6550 SE class project (continued) - diyAudio
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Old 3rd May 2008, 08:28 PM   #1
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Default 6550 SE class project (continued)

I have recently completed the project shown in this thread:

SE 6550 class project (continued)

I have to present it before the class on Tuesday.

In my opinion, the amp sounds wonderful (even through my dorm-mates non-hi-end KLH bookshelf speakers.)

Anyway, I found myself getting way too bogged down in things like clipping symmetry, when the voltage swing in the preamp stage (w/line level input) could never get that high in the first place!!

The assumption that the one-half 6sn7 configured as a CCS would protect the input from crosstalk and noise turned out to be dead-on. Channel seperation (not measured, just perceived) is absolutely fabulous.

The only thing I don't like is the 6mV of 60Hz hum appearing at the outputs. The only thing I can imagine it being is the fact that the filament supply has no center tap and is essentially floating. I am going to make one tonight out af a pair of 100-Ohm resistors and see if I have any luck.

After presentation, I will probably pull-apart most of the wiring I've done (as the layout is a bit sub-optimal.)

I highly suggest that you build one as it really does sound wonderful.
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Old 3rd May 2008, 10:03 PM   #2
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Quote:
The only thing I don't like is the 6mV of 60Hz hum appearing at the outputs. The only thing I can imagine it being is the fact that the filament supply has no center tap and is essentially floating. I am going to make one tonight out af a pair of 100-Ohm resistors and see if I have any luck.
Another thing you can try is to rig a voltage divider across B+ and ground. Set the divider for about 60 V. Connect the +60 VDC to 1 side of your filament winding. Connect that same side to ground via a high value/100 WVDC 'lytic.

Yes, it's definitely OK to combine the 2 techniques.
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Old 3rd May 2008, 11:08 PM   #3
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CD,

A 3rd option is to install a 250 Ohm linear taper pot. across the filament winding and ground its wiper. You adjust the wiper to achieve the lowest residual hum level.
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Old 5th May 2008, 09:55 PM   #4
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Default It worked....................

The ol' "faux center-tap" thing worked well. The hum went from about 7mV to about .9mV!!

Here is a pic of the finished beast.
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Old 5th May 2008, 11:39 PM   #5
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I took a quick glance at the pics and didn't see a schematic. Sorry if it's there and I missed it.

So from the pics it looks like a 6SL7 driving a 6550. ... any of the high gain driver tubes are more prone to AC power supply noise than their lower gain counter parts.

IF this is used with a computer or newer CD, you might be able to get by with a 6SN7 driver, which should have an easier time driving your output tube and have less noise as well... all without getting fancy... may not even have to change up the circuit...

Also, 6millivolts of noise is really not that bad... look/listen to some old school kit and you will be wondering how you thought it was loud! Not to mention at normal listening distance and levels - 8-10ft with 90db speakers... it's not loud enough to be heard... or your 6 millivolts reading is off. :^)

great looking amp... hope this has started years of tube enjoyment.

let's reiterate... that's .006V on your meter... thats not much.. and for the sound you should be getting, a quieter amp is not always better!
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Old 6th May 2008, 02:21 AM   #6
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Default Actually...................

Those are 6sn7's. One half of each of them is biased as a CCS @ about 8mA.

I got a chance to listen to the amp tonight on my friends 108dB Klipsch speakers- and I'm impressed so far.

There is a touch of "hand over mouth" muffling in general; I suspect that the culprit is the PIO coupling caps (they tend to round off a bit.)

I have some .056 uF Russian teflons that I'm going to try next.

I'll keep you posted.
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Old 6th May 2008, 03:28 AM   #7
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Yeah when I looked at the other thread again... I thought, eh, those plates are a little bigger than I thought... so I figured maybe as much.

Still, that noise figure seems reasonable to me... are you using feedback at all to get the hum down? was I wrong? Did I miss the schematic?
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Old 6th May 2008, 04:01 AM   #8
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Default Here's the schematic...................

There's no feedback.

This is the most current version of the amp. The only difference is a different PT with no center tap- and a bridge rectifier made with Hexfreds.

http://www.diyaudio.com/forums/attac...amp=1204136901


I am taking suggestions as to grid resistor sizing, however.

Any comments??
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Old 6th May 2008, 04:06 AM   #9
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Default Here's another pic.............

**
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Old 12th May 2008, 02:26 AM   #10
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Default diode differences

could you please explain how to wire in the hexfreds for a full wave diode bridge?
I'm presuming that you need 4 of them. What are the benefits and disadvantages vs. the ultrafast diodes and soft start recoverys?
Andy
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