click to expand...

Charlie,

This is ok for a test outside the amp. You wont know the true current through the bias until it is fitted to the amp as other things may have an effect such as your B+ value. IIRC Gingertube explains how to lower current by increasing the 16r resistor to 18r should this become necessary.

HTH Bill
 
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