Need help with an unusual problem

[Larpy]

Did some more snooping of my Eros with my new 'sillyscope.  I've concluded that the model I bought cannot detect AC signals below 100mV.  That means there's no way for me to adjust the level of an audio signal from my laptop into the Eros that matches the .4mv output of my cartridge. 

But my tests the other day seemed to confirm that my Sowter SUTs are indeed amplifying the signal by a factor of 10. 

So I decided to assume that the signal entering my Eros was .4mv and the signal after the Sowters was 4mv.  Can't measure either, of course, because they're too low for my 'sillyscope to detect.

I plugged my turntable into the Eros and played the first few tracks of the Cardas test LP:  a 1KHz tone and a 10Hz-30KHz sweep, both recorded (mastered?) at –14 dBs.  As expected, my little scope detected nothing at the input of the Eros and nothing after the Sowter 1:10 SUTs, but I got a clear signal at the outputs:

1KHz tone: 1.8v p-p
frequency sweep: 1v p-p from 10Hz to about 150Hz, where it rose to 1.5v p-p.  Stayed there until 7KHz, where it jumped to 2v p-p.  2.9v p-p at 11KHz, 3.5v p-p at 14KHz.  Then back to 1v p-p at 15K and then rising to 1.5v p-p at 20K.  I have no idea if this is expected behavior or not.

Then I decided to play a regular LP and measure the output of the Eros.  Highest measurement after a couple of minutes of watching the 'sillyscope was 6v p-p.  Is that normal and/or expected, given my cartridge's .4mv output?.

 

[2wo]

I would have a hard look at the RIAA, a misswired Or wrong component would mess up the eq.
Also if you need to continue it might help to jump out the transformers or injected the signal after. This would rule them out and you can raise your signal level up a bit, get it out of the weeds so to speak... John

 

[Larpy]

Why did the input level go down?  That may be something that has to be looked at on its own. 

I assume that's simply because I used test tones from different "channels" on YT.  The first two I used came from the same YT source, but the third came from a different source that presumably put out a lower sine wave amplitude.

Today I'll follow PB's and John's advice and desolder, remove, and reinstall the components in my RIAA EQ stage.  See if that makes a difference.

Thank you for your patience.

 

[Doc B]

Don't remove and replace EQ components unless they are determined to be in the wrong place. Just confirm.every position and reflow the connections first.

 

[Larpy]

OK, I'll exercise caution and restraint.

To clarify, I'm looking to measure the output of my Eros at 300hz, 1khz, and 4Khz, with the voltage at 300hz double that of the voltage at 1K and the voltage at 4Khz half of what it is at 1khz?  Getting such results will indicate that my RIAA eq is functioning properly?

 

[sl-15]

I read that you are using youtube videos for the tones. Here is a website that will work a bit better. It is a tone generator and you can choose the output and frequency. That way all the volumes for the different frequencies are the same. Hope it helps. https://www.szynalski.com/tone-generator/

 

[Larpy]

OK, my latest update: I reflowed the solder connections in my RIAA network and got the following measurements at the Eros' output jacks using sine waves from the site Stefan recommended (thank you for the recommendation Stefan; it's a great site and much easier to use than YT).

300Hz  8.7v p-p Right channel; 7.7v p-p Left channel
1KHz    4.7v p-p Right Channel;  4.1v p-p Left channel
4kHz    2.37v p-p Right channel;  2.0v p-p Left channel

These measurements were taken with a feed from my laptop with its volume set as low as it would go and the volume output at the tone generator site set at 9%.  If I turned the volume of the tone generator any lower I couldn't get a reading at 4KHz on the oscilloscope.

I played a sine wave and checked the difference in AC voltage between the input to the EF86 and the output jacks and in order to get a measurement on the oscilloscope at the EF86 I had to turn the volume of the input signal up quite a bit, but what I measured was 574mV p-p at the EF86 grids and 102 p-p at the output jacks.  Is that expected behavior?

And that reminds me: I've been regularly injecting far larger AC signals into my Eros than it would see from playing an LP.  Could that harm the circuit?  The Sowter SUTs?  I checked the resistance across their primaries and secondaries and they still measure in spec.

 

[Doc B]

The issue with signals that are 100x what is intended is that you are looking at severely distorted (clipped) waveforms to get your voltage readings since the 6922 will be way overdriven. The results will not be very accurate. Sorry for giving a recommendation for a cheap scope that apparently doesn't have the needed sensitivity. I have only used mine for automotive and motocycle stuff, not low level audio signals.

That said, the result does sound like the preamp is amplifying properly. It could be that the low signal level you get once the Eros is put back in the system is a problem somewhere before or after the Eros. Maybe a cabling issue from the tonearm/cartridge or the wrong input setting on the Moreplay.

 

[Larpy]

Can a 6922 tube be damaged by being overdriven the way I have been doing?

 

[Larpy]

Hooked my Eros back up to the audio system and, unfortunately, I'm hearing the same distortion on loud passages I've been hearing for months now.  I recorded about a minute of an LP and once again got an unsymmetrical waveform.  See attached.

I watched my little 'sillyscope while I listened to the LP and the massed strings crescendo that sounded distorted measured around 9v p-p.

So I'm afraid I'm back at square one.  Assuming a 9v p-p signal doesn't indicate the Eros is being pushed into overload, that is.

 

[Larpy]

Would hearing an audio file help?  Is there a way for me to share a brief (40 second) audio file of me switching back and forth between the same music on LP and CD?  The difference in sound quality is dramatic.  I could also attach the waveform of the recording as captured on Vinyl Studio so you can how LP waveform is asymmetrical whereas the CD one is not. 

I'm technologically backwards enough to have no idea how to share a brief MP3 file with others on a forum.

 

[Larpy]

OK, I'll drop the audio file idea.

Today I tried bypassing the Sowter SUTs and listening to (and recording the waveforms of) an LP.  Had to turn the preamp volume up all the way and there was tube rush, but the LP seemed to sound distortion-free.  What’s more, the resulting waveform, though tiny and hard to magnify clearly, looked symmetrical.

Is that a clue?  Could the asymmetrical waveforms I’ve been seeing and the distortion I’ve been hearing be caused by the SUTs stepping up the signal from my cartridge too much?

If so, this wouldn’t make sense:  I have a cartridge (Hana ML) that puts out 0.4mV into Sowter 1990 SUTs wired 1:10.  That should put the input to the Eros circuitry at the low end of its 4-6mV target range.  And yet when I’ve recorded music from LPs in the past few days, the signal into my digital interface has clipped a few times.  That means the Eros is putting out a signal somewhat hotter than line level, yes  The badly distorted mass strings I heard the other day looked to measure 9v p-p on the oscilloscope.  From what I understand, that shouldn't be too hot an output signal from the Eros.

As far as I can tell, my Eros is wired correctly.  The oscilloscope measurements of my RIAA network now read as expected.  I’ve checked and rechecked my wiring of the Sowters (and I double-checked that they are indeed 1990s).  And yet could the AC signal into/out of the EF86s be too high?  Is there a wiring mistake associated with the 6922 tube that would explain my symptoms?

I’ve swapped out all of the tubes and that makes no difference.  Changed interconnects.  Surely a bad AC cable couldn't cause this.  Right?

I’ve been reading up on asymmetrical waveforms and, from what I’ve gleaned, DC offset can be the source but when it is the mean amplitude will be displaced from the central axis.  That doesn’t seem to be the case with my asymmetrical waveforms, so I’m thinking DC offset is not behind my problem.  From what I remember observing my oscilloscope while I play music through the Eros, my mean amplitude is typically –1 volt or so.  And the waveforms from music I’ve recorded on Vinyl Studio look on-axis.

If I correctly understand what I’ve been reading, an asymmetrical AC waveform where the mean amplitude is on-axis (or close, as in my case), the source of the asymmetry can be high pass filters or “aggressive low-end processing.”

Is it conceivable that I’ve somehow inadvertently created a high pass filter in both channels of my Eros?  What kind of miswiring would create such a high pass filter?

This is the most puzzling and frustrating problem I’ve faced in 20-25 years of tube DIY builds.  And I’ve done dozens.

 

[Paul Joppa]

Somewhere in the thread so far there is a statement that the voltages are all within spec. But some of the other observations make me wonder if the 6922 is not operating correctly. If you can measure the DC voltages at the pins of the 6922 (pins 1-3 and 6-8) that would help.  The correct voltages are only a little different from incorrect ones, so it's easy to mis-interpret them.

 

[Larpy]

Measurements on the 6922 tube:
pin 1:  160vDC
pin 2:  97vDC
pin 3:  98vDC

pin 6:  160vDC
pin 7:  97vDC
pin 8:  98vDC

 

[Paul Joppa]

That's pretty close to the design target, though I'd rather see 170v at the plates. The difference means your 6922 has a slightly higher transconductance than nominal, which is generally a good thing but can upset the operating point a bit in this direct-coupled circuit..

If you have another 6922 on hand, preferably a well-used one, it might be worthwhile trying it. If not, you can measure the voltage difference between cathode and grid (pins 2-3 and 7-8), which should be about 1.50 volts. I don't expect that to show a problem, but our expectations have been a poor guide so far!

 

[Larpy]

I don't have any well used 6922s other than the Bugle Boy 7308 I've been using for a couple years.

Difference between pins 2 and 3 on it is 1.5 vDC.  The difference between pins 7 and 9 is a mere 1 volt.

I have a reputedly NOS Siemens 6922 that measures 160 and 155vDC on their plates, and
the voltage between pins 2 and 3 and 7 and 8 is a bit below 1.3 vDC in both cases.

The Russian tube that came with the kit measures 162 and 156vDC on their plates.
The difference between pins 2 and 3 is only .7 volts.  The difference between pins 7 and 8 is a paltry 0.55 volts!

I'm consistently low with all three tubes.  Might this be an important clue?

 

[Paul Joppa]

It is indeed - those grid-cathode voltages are too low for proper performance.

The first thing to check is the 47K/2W resistors - are they the right resistance? Are they properly connected? You should be able to measure the resistance from the plates (pins 1 and 6) to ground, with the amp power off.

While you are at it, also check the resistance from the cathodes (pins 3 and eight) to ground, which should be 27K - this will take some time, as the 100uF/160v caps will have to charge up. Also confirm that those caps are properly oriented.

 

[Larpy]

The resistance from 6922 plates to ground is 44K in both instances.  With the power off.  I checked the 2 watt 47K resistors on the board above the 6922 socket and they are the correct 47K resistors.  Just to make sure, I desoldered one leg of one and measured its resistance out of circuit: 47K Ω.

I reflowed the solder connecting those two resistors but no change in resistance.

Am I right in guessing this suggests something is miswired elsewhere and pulling the 47K Ω resistance down about 10%?

Measuring the cathodes to ground proved easier if I disconnected the bypass cap (which is not the stock electrolytic but a Panasonic DC link cap and so not directional).  Resistance from 6922 cathodes to ground is 27K Ω on the nose.

 

[Paul Joppa]

I hate to say this, but things are pointing to that 100uF bypass cap. The circuit is behaving as if something is draining current in parallel with the 27K resistor, and the only candidates I can see are that cap or the EF86 screen grid.

To test this, with the cap disconnected, repeat the voltage measurements, including the grid-cathode difference as well as the plate, grid, and cathode voltages.

 

[Larpy]

Don't hate to say anything!  Any advice you offer is immensely appreciated, and I'll be ecstatic when you find my problem.  I haven't been able to listen to LPs in 5 months, so at this point I'd happily replace any component to get my Eros happy again.

Measurements with 100uF caps disengaged (they actually measure 110uF):
Pin 1: 162v
Pin 2:  98.8v and then drifted down.  After a minute or so it was down to to 97.7v
Pin 3:  99v

Pin 2 to Pin 3:  1.1v

Pin 6:  157v
Pin 7:  101v
Pin 8:  101.3v

Pin 7 to pin 8:  started at .4v and slowly rose (over a few minutes) to .85v and then began drifting back down.  After a couple of minutes it was .76v.  Now that is weird.