"partial" hum on a Plexi - JCM800 hybrid

[warioblast]

Hello guys,

I built a 1987 / 2204 hybrid amp. I mixed a few schematics and here's what ended up with. (See attached docs.)
Basically, it's a 2204 JCM800 with a switchable 2nd gain stage, in order to get close to a 1987 plexi.  Plexi is my clean and dirty is the 2204.

My problem is, I have hum on the clean channel when the gain isn't cranked up. The hum disappears on the very last part of the travel of the pot. When the gain is maxed out, the clean channel is quite silent.
I have zero hum problem with the dirty channel.
I switched my gain & master pots and it didn't change a thing.
I tried to lower C2 value.
I tried a few resistors arrangements between C2 & the gain pot.

What do you guys think ? 

[Leevi]

Sounds like a wiring issue. Have you tried to move the wires with chopstick? Can also be a bad contact.somewhere.
/Leevi

[warioblast]

I have already changed all the wires that go to my relays. I changed the locations of my shielded wires ground connexions.
I played a bit with a chopstick. 
This afternoon, I tried to bypass R9& C4. It didn't change a thing too.
Could it be a design problem ?
A tech friend told me to use a bigger value for C1. I'm going to try that.

[warioblast]

It turns out, my dirty channel has the same problem too. Until today, I have always had the gain dimed on the JCM800 channel.
I have just tried to lower the gain and got hum too. :(

Before that, I changed V1.
I changed the location of the ground of the clean channel gain pot.
I wired the amp, to get rid of the relays and only have the clean channel.
I used a chopstisck around V1 and I found some sensitive spots on the dirty channel triode. Those scrachy noises get louder when the gain is lower.

I'm going to bump C1 value.
Ultimately, I'm going to take the relays out; I'll have more room to work.

[d95err]

I suspect that the hum is picked up when signal level is low after the gain pots. I’d check around relay 2.

I don’t see the point of relay 2 on your schematic. You could simply connect the output of each gain pot to the following circuit without the relay and only use relay 1.

[warioblast]

I followed Bogner's schematic. If it works for him...
Having 2 relays give the ability to totally disconnect the 2nd gain stage. My switching kit has 2 relays, I didnt put too much thought into it. You're right I could use only 1 relay. I will test both methods ...
as I took the relays out this afternoon. 
I wired the amp as a single input, single channel 2204 JCM 800. The good news is I'm hum free, even at low gain. 

[warioblast]

I did a lot of experimentations since then. I might forget a few steps.

Next up was to add a preamp tube for the plexi side. That way I had less cables and could use only 1 relay. Once again I had hum on the clean channel.
...done lots of tests ....
I decided to replace the relay with a toggle switch. No hum, no pop, everything worked perfectly on both channels.
So I decided to ditch this heater powered 6v relay kit.

I've used a 24V relay, using the bias taps of my PT (http://el34world.com/Forum/index.php?topic=22124.msg235648#msg235648), with good results before. So I put this together yesterday.

Unfortunately, now I have a huge hum, on both channels. It always hums, even without the footswitch plugged in.
BUT when I touch the guitar and the plug of the footswicth, the hum disappears.

I checked for continuity, the ground of the 24v relay circuit isnt connected to the chassis ground. I have 26V after rectification.
I tried to move the wires of the power supply of the relay, trying to be as close as could to the spots I know were working, but without success.

I tried to power the relay with a 24V adaptator. I still had hum, but less than with the bias powered circuit.
And the hum also disappeared when I touched the guitar & the footswitch jack.

I really don't know what to do now :(

[shooter]

replace the relay with a toggle switch. No hum, no pop, everything worked perfectly on both channels.

you can still get back to above?

If you "float" a noisy source, it's still noisy.
try everything relay outside amp, run relay wires "into" amp  using shielded cable, ground shield to amp chassis ONLY, (don't use shield as relay ground or anything relay)

[warioblast]

replace the relay with a toggle switch. No hum, no pop, everything worked perfectly on both channels.

you can still get back to above?

Sure. If I'm not powering the relay, the JCM800 channel is active and working fine. I'm pretty sure if I replace the relay with a toggle switch everything is still fine.

If you "float" a noisy source, it's still noisy.
try everything relay outside amp, run relay wires "into" amp  using shielded cable, ground shield to amp chassis ONLY, (don't use shield as relay ground or anything relay)

I was browsing the forum using the search function... and yes I realized I didnt try to move the relay itself. For both the 6V and 24V I kept the relay in the front of the amp. I'll post a picture later. I wanted to have the shortest wires. It might not be the right idea here.

[tubeswell]

Where’s the relay PS grounded?

[warioblast]

Where’s the relay PS grounded?

Right now, it is grounded to the formerly unused Bias CT of my power transformer.

[tubeswell]

Where’s the relay PS grounded?

Right now, it is grounded to the formerly unused Bias CT of my power transformer.

And your output stage is fixed bias? (Going by the schematic you posted)


When you say to the CT of the  ‘formerly Unused’ bias winding, I’m trying to understand how this relates to the grounding in the rest of the amp. Is this ‘formerly unused’ winding used for anything else?
And if it’s just for the relay PS, then how is that grounded?

[d95err]

47u for the filtering of the relay power supply seems very small. I would use something like 1000u. Try increasing the value of these caps significantly.

Also consider using a voltage regulator rather than just an rc filter.

[d95err]

Another question: you say the relay supply is grounded to the bias winding center tap. Where is the center tap grounded?

[tubeswell]

Another question: you say the relay supply is grounded to the bias winding center tap. Where is the center tap grounded?

That's what I want to know. The schematic above does not show a ground, or where any ground that might be there (which we can only guess about at this stage) might be.

[warioblast]

The Center of my Bias tap 20-0-20 isn't grounded. I guess you could say it's floating. It was my understanding that the power supply of the relay couldn't be connected to the chassis ground.

[warioblast]

 

Many thanks guys.

I have just used an alligator clip, and sure enough, as soon as I grounded the power supply circuit to the chassis, the hum went away.

The amp is quiet; no hum, no hiss, ,no buzz...; but I'm going to put a toggle switch on the power supply of the relay to make sure my operating "noise" level stays the same when the relay is engaged.

[d95err]

Great news!

Glad to see the forum could help you!