I have a problem with my amp, please help...

[arsun]

Yesterday, I wanted to swap some tubes and I put some regular ones (not coin-based) to make burn-in faster with some other, probably non-matching tubes. Here is what happened: After putting the new tubes, I realized that the sound is not coming from one channel, so I assumed that tube was faulty (although I must say that I did not try it with old tubes before the swap, however it was working fine before). I swapped the two tubes, but the problem remained. I put the old working ones, no difference. There wasn't any sound coming from one side. This was at the night time and I was really mad. I postponed opening the amp and checking the resistance and voltages to today. I just finished testing just a while ago and to my surprise, everything checked ok, both voltages and resistances. Only thing to mention will be that I read 1.5V AC in power terminal 2 instead of 0, but I am guessing this is ok as well. Every reading is within limits (the differences are usually less than 10% of the given values in the manual). With the resistances I read 220Ohm at A4 and B4, which is not matching the manual, but I remember reading somewhere that these values might be wrong, I am not sure where though... However, the readings in both channels are matching to each other anyway... This actually surprised me a lot. I was expecting to find some mismatch in both channels. Anyways, after this, I decided to use a more sensitive pair of headphones to test if there is even some sound in the left channel (I had problems with left). I used some crap earphones, which came with an mp3 player, however I knew that they are much more sensitive, so they were suited for this last test. Interestingly, there was some sound in the left channel. I could hear from both left and right, although left one is attenuated much more than the right channel so that I thought at first, there wasn't anything coming from there. The speaker posts behave the same probably, but again I can only hear from one side, my speakers are not sensitive at all...

So, what might be the problem, what do you recommend?

[Paul Birkeland]

I would start by checking the condition of the brown tubular carbon composition resistors that attach to each tube socket.  If you measure any (more than a few mV) DC voltage on the two pins per socket that have the carbon composition resistors attached to them, you will want to replace that resistor. 

I recommend this because you say your voltages are all spot on.  If you have a clip lead with alligator clips on it, you can clip the output of the potentiometer from both channels together (two runs of STP leave the pot and go to each tube socket, clip the red on each tube socket together) to test functionality of the audio circuit. 

Let us know how it goes...

[arsun]

Thank you for the reply.

Are we talking about the locations A1/B1 and A4/B4? The resistors of the value 220Ohm? I took some notes about my measurements, when I did them:
Resistances:
A1/B1 are very high in Kohm/Mohm range, I did not write the value when I tested it, the manual indicated that it was not important as long as it is high.
A4/B4 220Ohm (is this the correct value for these terminals by the way?)

Voltages:
A1/B1 are -17V as indicated

I need to check A4/B4 for small voltages then (I assumed a zero value when it was around 0.1-0.2V, I remember that, but I am not sure if this happened particularly at these locations). Probably perform another test sometime tomorrow and record everything. It is getting late, I do not want to rush things now. Just to be sure, we are talking about those resistors right?

[arsun]

I just completed the resistance checks, I just want to post them until I perform voltage checks:
I have to mention that I have C4S upgrades and MQ irons, so some values are not standard ones, but they match the correct numbers:

For most of the terminals indicated with *, I have fluctuating resistance values in the MOhm range. Some of them don't seem to fluctuate but are in hundreds of KOhm range, which I will post them anyway for the sake of information.

Terminal                                 Manual suggestion                                             Measured Resistances
Center pin input jacks               0 Ohm and 100 KOhm with volume change               100Kohm fixed independent from volume
5,25                                      0 Ohm                                                              both 0.5 Ohm
9,29,12,32                              600 Ohm                                                          all 616.4 Ohm
10,30,15,35                            0 Ohm                                                              all 0.3 Ohm
14,34                                    *                                                                     14: 249.8 KOhm  34: 249.9 KOhm (not fluctuating)
17,37,18,38,A3,B3,A7,B7          0 Ohm                                                               all 0.2 Ohm
19,39                                    0 Ohm and 100KOhm with volume change                 0 Ohm and 100 KOhm with volume change
20,40                                    258 Ohm                                                            20: 258.5 Ohm  40: 259.2 Ohm
A1,B1                                    *                                                                      both 250 KOhm (not fluctuating)
A4,B4                                    0 Ohm and 100KOhm with volume change                 220 Ohm with volume set to min and 100 KOhm with volume set to max
A6,B6                                    1.27 KOhm                                                          both 1.297 KOhm
A8,B8,C1,C2,C4,C5                  less than 1 Ohm                                                   less than 1 Ohm

I am curious about A4/B4 over there, since I read 220 Ohm at volume set to min, but both channels show the same value with one operating nicely, so I guess there was a mistake in the manual, or maybe not? Voltages will follow.
 

[arsun]

Voltage check is finished, I should mention that all the values which should be 0 as indicated in the manual are zero indeed, except at two places, which I will post. Here are all the results:

Terminal                                 Manual suggestion                                             Measured Voltages
1,21                                      183                                                                 1: 173.6  21: 174.5
2,22                                      183                                                                 2: 173.0  22: 174.5
4,24                                      183                                                                 4: 173.0  24: 174.7
6,26                                      384                                                                 6: 365.3  26: 366.5
9,27,12,32                              -17                                                                 9,12: -18.5  27,32: -16.8
14,34                                     -17                                                                 14: -18.3  34: -16.7
16,36                                     300                                                                 16: 273  36: 276
A1,B1                                     -17                                                                 A1: -18.2  B1: -16.6
A2,B2                                     345                                                                 A2: 331  B2: 336
A5,B5                                     72                                                                   both 79.5
A6,B6                                     2.5                                                                 A6: 2.486   B6: 2.450
A7,B7                                     0                                                                    A7: 0.019   B7: 0.011
A8,B8                                     6                                                                    both 6.87
C4                                         0                                                                    -0.006
C5                                         6                                                                    7.02

Power transformer terminals
1                                          120VAC                                                            122.2VAC
2                                          0VAC                                                               1.17VAC
4                                          3VAC                                                               3.3VAC
5                                          3VAC                                                               3.3VAC
6                                          163VAC                                                            154.3VAC
7                                          0VAC                                                               0.56VAC
9                                          163VAC                                                            154.3VAC
10                                         0VAC                                                               0.5VAC

Plate Choke Terminals
with dot                                                                                                       330VDC and 334VDC
without dot                                                                                                   344VDC and 347VDC

That covers it. I found A7, B7 and C4 the most suspicious ones I guess. What do you think, what should I do next? Please help

[Paul Joppa]

These voltages are all excellent. I conclude the tubes and the power supply are working  correctly.

There must be a failed connection in the signal path somewhere. You're going to have to be methodical unless you get lucky.

First of course, make sure the problem is actually inside the amp - swap the input interconnects to be sure it's not upstream of the amp, and measure the resistance across the speaker terminals on each channel to be sure they are connected to the output transformer.

Then it's down to looking for bad connections or solder joints in the signal path - RCA jack to gain control, gain to driver grid (the resistance check has covered that one), driver plate (A5, B5) to coupling cap (0.1uF) to output gridA1, B1), output plate (A2, B2) to parafeed cap to output transformer, autoformer wiring (T20, T40 resistance check covers this), transformer to T5, T5 to speaker binding post.

[arsun]

Thank you for the reply. I tested the source and swapped the interconnects before, the problem remained, hence it is in the amp. The speaker terminals show 0.5 Ohm value between + and - for both left and right. Is that right?

I will now look for bad connections in the signal path.

[arsun]

I was checking and resoldering some sections. Then I found out the following:

One of the 120 Ohm resistors are not showing that value on both ends. It is rather showing 0.5 Ohm. I also have to remind you that I used another terminal strip to be able to use the headphone jack I wanted. The thread is here: http://www.bottlehead.com/smf/index.php/topic,469.0.html and below is a summary:

I checked different locations for continuity, everything seems to be ok. T5/25 are connected to the strip correctly. Their location on the strip are connected to the appropriate speaker posts correctly. Then, from their locations at the strip, the signal are attenuated with a 120 Ohm resistor to another strip post for both right and left channels. Then from these posts, the connection goes to the headphone jack. At the terminal strip, I can see a 120 Ohm between T5 and the cable connecting the appropriate strip post to the headphone jack. However, the left side, T25 and the cable only shows 0.5 Ohm. I am guessing that 120 Ohm resistor is faulty, however I would have expected to hear more on that side, but the opposite of my expectation happened. What is wrong with that thinking, I am clearly making some mistake here. However, that was the only place, which I did not measure before and it seems to be on the correct left side with a mismatch of a value between the right side. Interesting. Bad point is, I guess I don't have a spare 120 Ohm resistor, so probably shipping will be more than the resistor itself, when I order another one. Maybe I order 2 . 

I would still like to hear your opinions about this finding before doing anything further.

[2wo]

In your resistance checks, you show a constant 100K from the input jacks, regardless of volume position. I would take a second look there. I don

[Doc B.]

I can see a 120 Ohm between T5 and the cable connecting the appropriate strip post to the headphone jack. However, the left side, T25 and the cable only shows 0.5 Ohm. I am guessing that 120 Ohm resistor is faulty, however I would have expected to hear more on that side, but the opposite of my expectation happened.

It's more likely that something has shorted and that you are reading the low resistance of the short rather than the actual value of the 120 ohm resistor. Check to see of something around the headphone jack is touching the chassis that shouldn't be, or if something is shorted to the ground buss that shouldn't be.

[arsun]

@2wo: Thank you for your reply. I think that value with the resistor is a mistake in the manual, I am not sure, but I remember reading it somewhere. Maybe someone can clarify this for us...

Thank you Doc for your comments. Now that makes sense. I knew something was wrong with my thinking about 120 Ohm being bad. Figuring this out is taking much more time than I anticipated... Hope I can find what's wrong with it...

[arsun]

BTW, while building it, I was afraid of that shorting issue, so I was very careful, at least I thought I was. I trimmed every end well, let's see how it turns out.

[arsun]

I think I found the problem. It seems to be with the headphone jack. The ground pin of the headphone jack is barely touching the left channel section. I think while soldering, I might have put a small solder in between them, I am not sure though, will clean it thoroughly and solder again, I will let you know.

Interestingly, it worked fine for quite some time...

[arsun]

Yeah, that was it. I also figured out why this happened after some time. A few days ago, after my last listening session, I realized the headphone jack was loose, so I tightened it. However, by close examination I did just a while ago, I realized the location of the pins for ground, left and right positives are changed by the torque I applied. Previously, the ground pin was not touching anywhere. Now it touches the left channel... Anyways, problem solved. I should thank anyone for their valuable time and comments... I really appreciate it... I should go back to listening now...

[Jim R.]

Congratulations!  Job well done.

-- Jim