Problem - muffled sound

[Paully]

There was a pop in my speaker while it was playing, wasn't turning it off or on.  Then I noticed that the sound sounded muffled, like there was little treble or the speaker was playing through a heavy wet blanket.  I swapped all the tubes, changed sources, and even changed preamps.  No luck.  I did change the speaker cables and the problem followed.  So I seem to be down to the left channel Paramount.  It still plays and the tubes light up, but there is occasional static and constant muffled sound.  Seems like this is the one where I had the problem with the short before and changed the diodes which fixed it at that time.  Any thoughts on where to begin?

[Paully]

Some voltages.  The amps are configured for 45 so I took voltages on both.  The first is the one with the difficulty.

1    453     459
5    226     198
6    226     198
9    188     152
10     0        0
11   120     233
12    50      162
14     0        93
15     0        24
16   465      470
17      0         0
18      0         0
19    188     152
20       0        0

Looks like the zener diodes went bad, sound right?  If so, would anything have caused them to go bad that I should go ahead and replace as well (or is it something else alltogether).

[Paul Joppa]

"The amps are configured for 45..." Tell us how - I'm sorry but I don't remember all the modifications we've discussed, and which ones have been adopted. Does it have the soft-start board?

Some of the voltages (in the manual I have) don't make sense, so I'll have to go through the manual and determine which terminal is where in the circuit, and what voltage should be there. This will take me some time since I haven't done it yet for the soft-start board, but it will eventually save me time once I write it all down...  :^)

[Paully]

Put in the 3K cathode resistors in paralel seemed like the main thing.  And it does have the new soft start board.  My memory is a lot worse than yours (migraines).  I am actually looking at the bottom of the amps trying to figure out if there is anything else I have done of significance.  I don't think so.

[saildoctor]

Hi Paully - you meant to say two cathode resistors in series right?  I'm sure you must have.  I actually just converted my Paramounts to use 45's a few days ago.  I've listened to them for a bunch of hours so far and I was going to recheck my voltages and bias tomorrow.  I'll post them in the forum post I had started about my own swap ("Converting to 2a3 mode").  I hope they could be of some use for comparison's sake.

[Grainger49]

We talked on the telephone right after Paul (Paully) posted the voltages above.  The first odd voltage is a high voltage on terminal 5.  If I am reading the manual right that is attached to the top of the cathode resistors (wiper of the hum balance pot).  I don't much like high voltage there.

Terminal 12 also jumps out.  It has 50V in the bad amp and 162 in the good one.  That is in the Zener string and that is what brought us to the idea that there is a problem with the Zener diodes.  And maybe the voltage coming from the power supply is low. 

I'll be driving the four hours to Paul's house on Wednesday.  While I'm there he will be putting in the resistors, tube sockets and 10 turn pots so the soft start on the amps can be adjusted without flipping them.  I just look over his shoulder and give guidance.

Hopefully he will know what is wrong with the amp and have parts available before I start the drive.  We can fix this if we know what it is.

Is there a way to test the Zener diodes?  I mean, they should read infinity in the reverse direction to a multimeter unless it puts out over 60V DC (I don't know of one that does).  And a Zener should be a low resistance, or small voltage drop, in the forward direction like a normal diode.  Right?

Are there any other things that should be checked.

[Paully]

Yes I meant in series (and I meant to spell parallel properly too).  We are going to install a 680 ohm resister in series with the cathode resisters also to get the resistance up a bit.  Paul Joppa posted on another thread replying to some of the voltages I had on the 45 and where they should be given what my line voltage are.  That post is worth reading.  Do post your resistances if you can, I would like to have the comparison.

Thanks for the post Grainger.  If we aren't able to figure out what is wrong for sure today I will go ahead and order the Zeners today so we will have them by Tuesday.  Might as well.  Hopefully it isn't something else causing the problem.

[Paully]

Ok, so I went ahead and changed the diodes.  Didn't work.  Not sure where to go from here.  Maybe I should take some more voltages on other areas on the amplifier.

Edit: Interesting symptom that I hadn't noticed that might help.  The amp works perfectly for about 5-7 seconds and then suddenly becomes what I am calling muffled.

Edit 2: Still plays fine for a few seconds but second thing I see is that the LED on the soft start board at D2 is not lighting up.  I changed that to a new one which also doesn't light up.  It is the LED nearest the big black capacitor and the letter B.

[Paully]

Thank you Grainger!!!!!!!!!!!!!  We re-wet the solder joints on the silver transistor between the two LEDs on the B side and the blue 147K resistor and the LED in question now lights up and everything is working.  Apparently just a bad solder joint.  Hopefully it stays working.  Thanks again!

Paul

[saildoctor]

Hurray Paully! I got to reading the soft start instructions when I rechecked my voltages and noticed it states something to the effecct of: the zeners will handle the voltage until the shunt regulator takes over.  This doesn't mention however if the zeners can regulate on their own indefinitely with the shunt regulator offline? That's beyond my current understanding of the paramounts design. I would be interested with seeing your new voltage readings. Did you install the extra cathode resistance?

[Paully]

I don't know about your first question.  Well beyond my understanding as well.

I am going to install the additional resistor between the cathode resistors today and tomorrow.  At the moment the plan is just to put a 680 ohm 25 watt resistor in.  I will be taking readings of the four pin socket at a minimum and will repost.

[Paul Joppa]

I don't know about your first question.  Well beyond my understanding as well.

I am going to install the additional resistor between the cathode resistors today and tomorrow.  At the moment the plan is just to put a 680 ohm 25 watt resistor in.  I will be taking readings of the four pin socket at a minimum and will repost.

5 watts or more is fine, up to 1000 ohms with 45s.

[Paully]

Ok, problem has resurfaced.  Pop in one speaker then muffled sound.  LED at D2 on the B side is out again.  This is the third time.  I can go back and re-wet the solder joints in question and see if that corrects the problem once again but I don't understand how a solder joint goes bad three times.  I suppose I can re-wet solder joints one at a time and see if I can isolate the specific location but I don't understand what is happening and why.  Any help?

Paul

[shelby1420]

Ok, problem has resurfaced.  Pop in one speaker then muffled sound.  LED at D2 on the B side is out again.  This is the third time.  I can go back and re-wet the solder joints in question and see if that corrects the problem once again but I don't understand how a solder joint goes bad three times.  I suppose I can re-wet solder joints one at a time and see if I can isolate the specific location but I don't understand what is happening and why.  Any help?

Paul

As silly as this sounds I went through the exact same thing with my seduction, re wetted three times, swapped this, changed that till I was friggin going mad!!! I decided to try and move every single piece ( resistor/cap wire end, tighten all the screws down for good contact, etc etc)  turns out it was a wire inside the Teflon covering that had snapped and was only buggering up once and a while, it looked perfect till I gave it a wiggle to find the wire inside had actually snapped in two about a half inch from the terminal strip, worth a try anyways.....

[Paully]

Certainly will look.  Thanks for the heads up.