JM Wonderland Build Issues

[W5FH]

Hello-
I built the JM Wonderland 50 w version per the attached schematic. Very seldom is one of my builds not dead quiet. There is a very low level hum that I cannot eliminate. It is originating in V-1B. If I touch a screwdriver blade to chassis and grid the hum disappears and amp is dead quiet. I cannot describe all of the work I did to try and eliminate this low level hum. The amp really performs well and is well liked amongst some of my friends. Recently I built an octal version of the Wonderland using 6SL7's in preamp and a 6SN7 push-pull reverb driver. This amp really sounds nice too however I am seeing the same thing involving the grid on that same stage (#2 dry stage). This triode is in a different envelope in octal build to facilitate the layout. My octal build layout greatly simplified construction and there are very few signal runs that need to be shielded or twisted pairs. There is a coupling network (220K R in parallel with 470 pF C) going into this grid (V-1B on Wonderland schematic). The hum reduces very slightly when I ground the side of this network coming from gain pot wiper. But when I ground the grid end of this network the hum completely disappears. Same effect on both 12AX7 and 6SL7 builds. Might someone have some insight into this? Thank you!

Two Rock John Mayer Production Schematic.jpg

[xtian]

Does the hum change with the Deep switch? Check resistance of V1b grid-to-ground.

[W5FH]

Thank you for the help xtian -
I will check the V-1B grid to ground resistance when I can get amp into my shop. The Deep switch causes an almost imperceptible reduction in hum. Anything upstream of the coupling network (shown in red) going to V-1B grid has a near imperceptible effect on the hum. Grounding the upstream side of this coupling network going to V-1B grid causes a slight reduction of the hum. Only by grounding the grid side of the coupling network kills the hum. I have tried many 12AX7's and though some were better than others the hum would not go away. Same thing applies to build with 6SL7 octals.
I read somewhere about an effect of whereby the hum was coming from cathode. I have the cathodes of this stage in both builds bypassed with 5 ufd Electrocube polycarbonate film capacitors and metal film 1% resistors (1.5K in 12AX7, 1.6K in 6SL7). Sure is odd how this occurs in both builds. I was originally thinking it was some form of ground loop being that V-1B and V-2A grids are tied together through grid stopper resistors and share grid return paths. I have experimented with grounding in all fashions regarding this.
Thanks,
Byron W5FH

[Stevem]

This is why I always have a 6 volt Lantern battery set up with clip leads to power a few preamp tube filaments to see if it’s better with pure DC.

V1a needs to be the most noise free gain stage in the whole amp if it’s a single channel deal.

Rectifying the ac to dc just for powering V1 takes 8 parts that total 10 bucks if that.

Is your B+ ac secondary ground going to the first filter stack and then to the chassis for ground?

[W5FH]

Thanks reply Stevem-
I respect and understand what you are saying about DC on filament but I am thinking my problem is from something otherwise. However my understanding of these circuits is only in developing stage so I cannot say with any authority. I use a ground bus running straight through amp, originating in power supply and grounding to chassis at input jack. There are some pics of this build under another post done very recently by me regarding 6SN7 Push-Pull Reverb Driver. To clear up any confusion in past I built the Wonderland per schematic using 12AX7's. Very recently I built an octal version of Wonderland but simplified the build using a Fender TMB tone stack and did not incorporate the local feedback networks, plus I used a 6SN7 push-pull reverb driver into a MPS 2 watt 22.5K to 8 ohm transformer. I was surprised to see I had the same issue with the grid of second dry stage (V-1B on Wonderland schematic). Evidently there is something about this Wonderland circuit that causes this.

[Guy77]

Thanks reply Stevem-
I respect and understand what you are saying about DC on filament but I am thinking my problem is from something otherwise. However my understanding of these circuits is only in developing stage so I cannot say with any authority. I use a ground bus running straight through amp, originating in power supply and grounding to chassis at input jack. There are some pics of this build under another post done very recently by me regarding 6SN7 Push-Pull Reverb Driver. To clear up any confusion in past I built the Wonderland per schematic using 12AX7's. Very recently I built an octal version of Wonderland but simplified the build using a Fender TMB tone stack and did not incorporate the local feedback networks, plus I used a 6SN7 push-pull reverb driver into a MPS 2 watt 22.5K to 8 ohm transformer. I was surprised to see I had the same issue with the grid of second dry stage (V-1B on Wonderland schematic). Evidently there is something about this Wonderland circuit that causes this.

Try shielding the long run of wire that runs from your Treble pot to your Volume pot if you have them setup as the original JM with the Volume a great distance from the Treble pot ( the Vol at the far right of the amp next to the Master vol)

You could also inject some DC into your AC heaters to elevate them.

Cheers
G

[W5FH]

Thank you very much for replies and help! I really thank you for taking the time here to help. I have a lot of additional info under the same time-frame topic of "6SN7 Push Pull Reverb Driver" that pertains to this. Sorry for the confusion I created by mixing these two topics.
Byron W5FH

[W5FH]

I wanted to post this info as it may help others dealing with same issue - the issue was a low-level hum in speaker independent of control settings; hum level was acceptable to some players but I am not happy until I can achieve a dead quiet amp.
I experienced this issue with a low-level hum pickup in two JM Wonderland builds, at grid (pin 7) of V-1B. The signal (from wiper of 1 meg gain pot, plus bright and deep switches) is passing through the parallel combination of a 220K resistor and 470 pF capacitor on to the grid of V-1B. If I grounded the grid-end of parallel 220K/470 pF directly with a screwdriver tip the low-level hum completely disappeared. If I grounded the other side of parallel combo there was just a barely perceptible decrease of low level hum. This occurred in two builds, both with DC elevation on heaters and good heater wiring practices of tightly twisting and down against chassis floor. Grounding was a ground bus originating in power supply and chassis connected at input jack.
If I removed the 220K/470 pF parallel combo and replaced it with a 68K resistor the hum went away completely, both amps. Referring to the JM Wonderland schematic the signal (from gain pot wiper and boost switches) splits and feeds V-1B pin 7 grid (through parallel combo of 220K/470 pF) and V-2A grid pin 2 through a 68K resistor. Evidently there is a form of a ground loop going on here that is influenced by the grid return resistance. I don't like changing the characteristics of an amp, such as I did here by replacing the parallel combo with a 68K resistor, however I can't tell any difference by ear in playing the amps. Since I am in the learning stages of these type things I thought I would pass this info along and perhaps someone can shed some light on it. In particular I am curious if the JM Wonderland builds based on the PC boards of Brian Colgan (The Tone Geek) exhibit this same very low-level hum. I am very thankful to Brian for providing the Wonderland info.

[W5FH]

One thing I forgot to mention -sorry- grounding the grid pin #2 of V-1A had no effect on the hum. I meant to include this to emphasize that the hum was coming from grid pin #7 of V-1B strictly.

[Stevem]

There are two possibilities that come to my mind.

One is have you tried reversing the connection of that 470 pf end for end, that is if it’s not a disc type cap.

Can you draw and post a schematic of all of the components in these two gain stages?

[W5FH]

Hey Stevem-
Schematic is posted at beginning of this thread. I am using a CDE silver mica 470 pF cap.