Tube layout question

[Paul P]

I'm not sure how close tubes can be to each other and to transformers
and chokes without causing problems so I'd appreciate opinions on my
current design. In particular, I'm concerned that my rectifier tube might
be too close to my tremolo tubes.



Paul P

[MWJB]

In short, I think you're pushing your luck trying to cram all that into a 13" long chassis. Go longer.

Ideally, you want the hi voltage DC from the power supply coming in at one end of the amp and the preamp signal coming in from the other end. You don't want AC signal and DC criss crossing back and forth, which looks pretty inevitable in your layout.

Preamp tubes can be pretty close together, you could stagger them if you have enough depth in the chassis and are wiring point to point/terminal strips. If you're using a circuit board you will most likely end up with problems with wires to and from tubes criss-crossing accross the board. Ideally, you want the related components to line up with their tube.

Rectifier, power tubes & PT can all be relatively close, check out some 5E3 pics to give you an idea.

[Paul P]

In short, I think you're pushing your luck trying to cram all that into a 13" long chassis.

Well, there's more to this picture . I'm already up to a 17" x 10" chassis,
which is the biggest I can find (from Hammond). Here's the rest of it (it
still hasn't been 'wired', is missing some parts, and may contain errors) :



The view is from underneath inside the chassis. The transformers and tubes
are on the other side, on top, as in a hi-fi amp.

Starting at the top left the boards are preamp (normal), preamp ('vibrato'),
tremolo circuit. The small output board is for a single-ended output, to be
replaced with the pi-output board if and when I go to push-pull. The circuit
at bottom left is reverb.

There shouldn't be too much criss-crossing of wires since each tube is
close to its circuit.

Paul P

[Mark Ingram]

I'm not the one to advise you on layout issues, but I like the look of your diagrams. What did you use?


Mark

[stokes]

It may or may not make a difference,but I would switch the PT and OT so the OT is close to the power tubes and the PT is close to the rectifier.Maybe move the reverb tranny just north of where it is now as well.Just my 2 cents worth.

[Paul P]

Thanks for your help. Here is a new version :



I liked the idea of having the transformer in a corner because I felt the
chassis would carry the weight better but I agree that the output transformer
goes better there otherwise. I still have to add the bias supply and a few odds
and ends to the drawing.

I would have liked to have everything in a row, like in Fenders, but the
chassis would be close to 30 inches long (that tremolo circuit takes up a
lot of space) and it would probably have to be custom built. There was a
chassis builder on the net but he seems to have come upon some rather
hard times and is no longer in business.

I'm starting to think maybe I should piggy-back the reverb on top of the
tremolo circuit. It's pretty far away from its controls, which come after
the tremolo. There appears to be enough room to squeeze the five
tubes together. Something like the following, in which I've also re-arranged
the power supply and output section, trying to get the power transformer
as far away from the input tubes as possible :



As far as which drawing program I'm using, it's an ancient one that I first
bought for Windows 3.1 but is the best drawing program I've ever used,
called "Designer" by Micrografx. It's no longer supported but it still runs
fine on Windows XP. Drawing each part is what takes the most time but
I enjoy doing it. You could probably do similar with any good drawing
package.

The drawing is a lot clearer in real life, it seems to go out of focus once
I've exported an image and sized it down to fit here.

Paul P

[Paul P]

[edit : please ignore this post as the top of the chassis is backwards to what it's supposed
to be. This has been fixed a couple of posts further down in this thread. Sorry.]


Here is an updated version of what my amp looks like 'on paper' at the moment :




I think the only concerns at this point are the distance between the preamp
tubes (2 small tubes front right) and the power tubes nearby, if this is a
problem at all, and the fact that the power tubes are quite a ways from the
PI tube and its circuitry (front extreme left tube). The wires to the power tube
grids represent the only really long signal wires in the amp.

If anybody sees anything wrong with this design I'd appreciate hearing from
you as I'm about to 'wire' all this together.

Thanks,

Paul P

[Bruce / Mission Amps]

I don't understand why you have the PI so far from the power tubes and the critical, high gain preamp tubes sitting on top of the power tubes.
Throw in some novice wiring and or lead dressing errors, ... especially those long leads from the input jacks to the preamp tube's grids and those same preamp tubes sitting in front of the power tubes... that could be a recipe for a some serious oscillation issues and a fight you don't want.
I think you should swap the power and output transformers and flip the whole preamp section around with the PI tube in front of the power tubes... keeping your power transformer further from the preamp tubes is good.
Look up Faraday shielding... with a layout like this, you'll quite possibly need this info too. Many vintage guitar amp builders from the past who used unusual layouts needed to use Faraday shields to control the oscillations.
Check out the way a vintage Hiwatt chassis is wired, or for that matter look at the way a vintage VOX AC50 is done.... or even a simple 50 watt Marshall.

[Paul P]

Thanks for the comments Bruce. I'll need a bit more time to digest all that
you've provided but wanted to at least show you a possibly better version
that I just did after I realized that the top of my chassis was backwards
with respect to what was inside the chassis.

I've just put the PI in front of the output tubes between the preamp tubes.
But (ah ! that's what you meant by long leads between jacks and preamp
grids. Yes, that was the above mistake, the jacks and tubes are real close
now.)

So here's a hopefully better version :




Paul P

[Paul P]

I took a look at some Hiwatt's. Here's one :

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120191165965&indexURL=4#ebay photohosting

Does this mean that output tubes are more of a problem close to preamp
tubes than big transformers ? I thought the electromagnetic field put out by
the transformers was the biggest worry. What are output tubes generating ?

I could move my choke forward and stick my power tubes at the back like
the Hiwatts.

It seems that some Hiwatts, perhaps older ones, didn't have the shield :

http://amp8.com/guitar/hiwatt/jpeg/hiwatt-l10.jpg

or maybe it was removed at some point.

Paul P

[Steve Conner]

Coupling between tubes and transformers isn't a very strong effect that I know of. The main problem in tube circuits is capacitive coupling between unshielded wires, and between the plates of nearby tubes. The output tubes in particular have several hundred volts of signal frequency AC on their plates, that will couple capacitively out through the glass. Try holding a scope probe tip against the glass, or even near it, if you don't believe me...

Coupling between tubes isn't a problem if you use sockets with tube shields. They also stop your preamp tubes from falling out, so you might as well use them

Coupling between wires can be fixed by using screened cable in the offending places, though you might not want to use it everywhere, as the extra capacitance to ground might kill some treble and lead to a dull-sounding amp.

I successfully built a 50W high-gain Boogie-style amp with the output transformer about 1" away from the first preamp tube. I just used shielding to make sure that the preamp tube and its wiring couldn't "see" the wires coming out of the O.T., or the power tube plates.

Ever since, I've put my O.T. at the preamp end, because I think it's more important to keep the P.T. and its associated wiring away from the O.T. and the first gain stages.

P.S. That shield in the Hiwatt is probably to keep the heat from the output tubes away from the capacitors. After all, it has shields on the preamp tubes.

[Paul P]

Coupling between tubes isn't a problem if you use sockets with tube shields. They also stop your preamp tubes from falling out, so you might as well use them

This is reassuring. I was planning to shield all my preamp tubes.

Thanks for the reply. I think I'm close to the point where I'll just the build
the amp and find out if it works or not. I can always change the chassis later.

Paul P

[MWJB]

It may take you 2 or 3 days to build it...2-3 years to debug and lose the oscillations & unwanted noises.

You still have the PI between the preamp tubes, to me this looks like you are asking for trouble.

The reverb comes before the vibrato in the signal chain, this ought to be reflected in your physical layout.

Bear in mind that many big amp manufacturers had test shops. If they had a "quirk" show up on one in >50 amps, then they had the manpower, experience & facility in place to make do. You're just building 1 amp, you want it to work right, you don't want a 100% failure rate.

I don't like having the OT at the input end of the amp, but hey a lot of successful designs have done this. So, if you are persisting with this chassis (I still think you would be better off with a longer one), move your OT right up to the input end of the chassis, mounted as close to the rear as possible.

Rotate your choke thru 90deg, it might even come forward. Move your power tubes to the rear of the chassis, as close to the rectifier/PT end of the amp. Rearrange your preamp, reverb, vibrato & PI tubes so that they run in that order down the chassis.

[Paul P]

Thanks MWJB. I really appreciate the input from yourself and others.

I've re-arranged things in two steps. The first was to get the phase inverter
out of there and send the power tubes to the back :



This puts the PI really far away so I switched the power and output
transformers which gives :



(It's starting to look like a Hiwatt )

I know that Fender puts the reverb in front of the tremolo (vibrato) but I'd
like to try it the other way around. I want to sound like I'm playing a
tremolo'd instrument in a church and not sound like an instrument in a church
that has been tremolo'd. I can always switch them later.

I notice that Fender put the reverb transformer at a 45 degree angle
(at least in the Deluxe Reverb). Was that for magnetic field purposes ?

Paul P

[MWJB]

Hi Paul,

The problem now is that you have the PT & inputs at the same end of the amp...this is generally a bad idea. Could you not bunch up the rectifier & power tubes some, there is still probably more space than necessary.

Having the PI so far from the power tubes is not great as you have gathered. Reverting to the first layout in your last post would be better. Maybe moving the choke to a more central position so that you can bring the PI & 6V6s closer together. In fact, if space is an issue I would be tempted to dump the choke altogether (use a 2.7K 5W resistor instead). The benefits of a choke may be outweighed by the layout difficulties.

My reservations about trem then reverb are that regeneration of the trem by the reverb may cause problems?