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Some more beginners questions about SSE

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Seems, that it is not possible to edit earlier posts, so I start here new one with a bit wider subject.

Supplemental capacitor, is it OK to use 300 uF 400V cap? Another thing, probably stupid question- is it possible, that using supplemental cap improves signiffically sound quality? I could swear there is mutch more bass than earlier, without this cap...

STB switch is installed, lets see, if it will blow away rect. , for now everithing is OK.
 
Yes, It is electrolytic cap, marked as 400WV 300uF with +/- polarity. B+ is actually around 450-460V, so not so good for cap, I guess... If it works near limits, should it warm up or is there other symptoms I should look for? Is 300uF OK, or will it start to give me somekind of troubles?
 
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.
 
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If it works near limits, should it warm up or is there other symptoms I should look for?

As I recently found out, sometimes the first indication that something is wrong is a BANG! A capacitor that big stores a lot of energy if it developed an internal short, it could explode rather violently. I would recommend disconnecting it immediately. See post #149 in this thread:

http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html

Is 300uF OK, or will it start to give me somekind of troubles?

300 uF may make life difficult for your rectifier tube. I would look for something close to 100 uF. If it is intended for DC voltage it should be rated for 500 volts or more. If it is an AC cap it should be rated for 370 VAC or more.

If you happen to have two of your 300 uF 400 V caps they can be wired in series to make a 150 uF 800 V cap. Put a 100K to 470K resistor in parallel with each cap to equalize the voltage.

Another thing, probably stupid question- is it possible, that using supplemental cap improves signiffically sound quality? I could swear there is mutch more bass than earlier, without this cap...

That is the reason that I recommend their use. It lowers the effective impedance of the power supply, so that more of your sound energy goes toward the speakers instead of going back into the power supply. I have found that a polypropelyne film (motor run) cap works better than a big electrolytic in this regard.
 
Thank you for saving me from explosion :) Now caps are installed, and as result the sound is unbelieveably improved- excellent got even better! To be a rookie has some advantages- so many things can surprise :D

One more question. I use CFB pretty seldom, but today, after finishing installation of caps, I tested all functions of amp. Everything was OK, except CFT- switcing ON and OFF does not make any difference. I checked wires and solderings, they seem to be OK. Bad thing is, that since last time I used CFB I have done some replacements (Volume pot), then 5AR4 started to arch- short connection somewhere (dissapeared after disconnecting and connecting wires) and I have no slightest idea when CFB dissapeared (or perhaps it is constantly ON?). Is there any simple way to find out where CFB gets lost? I have a multimeter and dont affraid to use it ;)
 
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
 
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.
 
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...?
 
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?
 
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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.
 
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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