Yet another 124 build

[GAStan]

Swapped in another tube for V1 and hum is MUCH better. I can vary the hum by touching the chassis or just waving my hand over it.

But this tube is microphonic. I'll continue to roll tubes until I find the best combination of the tubes I have available.

What is an acceptable loudness level of noise at full volume? Whisper? Normal conversation? Shouting level?

ideally with nothing plugged, low to very low noise, on clean must be almost nothing

Thank you bepone. I will make this my goal.

[martin manning]

Are you running a ground from the input jacks or relying on the chassis contact as a ground? You may want to try isolate both NOR and FET inputs and run a dedicated ground wire.

It's a good idea to run a ground wire from the input jacks to the input ground lug. No need to isolate the jacks from the chassis when you do this, it's a redundant ground, but a very small loop.

A low noise, low hum, and low microphonic tube for V1 is essential. Quiet JJ 12AX7S have worked well for me.

[ijedouglas]

No need to isolate the jacks from the chassis when you do this, it's a redundant ground, but a very small loop.

Just mentioned it as that is what HAD did In fact, he runs a separate ground for the NOR and FET

[martin manning]

My 124 has non-isolated jacks, with soldered wire from sleeve lug to sleeve lug and then to input chassis ground lug, and it is very quiet.

[ijedouglas]

My 124 has non-isolated jacks, with soldered wire from sleeve lug to sleeve lug and then to input chassis ground lug, and it is very quiet.

Great, as I said, that is what HAD did

When building my last amp, there were a couple of things that I noticed HAD was VERY particular about. Grounding was one of them. I'm sure isolating the jacks and running separate grounds was important to HAD otherwise he would have done the same as you...

[GAStan]

The non-isolated grounds are working ok, I did have one tube that is quiet in V1 but was a little microphonic. They ohm out good, 0.1 on my meter which is the same as I get shorting the probes together. One of the first things I did was install the grounds and ohm them out, then as I installed other items such as jacks, pots, switches etc. I checked continuity on them too.

The redundant ground sounds like a good idea and isn't much trouble to add so I will. I believe right now noise source is the tubes I have, just need to find the best one.

The amp is currently inverted on a small wood table so I can easily swap tubes. Hum is completely gone. Just static hiss. I'll keep trying different tubes. I do have some JJ's, I'll try one next.

[ijedouglas]

The redundant ground sounds like a good idea and isn't much trouble to add so I will.

I don't think this is a good idea at all. This is the most sensitive part of the amp. Either ground at the chassis or use a dedicated ground. My vote is a dedicated ground.

[martin manning]

124 has non-isolated jacks, and relies on the jack bushings alone for the grounds. An extra ground lead to the input ground lug insures that a loose or corroded bushing connection doesn't foul things up. If the bushing connection is new and tight, I'd bet it's not the source of a noise problem.

[GAStan]

I used star washers on the jacks, pots and switches. They all test good continuity to chassis ground.

I'll continue trying various tubes and report back what I find. IMO much of the noise seems to be from the tubes, but some is definitely originating either at the input or in CL1 prior to the CL Volume. Also part of it may simply be me being too critical of what I hear.

The noise definitely comes from the preamp. I can plug my guitar cord in to the effect return and it's VERY quiet. If I put my ear about 6" from the speaker I can barely hear a hum, most likely 60 cycle. Master Volume does not affect it. Master volume does affect a VERY SLIGHT hiss, again I must be within about 6" to hear it. IMO that's a pretty quiet power section

With OD off, TMB all at 12:00, PAB off, and both volume and Master volume at 50% there is a hiss I can whisper over. With both volumes maxed hiss is louder but I can still carry on a normal conversation. Either volume will affect the noise level. These are both standing next to the speaker. This is with a well burned in JJ 12AX7 pulled from my small kit built amp. Of course different tubes give different results.

My plan is to find the best (quietest) tube I have for V1, then find the best I have for V2. Then I'll work on further quieting the amp if necessary. Once the noise is under control I'll begin work on the tone. Does this sound like a good plan?

[67plexi]

It's too bad gut shots of #124 amp are no longer available When I built two 100 watt #124 amps
we made our own PC boards. What Gage wire are you using for the power supply filter grounds ??
Mr. Dumble used 18 AWG solid core wire filter and filament and solid core 20 AWG solid core wire on the preamp section
the AC wires 1 3/4" away from the DC wires Gut shot of my last ODS build mid plate high plate skyliner KT77 build.

amp 002.JPG

After fine tune

amp 002.JPG

[Charlie Wilson]

Guts

https://www.thesubjectmatter.com/dumblearchive.html

CW

[67plexi]

Thank you Mark

ODS_124049.jpg

ODS_124043.jpg

[GAStan]

It's too bad gut shots of #124 amp are no longer available When I built two 100 watt #124 amps
we made our own PC boards. What Gage wire are you using for the power supply filter grounds ??
Mr. Dumble used 18 AWG solid core wire filter and filament and solid core 20 AWG solid core wire on the preamp section
the AC wires 1 3/4" away from the DC wires Gut shot of my last ODS build mid plate high plate skyliner KT77 build.

The chassis and boards came from Erwin here on the forum. Very nice set he provides.

18 AWG stranded for grounds, B+'s, 12 DC and heaters. 20 AWG overcoat (fully pre-tinned stranded) for all other. Had trouble sourcing 20 AWG solid for reasonable price. Insulation on 20 AWG is thick so it looks bigger than the 18 AWG.

Nice builds, very clean. Thank you for posting them.

[GAStan]

Right now the biggest issue I have is with microphonics. I tested my tubes in another amp and all are at least a little microphonic when tapped, some very little, some ring like a glass bell, the rest somewhere in between. The seasoned JJ tubes I have are the best I have.

All sockets are Belton Micalex.

The JJ's, when installed in V1 of the ODS, give a slight low pitched "thunk" when tapped. The resistors and wiring to the grids are very sensitive to chopsticking. CL2 is most sensitive. I removed the 500pf ceramic capacitor with no change. Are CF resistors known to be microphonic? The orange drop cap can be heard when tapping it, but at a much lower level as if it is just the vibrations transferring through the leads. This cap is siliconed to the chassis. CL2 exhibits this behaviour with the CL Volume at zero. CL1 is similar to CL2 when CLV is turned up. All of the above is with OD off.

Moving to V2 the OD entrance trimmer was turned to zero basically grounding the OD input. Grids are microphonic but not nearly to the extent of V1. V2 plates are microphonic and tapping the plate wire can be plainly heard.

V3 is good, even with a tube in it that rang like a bell when tested in the other amp.

All power tubes are good as are the components.

I'm at a loss, what should I check next and how do I check it?

[ijedouglas]

Right now the biggest issue I have is with microphonics. I tested my tubes in another amp and all are at least a little microphonic when tapped, some very little, some ring like a glass bell, the rest somewhere in between. The seasoned JJ tubes I have are the best I have.

All sockets are Belton Micalex.

The JJ's, when installed in V1 of the ODS, give a slight low pitched "thunk" when tapped. The resistors and wiring to the grids are very sensitive to chopsticking. CL2 is most sensitive. I removed the 500pf ceramic capacitor with no change. Are CF resistors known to be microphonic? The orange drop cap can be heard when tapping it, but at a much lower level as if it is just the vibrations transferring through the leads. This cap is siliconed to the chassis. CL2 exhibits this behaviour with the CL Volume at zero. CL1 is similar to CL2 when CLV is turned up. All of the above is with OD off.

Moving to V2 the OD entrance trimmer was turned to zero basically grounding the OD input. Grids are microphonic but not nearly to the extent of V1. V2 plates are microphonic and tapping the plate wire can be plainly heard.

V3 is good, even with a tube in it that rang like a bell when tested in the other amp.

All power tubes are good as are the components.

I'm at a loss, what should I check next and how do I check it?

In my experience, the whole of V1 is super sensitive and will definitely be microphonic no matter what you do. Even touching the V1 grid wire with a chopstick will produce a sound. I am unsure what the level is of microphonics you are experiencing though and whether this is an indication of some other problem. I also noticed that your hookup wire is stranded. I always use solid hookup wire (as did HAD) so not sure if that is adding to the microphonics?

The bigger question would be, are the microphonics negatively impacting the sound of the amp? Can you hear artifacts while playing or is it only when you chopstick? Without having the amp in front of me and hearing/seeing the issue it is difficult to diagnose.