12v transformer for relays

[lowell]

I'm having a hard time troubleshooting this as I cannot get it to happen at the shop. I'm playing all weekend and gigs are 3-4 hours long. Last night the hum came back in second set. I have modified the supply with a regulator fyi. I have not tried the battery trick because I cannot get the problem to happen on the bench. I do however have some more insight on details. The hum is only audible on the OD channel, the hum is only there if a guitar is plugged into the input.

It's my belief that the hum is there on both channels but just not audible on the clean channel because the gain of that channel is so much lower. As said before I have the coils grounded where the DC supply is.

I will play the amp for a couple hours at the shop on Monday to try to replicate issue and troubleshoot. If any of this has spurred any thoughts let me know.

[Enzo]

Wait. The hum is only present when the guitar is plugged in? Your relay has no idea whether a guitar is plugged in or not, nor does its power supply. if you switch to the dirt channel and hum results, but pulling the guitar cord out of the input stops it, then your relay is not involved.

If you turn the guitar's volume control to zero, does the hum go away?

[lowell]

Ok so happened again last night as usual. I pulled the guitar cable and it went away. Then I plugged back in and turned my guitars volume down but it did not change, the hum remained. I then turned the gain pot of the dirt channel down and the hum went away. Seems to be something in the first or second stage in the signal path that's overheating maybe? Also, and maybe this will be fixed when the hum source is fixed, the channel switch is popping very loud when switched. I find this odd as it was not doing that when the relay supply was just 1 simple cap.

[Enzo]

SIncerely doubt it is overheating. I'd be betting on a poor ground, probably at the input jack.

Relay popping is almost never about the power supply. The coil itself should have a diode across it wired in reverse. That will help remove any inductive kicks when the circuit opens. But a lot of relay popping is cause by unterminated contacts.

[lowell]

ok what do you mean by unterminated?

[Enzo]

Let's say there is a cap coming from some point in the circuit, and it goes to a contact on a relay - or a toggle switch, doesn't matter - and the other side of the relay continues on to a grid or whatever. When the contacts close to complete the circuit, that cap has to come up to charge. That brief moment of current results in a pop.

What we need to do is terminate the cap - provide it with a destination at all times. SO we would add a resistor from the free end of that cap (the relay contact) to ground. It would be a high value like 1 meg maybe. Now the resistor completes a charging path for the cap, so it is always charged. When teh relay closes, it doesn't make a pop.

Sometimes you see a cathode bypass cap switched in and out of a stage as a gain boost. You will see the end of that cap is not left floating, it will have a high value resistor to ground, and the switch in and out simply shorts across that resistor. Same deal.

[lowell]

Ok right on. And this happens w/ coupling caps too? It must because there are a... well, couple..... of those connected to the relay contacts.

[Enzo]

The cap I described is a coupling cap. ANy coupling cap has the potential to pop. it doesn;t take high voltage or high power or something.

It doesn;t have to be a cap either. Lets say you have a volume control, and the wiper connects right to the grid of the next tube. No problem, we see that all the time. Now lets say you have two channels each with a volume control, so you add a relay to select the wiper of one or the other volume control to go to that grid. When the relay contacts leave the first control for the second, there is a moment when it is connected to neither, and the tube grid is connected to nothing. grid leak voltages can build up on that grid and make a pop. SO you would add a high value resistor to ground from that grid. Now it ALWAYS has a path to ground, even in the moment between channels as the relay moves.

[lowell]

Ok cool, hey thanks for the info and sticking w/ me on this.

[lowell]

Ok the hum WILL not happen at the shop. I've had the amp on for 1 1/2 hours. Maybe I need it on longer.

Anyway, of more importance right now is the popping noise. I've been alligator clipping some 1m's in there to find which relay pin will benefit from this. Most pins the popping actually more than doubles in volume and is obnoxious. FYI the popping is really only bad when switching from OD to normal channels. The only spot that helps, and quite a bit, is to add a 1m in parallel w/ the Cathodyne grid leak to ground. I'm thinking this might be messing w/ the cathodyne's balance but not really sure. Here's an updated schem if this helps. Of note is that I added a 1m grid stopper to the cathodyne to minimize unwanted distortion elements per Valve Wizard site's suggestions.

[Enzo]

In my view, and I'd love input from others here, I see the grid of V2A untern\minated. Every time RY1 switches, there is a moment when that grid is floating. I'd give it a grid return resistor.

C14 and C21 are both unterminated.

Look at the phase inverter tube. What voltage is on its grid at R48? 50? 80? C21 sits there at the output of the OD channel at zero volts. As soon as RY2 switches, BAM, C21 is hit with the 50v. There is a rush of charging current and the amp makes a pop.

Likewise, on the input side of the OD, C14 is chargfed up to that 50v for the phase splitter, then when you switch to OD< that cap discharges through R33,R34. Right into the OD input. Due to the size of R33, I think that won't be so sudden a pop.

But I think some redesign around RY2 is needed. For example, move C21 to the other side of the switch. I might wire it from the middle to bottom relay terminal on the right side of the relay. Then add a resistor to ground from the OD end of C21. That way the cap is always charged up, and you are switching the OD signal into it or out.

And i would also say that when experimenting around grids, it is better to tack solder a resistor in rather than have a couple feet of clip wire acting as hum antennas.

[lowell]

In my view, and I'd love input from others here, I see the grid of V2A untern\minated. Every time RY1 switches, there is a moment when that grid is floating. I'd give it a grid return resistor.

I put one in here and it made the pop worse.

C14 and C21 are both unterminated.

Tried C14 and it was worse... don't think I tried C21. Will do.

Likewise, on the input side of the OD, C14 is chargfed up to that 50v for the phase splitter, then when you switch to OD< that cap discharges through R33,R34. Right into the OD input. Due to the size of R33, I think that won't be so sudden a pop.

Yeah switching from Clean to OD there IS a pop but it's bearable. I'd like to completely get rid of it though.

But I think some redesign around RY2 is needed. For example, move C21 to the other side of the switch. I might wire it from the middle to bottom relay terminal on the right side of the relay. Then add a resistor to ground from the OD end of C21. That way the cap is always charged up, and you are switching the OD signal into it or out.

You just led me to an easier solution. I'll put C14 after the relay and ditch C21 altogether. Then add a 1m to the OD side of C14.

And i would also say that when experimenting around grids, it is better to tack solder a resistor in rather than have a couple feet of clip wire acting as hum antennas.

Agreed, I noticed today that alligator clip leads are quite hummy.

I'll try these mods tomorrow and report back.

Oh and the hum came back at my gig tonight... I'll leave the amp on for couple hours tomorrow. Only difference b/t today and gig tonight w/ amp was that the footswitch was plugged in tonight, but not during day when hum would not rear its head. I'll try it again tomorrow.

[lowell]

Ok so I moved some caps around. Below is updated schem. The pop overall is better and is equal whether switching OD-Clean and vice versa.

I tack soldered in some 1m's from C14 to ground on the relay side and from C25 to ground on relay side. This made no audible difference in the popping. I then turned ALL the tone and volume knobs off and the popping is still there, leading me to think it's a supply/ground issue. ??

[defaced]

Two thoughts (this seems oddly familiar)...

Have you tried leaving the input side of the OD channel connected and simply switching the output in and out? Similar to the design in the attached schematic. This is from an amp I'm building and it does not pop.

Second thought, because you have the 100K plate resistor going into the center pole of the relay, I don't think tacking resistors to ground on C25 will help you. If you do that, you'll have the center pole at B+ potential and the C25 cap side at ground potential, that will create a pop. So keep C25 where it is and add a cap between the plate resistor and the relay. This way, every end of the relay is isolated from DC by the coupling caps. Then if that doesn't work, add in big resistors to every relay pole but the master volume side pole (the MV is basically the big resistor there). Then if that doesn't work, cuss profusely and take up a drinking problem.

[lowell]

Right on, that fixed it. The input of OD is always connected now and the output switches.

Ok, now onto the hum. I've had the amp here at shop for couple days and left it on w/ and w/o footpedal plugged in. No hum. At EVERY gig I play the hum starts in the second set... usually after 1 hour of loud playing and an hour in stby. (2 hrs). I'm starting to think it's not the amp, maybe one of my pedals? The thing that's throwing me off is that it's only when I have the OD channel on in the amp, however perhaps the higher gain of this channel causes the hum from another sources more apparent thus making me think it's the OD channel. Tonight WHEN it happens (not if ) I'll try going direct w/ the guitar. Thought I tried that but it's hard to remember what I have tried at this point.