Troubleshooting reverb circuit - solved

[Stephen1966]

So, just to tie up any loose ends in this topic I've made a few adjustments and now the reverb is working fine. The adjustments included dropping the mixer resistor on the input side of the circuit from 470k to 330k (though you might drop it to 220k). Honestly, that didn't have the significant effect I thought it would - it represents something like an 8% swing to the wet side if we think of it as a voltage divider paired with its matching mixer on the dry side. Messing around with caps though, I eventually landed on a .015uF ceramic cap across the RETURN pot to ground - this had a noticeable effect and brought the level of noise down to barely noticeable at normal reverb settings. Still present when the return was all the way up but nothing too obtrusive. The final change was to modify the dropping string feeding the driver. Following the #060 circuit design, I had been using a 47k which dropped my driver's plate voltage to around 290V. This may have been part of the problem all along because since reading around I've seen Fender using up to 440V on the plates of the 12AT7 (I'm using the same). The datasheets all suggest the optimum plate voltage should be around 300V but I thought to hell with it, I would bump it up to 400V. Using a decade box to hone in on the voltage, I replaced the 47k with a 6.8k and nothing caught fire. I can turn up the return all the way now and there is no hum at all. Meanwhile, the cathode voltage has settled at 8.2V which is similar to the Fender figures so I'm a happy bunny Everything from public toilet to in your face.

Thanks to all! You are a great bunch of guys.

Stephen

[Stephen1966]

Speaking too soon

I was mighty surprised at the difference in sound and tone when I buttoned up the amp and put the chassis in the head. Can't say why but the audible experience is different. The amp is as quiet as a church mouse and overall, it is almost at 100% with regards the various tweaks.

Reverb then, I spent an inordinate amount of time hunting down the source of the hum and thanks to the members here I arrived at a solution which gave me (just a little noise) with the reverb return up full - perfectly livable - problem solved you might say. Unfortunately, my brain doesn't rest until it finds answers, and the biggest question I've come to realise, is 'what was the cause of the hum?' Early investigation looked at components in the RVB circuit. I swapped out a couple of resistors and a cap with vintage parts I found on ebay and they helped a little but that wasn't the source of the hum. The hum itself, had the character of the 50Hz mains cycle though through ineptitude or inexperience, I wasn't able to confirm that with my scope. In the end, I arrived here and then it was suggested placing a cap across the return pot to ground. Big improvement, it pretty much killed the reverb effect the higher the cap value though. .1uF no hum but no reverb either; .01uF is about right. So, I buttoned it up and now, after playing with this I am looking at a weak reverb effect. This hum, you may recall, was present with or without the tank being connected and it is a 9AB tank, not the more common 4AB type. I think the strength of the reverb (or lack of it) is partly down to the tank (an Accutronics) but the input impedence is only slightly different: 8 ohm for the 4AB, 10 ohm for the 9AB. The input impedance may be a factor here as well. I would be interested in hearing your thoughts on this.

But then, I noticed there was improvement when the plate voltage feeding the driver was increased from ~300V to ~400V as well, but without the cap across the return pot, the hum is still there.

So finally, I came across this... talking about the Princeton Reverb.





Like the princeton, my filtering for the RVB transformer is not directly from the screen node, but just after it with its own filter of a pair of 47uF caps in series. And mine, follows a choke (not present in the Princeton) as well.

PSU screen grab.JPG

When I arrived at this design for the PSU I didn't think that the level of filtering for the RVB would have been inadequate but after watching the video on 'Reverb tips' and looking again at the schematics I am wondering if this could be the cause of the hum. Might the answer, be to take the branch off for the RVB transformer, not from the screen node, but from the PI node? I might look at increasing the value of the caps in the RVB filtering as well. But after watching this video, and hearing the hum he got and identifying that it was very similar in mine, I am starting to wonder if this not a component issue, or a lead dress issue, or even a grounding issue, but rather the supply.

For the moment, I'm just playing through the amp and getting used to all its potential - and having great fun it must be said. But I intend to get it back on the bench at some point, and when I do, I would like to have a clear plan of action.

This is not an isolated problem, limited to my design, though my design is like many others in parts. I am wondering if this a systemic problem of the PSU design and if so, if it might offer a solution to other builders having similar problems.

Your thoughts, as always, are much appreciated.

Stephen

[Bombacaototal]

I have been struggling with a reverb issue for the past 4 days and thought this could be a good thread to share the issue. The reverb is exactly like Mayer Sig (attached)

I am getting pops, crackles and scratches coming from the Reverb circuit (audio attached - called hum). This is usually a tube issue but I've tried 5 different ones on the reverb (send/return) and the mixer. I have also removed the reverb driver tube and still get the same issues. I then grounded the rca return (at the 220K shown in schematic - which is PIN 7) and the pops and crackles are still here. If I turn the reverb return to zero or ground the coupling capacitor (.002uf) there is no pops or crackles. I have measured heaters (6.44V), cathodes (1.74V) and grounds (between .02V and .04V) along with supply and plates (dont recall those exacly) on the return recovery tube. I then assumed the issue can only be between the tube and the reverb return, so I replaced the plates (100K) and the coupling cap (.002uF) with no luck. I also tried jumpering the two lugs on the reverb return but issue was still present.

I honestly ran out of ideas, but wondering if anyone may have come across a similar noise issue which could potentially put me in the right track.

The only thing left would be to replace the sockets, in case there is a connection issue on one of the pins? I have removed all solder and resoldered all joints and I don't think its a cold joint. Or maybe to replace the return pot, but I am out of 100KB for now, will need to put an order in.

[sluckey]

Certainly appears that V2B circuit is the culprit. Try removing the 4.7µF cathode cap. Try replacing the 1.5K cathode resistor.

[Bombacaototal]

Certainly appears that V2B circuit is the culprit. Try removing the 4.7µF cathode cap. Try replacing the 1.5K cathode resistor.

You nailed it! Leaky 5uF, I removed it and and issue was no longer there. I then went on to replace it and boom, amp is fine now! I really appreciate the help.

Never thought a bad bypass cap would cause all these issues, you learn everyday

[Bombacaototal]

Just been running the amp and unfortunately the issue is still here

Will try replacing the resistor. Attached is how it sounds now, almost same as before

EDIT: just turned the amp again and now no more issue. So it seems something that comes and goes...

[Bombacaototal]

I had no issues with the amp this morning. In the recording you can hear me tapping with my finger strongly the underside of the chassis as the amp is with the opening facing down. Towards the end I also wiggle with my hands the reverb send/return tube. Recording is calle no issue

Second time around when I had turned the amp off and on, after leaving it on for a bit you can hear a louder pop after one of my taps. Seems like a charge build up? The audio recording is called pop. It is looking like a potential cold joint

[Stephen1966]

Hi Bonbacoatotal/Sluckey... I appreciate the input and I'm sure others will find it informative too. For my part, I'm hunting down the source of hum in the filtering supply. I've discovered a simple mod I could make that would take the reverb transformer supply not from the screen node, but from the PI node. Currently enjoying all the other dynamic possibilities of the amp and building a "sound picture" of its range. Eventually, will get it back to the bench and report back with the difference - if any - of moving the rvb to the next filtering stage. The cathode bypass cap on the output of the return tube is an interesting solution though and I had not thought of that. Was thinking it might be endemic to the circuit, given the number of similar complaints, but it could be down to a component after all. One way to find out, I guess...

Just a quick update, the hum you got is very similar to mine I didn't get any pops like you did though - that could be a cold solder joint speaking, as you suggest...

[Bombacaototal]

Today I wired the coupling cap of the send triode straight into the return pot. I basically removed the reverb send input lead, and took the return lead and placed there. No issues. With this test I am assured that the supply is fine (as send and return share the same anode), the reverb return pot is fine, as is the mixer circuit.

I also replaced the reverb pan cables just to be sure and no difference. I will start from the input of the driver tube and go all the way to the return pot and see if I have any luck, as the issue is for sure between driver and return pot.

EDIT: ok, back to the beginning, grounded the 220K of the reverb return (PIN7) and the issue is still here. There is so little circuit left and I already replaced the plates, coupling cap and the bypass cap. Also confirmed the return pot is not the issue . Will now replace the bypass resistor

EDIT 2: replaced the resistor, no luck. Only the socket left

[Stephen1966]

Today I wired the coupling cap of the send triode straight into the return pot. I basically removed the reverb send input lead, and took the return lead and placed there. No issues. With this test I am assured that the supply is fine (as send and return share the same anode), the reverb return pot is fine, as is the mixer circuit.

I also replaced the reverb pan cables just to be sure and no difference. I will start from the input of the driver tube and go all the way to the return pot and see if I have any luck, as the issue is for sure between driver and return pot.

EDIT: ok, back to the beginning, grounded the 220K of the reverb return (PIN7) and the issue is still here. There is so little circuit left and I already replaced the plates, coupling cap and the bypass cap. Also confirmed the return pot is not the issue . Will now replace the bypass resistor

EDIT 2: replaced the resistor, no luck. Only the socket left

Hi Bombacaototal, did you try putting a .01uF cap across the return pot (pin1 to pin3)? That definitely suppresses the hum in my circuit which is similar to yours. A .1uF cap kills the hum completely but unfortunately it kills the reverb effect along with it, so the advice to go as small with this cap as possible. I'm starting to believe the hum originates in the power filtering feeding the reverb transformer and I am assuming you have the same (or at least a very similar) filtering scheme when your B+ for the transformer is marked '2A'. That would take the B+ from the screens node. Like mine. There isn't a whole of filtering at that point. However, I got the hum even without the tank hooked up so if my filtering mod makes any effect it could be because of the ground potential between the transformer and the return tube grid stopper (220k for V2B in yours, 150k for V4b in mine) which share the same ground point (J9 - the grounded OUT lug for the RVB cricuit). It would only take a very small amount of noise in the signal hitting the grid of the input side of the return tube for it to be amplified and then sent on to the Return pot...

Skyliner 2.5 RVB circuit.jpg

Going down your path, I also swapped out a lot of components but all to little avail. Some made a slight improvement but didn't make a dint in the hum. Regarding the pops and squeals you got as well, is it possible these could be originating in filter caps? Definitely though, the cap over the Return pot makes an audible difference.

Stephen

[Bombacaototal]

Hi Stephen, thanks for the input but I actually have no hum. My issue is different if you check the recordings. And sorry for hijacking your thread. I have replaced the pot and issue is still here. I noticed that if I remove the RCA return cable there is no issue. But if I ground the output of the RCA return the issue is still here. So odd!

Are you sure your issue is not a ground loop? I found that the grounding of the reverb driver supply sometimes needs to be separated. You can try at an independent point like the one of the power tubes ground or with the power supply. I had success with these two in different amps

I also had another amp I had to amend the reverb pan ground from input to output and that worked. Another one to try

[Stephen1966]

Hi Stephen, thanks for the input but I actually have no hum. My issue is different if you check the recordings. And sorry for hijacking your thread. I have replaced the pot and issue is still here. I noticed that if I remove the RCA return cable there is no issue. But if I ground the output of the RCA return the issue is still here. So odd!

Are you sure your issue is not a ground loop? I found that the grounding of the reverb driver supply sometimes needs to be separated. You can try at an independent point like the one of the power tubes ground or with the power supply. I had success with these two in different amps

I also had another amp I had to amend the reverb pan ground from input to output and that worked. Another one to try

Thank you. And forgive, I thought I detected a hum in the first recordings you posted. A ground loop? It's possible. For instance, MrD didn't isolate the jacks for the reverb in and out - which I followed - and that would create a loop with the pan installed. I'm thinking another simple thing I might try is to take the ground lead from the reverb transformer and move it over to the ground used by the screens (GND4) if I remember correctly.

By the way, no problem with your contribution here...

[Bombacaototal]

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

[erwin_ve]

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.

[Bombacaototal]

Hi Erwin, thanks for the help. I am only grounding the coax on the RCA ground, the PIN7 side is not grounded, although it's the same shield, so I'd think one ground is enough? By the way I get the same issue without the coax and just using an alligator clip between the RCA and the PIN7.

My issue is not really a hum or a ground loop, it's like a popcorn in the microwave, lots of pops and scratches sounds. It's something new, I have had the amps for a number of months without this issue