Volume/Tone pot interaction affecting voltage & creating noise

[yello]

I have a projector amp converted for guitar use (oscillator circuit removed, guitar input added, re-capped, pots and tube pins cleaned - basic servicing). Other than replaced caps and jacks, and 3-prong, it is stock.

The amp is 99% functioning, but has a strange issue where the volume and tone pots interact and affect voltage (I measured plate voltage in the video but voltage at B+ nodes also affected) and create lots of loud noise. This only happens on the top 5% of the volume pot, when tone pot is also turned all the way up. With tone rolled back, but volume at full no issue. With volume rolled back and tone turned all the way up, no issue.

The following are the original schematic, a voltage chart comparing 2 of the same type of amp - but one working and without this issue (#1), to this amp (#2). The voltage chart says ef86 xtra, but this isn't that circuit, they just have the same tubes so I used that chart. And a video showing the problem using my DMM measuring plate voltage at the el84 v4.

Any ideas of what it can be? Any tests I need to complete to explore the problem?





https://youtu.be/thw_kuXHLIo

[JMPGuitars]

Sounds like a parasitic oscillation. Start by checking your filter caps. Then check the value of the power tube grid stoppers, and if needed, try increasing their value and see if that helps or not.

[Bieworm]

Isn't it also worth replacing the pots? When the carbon trace is compromised at the end of the sweep things would also get weird.

[yello]

Sounds like a parasitic oscillation. Start by checking your filter caps. Then check the value of the power tube grid stoppers, and if needed, try increasing their value and see if that helps or not.

I have checked the filter caps wiring extensively, but can give it a fresh look, measure caps, confirm grounds and wiring.

The amp does not currently have grid stoppers in stock form. The working Krinard modded amp also doesn't have them, so the voltages you see on the chart for both amrdps are without grid stoppers. I can easily add the 1k, but is that a possible cause of this issue?

Isn't it also worth replacing the pots? When the carbon trace is compromised at the end of the sweep things would also get weird.

Replacing the volume pot would be easy, 1M. Replacing the tone pot would be a nightmare given the number of components on it, and that it is a single knob dual pot with weird values, they measure at 2.7M and 3.3M. Not sure why the schematic says 1M, all these amps have the aforementioned pot.

The volume pot on the problem amp does measure a little weird - it measures 849k, then sweeps up to 1.2M at 1/4 turn, then back down to something like 10 ohms at the end.

[JMPGuitars]

Sounds like a parasitic oscillation. Start by checking your filter caps. Then check the value of the power tube grid stoppers, and if needed, try increasing their value and see if that helps or not.

I have checked the filter caps wiring extensively, but can give it a fresh look, measure caps, confirm grounds and wiring.

The amp does not currently have grid stoppers in stock form. The working Krinard modded amp also doesn't have them, so the voltages you see on the chart for both amps are without grid stoppers. I can easily add the 1k, but is that a possible cause of this issue?

I didn't mean check the filter cap wiring, I meant check if the caps are good or bad.

Not having grid stopper resistors on the power tubes could cause that issue. The schematic you posted DOES have grid stopper resistors. If your amp doesn't, then place the 22K grid stoppers recommended in the schematic.

Thanks,
Josh

[yello]

I didn't mean check the filter cap wiring, I meant check if the caps are good or bad.

Not having grid stopper resistors on the power tubes could cause that issue. The schematic you posted DOES have grid stopper resistors. If your amp doesn't, then place the 22K grid stoppers recommended in the schematic.

Thanks,
Josh

Caps are brand new, tested before install.

Sorry, I thought you meant screen grid stoppers. The stock 2.2k grid stoppers are there.

[Bieworm]

The pot swap is an easy 10 minute operation.

[JMPGuitars]

Sorry, I thought you meant screen grid stoppers. The stock 2.2k grid stoppers are there.

If the lower value isn't working, use a higher value. Try 10k.

[yello]

Sorry, I thought you meant screen grid stoppers. The stock 2.2k grid stoppers are there.

If the lower value isn't working, use a higher value. Try 10k.

Is this type of parasitic oscillation a common symptom related to volume/tone pot interaction when amp reaches full volume, for grid stoppers to be at play as the problem area? I'm trying to understand the "why" - replacing the parts will be fast and easy. And then wondering why it isn't happening in the other same type of amp?

[JMPGuitars]

Is this type of parasitic oscillation a common symptom related to volume/tone pot interaction when amp reaches full volume, for grid stoppers to be at play as the problem area? I'm trying to understand the "why" - replacing the parts will be fast and easy. And then wondering why it isn't happening in the other same type of amp?

One example of a similar issue:
https://www.youtube.com/watch?v=bCN4LH4urns


If you have a scope, you can try setting up the same way he did and see what happens.

[yello]

Is this type of parasitic oscillation a common symptom related to volume/tone pot interaction when amp reaches full volume, for grid stoppers to be at play as the problem area? I'm trying to understand the "why" - replacing the parts will be fast and easy. And then wondering why it isn't happening in the other same type of amp?

One example of a similar issue:
https://www.youtube.com/watch?v=bCN4LH4urns


If you have a scope, you can try setting up the same way he did and see what happens.

Awesome reference video, thanks!

[yello]

I looked and have a 1M pot, and the screen stopper resistors, but not 10k for the grid stoppers (closest I have is stock value of 2.2k but new, or 22k). I'll order some 10k.

The reason I was working on the amp for a friend is he wanted an IEC instead of the power cable, and when I tested it, it was acting funny, power tube was redplating. I looked inside and discovered someone in Brazil had recapped it incorrectly, using 80uf for the first node B+, and installed a Brazilian 5k OT.

I recapped it according to the schematic, and installed a proper 8k OT. Only then did I discover the volume/tone interaction thing. It may have been present before, but I didn't notice it.

Once I recapped and put in the replacement OT, I popped the original **** el84s back in, but they still red plated under current conditions, so I think they are toast. I think it was only one tube red plating, not sure if it followed the tube or socket as I didn't swap to find out. I popped in another set of vintage el84's and I do not notice any red plating.

Looking forward to seeing if grid stoppers fix the oscillation. And if adding screen stoppers helps. And if needed a new volume pot given it was measuring weird.

[JMPGuitars]

I looked and have a 1M pot, and the screen stopper resistors, but not 10k for the grid stoppers (closest I have is stock value of 2.2k but new, or 22k). I'll order some 10k.

Use any higher value there. 8.2k, 10k, 15k, 22k, 47k, 56k. Anything in between those values is fine too. If you have a pair of 22ks, I'd start there.

[yello]

I looked and have a 1M pot, and the screen stopper resistors, but not 10k for the grid stoppers (closest I have is stock value of 2.2k but new, or 22k). I'll order some 10k.

Use any higher value there. 8.2k, 10k, 15k, 22k, 47k, 56k. Anything in between those values is fine too. If you have a pair of 22ks, I'd start there.

I'll start there then - and report back when done, with results.

[yello]

I just took a look inside the amp to prep for installing the new grid resistors (I found I have some 2 watt cement 10k, or 1 watt 22k film), and noticed something I hadn't before - the OT is installed in reverse orientation. The wiring is correct, but it is turned 180 degrees from the one that would have been on it stock. Meaning the secondary and primary sides are oriented 180 degrees from typical install on this amp. I pulled this replacement OT from a similar model amp, and its the exact right part number OT, but the amps each attach the OT differently and I didn't notice until now.

I'm wondering if correcting/flipping the OT orientation would be a potential fix, or worth exploring? It's doable, I just need to unsolder, adjust which mounting flange gets bent forward versus backwards to mount it, then solder it up. I will do that and report back.

[yello]

I ended up rotating the OT 180 degrees and the problem reduced by about 50%. I then chopsticked wires in that area, and got the OT primary wires into a better position, and the problem is fixed. Filmosounds are very tight, and wires are all running close, so getting the OT primary wires tight to the chassis, close to the el84 sockets but away from other things as much as possible did the trick. There is still a plate voltage drop when both the volume and tone pots are all the way up.

So, now I am wondering if it is worth increasing the stock 2.2k el84 grid stoppers to 22k? Maybe that is the ticket for addressing the plate voltage drop?

Or needed to add screen stopper resistors? Stock it doesn't have them, and it is likely a good idea.

Or replace the volume pot, if that is related to the voltage drop?

I'm doing my best to answer my own questions to some degree by gathering info, and trying things. I will likely try the 22k grid stoppers first, then add screen stoppers. Then assess if volume pot replacement is needed, I'm leaning towards not but we will see.

[Bieworm]

Grid stoppers won’t affect the plate voltage imho.. they will prevent the crossover distortion. Screen resistors will drop the screen voltage a little. Plate voltage will be tweaked with dropping B+, or change the cathode resistor

[yello]

I ended up changing the el84 grid stoppers to 22k. Works well. And added a 1k screen resistor for each el84. That dropped the voltage on the screens by about 5 volts, so now screens are slightly lower than plates, instead of being 3v higher.

Left the volume pot, works fine.

I am wondering overall about the voltages in this one as measured and shown in my chart, compared to the Bill Krinard modded one as voltages are higher in this one even though I used the same filter/resistor values. If all the voltages are within a tolerable spec, I'm fine with it as the amp sounds good, but interesting to compare. On the chart this amp I've been working on is amp #2, the Krinard modded amp is amp #1.

[JMPGuitars]

I ended up changing the el84 grid stoppers to 22k. Works well. And added a 1k screen resistor for each el84. That dropped the voltage on the screens by about 5 volts, so now screens are slightly lower than plates, instead of being 3v higher.

Left the volume pot, works fine.

I am wondering overall about the voltages in this one as measured and shown in my chart, compared to the Bill Krinard modded one as voltages are higher in this one even though I used the same filter/resistor values. If all the voltages are within a tolerable spec, I'm fine with it as the amp sounds good, but interesting to compare. On the chart this amp I've been working on is amp #2, the Krinard modded amp is amp #1.

Glad you sorted that out.

Your voltages don't mean much to me since it's not a circuit I deal with, and I don't know what your targets are.

Beyond that, you should calculate your tube dissipation for the power tubes and make sure they're not crazy hot. My guess is they are hot as is, but you can calculate that and decide if you want to up the cathode resistor value. I think your schematic had a 130 ohm cathode resistor, I don't know what you actually used.

[yello]

I ended up changing the el84 grid stoppers to 22k. Works well. And added a 1k screen resistor for each el84. That dropped the voltage on the screens by about 5 volts, so now screens are slightly lower than plates, instead of being 3v higher.

Left the volume pot, works fine.

I am wondering overall about the voltages in this one as measured and shown in my chart, compared to the Bill Krinard modded one as voltages are higher in this one even though I used the same filter/resistor values. If all the voltages are within a tolerable spec, I'm fine with it as the amp sounds good, but interesting to compare. On the chart this amp I've been working on is amp #2, the Krinard modded amp is amp #1.

Glad you sorted that out.

Your voltages don't mean much to me since it's not a circuit I deal with, and I don't know what your targets are.

Beyond that, you should calculate your tube dissipation for the power tubes and make sure they're not crazy hot. My guess is they are hot as is, but you can calculate that and decide if you want to up the cathode resistor value. I think your schematic had a 130 ohm cathode resistor, I don't know what you actually used.

I used a 150 ohm cathode resistor as that is what Krinard used. I need to do the math and see where I stand.