No sound after upgrade install [solved]

[Paul Joppa]

In this post I will just address the channel imbalance.

The measurements at A1 and B1 (the grid of the power section of the tube) are 2.6dB different, indicating that the gain of the driver section is different. A difference is not unusual, but yours are at the normal limits of plus and minus. It's possible that is the entire problem, but let's make sure. As a first test, swap the tubes from side to side to see if the level difference follows the tubes. If the level difference switches channels and retains the same magnitude of difference, then a replacement set of tubes is the answer.

The above test assumes the power section gains are both equally matched. However, the AC voltage measurements were made at such a high input voltage level that the power sections are strongly overdriven, so the measurements at A2 and B2 (the power section plates) don't mean much. To get better information (hey, I'm an engineer, I LIKE more and better information!), provide the 60Hz tone but turn the volume control down until the middle terminal of the controls is at about 100mV (0.10 volt) AC. You should see around 2.0 volts AC at A1/B1 then, and around 30 volts AC at A2/B2, and around 1.0 volts at the speaker terminals. This will be a complete test of the gain structure of the amp in its linear range - specifically, the driver stage gain, the output stage gain, and the transformer ratio.

If the hum persists after following PB's suggestions, we'll address that next.

[mcwhart]

Thank you PB and PJ.  I will try what you suggest when I get home from work.

 I have already tried switching the tubes, and there was no difference in hum or difference between right and left channel output.  The low-level hum remained, independent of volume level, and the right channel remained louder than left, regardless of placement of tubes.

[Grainger49]

Since this has rolled to four pages I'm not going to read all the posts. 

We have seen some builders get an AC wire too close to the signal wire and pick up hum this way.  Just check that your wires are dressed like the manual.  Bottlehead is very careful that they layout the two such that they won't interfere. 

[mcwhart]

OK.  This is in response to PJ's instructions for addressing channel imbalance. 

Something is not right here.

With the 60 Hz tone, and the volume adjusted so that the top center lug on the volume pot measures 100mV (bottom center lug is more like 60 mV)  I get the following readings:

A1:  10.9 VAC
A2:  Blinking between 250 and 340 VAC

B1:   1.75 VAC
B2:   22.7 VAC

Speaker terminals on A side, red wire: 5.6 VAC
Speaker terminals on B side, red wire:  .88 VAC

So the B side readings seem somewhat close to what PJ indicated they should be, while the A side readings are 10 times what they should be.

I will power down and recheck all connections.

Thanks





In this post I will just address the channel imbalance.

The measurements at A1 and B1 (the grid of the power section of the tube) are 2.6dB different, indicating that the gain of the driver section is different. A difference is not unusual, but yours are at the normal limits of plus and minus. It's possible that is the entire problem, but let's make sure. As a first test, swap the tubes from side to side to see if the level difference follows the tubes. If the level difference switches channels and retains the same magnitude of difference, then a replacement set of tubes is the answer.

The above test assumes the power section gains are both equally matched. However, the AC voltage measurements were made at such a high input voltage level that the power sections are strongly overdriven, so the measurements at A2 and B2 (the power section plates) don't mean much. To get better information (hey, I'm an engineer, I LIKE more and better information!), provide the 60Hz tone but turn the volume control down until the middle terminal of the controls is at about 100mV (0.10 volt) AC. You should see around 2.0 volts AC at A1/B1 then, and around 30 volts AC at A2/B2, and around 1.0 volts at the speaker terminals. This will be a complete test of the gain structure of the amp in its linear range - specifically, the driver stage gain, the output stage gain, and the transformer ratio.

[Paul Joppa]

Swap the tubes, if you haven't already.

[mcwhart]

With tubes swapped:


With the 60 Hz tone, and the volume adjusted so that the top center lug on the volume pot measures 100mV (bottom center lug is more like 60 mV)  I get the following readings:

A1:  13.06 VAC
A2:  Blinking between 182 and 326 VAC

B1:   2.8 VAC
B2:   38.5 VAC

Speaker terminals on A side, red wire: 6.56 VAC
Speaker terminals on B side, red wire:  1.54 VAC

[Paul Joppa]

Thanks, that helps!

This is a puzzler for sure! Here are three things I see:

1) the A-side driver cannot have enough gain to turn 100mV into 10-13 volts, so I suspect some wiring error in or around the volume control - specifically, the middle lug of the control on that side should go to A4 but perhaps the hot lug does instead.

2) The high and unsteady output at A2 suggests to me that the tube is oscillating. First check is the integrity of the 220 ohm resistor at A1 - they are kind of fragile mechanically. Perhaps a picture of the A socket would help, if you can get a good close-up.

3) The volume controls are not well matched given the 60mV/100mV difference. A mismatch at very low settings is not uncommon but that seems a little more than usual, and at a higher setting than usual. I'd suggest getting the wiring sorted out before going for a replacement; spareparts will send you another control if you email.

[mcwhart]

I checked wiring around the volume control and it appears to be correct.  Also checked the wiring and connections throughout and reflowed a few connections.  Plugged it in and turned it on and now I am getting low level hum on both channels, with no signal from the right channel and a very weak signal from the left channel.  Ugh.  Do I go back to resistance and voltage checks from here?  Frustrating.

[Paul Joppa]

Man, I'm sorry - I hate when that happens, that point where nothing makes any sense and every thing you try makes it worse. It's no comfort, but we've all been there, or will eventually!

Yeah, there's something wrong and it's invisible. Either it's hidden - like a broken wire that is covered by insulation - or you can't see it because you've passed it over too many times. Don't discount the latter, it's a real thing that happens to us humans, I've been there more times than I care to admit. Whatever, you can't see the problem and it's not for a lack of looking.

So anyhow - start with the resistance checks again. Do them carefully and slowly. Every time you look at a wire, check to see if the wire got damaged when the insulation was stripped off - we're looking for hidden problems here. Get a bright light!

Then, I'll still say it looks like there's a problem in the input connections. So trace the signal path. Set the volume control about halfway, and measure each volume control resistance to ground. The highest resistance should be about 100K, that's the input to the volume control so make sure the resistance from that point to the RCA jack center pin is zero ohms. Then measure resistance from T14 and T24 to ground. Then A4 and B4. These should all be the same. Here I'm just trying to check all the input wiring and volume control action. I don't know what is wrong, so I'm just trying to determine that some things are right so we can cross them off the list.

[mcwhart]

I can't believe I am writing this, but it works!  Before running resistance and voltage tests I went through everything one more time, this time looking at it from a different angle, and I discovered a loose red wire on one of the bottom lugs on the volume pot.  I also noticed a red wire in the headphone jack assembly that was possibly touching two terminals.  Clipped off the end of that, flipped the chassis over, held my breath and powered on.  It sounds superb as a headphone amp for my Grado 325is phones.  For speakers I am using new Blumenstein Orcas with only about 150 hours on them, and as good as it all sounds now I am sure it will improve with time.

Thank you so much, PJ, PB, Grainger, and Doc B for all of your help and PATIENCE.  This has been a rewarding project, if challenging at times.  I learned a lot, and I have a hell of a nice amp.  I really appreciate all you have done.

Best regards,

Steve

[Paul Joppa]

Congrats!! So glad it's working correctly now.

Yeah, it usually turns out to be something really simple. Not always of course, but pretty darn often. We'd all make fun of you, but every single one of us has had the same experience, more than once! Real life has this way of keeping us humble ...