Carvin Nomad 112 Crackling and Hissing

[vintagekiki]

If you say so, then so be it.

[Perkinsman]

If I read this in order it seems to indicate that with R3 lifted that there is still 190V on pin 6 of V1. Is this correct? Because it sure shouldn't be!!! It should be 0V. And there should be NO TUBE IN THERE. With no tube in there the voltage on that pin should be over 400V if R3 is still connected. Which it shouldn't be.



How is C6 going to make noise with no voltage applied to it?

How many things can possibly be wrong with the same tests in the same place at one time!?!
[ATTACH]55734[/ATTACH]
I'm starting to wonder if we're not being F'd with here.

I'm not sure where this thread has gone off the rail but I'm hoping to get back to solving the problem. Obviously I'm not an EE, just a guy trying to follow the experts lead & fix a friends amp & learn something along the way. Much respect and gratitude for everyone's assistance. I'm attaching an audio file with V1 in. Voltages at pin 1=180, pin 6=190, pin 8=1.2vdc, R3=365 & 186, C2=189 & .40, C6=.3 & .1. The only voltage listed on the schematic is 190 at pins 1 & 6 so I'm not sure if the others I've mentioned are within spec. As stated, I have the same amount of hiss on the clean channel with V1 in or out, the volume controls hiss with V1 in or out. P4 (treble) decreases the volume a bit but not the amount of hiss. The soak channel doesn't have any noise until it's volume is turned up past 8, so it doesn't seem to impact the soak channel. I've lifted the legs of R3 & C2 with no change in hiss. I've resoldered all V1 pins, R3 & C2. I have noticed when testing some of the leads there will be a slight pop noise and then then the hiss quiets but always comes back. Also, I have not pulled C6 yet but was hoping to hear from the group as to whether the voltages I mentioned are even close to being correct.

[vintagekiki]

I'm not sure where this thread has gone off the rail but I'm hoping to get back to solving the problem. Obviously I'm not an EE, just a guy trying to follow the experts lead & fix a friends amp & learn something along the way. Much respect and gratitude for everyone's assistance. I'm attaching an audio file with V1 in. Voltages at pin 1=180, pin 6=190, pin 8=1.2vdc, R3=365 & 186, C2=189 & .40, C6=.3 & .1. The only voltage listed on the schematic is 190 at pins 1 & 6 so I'm not sure if the others I've mentioned are within spec. As stated, I have the same amount of hiss on the clean channel with V1 in or out, the volume controls hiss with V1 in or out. P4 (treble) decreases the volume a bit but not the amount of hiss. The soak channel doesn't have any noise until it's volume is turned up past 8, so it doesn't seem to impact the soak channel. I've lifted the legs of R3 & C2 with no change in hiss. I've resoldered all V1 pins, R3 & C2. I have noticed when testing some of the leads there will be a slight pop noise and then then the hiss quiets but always comes back. Also, I have not pulled C6 yet but was hoping to hear from the group as to whether the voltages I mentioned are even close to being correct.

for Ud =365 VDC
pin 1=180, Ia=0.84 mA
pin 6=190, Ia=0.79 mA
pin 8=1.2v Ia=0.8 mA
The voltages that you measured are in the range of +/- 10% voltages at schematics and correspond to anode current through tube of about 0.8 mA
To identify the source of hiss, put an empty male jack (without cable) in Return (J3) to disconnect the preamp from the power amp. Do you still have hiss?
If you still have hiss, hiss generates in power amp. Since tube V3 locate in power amp and in clean ch, check (replace) tube V3

[Helmholtz]

Audio file seems to be corrupted.

[Chuck H]

for Ud =365 VDC
pin 1=180, Ia=0.84 mA
pin 6=190, Ia=0.79 mA
pin 8=1.2v Ia=0.8 mA
The voltages that you measured are in the range of +/- 10% voltages at schematics and correspond to anode current through tube of about 0.8 mA
To identify the source of hiss, put an empty male jack (without cable) in Return (J3) to disconnect the preamp from the power amp. Do you still have hiss?
If you still have hiss, hiss generates in power amp. Since tube V3 locate in power amp and in clean ch, check (replace) tube V3

The volume control would not mitigate the hiss if it were being generated in the power amp.

I think Helmholtz's suspicion of V3 must be accurate. Grounding the grid of V3 with the volume control stops the hiss. Strange though that only V3B seems to be affected since a noisy tube is usually compromised in both triodes. The soak channel being quiet would indicate that V3A is not hissing because both channels use V3A as their final stage. Things that should be noted...

Provided the relays are switching correctly there is nothing but voltage dividers between V3B and V3A in the clean channel.

It has been reported that pulling V2 silences the noise. But V2 is exclusive to the soak channel which is reported to be quiet.

EDIT: I recently did some experiments with emission noise in preamp tubes for one of my designs. I think it would be a good idea to replace C8 on the off chance that this is the problem. It doesn't need to be 10uf. Anything that value or greater would be fine.

[Helmholtz]

Strange though that only V3A seems to be affected since a noisy tube is usually compromised in both triodes. The soak channel being quiet would indicate that V3B is not hissing because both channels use V3B as their final stage. Things that should be noted...

But the output (noise) of V3B gets further amplified by V3A by maybe a factor of 60...
V3B is only used in the clean channel (channel 1).

[Chuck H]

But the output (noise) of V3B gets further amplified by V3A by maybe a factor of 60...
V3B is only used in the clean channel (channel 1).

Ah, right. I had my A and B reversed. I edited above.

The point of amplification A to B is something I considered. I would start by replacing V3A with a know quiet tube.***

If that didn't work I would replace C8 on the off chance it's compromised and making some noise as it fails, injecting that into the cathode circuit.

*** New does not automatically mean quiet, or even good. I and many others here have had many new tubes, fresh out of the box that were noisy or bad.

[Helmholtz]

I would start by replacing V3A with a know quiet tube.***

I think the suspect is V3B. Noise produced by V3A wouldn't produce much output signal because of the low PA gain. But as V3B and V3A should be contained in the same tube (envelope), replacing V3 is what I recommend.

[Perkinsman]

Audio file seems to be corrupted.

I think the suspect is V3B. Noise produced by V3A wouldn't produce much output signal because of the low PA gain. But as V3B and V3A should be contained in the same tube (envelope), replacing V3 is what I recommend.

Sure enough, I swapped V3 with another known tube & so far NO noise!! I’ll let it run & make sure, then I’ll study the schematic & try to understand why. Thank you guys!

[Chuck H]

P.S.

I want to formally apologize to Perkinsman for implying that he failed to provide accurate information. Since the hiss seemed to terminate with the volume control I interpreted this to mean it terminated AT the volume control. Which didn't make sense at face value. So that was MY mistake. It was, of course, the grounding of the grid at the following gain stage and not anything behind the volume control.

[Perkinsman]

No apology necessary, I probably didn't explain things as clearly as I should have. Btw, I tested the V3 tube and it was the b side! Nice call guys! Now can someone explain how you narrowed that down to V3b??

[Chuck H]

No apology necessary, I probably didn't explain things as clearly as I should have. Btw, I tested the V3 tube and it was the b side! Nice call guys! Now can someone explain how you narrowed that down to V3b??

You stated just what was going on as per the tests requested. I made a social error when I assumed you got something wrong instead of me. I insist on the apology and wish to keep my shame on the issue

We narrowed it to V3B because the volume control grounds that triodes grid. With the other test reports this was the only thing that could possibly have been making the difference.

[Perkinsman]

You stated just what was going on as per the tests requested. I made a social error when I assumed you got something wrong instead of me. I insist on the apology and wish to keep my shame on the issue

We narrowed it to V3B because the volume control grounds that triodes grid. With the other test reports this was the only thing that could possibly hav,e een making the difference.

Ok, to recap in my mind.....so due to the fact that the hiss was only coming from the clean channel, we started looking at the problem from the 1b side, which is where the clean channel circuit begins. We see the 1b cathode was grounded so we had to look at the circuit from the plate on, which is why R3 & C2 and the power supply were suspect....until removing them from circuit didn't eliminate the hiss and the voltages were correct. Next in the clean channel circuit was the treble, which also didn't eliminate the hiss. We knew that the hiss went up and down with the volume so we had to look all the connections to the volume, which led to the 3b. Is this how your line of reasoning would work and would a scope have got us there sooner?

[Chuck H]

I don't think a scope would have been faster. Maybe? If you're thinking of getting a cope, just do! Super fun to have and if you pursue this work it will absolutely make itself useful. *bay is rotten with good scopes for under $200

As to the trouble shooting. Everybody does it different, but I never suspected the treble cap. And though I had considered the grounding of V3B back in post #24, I didn't report on it (insert forehead slap emoji). Really it just came down to this... The soak channel was fine, so everything involved in the soak channel was ignored. All the circuits for the clean channel other than V3B were quickly eliminated by removing V1 and then R3. Helmholtz gets credit for being the first to pin down V3B. Though I think he suspected it even before he pinned it down.

[Ballalloi]

Hi all,

I have a similar problem with my Nomad 112. It produces 2 types of noise, one of which is hiss, the other is more like a low level hum (not the 50/60 cycle hum).

The humming is in both channels, the hissing only on the clean.

Both are cut out or increase with volume, this is more marked with the hissing.

I have already made the test with the jack in the return socket, and the hiss stops, so I guess it must be the preamp.

If I remove V3, the hiss stops altogether (the hum is still there). I tried replacing V3 (with some 7 new tubes), with no results.

If I perform the chopstick test, it seems that every item I touch in the preamp (caps, resistors, relays) produces a crack. The same happens if I tap on the PCB of the preamp. The cracks occur also without V3 in place.

Please note that I am a complete noob, so any instruction has to be foolproof.

Thanks for your support