Help Troubleshooting volume loss PLEASE

[sonolink]

Hi guys,
Sorry for the delay, I was away gigging.
I decided to replace the caps since most of them were "recycled" so they could have been overheated or just toasted when unsoldering them. So to be certain, I replaced all the caps on the board and connected the unconnected opamp half as suggested by usbdevice.
The volume loss seems to be gone, so that's working fine.
BUT I still have 2 issues:
-the Gain pot is still scratchy
-there's an overall hiss. It's not very loud but it's there. I tried with a different tube (an ECC81 instead of an ECC83) but there's no difference.

This thing actually sounds quite good but I think it has got a HUGE output, so maybe there's a signal o noise ratio issue and I should tame it a bit?
I hope that makes sense

Here's another recording for your ears
Guitar is a Strat on bridge position. First clean sound, then both diode overdrive positions, after that the scratchy pot, then I change to neck position and finally with the Gain to about 12 o'clock. Here you can tell how powerful is its output.

View attachment GTR01.mp3

I took a pic also for you to see the settings. Any ideas most welcome



Thanks
Cheers
Sono

 

[usbdevice]

Glad to hear the volume loss issue is gone.
Could you measure the voltages on the gain pot terminals again to see if they've come down? It does sound like the scratchiness in the latest recording is not as bad as before although it's hard to tell.
And since you mentioned the recycled caps, are the pots brand new? Maybe a squirt of contact cleaner for the gain pot helps? A dirty pot wouldn't explain the DC you measured but it's worth a try.

Lenny

 

[sonolink]

Glad to hear the volume loss issue is gone.
Could you measure the voltages on the gain pot terminals again to see if they've come down? It does sound like the scratchiness in the latest recording is not as bad as before although it's hard to tell.
And since you mentioned the recycled caps, are the pots brand new? Maybe a squirt of contact cleaner for the gain pot helps? A dirty pot wouldn't explain the DC you measured but it's worth a try.

Lenny

Thanks for your reply Lenny

I've got bad news though
The volume loss is actually still there. It just took a long time to show. This thing is really puzzling me. The board is now all made of brand new components.

I think that separating the three circuits (tube preamp, Cab sim and balanced output) and test them separately will pinpoint the problem. So, I tested the preamp using the unbalanced output into a DI box and into the soundcard and everything works fine. There still is that hiss I mentioned previously and the scratchy gain pot so I will measure the voltages you suggested and report back.

Next, I'd test the guitar directly into the cab sim with its output connected to the external DI box and into the soundcard and check what happens.

Finally, I'd test the guitar connected into the balanced output circuit directly into the souncard.
I'll try to do all that tomorrow and report back.

Thanks a lot for your support
Cheers
Sono

 

[sonolink]

Ok, so I disconnected the cab sim and the balanced output from the preamp, and measured the following voltages on the preamp:

Master: p1=0v, p2=0v, p3=0v
Gain: p1=0v, p2=-0.01, p3=0v
Input Jack T=-0.7, R=0v, S=0v
Output jack T=0.7v, R=0v, S=0v

Any ideas?
Thanks
Cheers
Sono

 

[usbdevice]

Hi Sono
So the the DC voltage across the Gain control is indeed more or less gone. But again I notice an inconsistency with the measurement: Master P2 and Tip of the output jack should be at the same voltage and yet there is 700mV at the latter. Was the preamp in bypass by any chance, because then you'd basically be measuring a floating wire.
Did you get to try out playing through the indivdual sections as you mentioned above? Any conclusion regarding the volume loss?

L

 

[sonolink]

Hi Sono
So the the DC voltage across the Gain control is indeed more or less gone. But again I notice an inconsistency with the measurement: Master P2 and Tip of the output jack should be at the same voltage and yet there is 700mV at the latter. Was the preamp in bypass by any chance, because then you'd basically be measuring a floating wire.
Did you get to try out playing through the indivdual sections as you mentioned above? Any conclusion regarding the volume loss?

L

Hi Lenny
Thanks for your reply

I must say this thing is driving me a bit crazy now. So I will try to be as rigorous as possible.
As I see it (I might be wrong) there are 3 issues here and maybe they are related (or not):
1-volume loss
2-scratchy Gain pot
3-inherent hiss (not huge but it's there).

1-volume loss:
I have physycally disconnected the tube preamp from the cab sim/balanced output section by pulling out a jumper, plugged my guitar inand connected the output jack into my soundcard. Evrything seems to work fine and sound is even great for a direct connection into a soundcard.
After NINE MINUTES of playing, the volume loss happened.
So I switched off the preamp and switched it back on there and volume would happen again but only for a few seconds...

2-scratchy Gain pot
I have measured voltages again with the Gain pot ALL THE WAY UP, and making sure that the amp was NOT in bypass mode.

Gain: p1=0v, p2=-1.7, p3=-1.7v
High: p1=-0.7v, p2=-1.7v, p3=-1.7v
Mids: all pins 0v
Bass: p1=-0.2, p2=-0.7, p3=-0.7
Input Jack T=-0, R=0v, S=0v
Output jack T=0v, R=0v, S=0v

As for the Master pot, voltages between Master pin 2 and input jack tip are 0v IF the Gain pot is at 0. As you crank up the Gain pot Master pin 2 voltage goes up to 3v and when all the way up the meter says "OL".

Any ideas as to what is going wrong?
Cheers
Sono

 

[Khron]

1-volume loss:
I have physycally disconnected the tube preamp from the cab sim/balanced output section by pulling out a jumper, plugged my guitar inand connected the output jack into my soundcard.

That right there is kinda vague. So during that test, you were playing through... only the tube preamp section, the output of which was connected to your soundcard?

PS: If by "soundcard" you mean the on-board audio of your computer's mainboard, the "inherent hiss" should not come as a surprise.

As for the Master pot, voltages between Master pin 2 and input jack tip are 0v IF the Gain pot is at 0.

Why would you measure that? Or what exactly is that supposed to be measuring? And is that DC? Or with some sort of signal playing through the preamp?

 

[sonolink]

That right there is kinda vague.

No, it's not. Actually it's quite specific...

So during that test, you were playing through... only the tube preamp section, the output of which was connected to your soundcard?

Yep, that's exactly what I meant.

PS: If by "soundcard" you mean the on-board audio of your computer's mainboard, the "inherent hiss" should not come as a surprise.

Nope, it's a Motu Ultralite mk3. You know that card quite well AFAIK ...
The "inherent hiss" also happens when I plug it into my tube amp combo or its FX return, btw...

Why would you measure that? Or what exactly is that supposed to be measuring? And is that DC? Or with some sort of signal playing through the preamp?

As you can read in previous posts, there's a scratchy Gain pot, leading to believe there's DC across it. So after replacing all the caps on the board with new ones (because some of them were "recycled" ones, so maybe faulty), and still encountering the same scratchy pot, I thought it was a good idea to measure voltages across pots, etc, to try to figure out why the hell is that pot scratchy and/or having DC across its pins abd if that eas in any way related to the volume loss ...

Any better ideas?
Thanks
Sono

 

[Khron]

I thought it was a good idea to measure voltages across pots

voltages between Master pin 2 and input jack tip are 0v IF the Gain pot is at 0.

It's the bolded part i have a (logical/rational) problem with...

That being said though, you might've just answered your own question after all, by accident.

Is your guitar equipped with active pickups, or an active preamp? Because it sure sounds like it's putting out DC, which leads to the so-called "blocking" of that first preamp tube the signal encounters.

Move R12 from where it is, to being connected between the input jack tip and ground. Then add a 100nF (film) cap between the jack tip/R12 node, and A2 of the bypass switch, and see if it improves things. You could then even remove R20.

 

[sonolink]

It's the bolded part i have a (logical/rational) problem with...

I was measuring voltage across the Gain pot and decided to measure the other pots too. Since the input and output jacks are related I also measured those pins...

That being said though, you might've just answered your own question after all, by accident.

Is your guitar equipped with active pickups, or an active preamp? Because it sure sounds like it's putting out DC, which leads to the so-called "blocking" of that first preamp tube the signal encounters.

Move R12 from where it is, to being connected between the input jack tip and ground. Then add a 100nF (film) cap between the jack tip/R12 node, and A2 of the bypass switch, and see if it improves things. You could then even remove R20.

My guitar is a 63 strat with single coils, so no active circuit as far as I know but I'll try what you say and report back.

Thanks for your help
Cheers
Sono

 

[Khron]

My guitar is a 63 strat with single coils, so no active circuit as far as I know but I'll try what you say and report back.

As you crank up the Gain pot Master pin 2 voltage goes up to 3v and when all the way up the meter says "OL".

You still haven't mentioned if that was supposed to be DC, or AC, or both, or what range you were measuring that on.

Got any other tubes you could try in this circuit? In case the one that's in there now, is a dud...

You don't happen to have (or can borrow) an oscilloscope, do you?

 

[sonolink]

You still haven't mentioned if that was supposed to be DC, or AC, or both, or what range you were measuring that on.

Got any other tubes you could try in this circuit? In case the one that's in there now, is a dud...

You don't happen to have (or can borrow) an oscilliscope, do you?

Sorry, what I measured is DC.
I already tried with a different tube. They're used ones but should be sound. I'll try with a new one just in case and report back.

Cheers
Sono

 

[Khron]

Sorry, what I measured is DC.

And is that DC? Or with some sort of signal playing through the preamp?

You don't happen to have (or can borrow) an oscilloscope, do you?

And have you already replaced C6 with a known-good one?

 

[sonolink]

And have you already replaced C6 with a known-good one?

I have, with a brand new one...

 

[Khron]

Would be interesting to monitor the HV DC supply to the tube, when it sounds ok versus when it doesn't. May well be that boost-converter "runs out of steam".

I "like" how you seem to cherry-pick which questions you answer, though (one out of three, best case) Not exactly motivating...

 

[sonolink]

Would be interesting to monitor the HV DC supply to the tube, when it sounds ok versus when it doesn't. May well be that boost-converter "runs out of steam".

Ok, I can do that and report back the result.

I "like" how you seem to cherry-pick which questions you answer, though (one out of three, best case) Not exactly motivating...

I sincerely apologize Khron, I'm on my mobile phone and at the studio in the middle of a session (deadly boring one btw) and didn't see your question about the scope...

I can borrow a scope form a friend. It's a small Owon. And it also has a High Voltage probe IIRC. If you could tell me how to use it I'll be glad to do it

Cheers
Sono

 

[Khron]

If you could tell me how to use it I'll be glad to do it

You'll want to (at least initially) be probing the plate / anode of that first triode - if it's a digital scope, maybe even "record" (single-shot capture at a slow timebase, 1sec/div or even slower, if it allows you; we want to see if/how the waveform clips, when the volume drops).

At the same time, use crocodile-clips from your multimeter to monitor the HV DC supply. On that note, what sort of 9V supply are you using, to test this?

You might also want to add another 100nF between pins 4 & 8 of IC1 - i see that only IC2 has one, for whatever reason.

 

[sonolink]

You'll want to (at least initially) be probing the plate / anode of that first triode - if it's a digital scope, maybe even "record" (single-shot capture at a slow timebase, 1sec/div or even slower, if it allows you; we want to see if/how the waveform clips, when the volume drops).

Ok, so that would be connecting the probe between v1 pin1 and GND, right?

At the same time, use crocodile-clips from your multimeter to monitor the HV DC supply. On that note, what sort of 9V supply are you using, to test this?

It's a One Spot 9v/1700mA filtered PSU
https://truetone.com/1-spot/

You might also want to add another 100nF between pins 4 & 8 of IC1 - i see that only IC2 has one, for whatever reason.

That part of the circuit belongs to the cabsim and right now is being left out.

I'll post an updated schem of the tube preamp only in a sec.

Thanks
Sono

 

[sonolink]

Here goes:

 

[sonolink]

You'll want to (at least initially) be probing the plate / anode of that first triode - if it's a digital scope, maybe even "record" (single-shot capture at a slow timebase, 1sec/div or even slower, if it allows you; we want to see if/how the waveform clips, when the volume drops).

One question: what signal shall I use? Still my guitar or a sine wave? If a sine wave would 1k be fine?

Cheers
Sono