18w lite P2P layout design.

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
Ronno25
Unrated
Posts: 2
Joined: Sun 03/14/21 10:45 pm

18w lite P2P layout design.

Post by Ronno25 »

Hey all - new poster here. But I've been reading this forum for a while now and have gathered some great information from you all.

I'm planning an 18w lite build. I'll be using a chassis that already has a PT and OPT, a few tube sockets, and an appropriate place for a cap can. It's small at about 9" by 4.75". I've worked up a layout diagram in DIYLC and my layout has been influenced by the chassis I'm using.

I'd sure appreciate some extra eyes on the layout. I'm most specifically concerned about avoiding noise issues from lead dress and component placement such as hum or parasitic oscillation but if there are other issues such as missing components or wiring mistakes that you see do let me know.

Also, I wanted to do all grounds in two places only: near PT and near preamp. But I couldn't really avoid having a third ground between the two el84 power tubes. Alternatively, I could route that ground with a longish wire to the preamp ground point. Thoughts on what the better option might be with regard to lowest noise?

I mostly followed the Mark Huss' Lite IIb schematic but deviated in small ways that are pretty obvious with component value and output valve screen resistors, etc.


Thank you in advance for any help here.
18w lite layout.pdf
18wattLite2b.gif
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3954
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: 18w lite P2P layout design.

Post by JMPGuitars »

I don't have time right now to go in depth on the component values, but there's a few 'best practices' you should be following. First, read the grounding scheme thread in my signature. Second, use isolated Cliff style jacks. Finally, you're asking for trouble wiring your heaters that way, especially with elongating them around that terminal strip.

With a PTP build, there's little more important than the quality of the lead dress and soldering. So you might want to also watch the soldering videos.

Thanks,
Josh
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

Ronno25
Unrated
Posts: 2
Joined: Sun 03/14/21 10:45 pm

Re: 18w lite P2P layout design.

Post by Ronno25 »

Thanks for the tips. I redesigned to ground the preamp at the input jack.

What's the decision making behind grounding the output tube grid leak resistors with the power amp or the preamp? I see many designs where they are grounded with the preamp, not the power amp.
18w lite.pdf
You do not have the required permissions to view the files attached to this post.
0 x

ewizard
Unrated
Posts: 11
Joined: Sun 05/14/17 4:19 pm

Re: 18w lite P2P layout design.

Post by ewizard »

It has been a while since this post was made, so this may be irrelevant, but just to add some rhetoric, I will give you some data.

In most cases, especially with simpler circuits, you can get away with some bad practices on grounding schemes and avoid noise. There are best practices and there are practical adaptations that are just as good, and then there are ones that don't work at all.

Technically speaking, grounding EVERYTHING at the input is best, BUT has its challenges. I don't like grounding at the input myself. I tend to go for a ground buss approach where all the grounds cascade and tie to ground at only ONE point with EVERYTHING in the amp except the safety ground. Amps with center taps tend to work better with this approach. It is NOT good practice to place center taps into ground schemes that cascade to the input of the amplifier. Since having center taps grounded near the transformer and the rest of the grounds going to the input breaks away from the idea of ONE grounding location, that is why I prefer to cascade my grounds on a buss.

Another very good and technically good way, is to have multiple star ground points. Each section of the amplifier generally has its own node ( dropping resistor and decoupling cap ). If you keep these nodes near each amplifier section and make each section its own star ground and then tie this star ground to the chassis, you can generally avoid problems so long as each connection to the chassis descends in path length to the safety ground in the amp and is always longer and longer in path length from the safety ground to the input. I.E. you ground the input at the chassis with all the components in the V1 circuit going to that point. The next node off of V2 and all its circuitry is grounded to the chassis at another point in the chassis that is closer to the safety ground than V1's circuitry, but further away than anything in the next tube's circuitry. Since you are using the chassis as a " Buss " you simply try and make it so all path lengths descend in length to the safety ground. If path 2 is longer than path 1, you can end up with a loop etc. etc.

This is why cap cans are not as desired. They generally share a ground lug for multiple sections, adding ground loop potential. This is why the power section is usually the only nodes that use a cap can, the sections have enough separation and are in the section of the amp that is least susceptible to noise. The V1 input tube is generally the only section of the amp that is susceptible to noise and requires the most stringent noise abatement. If you get that section right, noise elsewhere generally becomes of little concern assuming you follow the practice applied to V1. There really isn't a best practice so to speak, so long as there is no noise. If you follow the best practices within reason, you just tend to get away with it is all. In amps that are much more complicated and have many circuits that tie back and forth, that is when it becomes critical.
0 x

Post Reply