# First crack at P2P layout



## Todd MacCulloch (Mar 8, 2018)

I am first time builder.

I am building a Two-Stroke based on Dave Hunter’s book. 

The Tube Amp Network hosts a copy of the layout and the schematic

Inspired by Rob Robinette's site and using DIY Layout Creator I came up with a layout for my *Two Stroke* Amp Head.










I plan to use a 12 X 8 X 2 Hammond aluminum chassis.

The black and grey twisted pairs represent a *shielded cable*.

I am using a separate _preamp bus_ and _power-amp bus_.

The main modifications from the original schematic are:

* Mod-1*. Added a 470Ω _screen resistor_ to V2 (see forum discussion), in my schematic this is R15.

*Mod-2*. Added some SPDT switches to pick 3 of the 4 output transformer impedances.

*Mod-3*. Based on Uncle Doug's videos, some helpful forum members, and Merlin's book , the 68KΩ _grid stopper_ on V1 should be smaller. Fender amps would have the equivalent of 34KΩ and Merlin suggests that 10KΩ + 680pF capacitor will be quieter. I have not updated the layout yet but I am considering it.


*Questions:*

What would you change?
The layout is roughly scaled so I think the chassis will be big enough.
How about the tube layout?
The transformers position?
The filament wiring?

Thanks


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## keto (May 23, 2006)

I didn't walk it step by step, and I haven't done PTP. But I walked thru the Xformers, the power and outputs, and those all make sense to me...which isn't proof of any sort, but I did build a couple amps, a kit and a full parts acquired turret board build.


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## greco (Jul 15, 2007)

I can't help with your questions but am writing to ask if you will consider a build thread with some pics. Thanks.



Todd MacCulloch said:


> The layout is roughly scaled so I think the chassis will be big enough.


My only comment would be to possibly reconsider going to a chassis with more internal real estate. Working inside these smaller dimensioned ones seems to be a typical and logical frustration. In addition, you are not building an eyelet or tag board outside the chassis and then mounting it into place. Soldering and working in cramped spaces is no fun and might take away from the enjoyment of the build. Just a thought.


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## modern (Mar 2, 2007)

I built one of these using a Hammond chassis that was about the same size Fender used for the Champ and a 5F2-A board. I was pretty happy with how it turned out but I am planning to put a choke in it.

The only point to point amp I own is a Pepco that is very similar to a Princeton and the chassis is quite large considering how simple the circuit it. I don't think you'll enjoy working in a small chassis and it will make it difficult to modify anything in it later.

The fuse and power switch should be on the hot black wire not the white neutral.


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## mhammer (Nov 30, 2007)

I have absolutely no basis for offering comment on layout. But I will note in passing that, in his book on Fender and Marshall amps, J.C. Maillet (vivaanalog) provides a list of resistances between what is point A and C in the schematic you link to, for a range of amplifiers. His conclusion is that the higher the total resistance between transformer and preamp section, the "raspier" the amp's tone. Where the manufacturer opted to use chokes instead of resistor/capacitor combos, that lowers the total resistance even further. So J.C.'s urge seems to align with Merlin's recommendation to reduce R2.

I will suggest a few things: 

- instead of a switch and R13/R14, consider tying the outside lugs of a 25k-100k linear pot directly to C8/C9, and the pot wiper to ground, to let you dial in degrees of frequency boost. Note that bypassing R6 with a cap provides a boost to AC, for that range of frequency content dictated by the value of the bypass cap. Smaller cap values move the boost point higher up the frequency spectrum. The degree of boost depends on the value of the resistance between the cap and ground. So when that resistance is noticeably greater than R6, there is no boost. Ideally, one would use a W-taper pot for this (log in each direction from the mid-point), but those may be difficult to get.

- the circuit is pretty darn close to my 5F2-A Princeton, which I've tinkered with over the years. R12 provides the negative post-transformer feedback to keep it cleaner, in effect becoming a sort of fixed presence control. You can vary the degree of negative feedback and "presence" by inserting a 5k pot and .1uf/200V cap in series, and placed in parallel with R7. In my own case, I simply experimented with lifting the feedback resistor, since I didn't want to start drilling holes in (what was then) a 50 year-old chassis. It gave ice-pick-through-the-forehead tones. In your case, you're not defacing a relic. Plus, you'd also be machining an aluminum box, which is a LOT easier.

Note that the Tone control works for treble-*cut* at ALL Volume settings, but the treble-_boost_/brite function only works at low Volume settings. C4 functions exactly like the compensating cap on a Tele or Strat volume pot. Of course, we tend to leave guitar volume controls up full most of the time, while we leave amp volume below 10 (12) most of the time.


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## Todd MacCulloch (Mar 8, 2018)

*1. Chasiss Size*
I know - a bigger chassis makes more sense, but I am soooooo drawn to the Greer and Benson heads.

However, when I sit back and think about it:

This is my first build and it's easier to work on a bigger chassis.
This is kind of hot-rod, experimental amp - it should be a platform to try ideas like @mhammer suggestions. Bigger chassis gives you more options.
The next chassis size up is not too big and not more expensive 16 x 8 x 2.
I'll play around with a layout 16 x 8 X 2.
It's also easy (and fun and cheap) to cut out some cardboard templates and try some test assemblies ...
Plus my next build could be a smaller chassis ;-)

*2. Build Thread*
You are are right. I was planning on starting one after I get some of the up front decisions made. I am also going to capture some of the details references in blog and I will share links to that in the build thread.

I will link the build thread to this post and others where we discussed some of the options.

(from the future) here is the build thread


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## oldjoat (Apr 4, 2019)

when doing p2p , leave a small bump / hump in the cap wires ----^----[]----^---
electrolytic caps' construction have a small contact point inside , 
after many heat / cool cycles and tight wires, the inner contact pulls away
you then get internal arcing / buzzing and extra noise to track down.


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## mhammer (Nov 30, 2007)

Interesting tip. That's a bit like the small spaces left between sections of a bridge/overpass to allow for expansion during hot weather.


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## jb welder (Sep 14, 2010)

Just to nitpick a bit about the schematic (I know it's not your drawing) in case anyone is scratching their head looking at it.
L1 is shown as shorted out, there should not be a connection where the 2 lines cross. The 5y3 is drawn wrong, the 2 sides of the filament should connect to L1, the cathode is not usually shown on rectifier tubes.
And, as @modern pointed out about the layout drawing, it is important that the fuse and switch are on the 'hot' side (black). The neutral (white) should go straight to the power transformer.


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## Todd MacCulloch (Mar 8, 2018)

oldjoat said:


> when doing p2p , leave a small bump / hump in the cap wires ----^----[]----^---
> electrolytic caps' construction have a small contact point inside ,
> after many heat / cool cycles and tight wires, the inner contact pulls away
> you then get internal arcing / buzzing and extra noise to track down.


I read something about that in one of the NASA soldering guides. This site also has pictures illustrating similar ideas

Mounting Guidelines: PCB Board Design and Handling | doEEEt.com


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## PTWamps (Aug 5, 2016)

If you are going with the single input jack, reduce the 68K grid stopper resistor (R4) to 33K. That will make your input act like the "Hi" input on a Fendery type amp and give you a little more volume at the preamp stage. In a dual input amp (like a Fender Deluxe or Bassman, etc.), the two 68Ks operate in parallel on Input 1, making approx. 34K (33 K is just the standard value you will find available in modern carbon film resistors). Only Input 2 (the low impedence one, rarely used by most players) has 68K on the grid.

Another thing, on top of jb welder's comments about proper AC wiring, is to put the safety ground (green) on it's own dedicated bolt with a ring terminal and lock washers on both sides. This is standard practice (and legally required in some territories) and ensures that the safety can never jiggle loose through transformer vibration, etc.


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## oldjoat (Apr 4, 2019)

(x2) go through the trouble of a 3 prong power cord... ( not a 2 wire , just in case you're tempted ) 

never use cheaters ( that bypass the 3rd prong )


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## loudtubeamps (Feb 2, 2012)

Power and Output transformer orientation is at the top of my list for a quiet-"hum free" build. I use a pair of headphones plugged into the speaker jack to place the transformers. Info on "how to" is out there. Also keep primary leads and B+ voltage away from the audio stages, especially the first stages...whenever possible. I like to use shielded cable throughout the entire audio path.


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## Todd MacCulloch (Mar 8, 2018)

The fun is continuing on the build thread https://guitarscanada.com/index.php?threads/p2p-2-stroke-amp-head.239702/


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## DavidP (Mar 7, 2006)

Try to keep those heater wires away from as much as possible from the circuit components; right down flat on the chassis is tweed-era vs raised along the run of tubes is blackface-era format. Also if your other wires must crossover each other, try to do the crossover at right angles to wrt each other. That's my $.02 at this point.


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