Sp14 issues. Your thoughts please.

Note: I emailed back and forth with Roy about this yesterday, he suspects it's possibly a weak rectifier, but while waiting on a replacement, I thought I'd run it by the group here just to see if there any other thoughts or ideas.

I built this unit last December.
Other than a microphonic tube needing to be replaced, all has been well, for the most part.

I am not a tech, and probably know just enough to be dangerous.
I'm pretty confident in my soldering skills.

I haven't spent much time at all lately doing any critical listening, I mostly just have my stereo on for background.
The last few times I had it on, I thought I heard some scratchiness, but on the other hand I have some mp3's that were made from vinyl, and I'm not always certain which versions I'm listening to, and then there's the I don't want to believe I hear something wrong factor.

I turned on the system yesterday, and the volume was very low.
I hardly ever go past noon on the master volume.
The right and left are always at full.
Yesterday 12:00 on the volume equalled what would normally be about an eighth or so, with the left channel way lower than the right, with some scratchiness emitting from that channel as well.
I also noticed as I turned the master volume control there was a clicking or popping noise emitting from the speakers on each graduation of the dial.

I checked all my connectors, and verified them good.
I tried a different source as well, also trying different inputs on the preamp.

I then bypassed the preamp altogether to verify it wasn't the amp.
The amp worked fine, and was quiet at idle.

After chatting with Roy, I opened it up and checked voltages.
All the voltages were within spec, except at D13.
Specs call for 20-40 vdc at D13, I'm getting a fluctuation between 9-14 ON BOTH SIDES.

The only other thing, and I'm not sure if it means anything, but at the point marked 300V on the board, I'm showing 288.

I'm just having a hard time believing it's the rectifier with all the other voltages being in spec.
But hey, what do I know, this is my first venture into the tube world.

Unbelievable.
I got it working.
I went in for a serious look, looking for bad joints or anything.
Believe it or not, I found a dead bug or something across the pins on the back of the source/tape switch.
I dug it out, and it's working again, although the voltage is still off at D13 on both sides. (9-14 volts)
And a hum is present, albeit not real loud, and only noticeable between tracks, but it's been there awhile and I didn't think much of it.
So I guess I'll measure it again after I put it a new rectifier sometime next week.

Please read this post carefully re: sp14 hum.

https://dynacotubeaudio.forumotion.com/t4071-sp-14-hum-elimination

bbqjoe wrote:Unbelievable.
I got it working.
I went in for a serious look, looking for bad joints or anything.
Believe it or not, I found a dead bug or something across the pins on the back of the source/tape switch.
I dug it out, and it's working again, although the voltage is still off at D13 on both sides. (9-14 volts)
And a hum is present, albeit not real loud, and only noticeable between tracks, but it's been there awhile and I didn't think much of it.
So I guess I'll measure it again after I put it a new rectifier sometime next week.

Glad you were able to "debug" your preamp

Regarding the D13 voltage, this is a bit low and can lead to hum. Roy will probably explain to you how to increase it. This power supply design depends on having at least 20 Volts across D13 to work with, and it is a simple matter to increase this voltage by changing one of the resistors. In my SP12 I had to decrease R23 from 313 to 274 K. This brought my D13 voltage from 8V up to 27V. I also found that adding a little more capacitance after the 12x4 decreased the 120Hz hum by an additional 10dB.

rjpjnk wrote:



Glad you were able to "debug" your preamp

Regarding the D13 voltage, this is a bit low and can lead to hum. Roy will probably explain to you how to increase it. This power supply design depends on having at least 20 Volts across D13 to work with, and it is a simple matter to increase this voltage by changing one of the resistors. In my SP12 I had to decrease R23 from 313 to 274 K. This brought my D13 voltage from 8V up to 27V. I also found that adding a little more capacitance after the 12x4 decreased the 120Hz hum by an additional 10dB.

What I don't get is that the proper voltage was there when I closed it up.
What changed?
After some further testing, I found that the 300 volt spot (pin of the rectifier on the board was only coming in at 285, whereas all the other voltages (except D13) were right where they're supposed to be.
Again, Roy thinks it's the rectifier.
So I'll replace that soon, test, and we'll go from there.

Thank you for your input.

Oh, I didn't realize the voltages used to be higher. If this is the case than yes, something changed. Indeed it could be the rectifier tube.

Was the D13 voltage normal before, or just the 300?

rjpjnk wrote:Oh, I didn't realize the voltages used to be higher. If this is the case than yes, something changed. Indeed it could be the rectifier tube.

Was the D13 voltage normal before, or just the 300?

You know, I don't recall testing the 300 volt point when I built it, I'm not sure that's even in the start up instructions.
It could be, then again I just recently learned if I use both hands, I can find my ass.
But I do know D13 measured properly, or I would have been in touch with Roy immediately.