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Old 26th October 2003, 08:27 PM   #1
jazz is offline jazz  Netherlands
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Default opt windings, need some help

hi there,

I'm currently building a little power amp using an ecc83 and two 6v6's in puhs pull mode. The output trannies came out of an old lafayette 224 and unfortunately i'm not quite sure about the winding layout. I know this trannie has a secondary of 0-8-16 ohms layout but don't know which is which. Right now the unit gives music but is very limited in power output before distorting and worse, when i connect the secondary "ground" to the signal ground of the system i get hum like crazy.

I've measured resistances between the windings and they are as follows: 1-2 / 0.4 ohms 2-3 / 0.9 ohms 1-3 / 1.5 ohms. Right now the speaker is connected between 1-2 and feedback is taken from 3. Also hum is only there when earth is connected to winding one not with winding 2.

Is there someone wise enough to shed some light to this and help me out?

thanks in advance
Joris
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Old 27th October 2003, 12:55 AM   #2
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First of all, disconnect feedback, that can cause problems.

Second, with that disconnected, there should be absolutely no change (except perhaps a slight difference in HF peaks, for you pedants out there ) between grounded and floating output.

What is the resistance of the primary side, and it to the secondary?

Tim
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Old 27th October 2003, 03:28 PM   #3
jazz is offline jazz  Netherlands
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hi,

I'm actually a bit reluctant to disconnect the feedback given possible stability problems. It looks as if there is a frequency/ffilter circuit in there as well, 100pf in series with 180k from the first (common cathode) stage anode to the hv. I'll give it a try tough.
What about the gain tough? the amp is quite sensitive as is, ain't i gonna oversteer the output stage?

The impedance of the primary is 8k anode-anode. I'll have to look up the resistance but seem to remember it was higher. the resistance from primary to secondary is at least more than two megs.

Does it look like i'm doing something wrong or have i got the windings right?

thanks
Joris
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Old 27th October 2003, 04:07 PM   #4
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Try attaching the feedback return to the other end of the secondary winding. If you have it wrong you get instability, this may be what the hum is that you experience, so you must try the opposite to what you are doing to eliminate that.
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Old 27th October 2003, 04:16 PM   #5
jazz is offline jazz  Netherlands
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Hi,

Ok I'll give it all a try tonight I'll report back about it tomorrow.

Thanks
Joris
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Old 27th October 2003, 04:24 PM   #6
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That's a good point. You want negative, not positive feedback.

The R+C (Zobel network) on the first stage is to reduce gain at HF, without causing phase shift (such as a straight capacitor would). This is generally above hearing (ultrasonic), and stabilizes the amplifier when NFB is applied. Similarly, there is often another network across the OPT primary, for the same effect. On the NFB resistor there might be a small capacitor, this is again for HF stability.

Testing without NFB is useful because it can cause many problems if it isn't hooked up right. Like the motorboating and whatnot that you were getting.
Think of it this way, NFB feeds back some of the output to the input; if the amp is weird, the weirdness is compounded throughout the entire circuit, making troubleshooting difficult. By opening the loop, the weirdness (which may appear at one point in the circuit) is easy to find, going stage by stage.

Tim
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Old 28th October 2003, 12:56 PM   #7
jazz is offline jazz  Netherlands
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Hi,

I reversed the secondaries and now it works as it should. As for that hum, it was oscillation as i should have known of course. Never heard it at such a low frequency before.

Thanks for the help guys
Joris
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Old 4th November 2003, 10:02 PM   #8
jazz is offline jazz  Netherlands
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Hi there,

Just to let you know, i've had the amp singing for a week now and i love it. I took it over to my dad and played along with an fvp5 on his AR3 loudspeakers. I didn't expect it to be able to drive these but it had both me and my dad totally blown out of our socks. Guess i'll be going along this path a little further...
Here's a pic.
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Old 4th November 2003, 10:13 PM   #9
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Hi,

Quote:
I didn't expect it to be able to drive these but it had both me and my dad totally blown out of our socks. Guess i'll be going along this path a little further...
Replace the ECC83 with a good NOS 6SL7 and I swear you'll never even find those socks again.

Nice caps, BTW. Look like some Philips I've known.

Amazing what a few quality watts can accomplish, he.

Cheers,
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Old 5th November 2003, 05:23 PM   #10
Raj1 is offline Raj1  United Kingdom
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Quote:
Originally posted by fdegrove
Hi,



Replace the ECC83 with a good NOS 6SL7 and I swear you'll never even find those socks again.

Cheers,
Interestingly enough, 2 pairs of socks were seen travelling at high speed (in perfect unison), at various places in the UK.






Thanks
Raja
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