Troubleshooting reverb circuit - solved

[Guy77]

Maybe you have tried this already but have you placed a cap across the reverb return pot, pin 1 to 3?
Something in the 0.01uf range to start with.

Cheers

No I haven't tried that yet. My understanding is that this would create something like a high cut/low pass filter. I think have the higher frequencies under control - are you saying this can minimise the low frequency hum as well? I guess I would be using something like on the OD (V2): a ceramic cap. X5F? Pins 1 and 3 are on the outbound side of the tube.

Hi, I was actually referring to the leads on the Reverb return pot when I mentioned pins 1 and 3. I didn't realize how many preamp tubes you had in this amp. Your idea of replacing one of the 12ax7s with a 12AY7 sounds like a good idea.

Cheers

Guy

[Stephen1966]

Maybe you have tried this already but have you placed a cap across the reverb return pot, pin 1 to 3?
Something in the 0.01uf range to start with.

Cheers

No I haven't tried that yet. My understanding is that this would create something like a high cut/low pass filter. I think have the higher frequencies under control - are you saying this can minimise the low frequency hum as well? I guess I would be using something like on the OD (V2): a ceramic cap. X5F? Pins 1 and 3 are on the outbound side of the tube.

Hi, I was actually referring to the leads on the Reverb return pot when I mentioned pins 1 and 3. I didn't realize how many preamp tubes you had in this amp. Your idea of replacing one of the 12ax7s with a 12AY7 sounds like a good idea.

Cheers

Guy

Ah! Sorry, my misunderstanding. I get it now. I can certainly try it.

[talbany]

As a little side note as well
Back when I was at VVT Amps (12 years ago) we would buy ECC-83's by the case's JJ's (I think it was 50) To get the deal we had to order at least 100 at a time. We had a tester that let you audibly hear the noise floor of each tube as it's being tested. Roughly 1/3 of those tubes were unusable due to noise alone!.( I believe Gary at Glaswerks also had a similar fail rate at the time)
All the tubes we tested had some level of noise, some were just worse than others. Anytime you start loading up an amp with more accessories like reverb recovery/EFX loops/mixers w/ more gain stages, it doesn't matter, even if you did everything correctly your always? always! adding more noise into your signal path. 002 has a total of 7 with 5 in the preamp section
BTW. If I had to guess I imagine quality control on 12AX's are even worse now

Tony

Interesting! I've been made aware of the noise added by the ascending gain stages - My amp has 10 tubes with six in the preamp section. Not having the resources to buy anything in bulk like that, I have a "selected" 12AY7. I am thinking that if the hum is simply a feature of the design then a way to mitigate it might be to use a lower gain tube, in the recovery section (V4) like the 12AY7 (mu of 40) and see where that gets me. I saw that in the #060 MrD was using 7025s which I have in my CL and OD sections (no complaints there). As I said, these are JJ 12AX7s at the moment and though marginally quieter it still sounds like an AC hum.

What do you think of this idea for chasing down rogue components. I could place a lead on the input of the PI and working from the entrance of the rvb (avoiding the HT areas around plates) follow the signal path to see where the hum starts. Do you think it would be effective for routing the signal from the entrance to the PI (bypassing any later stage with a faulty component)?

I have not had any luck with going to a lower gain tube here. The problem being that with a lower gain tube you have to turn the reverb return higher in order to get the same mix of wet/dry as the 12AX, sometimes (Depending on the tube) it can make the problem worse.

As far as chasing down rogue components like you describe is? The Hum/Noise can infest the entire reverb section. It's usually best to simply try to isolate the section determine the one with the most noise (Generally Recovery or mixer) and start from there. If you find they all have about the same noise floor you know it's not any one section but a combination of several contributing factors. So the question is? How bad is it? Everyone's threshold for noise is different

Tony

[Stephen1966]

As a little side note as well
Back when I was at VVT Amps (12 years ago) we would buy ECC-83's by the case's JJ's (I think it was 50) To get the deal we had to order at least 100 at a time. We had a tester that let you audibly hear the noise floor of each tube as it's being tested. Roughly 1/3 of those tubes were unusable due to noise alone!.( I believe Gary at Glaswerks also had a similar fail rate at the time)
All the tubes we tested had some level of noise, some were just worse than others. Anytime you start loading up an amp with more accessories like reverb recovery/EFX loops/mixers w/ more gain stages, it doesn't matter, even if you did everything correctly your always? always! adding more noise into your signal path. 002 has a total of 7 with 5 in the preamp section
BTW. If I had to guess I imagine quality control on 12AX's are even worse now

Tony

Interesting! I've been made aware of the noise added by the ascending gain stages - My amp has 10 tubes with six in the preamp section. Not having the resources to buy anything in bulk like that, I have a "selected" 12AY7. I am thinking that if the hum is simply a feature of the design then a way to mitigate it might be to use a lower gain tube, in the recovery section (V4) like the 12AY7 (mu of 40) and see where that gets me. I saw that in the #060 MrD was using 7025s which I have in my CL and OD sections (no complaints there). As I said, these are JJ 12AX7s at the moment and though marginally quieter it still sounds like an AC hum.

What do you think of this idea for chasing down rogue components. I could place a lead on the input of the PI and working from the entrance of the rvb (avoiding the HT areas around plates) follow the signal path to see where the hum starts. Do you think it would be effective for routing the signal from the entrance to the PI (bypassing any later stage with a faulty component)?

I have not had any luck with going to a lower gain tube here. The problem being that with a lower gain tube you have to turn the reverb return higher in order to get the same mix of wet/dry as the 12AX, sometimes (Depending on the tube) it can make the problem worse.

As far as chasing down rogue components like you describe is? The Hum/Noise can infest the entire reverb section. It's usually best to simply try to isolate the section determine the one with the most noise (Generally Recovery or mixer) and start from there. If you find they all have about the same noise floor you know it's not any one section but a combination of several contributing factors. So the question is? How bad is it? Everyone's threshold for noise is different

Tony

True. I'm going off to try to record it now. I can try the lower gain tube - what's the worst that can happen?

[Stephen1966]

So just for giggles I thought I would try recording the sound of the amp before trying anything. A couple of recordings here... The first is just of the background noise (hum) with a bit of commentary to give you an idea of the noise levels. The second is a recording of the guitar at the acceptable level of noise - sorry for that, it was 13 deg. C. in the cellar and I only watched a guy teaching this tune last night. That's my excuse course, I'm a terrible guitar player.

Voice 057.mp3

Voice 059.mp3

Then, I started with the cap idea of Guy's. .01uF to start... reduced the hum a little but then tried a .1uF. That took the noise away completely. However, I'm thinking this is a low pass filter so why this works is a bit of a mystery to me. Perhaps you could educate me Guy? I feel convinced the noise I am hearing is low frequency. I am prepared to live with an acceptable level of noise - its the nature of the beast - so I tried a few other values of cap. .022uF which was better and .047uF which seemed to represent a happy medium. This will allow me to have the reverb dialled in and ready to go without colouring the clean sound noticably. It's a significant difference. But am I imagining this, or is the overall volume of the amp reduced?

I put a film cap in there [https://cz.mouser.com/ProductDetail/CDE ... vwFbTXM%3D] I have others of the same value but i don't know what the accepted wisdom is of cap type there. I tried a 6ps and polarity matters. This film cap seems to do the job though. I couldn't detect any difference in tone (bass or high frequency response) so maybe the question is moot.

[Guy77]

So just for giggles I thought I would try recording the sound of the amp before trying anything. A couple of recordings here... The first is just of the background noise (hum) with a bit of commentary to give you an idea of the noise levels. The second is a recording of the guitar at the acceptable level of noise - sorry for that, it was 13 deg. C. in the cellar and I only watched a guy teaching this tune last night. That's my excuse course, I'm a terrible guitar player.


Voice 057.mp3
Voice 059.mp3


Then, I started with the cap idea of Guy's. .01uF to start... reduced the hum a little but then tried a .1uF. That took the noise away completely. However, I'm thinking this is a low pass filter so why this works is a bit of a mystery to me. Perhaps you could educate me Guy? I feel convinced the noise I am hearing is low frequency. I am prepared to live with an acceptable level of noise - its the nature of the beast - so I tried a few other values of cap. .022uF which was better and .047uF which seemed to represent a happy medium. This will allow me to have the reverb dialled in and ready to go without colouring the clean sound noticably. It's a significant difference. But am I imagining this, or is the overall volume of the amp reduced?

I put a film cap in there [https://cz.mouser.com/ProductDetail/CDE ... vwFbTXM%3D] I have others of the same value but i don't know what the accepted wisdom is of cap type there. I tried a 6ps and polarity matters. This film cap seems to do the job though. I couldn't detect any difference in tone (bass or high frequency response) so maybe the question is moot.

Glad to hear the cap worked!
I have been building amps with overdrive and reverb combined for a while now and when I first started to get a little noise on the Reverb the cap fixed it.
If you look at the JM/Wonderland layout it also has a .01uf cap across the Reverb return pot.
Basically its a balance between how much noise you can tolerate and how intense you want the reverb to be.
If you just need to take away a little noise then go with a small cap like a .001uf. If you need to take away a lot of noise go with a larger cap like a .01 or in your case a .047uf. The larger cap will make things quieter but at the same time it will take away more of the reverb intensity.

Cheers!
Guy

[Stephen1966]

...

Glad to hear the cap worked!
I have been building amps with overdrive and reverb combined for a while now and when I first started to get a little noise on the Reverb the cap fixed it.
If you look at the JM/Wonderland layout it also has a .01uf cap across the Reverb return pot.
Basically its a balance between how much noise you can tolerate and how intense you want the reverb to be.
If you just need to take away a little noise then go with a small cap like a .001uf. If you need to take away a lot of noise go with a larger cap like a .01 or in your case a .047uf. The larger cap will make things quieter but at the same time it will take away more of the reverb intensity.

Cheers!
Guy

Sure! I'm still not really any wiser about how this works but larger cap less noise is exactly what i got. Didn't do anythign more than try a handful of caps only listening for noise reduction. Maybe I will put something on the looper and listen as I do the same thing again. Had the feeling that my overall volume was down, but unlikely so, and more likely, it was difference in intensity of the rvb I was hearing.

In any event, thank you Guy. You have no idea how many hours of banging my head against a brick wall this has given me. Very impressed and pleased. Chuffed to bits

[ijedouglas]

Great job! Always satisfying when you find and fix it.

When I saw Guy mention the cap across the return I remembered the JM sig. The layout has a 0.1uf but I think the general consensus was that it is a 750pf. I updated mine to a 750pf and really like the reverb balance.

[Stephen1966]

Great job! Always satisfying when you find and fix it.

When I saw Guy mention the cap across the return I remembered the JM sig. The layout has a 0.1uf but I think the general consensus was that it is a 750pf. I updated mine to a 750pf and really like the reverb balance.

Many thanks Ian More work to do to find the magic ingredient for sure. I have a film cap in there at the moment but I will try to find a ceramic - too early to say if this has any tone shaping effect. More testing needed. This is all helpful information.

[dbharris]

If you find this dampens reverb signal too much, you can adjust your voltage divider at the reverb mixer to rebalance the signals.

-Dan

[Stephen1966]

If you find this dampens reverb signal too much, you can adjust your voltage divider at the reverb mixer to rebalance the signals.

-Dan

Hi Dan, do you mean something like this?

Reverb circuit 2 - Skyliner.jpg

[pompeiisneaks]

That's correct, that and the other 470k mixer. Basically if the reverb is softer, you could lower the other 470k just below and to the left and it will not attenuate the signal as much. I'm not sure how to balance those exactly... but those two are the mixers.

~Phil

[dbharris]

Yup, exactly. When I was tweaking my JM Sig amp, I found that those two parts were very interactive and sensitive to changing the vibe of the amp.

I would use the smallest cap possible to control your noise floor and then match the divider in the mixer to get the balance and feel you are looking for.

-Dan

[pompeiisneaks]

I guess it's possible to easily create a 'mixer' pot there by putting say a 220k resistor in the place of each, but instead of sending those on, put each side into pins 1 and 3 of a 500k pot and the wiper is what would go onto the next stage. This gives you a 'mix capability' to raise one and lower the other for a balance. Then at dead middle it would be like 470k to either side but you could go as low as 220k on one with 720k on the other either direction.

~Phil

[Stephen1966]

I guess it's possible to easily create a 'mixer' pot there by putting say a 220k resistor in the place of each, but instead of sending those on, put each side into pins 1 and 3 of a 500k pot and the wiper is what would go onto the next stage. This gives you a 'mix capability' to raise one and lower the other for a balance. Then at dead middle it would be like 470k to either side but you could go as low as 220k on one with 720k on the other either direction.

~Phil

Hi Phil, nice to hear from you. As much as I like this idea of using a trimmer pot it's a little outside the scope of the present build. I would need to make a new board perhaps with some design changes to accommodate it in the given space. The next build perhaps... Being an essentially set-it-and-forget-it, once only operation to balance the character of the rvb in combination with the cap to ground to reduce noise, it seems to more appropriately call for fixed resistors.

The size of the cap on the Return pot has a tremendous effect on the amount of noise and at these low capacitances, little audible effect on the tone. Rather than reducing the value of the cap down to .01uF (or lower) I am inclined to go higher. Because at 0.1uF the noise was gone altogether (indiscernible from when the Return pot is at at 0%). Unless there is some other impact I haven't heard yet, I might be able to increase the size of the cap and reduce the size of the mixer resistors for increased gain to both wet and dry portions of the signal but maximum noise reduction. Or maybe it doesn't work like that!

I am curious to see what the effect would be to reduce the mixer resistors both equally, perhaps down to 220k and what the effect would be of attenuating one side more than the other.