Understanding where shielded wire and twisted wire should be used

[munkeyboy]

I have a trainwreck rocket project (using peavey classic 30 transformers and a tweed 5f6 style cab/chassis).  This is my first wreck and/or ac30  build.   I've finished the build, but running into two issue.  Everything works and I actually love the sound, except there is a hum and a kind of oscillation.   I know these high gain amps have more noise, but I still think the hum is excessive.  The oscillation is worse, however.  Here is the scenario where oscillation comes in.  I can turn the volume all the way up, with the lar mar master volume around 3.  Sounds great, no issues, nice sweet breakup.  But once I start turning up the MV, i get an oscillation.  Kind of like a low oscillating squeal. if I turn down the volume to 3/4 and turn up the MV to 10, everything sounds great. 

My question is where should I be using shielded wire in this amp without going overboard.  And where would I just get by with twisted wiring instead?   Right now I just have shielded wire on the input jack to preamp.  I have twisted pairs on the heater wires where necessary.  I also tried twisting the MV to el84 control grids.  It might have helped slightly.

Attached is my layout. [edit: updated layout a few posts down]

[BrainDeadAmps]

try from you preamp vol pot wiper to v2 p7 grid. If you can eliminate noise from the very beginning of your signal, it won't be amplified by the succeeding stages of amplification. Also while your layout is very neat it's easier to judge your issue with photos of the guts.

[AlNewman]

You may want to try a larger grid stopper off of the input.  30k plus instead of 10k.  Limits radio frequencies from your guitar.

[munkeyboy]

try from you preamp vol pot wiper to v2 p7 grid. If you can eliminate noise from the very beginning of your signal, it won't be amplified by the succeeding stages of amplification. Also while your layout is very neat it's easier to judge your issue with photos of the guts.

Here is the guts from a two angles

[kagliostro]

To me seems that the diode bridges are correct on the photo but wrong on the Layout


Franco

[munkeyboy]

To me seems that the diode bridges are correct on the photo but wrong on the Layout


Franco

You are correct.  There are a few mistakes I corrected but didn't seem to save the file last time.  I think there is a wrong 470pf as well. here is the corrected file.  Pay no attention to resistor color codes.  They are not tied to label value.   I also need to double check the input grid stopper.  I don't think it is 10k or if it is I will put a 33k in there.

try from you preamp vol pot wiper to v2 p7 grid.

Ok, i just added shielded wired on v2 p7 but it didn't help. 

More info.  Even with no guitar and the input shunted, this oscillation occurs. Chop-sticking doesn't really help, but the vibrations from chop-sticking can sometimes make it worse... or feed it if that makes sense.  I may just have a bad tube somewhere.  They aren't microphonic that I can tell, but who knows.  I've rolled a few preamp tubes, but not output yet.  I also don't have a metal chassis bottom cover yet.  That could help with hum I guess.

[edit, added el84 heater to cap to ground.  notes this is the peavey classic 30 heater design due to the 36v tranny taps]
[edit, fixed incorrect heater wiring in layout. there has got to be a better way for this wiring]

[tubeswell]

My question is where should I be using shielded wire in this amp without going overboard.

Any long run of signal wire going to a preamp tube grid. (e.g. input stage, or a stage after a  control pot).

And also, use grid stoppers -soldered immediately on tube grid socket pin clamps. Go higher (rather than lower) resistance until the oscillation stops - if your squeal is in fact HF oscillation, and not a microphonic tube etc).

[PRR]

Some casual grounding there. 47V heater returns to input jack. The 300VDC first-cap has cathode resistor returned on the dirty side of the rectifer-cap and the MV returned not much further along. That's like making the children cross the super-highway to get to school.

[kagliostro]

Maintain the 10K at the input and add a 470pf, use this arrangement (obviously your connection pin will be on pin #7)


Franco

[munkeyboy]

Some casual grounding there. 47V heater returns to input jack. The 300VDC first-cap has cathode resistor returned on the dirty side of the rectifer-cap and the MV returned not much further along. That's like making the children cross the super-highway to get to school.

Thanks, help me understand a little better.   I don't follow where the heater returns to input jack.   I did attempt a star ground scheme, where does this fail?  Should I split the the rectifier ground to a new ground lug?  Then move the MV ground to the other pot ground tabs?

[munkeyboy]

Some casual grounding there. 47V heater returns to input jack. The 300VDC first-cap has cathode resistor returned on the dirty side of the rectifer-cap and the MV returned not much further along. That's like making the children cross the super-highway to get to school.

Thanks, help me understand a little better.   I don't follow where the heater returns to input jack.   I did attempt a star ground scheme, where does this fail?  Should I split the the rectifier ground to a new ground lug?  Then move the MV ground to the other pot ground tabs?

  Scratch the heater question.  I follow what you are saying.

[kagliostro]

May be you know it, but it worth to remember

The shield of shielded cables must be connected to ground at only one side

-----

EDIT

I forgot to show you a detail about the grounding of the shielded cable at the input

note that the shield isn't connected to ground and instead it is connected to the cathode of V1 (to the cathode of the triode it feeds)

Franco

[munkeyboy]

So do these changes to the ground scheme make more sense?

Also updated layout to show my current shielded cable runs (purple dotted lines) and a few other changes to hopefully more accurately show my current real layout (minus ground changes). 

[kagliostro]

Did you connected the Heaters the way you show on the Lyout ?

I don't like to use the chassis as path for the Heaters

Franco

[munkeyboy]

Did you connected the Heaters the way you show on the Lyout ?

I don't like to use the chassis as path for the Heaters

Franco

  not yet. right now preamp heaters are grounded at input like prr pointed out. I could run a ground wire all the way back to the other side on the PT bolt. 

[munkeyboy]

My question is where should I be using shielded wire in this amp without going overboard.

Any long run of signal wire going to a preamp tube grid. (e.g. input stage, or a stage after a  control pot).

And also, use grid stoppers -soldered immediately on tube grid socket pin clamps. Go higher (rather than lower) resistance until the oscillation stops - if your squeal is in fact HF oscillation, and not a microphonic tube etc).

So, I added a 470r to v2.  That definitely helped.  But like a noob I made more changes to my grounds per PRR before testing and now my hum is way worse.  I'll walk back the changes. 

But I had questions on the output filament heater circuit.  Tubeswell I think I saw you had peavey experience.  The classic 30, which this transformer is pulled from, had a fixed bias.  Since I'm now using cathode bias, do i change the circuit?   Mainly wondering what C56 in the attached schematic is doing and if I should eliminate that cap to ground.

[AlNewman]

This is a good read, if you haven't already.
Edit, forgot the link.
http://www.valvewizard.co.uk/gridstopper.html

[BrainDeadAmps]

in-case you haven't read this article, could be useful.

Credit to Valvewizard

[acheld]

It's always a good thing to have a schematic, and publish it. 

[munkeyboy]

It's always a good thing to have a schematic, and publish it.

Sure. See attached.

[AlNewman]

I notice there's a potentiometer with one leg not attached.  Is that your cut pot?  Should that be jumpered to the wiper or something? 
Perhaps, and don't listen to me....  But try moving the purple wire to lug 3 and installing a jumper from 3 to 2.  More experienced people here may disagree.  Listen to them.

[sluckey]

Fender has been wiring two legged pots for a looooong time. Look at the Bass pot on any AB763 amp.
I prefer to connect the unused leg to the wiper.

[72Blazer]

Fender has been wiring two legged pots for a looooong time. Look at the Bass pot on any AB763 amp.
I prefer to connect the unused leg to the wiper.

Sluckey can you please elaborate a little on this?  looking specifically for the the benefit of connecting the unused pot to the wiper and what this does for the amp/sound?

Thanks
Vr
J

[sluckey]

Fender has been wiring two legged pots for a looooong time. Look at the Bass pot on any AB763 amp.
I prefer to connect the unused leg to the wiper.

Sluckey can you please elaborate a little on this?  looking specifically for the the benefit of connecting the unused pot to the wiper and what this does for the amp/sound?

As long as the pot is OK there will be no effect on sound. But if the wiper fails on the two lug pot, the pot value goes to infinity and the effect on sound may not be predictable. However when the unused lug is connected to the wiper and the wiper fails, the pot value goes to max pot resistance, same as turning the pot to 0 or 10.

[AlNewman]

Good info, Sluckey, thanks.  When I look at schematics, it's always kind of confusing how and why they picture pots.  Once I figured out what CW means, it helped, but to know the benefits of wiring a certain configuration is beneficial.

[munkeyboy]

Good info, Sluckey, thanks.  When I look at schematics, it's always kind of confusing how and why they picture pots.  Once I figured out what CW means, it helped, but to know the benefits of wiring a certain configuration is beneficial.

   

Yeah, I always forget how pots on schematics work. I was confused by the schematic, but I just looked at a bunch of trainwreck/ac30 cut schem and layouts.  It seemed random.

[sluckey]

Once I figured out what CW means, it helped,

I worked with a lot of excellent schematics of military and FAA equipment throughout my career. Many of those schematics would use an arrow to indicate CW rotation of a pot. I find that very useful. Don't see that much in consumer electronics. Here's an example of what I'm talking about...

[Colas LeGrippa]

On my all original Traynor amplifier from '70 or so, the signal wires were not shielded. More: all the pots third legs were  tied to ground to their corresponding location. The noise was normal, not annoying and the amp coiuld be even used for recording.


If you're adding excessively high grid stoppers or small caps accross plate resistors or  put anything anywhere to eliminate weird noises  or oscillation, you,'re not curing the problem but masking it. And the.price to pay to rush to fix the damn amplifier is a loss of tone in general, high frequencies, definition and volume.


If you re working with known good tubes and sockets, you can do over your homeworks.


I would start by grounding correctly the preamp section and their  supplying capacitors. I bet you'll solve all the issues (if your solders are good too )


Colas

[munkeyboy]

I would start by grounding correctly the preamp section and their  supplying capacitors. I bet you'll solve all the issues (if your solders are good too )

When I started this thread, I was having an issue with oscillation.   I've corrected this now.  I reverted some changes to the grids to my original and reflowed my connections.   I think that corrected my issue (i think it was a loose HT wire to tube pin).

The issue I have now is 120hz hum after splitting the power supply ground.   This is an odd issue, so let me explain.  If I keep the rectifier board ground connected to the end of the preamp and power filter grounds, I have almost zero hum (no input, with input I have some hum).   If I split the rectifier board ground connection and move it to the PT bolt or the Choke bolt, it is horrible hum.  It doesn't make sense to me.  I don't mind keeping the connection as is, but this seems very odd.  Could there be a short in the PT or Choke?

Please see attached pic for a visual explanation.  I tried moving the highlighted rectifier ground wire around to the various ground locations.

[PRR]

Don't think "ground". A tranny bolt can not absorb infinite crap.

Think "return". You have lightly filtered (so some 120Hz) HIGH DC current through your heaters then meandering around the chassis past all your sensitive spots. No. Return the bottom of the heater string to the heater supply filter cap.

FWIW: I think "star ground" is more confusing than useful until you have a LOT of bitter insight.

[munkeyboy]

Don't think "ground". A tranny bolt can not absorb infinite crap.

Think "return". You have lightly filtered (so some 120Hz) HIGH DC current through your heaters then meandering around the chassis past all your sensitive spots. No. Return the bottom of the heater string to the heater supply filter cap.

FWIW: I think "star ground" is more confusing than useful until you have a LOT of bitter insight.

Ah.  Ok, so that is a long return.  Not as familiar with DC heaters, do I need to twist in-series DC wires, or just run a very long return wire along the chassis to the filter cap.

[munkeyboy]

Don't think "ground". A tranny bolt can not absorb infinite crap.

Think "return". You have lightly filtered (so some 120Hz) HIGH DC current through your heaters then meandering around the chassis past all your sensitive spots. No. Return the bottom of the heater string to the heater supply filter cap.

FWIW: I think "star ground" is more confusing than useful until you have a LOT of bitter insight.

So keep the Red highlighted connection as is (to PT Lug) and reroute the yellow highlighted HT Return like so?

[munkeyboy]

Well I pretty much ended up where I started except my hum issue is better.  Long story short, grounding wasn't the cause of the oscillation issue.   Matter fact, I don't think the symptom is actually oscillation, but vibrations in the cab.   I thought I had fixed it, but once the chassis was back in the cab, the noise came back.   Tilting the cab forward 30 degrees fixes it.  Any more than 30 degrees and it comes back.  Tilting it backwards makes it worse. I think the tube sockets move too much in their mounting brackets. IDK.

I now have one ground for the mains at the pt bolt and one ground for the rest of the amp starting at the input.   Zero hum without input, some hum with guitar plugged in.  Could be cable or guitar or something else.  I do occasionally get a pop or electrical noise that come and go, but sounds like environmental noise. 

Anyways here is my final layout and grounding scheme until I tackle guitar hum.