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Old 19th January 2010, 12:37 PM   #11
Junior is offline Junior  Singapore
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Originally Posted by whitelabrat View Post
The only concern I would have regarding a large capacitor would be the current rush at power on if you are using solid state rectification. A CL-90 thermistor should help there. Otherwise tube rectification should provide a soft enough start.

Correction: If you B+ is 450v your capacitor should be rated for 500v to be safe. If your running with 400v caps with 450v I'd expect cap life to be minutes and not hours.
to get away with the high inrush current, can one add a Soft-Start Circuit For Power Amps by "Elliott Sound Products"?

Soft-Start Circuit For Power Amps
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Old 19th January 2010, 03:42 PM   #12
rknize is offline rknize  United States
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Originally Posted by urmoo View Post
Hmmm, OK, I will try to rephrase the question. Is it possible, that CFB has gone because some el. components are blown up (C12 and C22 for example)? If yes, how to check?
If those caps were open, that would explain why the CFB switch isn't working. It's would also reduce the amp's gain.

Another possibility is that you have the pole from the switch that is supposed to be attached to the "positive" speaker terminal connected to the ground side instead. George mentions that you usually have to invert the polarity of the secondary winding for CFB to work properly as negative feedback. Perhaps you swapped the CFB switch wire as well? The CFB switch should be on the positive (ungrounded) speaker terminal, regardless of how you have the OPT secondary wired.
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Old 21st January 2010, 02:49 PM   #13
urmoo is offline urmoo  Estonia
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Thanks for your answer, Rknize. I had some hesitations about wires as well, so I switched places of OPT secondary wires. Some kind or very weak FB appeared, but so weak, that I had to try different styles of music and listen very carefuly to understand, if any FB is present. One strange thing- quote from SSE page:

The secondary of the OPT's are connected to ground on the PC board so a separate connection is not needed. Note the reverse connection of the output transformer secondaries. This is needed on many OPT's to get NEGATIVE feedback to the cathode of the output tube. If unsure of your particular OPT's try wiring the secondaries each way (swap the two secondary wires, leave the primary alone). Chose the connection that produces the lowest volume.

The problem is- previous connection gave no effect what so ever, current one gives weak FB. I remember when I made the connection first time (at the beginning), this louder sound was terrible...

Amp's gain seems to be as supposed to be, but is there any simple way to check the health of those caps? Measuring current somewhere or...?
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Old 2nd February 2010, 09:22 AM   #14
urmoo is offline urmoo  Estonia
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Because of the mess with wires I rewired everithing from scratch. Now the amp is more quiet, no noise from my TV set next to amp, but CFB is still barely noticeable... I read somewhere (perhaps Tubelab web?), that CFB is mostly ment to improve budget OPT poor sound. Could it be, that with massive OPT's (EDCOR CXSE25-8-5K) and cap the effect of CFB increases? Or is there anything concerning CFB I might have burned down?

Last edited by urmoo; 2nd February 2010 at 09:28 AM.
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Old 2nd February 2010, 02:50 PM   #15
rknize is offline rknize  United States
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Yes, the feedback loop is between the output tube cathode and the OPT secondary. The idea is to flatten the OPT response (and the tube as well), but it is done by reducing the overall gain of the final stage. So the bottom line is that CFB will reduce the gain, regardless of quality of the OPTs. On my amp, the difference is not subtle. Do you have any detailed pictures of the relevant wiring? As I said before, the only component failure I can think of would be the cathode bypass caps.
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Old 3rd February 2010, 09:17 AM   #16
urmoo is offline urmoo  Estonia
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Thanks for your answer! Wires for CFB were done from scratch exactly by schematics, also tried to change OPT-out wires. Caps seems to be also OK.
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Old 12th February 2010, 06:19 PM   #17
urmoo is offline urmoo  Estonia
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Erhm, as CFB is not realy important to me, I decided to leave it alone. Then week passed and it started to disturb me again- what the heck, there is a switch, but it does not work... I guess you know the feeling
All the wires were rebuilt from scratch exactly by schematics, switch of OPT sec wires have been done, still nothing- no difference if switch is on or off, perhaps this "weak feedback" was more wishful thinking than reality. Could someone (George?) tell me how can I find out, using multimeter, if the CFB is present or not? About caps- how to check them, if they are OK?
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