Thursday, November 12, 2009

Drawing up a component layout from a tube amp schematic

I'm building up a low wattage tube practice amp roughly
based on an old Gregory Gemini 700 / Mark 5.
Since the circuit is pretty simple, the process may help you to learn
how to translate a schematic diagram into an actual tube amp layout.
Here's a schematic for the No. 2 version of the Gregory:

So how do I get from that schematic to this:

It's not too difficult if you take it one step at a time.
I'll show you loosely how I approached it for this one.

First have a look at the schematic and try to divide it into functional blocks.
That will keep you from getting confused as you go along.
It'll also be much easier to troubleshoot if everything doesn't work on the first try.

Below I've boxed the preamp stage in red, the tremolo section in orange,
and the power and output section in black:

Now you need to figure out what the components are actually
going to be physically and electrically attached to.
I find that some kind of tagboard or turret board makes the planning
and subsequent wiring neat and easy to follow.

I picked this board from Antique Electronic Supply.

Now on a sheet of graph paper I drew in the components
for the individual amp stages in colors that match
the sections I indicated on the schematic.
Then I drew in all the connections that would need
to be made according to the schematic.

At this point check and double check you work.

This drawing is done imagining the tagboard oriented vertically.
Each of the rectangles relates to one resistor or capacitor in the circuit
and the lines represent connections between them.

I do this stuff pretty regularly, so you'll see I'm not super fussy about neatness or corrections.
You'd want to be a bit more careful yourself until you can rely on your instincts to catch errors:

There also components which will not be mounted on your board
(control pots, tube sockets, transformers etc).
I've been lazy here and only indicated the pots.
They're in green on my hand drawing and circled in green on the schematic below.
You should probably be careful to include them all if you haven't done this before.
Thoroughness will pay off in the end.

Now turn the layout drawing in the same direction as the tag board
and you're ready to start soldering:

I install all the components first and then move onto the connections.
Here are the caps and resistors installed on the board:

And here's the board with the electrical connection in place:

Now all that's left is to install the board and make the connections
to the chassis mounted components and fire it up to see if it works!

If you're paying very close attention, you'll notice that my layout
drawing doesn't match the schematic drawing exactly
(and the board doesn't exactly match the drawing).
I've made some changes as I been working on these amps
but I haven't drawn up a revised schematic yet.
The one here should serve well enough for the example though.


Anonymous said...

I noticed that the gregory was designed for 117v, my main power is going to be 240/250v 50hz, what should I change in the circuit? Sorry for the newbie question. Thanks.

akavalve said...

You'd need a step down transformer to give you 120 VAC. Not a big deal since this thing really should have an isolation transformer anyway. You can just sub a step down in instead and you still get the isolation.

A note though - this post was meant mostly to be illustrational, not instructive. So don't try building one from my drawing (or even from the schematic as even it doesn't actually show the whole circuit!)

I've also found them to be somewhat finicky circuits noise wise so it might be frustrating as a newbie project. If you're gung-ho to do it though, let me know and I'll try to provide enough detail.

And, as always, know how to be safe. Even a little amp like this can kill you if you don't take proper precautions.

Anonymous said...

Thanks for the info. If you have the time in the future, please do share a more detailed post with the complete circuit. I would love to start a project based on this design.

Anonymous said...

I'm interested in contacting you about some mods to my Champion 600. I can't find info on the blog--is your email posted somewhere? thanks.

akavalve said...

I believe it's in the "About Me" section.

But in case not...

Drop a line and we'll talk it through.

Anonymous said...

Thanks for the contact info--message sent.

esteman said...

hi, thanks for the info. I am going to modify an old japanese tape recorder amp that has these same tubes into a guitar amp. your comments about this mark v encourages me to do this one. i got a question: why are there 3 transformers ?. i know 1 isolation, 1 output. But the other??dominator

akavalve said...

Do you mean there a 3 transformers in the tape deck?

The third could be a choke (just 2 leads).

It could be for the bias oscillator circuit feeding the tape heads.

It's tough for me to say without seeing the schematic.

coolcool654 said...
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英文發音真難 said...
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Anonymous said...

thx u very much, i learn a lot

Daniel Dempsey said...

Hi, thanks for the great post! Did you use a 1:1 power transformer for this project? Is that what you'd recommend for AC/DC amps like this? Thanks so much!

Florian said...

Man, I read all your blog posts today and really learned a LOT! I'd say that the goal you described in the first post, wanting to offer good information for people just starting out working on tube amps, has definitely been achieved.

Still... I'm eager to know how the Champ 600 preamp bias posts continue AND I'd like to read more detail about your build of the Gregory amp. The schematic and the tubes that it uses look a bit out of the ordinary. Enough to learn there, I guess.

Anyway, I hope you'll find time/inspiration/whatever to continue your blog somewhere in the near future cause people are enjoying it!