Chassis Design

[Wilder Amplification]

Most of the amps I've worked on and built have had the OT right in the middle of the chassis perpendicular to the PT. As I'm drawing up the blueprints for the dual rail amp I'm building, I'm thinking about placing the OT clear on the opposite end of the chassis for weight/balance purposes plus I'd like to make the front of the cab see-thru so for aesthetics I don't want any transformers obstructing the view of the valves.

The question I have is with the OT clear on the input end of the chassis if there's a risk of oscillation issues with the OT being close to the input of the circuit. If so, is there a certain way to lead dress the OT wires to prevent the chances of this?

[Albert Kreuzer]

It has been done.
Hiwatt and Ampeg Portaflex come to mind.
Just keep the wires separated from the input stage.

[Chuck H]

Don't forget Soldano. They did it with a high gainer. May be worth checking out their design for tips/clues about how to keep it from oscillating.

I would think that a steel (rather than aluminum) chassis would help since it provides better shielding against EMF. And run leads OUTSIDE the chassis enclosure. Never tried it because it always seemed like a cumbersome design hurdle.

[Steve Conner]

I did it this way on two amps, both of which have a high gain channel.

Magnetic induction doesn't seem to be a problem, the main issue is capacitive coupling. So all you need to do is run the primary leads outside the chassis, or put them inside a tube of copper braid salvaged from some old coax cable, or make a shield out of some copperclad PCB stock, or simply keep them far away from the first few stages. Same applies to a lesser extent with the secondary leads and speaker jack.

Remember that any exposed windings will radiate just as bad as the primary wires. If the OT is a drop-through type, you may have to put a bell on the bottom inside the chassis.

[Hardtailed]

Soldano SLO is a classic example, but most brand new amp (including Marshalls) are this way. Of course, when it's PCB you can do a lot of things with whatever space you have available, but the SLO was successfully replicated with turret board construction. The SLO also has the preamp tubes up front and the power tubes in the back, allows a ridiculously short connection from the input jack to the first tube

[defaced]

I build my amps this way. I've both shielded the OT plate wires and left them out in the open and never had a probelm with either. I'll have to look at exactly how I did the lead dress when I didn't shield them, but I think I just ran them along the back corner of the amp. These were built with top mount OTs, 4 gain stage preamps, steel chassis and PT and OT 90 deg to each other.

[diagrammatiks]

according to morgan jones in building valve amps the ot should never be in the middle anyway. that's pretty much the worst place for it.

[clyde1]

I've run them outside or inside fixed flat to the chassis under the elevated tag board.

[redelephant]

Like this?
PT and choke on one side, OT on the other.
I have this layout for the same reasons you mentioned. No need for running wiring on the outside. The output tubes should ideally be as close as possible to the OT b/c it is in the plate to OT primary wiring you have potential transmission of unwanted signal. I have the output tubes in the rear of the chassis, behind the preamp section which is placed on the front of the chassis near the input. Never had any problems even with 4-stage high gain preamps.

[GregS]

according to morgan jones in building valve amps the ot should never be in the middle anyway. that's pretty much the worst place for it.

What is the reason given for this?

I would think that what is the best or worst location would vary somewhat depending on the rest of the layout.

[Steve Conner]

I've got a copy of the book, and I don't remember reading that in it. Just the usual about how it should be kept far away from the PT, rotated through 90 degrees, etc.

[Wilder Amplification]

Yep...was already planning on the perpendicular orientation of the OT relative to the PT. Sounds like many have done it a variety of different ways with no issues which is good news.

I've also thought about twisting the primary and secondary wires (each twisted with their own only) to cancel any potential EMI into adjacent circuitry as well. In theory this should help as well (at least that's what my brain is telling me but I could be wrong).

[Chuck H]

I've also thought about twisting the primary and secondary wires (each twisted with their own only) to cancel any potential EMI into adjacent circuitry as well. In theory this should help as well (at least that's what my brain is telling me but I could be wrong).

I started doing that after it fixed an oscillation I had on a modded amp. After raising the gain the proximity of the OT primary leads caused instability. I twisted the primary leads and since most OT secondaries have multiple taps I braided them tightly. It looks nice and neat too. I do it on all my builds now. Of course, if your running your leads through a copper braid or tube it hardly matters. I have one design with a run of filament lead that exits the chassis and re enters right at V1. I wouldn't be comfortable running HV leads outside the chassis in anything I was going to sell. You can never be sure what an amp owner will do. It can really leave you scratching your head wondering what was going through their mind. So, safety first.

[Gregg]

I build my amps this way.

+1.
All amps I've built so far (and they are all high gainers) have the transformers on the opposite sides. For some reason it never felt right to me to have them in the middle Marshall style.
Sometimes oscillations are possible but just in case I usually use a 15cm shield connected to the chassis in the middle between input and output. Also a tube shield/retainer for the first tube which is close to the OT can help.

[diagrammatiks]

My bad I interpreted it a bit wrong

Here's what Morgan says

"Because coupling decays with the cube of distance [1], as the distance between offending items is increased, the interference falls away rapidly. However, simply increasing the gap between two adjacent transformers from 6 to 25 mm does not materially reduce the interference, because the transformers are typically 75mm cubes, and the spacing that applies is the distance between centres, which has only changed from 81 to 100 mm, resulting in only 5.5 dB of theoretical improvement. In practice, when the transformers are this close, coupling does not obey the cube law very well because the transformers do not see each other as point sources, so a 3dB reduction, or less, is more likely."

Which I took to mean that simply moving them further away would not be sufficient...they would also have to be turned. I suppose that it could also mean that a few mm's of distance does not matter. However, 3db seems better then nothing.

Hey says this later though on page 24

"We should aim to attack all parts of this equation to minimise capacitance. Reducing plate area means keeping wires short and crossing them at right angles, whilst increasing plate dis- tance means keeping parallel wires apart. In terms of chassis layout, this means that the output valves should be reasonably close to the output transformer and the driver circuitry should be reasonably close to the output valves."

So it seems that in order of importance, turning the transformers so that they are angled in the least noisy way is primary...

How close the preamp tubes are to the ot seems to be of secondary importance.