Tube Phono Equaliser

“Scratch”

Page 3, Building Scratch

Last updated December 27, 2001

Preparations

I started by taking all pieces and positioning them in the case, just to see how we were doing with the available space.

Not that I would not sooner or later hit myself over the head because I forgot to reserve space something or for taking too much unnecessary space at one place so that the last 20% of scratch would need to be squeezed in 1% of the footprint.

As we say in Dutch: Experience is the best teacher. Unfortunately, I cannot start with my 5^th project, I have to do the first 4 also..

After trying several lay-outs for the components, I finally chose a lay-out where the power supply would fill up one part of the housing and the amplifier part with the tubes the rest.

I will mount the tubes using an aluminum L frame, so that soldering all connections to the sockets will be easy. I regretted that choice just a little after I finished making the three holes for the tubes in the aluminum frame. I promise myself that I will buy myself some special tools for this purpose for X-mas.

The one exception is the rectifier. I could not justify it taking so much space in horizontal position so I made something out of two pieces of aluminum L-frames. Even in vertical position there is enough headroom left for the rectifier and I’m not really worried about temperature problems.

The transformer and the coils are connected to the case using 3-5mm rubber rings. Our local hobby shop had a case full of them behind the counter and these rings are just terrific. For this job.

On the picture on the right the power supply section is almost finished.  The rectifier is shown close to the mains transformer. I had the intention of having very short AC lines. In the front a little (and only) print takes care of the 12.6 V heating for the three tubes. ON the picture it clearly shows that I did not provide any additional cooling for the voltage regulator. Not smart! Around the tubes I placed some soldering braces. Helpful, but more than twice the amount needed.

On the left the amplifier section is shown.  I anticipated far more soldering connections. Well, better safe than sorry. But to my surprise most connections are connections to mass anyway. Hmmm.

The two yellow Jensen caps of 0.33uF are directly connected to both the tubes and the cinch output connector. I’m still not convinced that this is the best solution, but the signal path is definitely short now. For the rest, I built Left and Right channel symmetrical: The 5751 tube is in the middle and on the left side of the picture the right audio channel is shown and on the right side the left channel.

Although I do not expect that something terrible might happen to scratch as long s I do not shake it in upside down position, I will position/fixate the two Black Gates capacitors with a little silicon kit.

RIAA network	EZ80 Tube Rectifier

Internals

On the a picture of Scratch when I thought it was finished. That means: Scratch worked and  it and me needed some time to prove that it would continue working. Therefore I connected a multi-meter to the B+ and the heating circuitry for a while. Good thinking, because after 10 minutes or so scratch begun loose power and ceased work completely.

With the meter attached debugging was easy: The heating circuitry of Scratch was losing power after a while. This was due to the fact that I did not supply enough cooling to the voltage regulator 7812.

An additional piece of aluminum L-frame did the trick. Scratch has not failed since.. I promise myself to build a more elegant solution next week and have the 7812 cool its feet to the chassis.

It is Finished ….

Scratch is finished. Well, in any case I will not modify it until the components have had some time to settle. In a weeks time or so I will probable open the case again and change the cooling of the voltage regulator, glue black gate capacitors and change something to the way scratch is connected to earth.

I still hear some hum, especially when opening up the volume on the UL40-S2. under normal operating conditions the amount is not enough to be noticed when a record is playing. The hiss of the groove is much louder. But anyway, scratch need to be perfect and I want to connect it to any amp I like without hum problems.

When connecting scratch to the Marantz PM-14 amp in the living room I do not get any Hum, so maybe it is a grounding issue (The Marantz is double isolated).

I’m pleased with the end result: Both cosmetics and the sound qualities. But the housing remains very uuhh blue.

The sound of Scratch was after one day of operation already very open and pleasing. Only there was this hum which I want to be reduced before using Scratch on my UL40-S2. L.

I did not yet make an on-off switch on the front. Mainly because I expect to leave scratch on for most of the time an I do no want someone to accidentally switch off scratch. So I consider making an on-off switch on the back .

What would I do different next time?

The housing I bought for Scratch was after all large enough. Well, in other words, it was too high. It has he advantage though that temperature issues are non-existent. But, I did no take into account that tube-amplifiers have large transformers and that the bottom plate of the housing was designed for solid-state designs. Therefore, I will add a 5^th foot o scratch making sure that he/she/it will not have a small belly. The weight problem is not something I’m going to solve I’m afraid.

Secondly, I really think it pays to visit flea markets and look for cheap electronics components. In my case, with my limited experience with tube projects the costs of this project have become just a little too high. I think that spending even more time in finding components cheaper will greatly reduce costs of making tube amplifiers.

I do want to get a better understanding of the principles of grounding components, understanding why hum is introduced in my projects. Etc. I discovered that by not connecting Scratch to the earth/ground that all hum disappeared instantly. Nice, but I want to know whether it would have been possible to avoid hum by building Scratch differently.

Finally, I think I got the spirit of building Hifi myself. And I think that building it yourself becomes even more fun if you have the right tools, therefore I will look for a scope, a vertical drill, etc. to make life of the DIY freak easier.

<< Part 2, Ingredients

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Part 4, Listening >>

© Maarten&Annemarie, December 2001