Amp not used for a month now has distortion [Resolved]

[Horatio]

Hi, strange experience with one of my Paramounts today and wanted to see if there were any specific areas I should focus investigations.

Background: Used my Bee Pre and Paramounts extensively on a daily basis. Then I had a 300B tube blow that had to be returned to the vendor under warranty. This has resulted in two months of downtime on the Bee Pre and Paramounts while I awaited the new tube arriving. Fitted the new tube tonight and was greeted with distortion from one of the Paramounts that I don't believe was present before - I can't rule out that it wasn't that Paramount that the 300B tube blew in.

Problem: One amp has distortion whenever the volume is turned up on my Bee Pre past -27db. It sounds ok until that point and then when I turn it up to -18 the distortion is immediately and dramatically apparent and turning it up higher makes it even worst with loud pops though the speaker.

Basic Troubleshooting: I have swapped the channels coming out of the Bee Pre and the problem stays with the same amp. I have swapped the two tubes in the amp and the problem stays with the amp. I have also swapped the power, speaker cables and phono input and the problem stays with the amp.

So at this point it looks like there may be a problem with the amp. I am planning on doing the full suite of voltage checks etc this weekend but I was wondering if there are any areas that I should be focussing on during the investigation given the symptoms?

Thanks,

Bill

[Doc B.]

You are on the right course.  It is possible that when the tube blew something else was damaged. Start by taking resistance measurements before you power the amp up again. If you see any readings that are off post them here. Once we check that off we can move toward determining that powering up the amp is not going to cause further damage, and we can move on to more specific areas to look at.

[Horatio]

Ok, I have carried out the resistance check, most appear to be bang on. The two that were most out were on the 9 pin socket:

B4 156K - I guess this can vary, however I was getting a pretty solid 156k

B7 248 (Still in the 10% to 15% range)

Close enough to move to voltage checks?

Bill

[Doc B.]

Yes, go ahead with the voltage measurements now.

[Horatio]

Ok, voltage checks carried out and results are as follows:

NOTE: UK 240V using a Sophia Electric Carbon Princess 300b

T1 Manual 440 Actual 438
T5 Manual 71 Actual 74
T9 Manual 175 Actual 185
T10 Manual 0 Actual 0

T16 Manual 457 Actual 463
T17 Manual 0 Actual 0
T18 Manual 0 Actual 0
T19 Manual 175 Actual 185

A1 Manual 74.1 Actual 73
A2 Manual 440 Actual 442
A3 Manual close to 0 Actual close to 0
A4 Manual 69.2 Actual 67

6V Red Manual 3.1VAC Actual 2
6V Black Manual 3.1 VAC Actual 2

Kreg (A Side) Manual 4.1 Actual 2.9
kreg (B Side) Manual 6.3 Actual 5.4
OA Manual 300 Actual 300
OB Manual 175 Actual 185

If it would be helpful I can take readings from the good working amp using the same tubes for comparison.

Any help or insights much appreciated!

Bill

[Doc B.]

Your 3.1vac at 2V measurements are a concern (is the meter switched to AC for those 6V measurements?). Measuring the good amp at those points would be helpful.

[Horatio]

Apologies, while I did have the meter set to VAC the resolution was not correct. I have re-measured the broken Paramount and the readings at the power supply are:

6V-Red Manual 3.1 Actual 2.7
6V-Black Manual 3.1 Actual 2.7


I have re-taken the voltage measurements using the good Paramount amp with the same two tubes plugged in as used in the broken amp readings above:

NOTE: UK 240V using a Sophia Electric Carbon Princess 300b

T1 Manual 440 Actual 436
T5 Manual 71 Actual 73
T9 Manual 175 Actual 177
T10 Manual 0 Actual 0

T16 Manual 457 Actual 462
T17 Manual 0 Actual 0
T18 Manual 0 Actual 0
T19 Manual 175 Actual 177

A1 Manual 74.1 Actual 77
A2 Manual 440 Actual 436
A3 Manual close to 0 Actual close to 0
A4 Manual 69.2 Actual 72

6V Red Manual 3.1VAC Actual 2.7
6V Black Manual 3.1 VAC Actual 2.7

Kreg (A Side) Manual 4.1 Actual 3.6
kreg (B Side) Manual 6.3 Actual 6.55
OA Manual 300 Actual 301
OB Manual 175 Actual 177

Hopefully the comparison may shed some light on things.

On further listening to the broken amp I actually think the distortion may be present at lower volumes - just a lot harder to make out.

Bill

[Doc B.]

I would suggest starting by trimming the blue 10K pot on the small PC board to get those 185V plate voltage readings down to 175V (measured at OB) and listen to see if the distortion goes down.

[Horatio]

Ok, gave it a go and I could not get it any lower. The range seems to be mid 185 to around 300 then the component makes a faint clicking sound at either extremity. The low point does seem a bit variable but I cannot seem to get it any lower - it is currently slowly creeping up towards 187 as it sits on the bench without me touching it.

Thanks,

Bill

[Horatio]

Hmmm, after leaving it off for another 20 mins I took the readings again. After being on for 1 minute it was sitting at 178 but slowly rising to over 180 and onward over a number of minutes.

Out of curiosity I hooked up the speakers and played some music. At first it sounded great, no obvious distortion, so I gradually cranked up the volume on the Bee Pre then when it was almost up to full volume the distortion came back and was then present at the lower volume levels again.

Bill

[Horatio]

Hi, just bumping the thread to see if any options for troubleshooting spring to mind. Should I try reflowing the solder joints, are there any specific components I should be testing that could deliver the distortion?

I can happily upload a sample of the distorted sound if that would help...

Thanks,

Bill

[Paul Joppa]

The driver heater power measurement (2.7+2.7vAC) is disturbingly low. It may be a meter anomaly, some inexpensive meters become inaccurate at low AC voltages, but a low heater voltage would give this issue so it's worth checking the voltages and wiring more carefully. Measure the voltage between the red and black wires at the power supply board first, then between pin 1 and pin 9 - and use a higher resolution meter scale if you can, so you can see three digits.

The fact that the 300B filament voltage is spot on implies the power line voltage is right, so the transformer should be supplying 6.30 volts AC.

You should be able to adjust the cathode voltage kreg(A) as low as 2.50 volts DC, but you did not get lower than 3.6v in the Nov 13 measurements. That may be the actual problem. Reflow solder connections on the 431 chip and the trimpot. (Yes, I know that's going to be a time-consuming hassle!)

[Horatio]

Ok, some progress...

1) I purchased a new more accurate meter to retake measurements
2) Now reading 3.2 and 3.2 VAC
3) Trim Pot adjustment shows 175 at OB
4) Reflowed the solder on the C4S board 431 and Trim Pot
5) None of this made any discernible difference so I swapped the C4S board with the one in my working amp - the distortion moved with the C4S board so thats progress!!!

So on reasonably loud music (the track itself being loud), even with the DAC connected directly to the amp I get obvious distortion at about 66% volume and above (it may be there at lower volume but it gets hard to tell.

I am going to reflow everything I can on the problematic C4S and then will post a full set of readings plus pictures

Bill

[Horatio]

Ok, reflowed just about everything I could on the C4S board and checked the resistors for obvious mistakes and didn't find anything. Distortion remains as the volume ramps over 50%.

Resistance

Terminal   Manual   Non Working Amp
1   *   *
2   no connection   no connection
3   no connection   no connection
4   no connection   no connection
5   1 KO   1 KO
6   1118 O   1116 O
7   249 KO   249 KO
8   no connection   no connection
9   124 KO *   241 KO
10   0 O   0
11   3.0 MO *   14.1 MO
12   5.6 MO *   O.L
13   0 O   0 O
14   9.2 MO *   O.L
15   15 MO *   O.L
16   *   *
17   0 O   0
18   0 O   0
19   128 KO *   241 KO
20   249 KO   248 KO
      
A1   1027 O   1030 O
A2   *   *
A3   249 KO   248 KO
A4   1028 O   1025 O
      
B1   0 O   0
B2   8.2 KO   6.41 KO
B3   249 KO   248 KO
B4   130 KO *   241 KO
B5   0 O   0
B6   177 KO *   245 KO
B7   220 O   253 O
B8   *   O.L
B9   *   .4 O

Voltage

Terminal   Manual   Non Working Amp
1   440   433
5   71   72.4
9   175   175.1
10   0   0
      
16   457   457
17   0   0
18   0   0
19   175   175
      
A1   74.1   76.5
A2   440   433.2
A3   close to 0   18mv
A4   69.2   72
      
6V-Red   3.1 VAC   3.1
6V-Black   3.1 VAC   3.1
      
Kreg A   4.1   3.16
Kreg B   6.3   6.81
OA   300   298.3
OB   175   174.8

Any suggestions would be much appreciated!

Bill

[Doc B.]

I would say at this point the best thing to do would be to verify that you get the same resistance measurements across each resistor on the distorting board as you do on the good board. It could be that a semiconductor on the board is somewhat damaged, though they usually either work all the way or they are totally dead. It's often difficult to spot one that is only partly shot and it might be best to replace the transistors and 431 or maybe even build a new board from scratch. The only other thing I can think of is maybe somehow the cap in the soft start circuit is acting up, but it seems like that would show up in the voltages.

I'm not saying that it makes sense that just leaving the amp off for a month would cause semiconductors to fail, but I don't have any other particularly bright ideas for what might be wrong.