Troubleshooting reverb circuit - solved

[Stephen1966]

I would definitely isolate the RCA jacks. Give it a try and let us know.

This is where I am at on mine: from the RCA return, I have a coax cable going to PIN7 of recovery stage and If I ground PIN 7 the issue is here. But if I remove the coax and ground PIN7 there is no issue. So strange! Also noticed that if I remove the RCA return cable (from the pan) the issue is not here too. I am so lost, such a difficult battle this time

Is the RCA coax grounded on both sides? In case of a ground loop caused between the coax shield and pin 7 ground.

Hi Erwin, I wasn't sure if you were replying in response to my problem or MrBs - in any case it's a useful line of inquiry in my situation... My coax cables are shielded at both ends to their respective jack sleeves. I would need to check, but I thought the female jack on the input side of the pan was isolated by design but if it isn't it will be creating a loop between the pan and chassis. As mentioned previously, my hum was unchanged whether the tank was connected or not. As a simple first line of investigation another member here has suggested I could simply measure for resistance with a DMM between the pan and the chassis. If I get OL between the pan and chassis then the pan is not part of a ground loop. I will report back...

[Stephen1966]

Hello Bombacaototal, another thing you might want to look at are your leads. I think it's unlikely in your situation but perhaps worth a look.

Here's a pic of one of my speaker cables from earlier...

IMG_20220403_154215.jpg

Didn't realise it at the time but there were a couple of things wrong here First it was instrument cable and the small core of the cable was carrying a lot of current and heating up quite considerably, melting it's insulation. Second, I hadn't read the instruction on the cable fitting properly and had left a second conductive sheath in place under the braided shield. When the insulation around the core melted, it started to make contact with the conductive sheath, creating a short, accelerating the process. There was a crackling with this too, before the amp simply appeared to die A nice puff of acrid smoke to along with it. The crackling sound I had been experiencing may have had this as its cause all along. Anyway, long story short, all this was fixed and the amp is now crackle free and back on its feet.

Stephen

[Bombacaototal]

Thanks Stephen, these amps ever now and then have ways to keep us in "check" and humble.

I am still carrying on the big fight with this one:
- If I plug the coupling cap of the reverb send to the master (before the send pot), no issue (this indicates that there is no issue in the supply or heaters as these are shared between send and return).
-If I plug the coupling cap of the return to the master (before the return pot) there is the issue. This indicates that it's probably something before the mixer circuit
- with reverb return set to zero there is no issue. Also indicates that the mixer circuit is fine
-If I remove the RCA return coax that goes to the recovery triode and connect PIN7 to ground, no issue but if I feed the reverb send pot OUT to the PIN7 of the return triode there is issue
-If I send the coax of the RCA return straight to the mixer PIN2 I have issues too even grounding the coupling cap of the return

**At this point it is showing that there are issues in both the RCA return and the recovery circuit**

-if I remove the RCA cable on the send side makes no difference
- if I remove the RCA cable on the return side is the only thing that cures the issue
- I have tried 2 new RCA cables and 2 new pans with no avail
- i have tried 5 different tubes in the return and mixer circuit, no difference
- I have replaced the RCA return female jack with no difference
- I have replaced the reverb return pot, no difference
- I have replaced the plates, coupling cap, cathode resistor and cathode capacitor of the return circuit, no difference
- I have removed the RCA coax and connected the RCA return to PIN7 with alligator clips and have the issue (so it means the coax cable is fine)
- I have removed the 220k of the RCA return and made no difference
- I have once again replaces the plate of the return recovery, no difference
- the RCA send and return are isolated and have a buss bar in between them. This is used for grounding the coax, driver tube cathode, the 220k after the RCA return and the reverb transformer. This is then grounded at the pre amp star ground. I have removed all grounds and moved them around, no difference

I have narrowed down to something between the RCA return and the return recovery
- the only thing that makes the issue stop completely is removing the RCA return cable, or to remove the coax from PIN7 and the ground it, or to turn the reverb return to zero
- the weird thing I still don't get it is why, if I ground the PIn7 with the RCA return coax, the issue is still here

I wonder if the issue is elsewhere and maybe only this part of the circuit is, for some reason, picking it up. I would not know where to start. With reverb return to zero the amp is absolutely fine

The amp was fine until I amended the voltage from 240V to 120V. I have reversed that and no difference

[Stephen1966]

Thanks Stephen, these amps ever now and then have ways to keep us in "check" and humble.

Amen to that

I think we can summarise with these notes:

I am still carrying on the big fight with this one:
- If I plug the coupling cap of the reverb send to the master (before the send pot), no issue (this indicates that there is no issue in the supply or heaters as these are shared between send and return).
-If I plug the coupling cap of the return to the master (before the return pot) there is the issue. This indicates that it's probably something before the mixer circuit
- with reverb return set to zero there is no issue. Also indicates that the mixer circuit is fine
-If I remove the RCA return coax that goes to the recovery triode and connect PIN7 to ground, no issue but if I feed the reverb send pot OUT to the PIN7 of the return triode there is issue
-If I send the coax of the RCA return straight to the mixer PIN2 I have issues too even grounding the coupling cap of the return

**At this point it is showing that there are issues in both the RCA return and the recovery circuit**

...

I have narrowed down to something between the RCA return and the return recovery
- the only thing that makes the issue stop completely is removing the RCA return cable, or to remove the coax from PIN7 and the ground it, or to turn the reverb return to zero
- the weird thing I still don't get it is why, if I ground the PIn7 with the RCA return coax, the issue is still here

I wonder if the issue is elsewhere and maybe only this part of the circuit is, for some reason, picking it up. I would not know where to start. With reverb return to zero the amp is absolutely fine

The amp was fine until I amended the voltage from 240V to 120V. I have reversed that and no difference

I understand better when I can see the problem, so I've tried to make a summary of your troubleshooting in this:

IMG044.jpg

So, to get to the heart of this, let's begin by ruling out what is not the issue.

• It isn't your cables or your pan.

• Grid stopper, plates and cathodes on the recovery side made no difference

• Neither did the Return pot.

• Different driver tubes also, didn't make a difference

It is telling that you say before you amended your mains supply voltage from 240V to 120V (and then reversed it) there was no issue. Does that suggest to you that there may have been a component failure either during or after the transition, given that it was present after you reversed it as well?
Removing the RCA return cable or turning down the Return pot is of course, no solution.

Either the problem is being allowed to propagate through the circuit once the reverb return cable is attached (meaning it could be occuring downstream) or the fault lies before the return jack. As a quick test, you might try injecting a small signal at V2Bs grid and seeing if the problem persists with the return cable detached. If it is fault free there, you might conclude that the return side of the circuit is not the culprit.

I also wonder about your choice of driver, the 5751, the datasheets suggest this is more akin to the 12AX7 and has similar current capabilities. I'm sure you will find others who say that a 12AX7 can handle that position fine, maybe you are of that mind, but have you tried a more robust 12AT7 instead? Popping, crackling and fizzing all sound like a tube issue to me but I've only ever had a couple of tubes go out on me like this and they had died very shortly after.

The key event here, seems to be when you moved over from 240V to 120V, so though you seem convinced it is not a supply issue I would keep looking there.

No mention, to my recollection either, that you replaced coupling caps in the reverb circuit, but the change to higher current supply, might also have stressed a coupling cap to the point (almost) of failure especially if it was already on its last legs or in less than prime condition.

If I have missed or misunderstood anything, please let me know.

Stephen

[Bombacaototal]

Thank you for your time and reply Stephen.

I was checking your notes and everything is correct except for the mixer tube. On that one I only changed the 100K plates and also removed the 10M to see if there was any issue on the LNFB. I have not amended the bypass resistor or capacitor. All test this far indicated that the mixer circuit had no issues.

And I also replaced the .002uF of the return recovery circuit which was not marked in your hand sketch

Other things is that I am using a 12AT7 on the reverb. The 5751 on the schematic is a typo, sorry for that. I did remove the driver tube and it made no difference and I also tried different tubes.

And just to clarify the issue indeed started after 240V to 120V conversion. Once I reversed back to 240V the issue still was present. I was thinking that maybe there was something wrong on the 120V tap, but after this that it points to something else. And therefore like you point out, can be that something failed after the conversion.

I did inject a signal on V2B grid, what I did was the sent pot OUT jumpered there. When I did the test I had removed the coax that goes from the RCA return to the recovery grid. And the issue was present. After the tests I could notice the issue in two places, independently, the return recovery circuit and the RCA return.

[Stephen1966]

Thank you for your time and reply Stephen.

I was checking your notes and everything is correct except for the mixer tube. On that one I only changed the 100K plates and also removed the 10M to see if there was any issue on the LNFB. I have not amended the bypass resistor or capacitor. All test this far indicated that the mixer circuit had no issues.

And I also replaced the .002uF of the return recovery circuit which was not marked in your hand sketch

Other things is that I am using a 12AT7 on the reverb. The 5751 on the schematic is a typo, sorry for that. I did remove the driver tube and it made no difference and I also tried different tubes.

And just to clarify the issue indeed started after 240V to 120V conversion. Once I reversed back to 240V the issue still was present. I was thinking that maybe there was something wrong on the 120V tap, but after this that it points to something else. And therefore like you point out, can be that something failed after the conversion.

I did inject a signal on V2B grid, what I did was the sent pot OUT jumpered there. When I did the test I had removed the coax that goes from the RCA return to the recovery grid. And the issue was present. After the tests I could notice the issue in two places, independently, the return recovery circuit and the RCA return.

So to clarify

We can also rule out the coupling cap between V2b and the return pot, the driver tube and by jumpering a signal from the send pot directly to V2Bs grid the driver tube and circuit, the transformer and the tank. Interestingly, your recordings presented the problem even with the tubes operating with no signal. Does the issue get worse when a signal is applied?

I am still thinking it could be a filter cap or dropping resistor issue since the 120V tap doesn't seem to be the source in itself. If it's a dropping string issue on B+3 then it could propagate along to B+4 and so on... later filtering stages (feeding your clean preamp) may be filtering it out to an inaudible level.

It would be useful to see the whole schematic (with any typos corrected) because, just a for instance and not suggesting this might be a cause, I don't know where your PI figures in the scheme. Also, perhaps, (just to yourself) think back to how you performed that conversion... you would have discharged all the caps first, but then, did you use a variac to bring up the voltage gradually or did you just throw the switch. Is there any chance the filtering caps experienced any kind of surge? The Reverb circuit, and the return part of the circuit in particular, is either the source of the problem or a symptom with a deeper common cause. You are seeing the problem in two places independently: the return recovery circuit and the RCA return. I would be asking myself - what circuits/connections do they have in common? And, if I can test those independent places by isolating them, can I rule them out as the cause and trace the problem further back, to its root?

Also, it might be useful to post pictures of that site. A stray copper filament from a bit of the shielding of a coax can be almost impossible to see (with my eyes at least) but it can wreak all kinds of carnage, shorting stuff! I do actually use my phone camera to take close-ups to check for any debris or build anomalies. And for a really close look, I use a jeweller's loupe when I can get it and my head, in the chassis

Stephen

[Bombacaototal]

Hi Stephen, just a correction, the recordings were done with signal, IE the circuit fully wired

In regards to the filtering, surely if it was affecting the return stage it would also affect the send stage, which doesn't seem to be the case

I didn't use a Variac to fire it up after the conversion. I actually never do even at first fire up in any amp.

The RCA return jack and return recovery stages don't share anything, except the coax that connects them

This is surely a odd one, but I think, as you gave advice, I will start looking at the rest of the circuit, who knows...

[Stephen1966]

Hi Stephen, just a correction, the recordings were done with signal, IE the circuit fully wired

In regards to the filtering, surely if it was affecting the return stage it would also affect the send stage, which doesn't seem to be the case

I didn't use a Variac to fire it up after the conversion. I actually never do even at first fire up in any amp.

The RCA return jack and return recovery stages don't share anything, except the coax that connects them

This is surely a odd one, but I think, as you gave advice, I will start looking at the rest of the circuit, who knows...

With B+3 feeding V2A and V2B you're right, if it were a filtering issue you could expect to see it on the send side as well. But you don't, so we might rule that out as well. I would look next at the cathodes on the return stage. And look at the ground scheme in general.

My feeling with this kind of circuit is that it is very good at amplifying any kind of ground floor noise which is kind of where I am with mine, I don't get the pops or the squeals but it does pick up quite a bit of the unfiltered ground noise (around 50Hz). Yours on the other hand, suggests a faulty component somewhere in the chain given its intermittent nature. How far does that extend in your ground scheme? If there is a fault in the filtering it could be riding on the ground potential of the reverb transformer which you might not see before it is amplified in the return side of the tube but that's a long shot... Maybe try replacing the cathode bypass resistors on the return side if you haven't already. I would certainly try relocating the ground from the transformer nearer to the screen grounds. That's what I am planning with mine.

[Bombacaototal]

I had replaced the RCA jacks with the ones I had I hand, but yesterday the matching set I used arrived and for tidiness I replaced RCA return jack again. Amp had no issue at all after the change, and I left in running for two hours. I tried to bang on it with a stick, couldn't make the issue come back. Didn't move or touch the amp except for turning it back on this morning and again no issue.

Next up will be disconnecting the RCA banging the amp from outside and trying again.

I am starting to think that it might have been a grounding issue with the jack itself and that is why sometimes it will not happen and sometimes, after plugging and unplugging the cable it will appear.

[Stephen1966]

I had replaced the RCA jacks with the ones I had I hand, but yesterday the matching set I used arrived and for tidiness I replaced RCA return jack again. Amp had no issue at all after the change, and I left in running for two hours. I tried to bang on it with a stick, couldn't make the issue come back. Didn't move or touch the amp except for turning it back on this morning and again no issue.

Next up will be disconnecting the RCA banging the amp from outside and trying again.

I am starting to think that it might have been a grounding issue with the jack itself and that is why sometimes it will not happen and sometimes, after plugging and unplugging the cable it will appear.

Hey! That's great news. Sounds like you are making real progress.

[Bombacaototal]

I think I found the issue. The amp has been running with no issues for a few days. Then as soon a I turned my computer on the issue came back. Then I plugged the amp into the power conditioner, with the PC ON and the issue is more mild, almost non existent, but still here. I am assuming there is something with dirty AC which impacts the reverb circuit, downstream from the PT. Besides the power conditioner, is there anything I can do to the amp to help on this? I might need a new PT!

I usually run my 240V amps from a power conditioner into an eurovolt into the amp's mains. But after converting to 120V, I was running the step down transformer off the wall, and hence why the issue only showed up at that stage

[Stephen1966]

I think I found the issue. The amp has been running with no issues for a few days. Then as soon a I turned my computer on the issue came back. Then I plugged the amp into the power conditioner, with the PC ON and the issue is more mild, almost non existent, but still here. I am assuming there is something with dirty AC which impacts the reverb circuit, downstream from the PT. Besides the power conditioner, is there anything I can do to the amp to help on this? I might need a new PT!

I usually run my 240V amps from a power conditioner into an eurovolt into the amp's mains. But after converting to 120V, I was running the step down transformer off the wall, and hence why the issue only showed up at that stage

Dirty AC! Interesting! Do you think a different PT would make a difference? I mean, it's possible, I guess, but it would be big money and a big disappointment if the new PT did the same thing. Your power conditioner on the other hand, does tame the effect so I would recommend looking in that direction. I find my amp picks up mobile phone signals quite easily - it's quite distinctive so it's easy to identify and basically goes away when I switch off the phone. At one time, I did toy with the idea of getting an isolation transformer for the mains supply in the workshop but more for safety reasons than power filtering. I don't have much experience of that. Others may be able to point you towards more effective power conditioners. But basically, I use a filtered IEC module for the amp.

https://cz.mouser.com/ProductDetail/XP- ... tuDg%3D%3D

I get lots of voltage swing during the day - 7 to 8 volts between morning and evening, but I couldn't say if the supply is dirty or not as I don't hear any problems with it. I either have no problems or the filter is doing its job. Maybe it's your transformer but I honestly couldn't say. Easy fix though, don't run your PC with the amp!

[ijedouglas]

is there anything I can do to the amp to help on this? I might need a new PT!

I don't think a different PT is going to help. This sounds like either your filtering is not adequate or the reverb circuit (or amp PS) is picking up interference from the computer(perhaps monitor?) I have a similar issue with my wireless router (I also run my amps through a power conditioner). I can lower the noise floor by changing the position/angle or the router. Try changing the angle of the monitor/computer to see if it changes anything.

[Bombacaototal]

is there anything I can do to the amp to help on this? I might need a new PT!

I don't think a different PT is going to help. This sounds like either your filtering is not adequate or the reverb circuit (or amp PS) is picking up interference from the computer(perhaps monitor?) I have a similar issue with my wireless router (I also run my amps through a power conditioner). I can lower the noise floor by changing the position/angle or the router. Try changing the angle of the monitor/computer to see if it changes anything.

Thanks for the contribution Ian, and noted with thanks in regards to the PT potentially making no difference. I have been testing the entire morning. I did check diodes too on the rectifier board and they are reading fine.

With the amp plug to mains via the setup down transformer, there is no issue, but actually as soon as I turn the power conditioner on, which powers my computer, studio speaker monitors, mixer desk, etc, but not the amp for the test, the issue starts in the amp. It's not the computer necessary. The power conditioner is plugged to a socket in one side of the room whereas the amp is in the socket on the other side, but they are sharing a common "circuit" line.

So you think it could be the supply cap feeding the reverb? I actually use a cap can (high grade CE manufacturing) which feeds the tonestack, send/return and mixer circuits, and another f&t cap in the supply board for the reverb driver. Or do you mean the mains supply cap? I use a quad of the 100uF blue Sprague 350V

The only thing I know this far is that this issue related to AC interference.

I just did a recording, amp's master at maximum. It was fine then I turn on the power conditioner and you can hear the issues

[erwin_ve]

Bad design filtering in switching powersupply do cause the interference you describe.
I had a 2013 Samsung laptop, whenever the adapter was out--> good sounding amp. With the adapter connected, a high pitched noise was present in my amp.