Lead dress twisting wires

[dscottguitars]

Hi,  I would like to confirm a couple things about lead dress:

Is it okay to twist the 5v wires from the tube rectifier with the high voltage wires?

Is it okay to twist ground wires when possible?

Is it okay to twist the cathode wires from the power tubes together?

I've attached a drawing of my layout but it is not complete.  It only shows the signal path and connections to the tube sockets.

[FranciscoPerez]

Hi,
I don't see any benefits, but yes I'd say it's ok!

[sluckey]

Is it okay to twist the 5v wires from the tube rectifier with the high voltage wires?   Yes

Is it okay to twist ground wires when possible?   Yes

Is it okay to twist the cathode wires from the power tubes together?   Yes

But why? There's no electrical reason to do so. It's a personal choice, but I think those wires look better/neater if not twisted.

[dscottguitars]

I thought it would look cleaner if the wires were twisted.  Every amp that I've seen in this forum does something like that.  That's the only reason, is to group them together so they're not so random and messy looking.

thanks...

[sluckey]

I've never twisted those particular wires in any of my builds.

[HotBluePlates]

If you're worried about lead dress, it would seem you would want to minimize the lead length of all grid wires.

The input 12AX7 looks okay, but rotating the other preamp tubes clockwise by about 90 degrees, and the 6L6's counter-clockwise about 90 degrees would reduce grid wire length all around.

Is this a known-good layout from an outside source? It appears to be too focused on compressing everything into the smallest space possible. That has much bigger implications for "lead dress" than twisted wires.

For example, the 82kΩ phase inverter plate load is folded over next to the 820kΩ grid resistors for the phase inverter. That places a relatively high-level output right next to its (high impedance) input, which introduces risk of oscillation in that stage.

I couldn't really check over everything, as not all connections are shown; however, it looks like you're short a ground terminal to use with your bias test points.

[Willabe]

The bias/test points are wrong.

You can put the 2 bias K 1R's across the test points jacks and then just run a single wire back to it's ground star point.


           Brad      

[Willabe]

Here's a drawing I did but haven't built yet.

1st bias cap gets grounded with the B+ plate supple node filter caps ground and the 2nd bias cap gets grounded with the B+ screen supply node filter cap.

The - D - on a few of the R's is a note to myself for Dale metal film R's as there leads are thicker.

         Brad    

[HotBluePlates]

The bias/test points are wrong.

Well, he's got points for the 1Ω and for a B+ measurement (so the chassis doesn't have to be pulled). That what I meant when I said he was lacking a ground test point.

[dscottguitars]

The ground for the test points are not drawn it there, they connect to a network that switches from cathode to fixed bias and that's why I didn't include it.  I may do what Brad suggested with the 1 ohm resistors, that looks like a better plan.  I don't know what you mean by the 82K resistor on the PI though.  Is it because it's close to the input resistors, or the wires?  I took some pics of what I've soldered so far.  

It's my own layout, not from something else.  It's just a plan to guide me and keep things as neat as possible.  It's tight because the printout for the board was not to scale, I thought I had a 14.5 inch board, but it came out as 12.5 inches.  And, this drawing is not 100% to scale, close but some things are estimates.  There's about an inch from the 82K resistor to the 820K ones.  

[Willabe]

Oh he wants a B+ test point so not to have to pull the chassis. I'm still not sold on that idea but it's his build.

I'd still move those 1 ohm R's off the board and put them on the jacks when he adds a ground jack. Less wires running back and forth.


           Brad    

[dscottguitars]

Ground test point is the bolts on the chassis or the chassis itself, it's anodized aluminum.  They work just fine.

[Willabe]

Ground test point is the bolts on the chassis or the chassis itself, it's anodized aluminum.  They work just fine.

Is this going to be a combo or a head?

I'm asking because if the idea is to bias up tubes without pulling the chassis how are you going to be able to reach the ground point/bolt without pulling the back panel off or if a combo reaching up around and behind/under the power/rectifier tubes? Seems to defeat the purpose.


           Brad   

[HotBluePlates]

Ground test point is the bolts on the chassis or the chassis itself, it's anodized aluminum.  They work just fine.

If you already have 3 tip jacks, why not spring for the 4th and have the scheme make sense?

It's my own layout ... keep things as neat as possible. ...  There's about an inch from the 82K resistor to the 820K ones. 

Remember the hum issues in your other build?

So this won't cause hum, but here's a simple "lead dress rule":
  - Keep outputs away from inputs.
  - Keep high level signal wiring away from high impedance circuit points (because the high-level wiring induces a signal in the high-impedance wiring).

Consider alternate ways to do what you're trying to do. When I built my 25L15, I used effectively a single row of turrets on either side of the row of tube sockets, wires runnign from turte to socket pin, and parts spanning the gap between turret rows over top of sockets. This put everything in a compact package, but still allowed for good lead dress, and such tricks as placing the heater wiring 90-degrees to all signal wiring (even 90 degrees to socket pins, if you look closely).

To give a sense of scale, it's only 4 inches from the top panel to the bottom panel in that preamp chassis; even less if you deduct the size of the lip on the chassis edge. Yet there's still plenty of room for the entire preamp and phase inverter (and power supply parts, but that's beside the point).

I know we tend to stick to the 3-1/8" model for a turret board, but sometimes it makes sense to do things differently if you don't also copy Hoffman's layout approach.

[dscottguitars]

Hey Brad, it's a head.  The entire chassis and any bolt on the socket or panel is ground.  I don't really want to touch the chassis and scratch it so I just touch the bolt holding sockets in place or the panels together.

I'm still a bit worried about the high level signal and the high impedance input.  Do I not have enough space between the two in the phase inverter?  I can move those resistors back a bit if necessary.

Why not put a ground test point?  I didn't think of it. And since the panels are done I can't label it.  It's really not necessary considering anywhere you touch on the chassis is a ground test point.  Maybe the next amp...  I have my panels made by Front Panel Express, the front, top and back.  I bolt them together using 3/4" aluminum angle pieces, attach sides and put a piece of sheet metal on the wood bottom to enclose the shield.

[HotBluePlates]

I'm still a bit worried about the high level signal and the high impedance input.  Do I not have enough space between the two in the phase inverter?  I can move those resistors back a bit if necessary.

I don't know. There's nothing written anywhere about what signal level & what distance are okay. The issue would be a bigger problem if it were the phase inverter output and an earlier gain stage input (then there's more gain in the loop formed by the output/input).

Since you already seem to have the thing built, try it and see what happens. If you get instability, or hash not resolved by any other means, it might be the phase inverter.

... And since the panels are done I can't label it.  It's really not necessary considering anywhere you touch on the chassis is a ground test point.  ...

Is that anodized aluminum, or black paint? Obviously, if it were painted, you wouldn't get conductivity through the paint.

I just noticed something else though... Are though banana jacks you're using for the test points? Not meter tip jacks? If so, the B+ jack is a hazard.

I argued for using a tip jack for B+ measurement in another thread. But that assumed the small opening of a tip jack, and the difficulty in accidentally poking something in there which could be a short. If you're using banana jacks (with a much bigger opening), the B+ jack seems like a very bad idea.

[dscottguitars]

It's anodize aluminum.  And yes those are banana jacks    I pissed now because I got them as test points from mouser, thinking that's what they were and what to use.  Thanks for letting me know.  I'm trying to get parts in bulk and less price but have you any idea how hard it is to find tip jacks online?  To buy them from this site I pay too much for shipping on top of it.  I guess I'll have to eat it this time, dammit!

[Ed_Chambley]

I have actually seen some test points made from ceramic and metal.  Very tough looking.  Anyone know where these are sourced?  Just thought of it, not intending to hijack.

[HotBluePlates]

And yes those are banana jacks    I pissed now because I got them as test points from mouser, thinking that's what they were and what to use. 

You can use banana jacks, you just have to use cables that have banana plugs on both ends to run from the amp to your meter. With tip jacks, you could just plug in your meter probes.

Still, the banana jack for B+ seems pretty large.

Mouser also has tip jacks. Yes, you can save a little over buying from Doug if you go to a big distributor like Mouser. But to find the 80 things you need, you have to search through 180,000 things you don't need. You also have to pay very close attention to data sheets posted with each item and do a lot of legwork to figure out what you really need, because you usually can't count on the pictures or drawings representing exactly what you're buying.

Unless you going into manufacturing (buying a couple-hundred of each resistor & cap), I think you'll find the savings swallowed up by the 2 or 3 follow-on orders you're gonna make as you find you forgot a couple items or ordered the wrong thing...