Saturday, 16 December 2017

EL84 Amp II: progress post 1

Progress is happening with the new amplifier... this design is more modular as I have decided to design standard boards for tone controls, headphone output, and phono RIAA.
Having standard boards for these means I can easily accommodate future builds, and provide a "menu" of sorts.

The process has not been entirely smooth sailing, owing to the somewhat hit-and-miss nature of home PCB fabrication. Until now, my method has been to print the PCB design onto an iron-on transfer which then gets pressed onto the board (and then touched up with the etch resist pen) before going into the etchant.

This process has been unreliable and time consuming, and expensive, owing to the high reject rate. So a new technique was called for.

I've decided to move to a photosensitive board workflow. The design is printed onto transparency, which is then plaed over a light-sensitive board and exposed under a UV light, thereafter a two-step chemical process: Developing then Etching.

The first board I designed for this project is the tone control. This is using the same circuit as the previous project, except I had two changes:
  1. I needed to reduce the size of the board 
  2. I needed to put the tubes on the copper side of the board
So, I re-designed it to be 120mm X 65mm (down from 150 X 75) and attempted to fabricate the board... with less than spectacular results

Yeah. Not enough light. 

This board failed because I did not expose the photosensitive layer sufficiently.

Lesson learned, I did a second attempt, which looks much better. So I went ahead and drilled and stuffed it.


There are four topside wire links on this board. I always try to design with as few of those as possible. It's a challenge!

The copper looks a bit more messy than I'd like because the balance of exposure and development and etching was still not quite right, but this board is usable at least.

The tubes are on the copper side because of the customer's preferred aesthetic of having the tubes visible. This design will be applied to the other boards in this amp as well.

In the process, I have become a lot more familiar with the operation of my PCB software: namely DesignSpark from RS. Also its quirks and foibles, such as less-than-ideal behaviour when moving things around, and its ability to have "invisible" track that isn't visible in design but is when you print. As a result of this, the board above needs to have one track cut with the dremel and re-routed with a short jumper on the track side. Yeah I hate doing that!

Lesson: Inspect the board VERY carefully in print-preview before fabrication.

Or, to use an appropriate engineering axiom: Measure Twice, Cut Once!

I also built the bias boards for the EL84s. Owing to the amount of heat these produce, I am not mounting them on boards, but the voltage divider and potentiometers for the negative bias voltage, and the cathode shunt, can be put on a board. So drawing on my earlier design, these are the bias boards, made using the same technique:

Next up: A two-triode RIAA stage, I'm planning this on a board on 100 X 65mm.

There's a reason I want these boards as small as I can get them: The size of the chassis

Internal Dimensions 300 X 225mm

This chassis is going to represent a challenge to fit everything into it... this design will have 13 tubes: The RIAA stage, tone controls, headphone stage, as well as the amplifier itself. And size is a consideration since it will be packed up and sent overseas when it's finished.

Next update when I have more boards to show...

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