added this fancy master volume...hum

[EthanB17]

Well folks i finally took my wife's advice and started installing master volumes in my amps. The only one that is giving me trouble is my JTM45+ from tube depot.  i'm not sure how to post a proper sized picture so i'm posting the link to the layout i used.... 
http://site.triodestore.com/MASTERVOLUMEJTM50MasterVolume-1.pdf 
that is the standard PPIMV layout that i see folks use. to note the only difference between this layout and the JTM45+(in regards to the MV) is my layout uses .022 microfarad coupling caps off the PI vice the .1 shown in the layout.

So the problem is the master works great, tone is wonderful and works better than i expected... except for the new extremely obvious hum that increases as you open up the MV.  I initially used dual conductor shielded wire as the layout depicted but when i got all that nasty hum i decided to just use standard 20 AWG wire to see what would happen.  Both types of wire yielded identical results.  As i'm new to this amp building business i could use a little help...HELP!!! I really want this MV to work as a JTM45 amp is completely pointless(IMO) unless you crank it... And i dont think the neighbors like AC/DC as much as i do( could just be my playing).

Cheers
Ethan
{EDIT- fixed URL --PRR}

[John]

That link doesn't work for me.


Your hum: is it a low pitched hum, like heaters (60 hz) or higher, sort of like a buzz?

[kagliostro]

For me the link didn't work   

---

Upload the image here (free and without registration)

http://imgur.com/

then post the link here

Franco

[sluckey]

Be sure you follow the wiring diagram EXACTLY.

[EthanB17]

Gentlemen, it looks like the post that Sluckey put up will provide access to the layout.( thanks!).  i'm confident that my wiring is correct.  It is not  a buzz. it is a very smooth hum. that increases with volume. i don't think the frequency increases with volume, just the audible sound.

[PRR]

Link had an extra dot at the end. Fixed now. Leave space both sides of a URL so the forum and the destination knows where it ends.

Agree that it looks correct in the drawing.

[SolidStateCircuitReject]

when you were using shielded wire, the shielding was grounded at the pot and clipped at the other end right?

[EthanB17]

I had it clipped at one end and the other end i had connected to the first leg on the pot(the outside leg the resistor connected to). should i have connected the shield to the back of the pot?

[SolidStateCircuitReject]

no, you did it correctly.

[sluckey]

should i have connected the shield to the back of the pot?

No! The shield has your negative bias voltage on it. Look closely.

[EthanB17]

well since we are back to square one i guess i can attempt to rewire it with the dual conductor shielded wire again and try and be neater. Is it safe to assume that the problem is most likely with the MV circuit since it didnt exist before the mod?

[sluckey]

That is a proven circuit. It should work just fine if wired correctly and neatly. Can you post a pic of your work?

[SolidStateCircuitReject]

take your time to be as neat as possible, it's easy to leave something not connected right when there are several items tying in within those two outer lugs. Let us know!

[ratgon]

Sorry to jump in here outta the blue but I'll be incorporating this circuit in a build I'm starting. So it uses two-conductor shielded wire? If so will using four single shielded runs be an issue or is it just best to locate the double. Or am I just looking at this all wrong. If so just consider this post a little comic relief.

[sluckey]

Single conductor shielded cable is fine. Shielded two conductor is just neater looking. Unshielded wire may be fine if your wire length is not too long and don't have to pass through any noisy areas.

[ratgon]

Thanks Sluckey, I'm taking my time and starting to populate the AC-15 board so I'll be looking at this closely as I get to the MV.

[ratgon]

By the way, why does the signal needed to be shielded from noise at this stage of the circuit and nowhere else beyond input?

[sluckey]

It doesn't ***IF*** you keep the leads short and don't mount the pot in a noisy area or run the short leads through a noisy area. I consider 4" to 5" short.

[ratgon]

Thanks. That makes sense. Plus it seems as though since the signal is amplified so much the farther it moves along the circuit that the s/n ratio becomes less an issue. Or at least that's why I've figured we shield the extremely small input signals.

[PRR]

User knob before the big output stage is more likely to wired be near the big racket off the power supply and the huge audio swings at the final tubes' plates. Preamp and tone knob wiring is usually in a more quiet area.

So It Depends.

[ratgon]

Ah! That makes sense too. Thanks.