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Old 4th April 2006, 11:35 AM   #101
EC8010 is offline EC8010  United Kingdom
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To cause audible oscillation, your output transformers would need to be stunningly bad, so I think it's more likely to be your power supply and earthing arrangements. Are you able to post a photograph of the underside?
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Old 5th April 2006, 04:32 AM   #102
ctaudio is offline ctaudio  United States
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Excellent point!!

That gave me a good place to start. I suppose it's not just a little parasitic capacitance or leakage inductance that causes such an oscillation. So I tried disconnecting the negative feedback. The first thing I noticed was the lower output without the NFB. That didnít seem right. It looked like I may have reversed the phase of the output transformer (?) (I had the blue and blue/yellow wires in the primary of the Hammond 1608 connected to the top tube in the schematics, and the black wire in the secondary connected to ground.)

Reconnecting the amp to the B&W 804 and the oscillation is gone. I tried reconnecting the NFB, but I couldnít get the oscillation back again to confirm that it indeed was the problem. I suppose things got changed just enough with me moving things around that the amp is now stable even with the positive feedback (?)

I tried reversing the feedback connection with the green wires on the secondary connected to ground. I got a slightly lower output as expected. But the amp runs fine without the feedback. Iím leaving it out for now.

Another issue I had was the fuses. The HT switch helps, but still not enough. I now use a few 33ohms in parallel to limit the current on power up and use another switch to short out the resistors after a minute. Iím going to order a CL08 inrush current limiter. Hopefully thatíll keep the fuses from blowing.

All in all, things well pretty well for my first attempt at DIY. Iím now thinking about building the other channel. Iíve put a few pictures of the amp on the following website. Iíd appreciate any comments on the layout, components, etc.

http://cheunglai.smugmug.com/gallery/1336901
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Old 5th April 2006, 05:03 AM   #103
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Default Toroid transformer

ctaudio - if that's your first DIY amp, colour me impressed! Obviously this isn't your first circuit on a perf board though. :-) Yeah, I was going to suggest a thermistor, but you've beat me to the punch...

Anyway, I was wondering about toroidial output transfomers, which I found on eBay (oh oh). I can get a really good price on some 10W 40% UL toroidials, but I hear they (toroid output trafos in general) have a problem when there's a DC current imbalance between the two tubes. Do we have any opinions on this? I don't have any idea how to tune current output of the tubes - is an imbalance caused by mismatched tubes, or mismatched resistors, or other parts? Can it be fixed with a pot somewhere? Careful parts selection? Could I simply put a DC blocking cap between the EL84 and the trafo, or is this a no-no? Should I just go away and read about circuits for a long time and figure it out for myself? :-)

Jeremy
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Old 5th April 2006, 06:26 AM   #104
ctaudio is offline ctaudio  United States
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Thank you, Jeremy. This is indeed my first tube amp (my first analog anything, for that matter). But you are right. I have done a good number of digital boards in the past.

I've been listening to the Baby Huey (in mono) in the past hour or so. I must say I am very impressed. I can't wait to complete the other channel. I may have to retire my ML...

I've also heard good things about toroidial output transformers. I'd be interested in hearing what you find out.
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Old 5th April 2006, 06:26 AM   #105
Giaime is offline Giaime  Italy
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Maybe a CCS (or a separate fixed bias voltage pot) for each tube helps to balance currents.

I'm gonna try with toroids, they're damn cheap.
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Old 5th April 2006, 06:39 AM   #106
ctaudio is offline ctaudio  United States
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This amp design uses a bias servo circuit to maintain dc balance for the toroidal output transformer.

http://www.normankoren.com/Audio/TENA.html
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Old 5th April 2006, 11:50 AM   #107
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The two bias circuit at the cathodes of the EL84 are providing a constant and equal current draw. The absence of DC in the OPT will help to produce the excellent effects on the bass response Gingertube is talking about!

Another option, simple to implement, is the use of an LM317 or LM337. John Broskie shows it here;
http://www.glass-ware.com/tubecircui...o_Biasing.html
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Old 5th April 2006, 11:59 AM   #108
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Quote:
Iíd appreciate any comments on the layout, components, etc.
Personally I got the impression that mox resistors don't help the sound. A little lifeless and cold. In my amp I had them in the cathode circuit only..and I prefered Dale RS-5 I tried subsequently.
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Old 13th April 2006, 02:37 AM   #109
ttan98 is offline ttan98  Australia
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I think EL84 is a very good tube/valves with many good qualities, cheap, excellent sounding and easier to drive hence design. There are many types available out there to experiment as well.

Those who are interested in high powered amplifier should look at making a total of 8 tubes(in 4 per side in PP mode) per channel giving a total of about 35watts per channel, close to class A mode.

This I think is a good project for those who are keen. I am thinking of making one.
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Old 13th April 2006, 05:05 AM   #110
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Quote:
Originally posted by ttan98
Those who are interested in high powered amplifier should look at making a total of 8 tubes(in 4 per side in PP mode) per channel giving a total of about 35watts per channel, close to class A mode.
[/B]
Hey I happen to have 8 EL84 do you know where i can find this design? Is there one that only uses 4 tubes per channel. I would be happy with 15W per channel.
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