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    VTA ST-120 - Born in the UK...

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    pichacker

    Posts : 13
    Join date : 2016-08-01
    Age : 52

    VTA ST-120 - Born in the UK...

    Post by pichacker on Wed Aug 24, 2016 5:43 pm

    Ok, so I know it's not quite the Bruce Springsteen song title but seemed somehow apt.

    After hitting a mid life crisis for the second time (First time I bought a fast Yamaha R6) I had an urge to build a valve audio amp. The idea had been brewing for quite a while since I had sold my Quad II mono blocks to pay for a holiday. At the outset let me say that at this stage I don't intend to make this a post describing how fantastic the finished amp sounds. This has been said many times in the past. (And it does...)



    I did the normal searching and came up with two contenders. The first one was an amp with a name similar to an inventor of the light bulb and the second the Latino special.  Emails were duly fired off and responses received. I reviewed the products, and the ease of making contact with the sellers and only really had one choice. Thanks Bob!! So PayPal did the deed and eventually I had a box of first class components arrive on my door step.

    Now the fun began. I had a few requirements from the amplifier in that it must be capable of being used by my other half and it had to be as safe as practically possible. There would be times when it "might" get left on unattended. As we all know valves can fail, sometimes quite dramatically and to limit any further damage to transformers etc. we would need to switch off the power as soon as possible. So this got me thinking. The basic design of the ST-120 amp was born in the days of Teddy boys and health and safety hadn't been invented. Add to this that fact that the mains supply here in the UK is 230V, the original design only had one fuse, the case wasn't grounded, I felt a few modernisations were in order. No disrespect to the kit as supplied, the beauty of a kit is that it allows the constructor to install a degree of their own personality into the end product. If I had done anything to the Quad II's they would have lost all their "snake oil" and become worthless.

    Unlike some of you that are reading this post I don't have a lot of room so the amp has to fit in with the other household items around it. This meant that it had to match the TV / Sky TV / CD in terms of appearance. It was therefore decided to move the inputs to the rear panel and cover the bias monitor socket to "modernise" the front look. I found that a 1U high 19" rack panel front was just the right height to fit both the front and rear panels. This was duly cut and screwed on. On the front I now have a push on/off button and four LEDs.



    On the rear I wanted to mount a filtered mains input socket, a switch to change from 4 to 8 ohm speakers, audio inputs, speaker connections and finally a multipole socket from which I could remotely turn the amp on and off.



    To allow for the remote switching, the front panel toggle switch, a delayed HT turn on and safety monitoring I employed a PIC processor (sorry did I swear in a valve section?). This PIC does the following:-

    1 ) Allows for a toggle on/off function
    2 ) Allows for remote on/off
    3 ) Monitors the voltage across the individual cathode resistors.
    4 ) Controls the mains power to the transformer
    5 ) Provides a delayed HT switch on feature.
    6 ) Drives each LED individually to indicate the correct standing current in each output valve. (fast flash too high, slow flash too low, on ok)
    7 ) Monitors the -ve bias rail and will not enable HT unless this is present.
    8 ) If excessive current is detected in any one of the output valves will shut off the amplifier.
    9 ) Special bias setting mode where the high/low tolerance of the LED indications is tightened to allow for precise setting to ~500mv as per spec.

    The basic circuit of the ST-120 is unaltered and follows Bob's instructions. I had to move the HT choke to create room for the control board. As an apprentice I learned the art of cable lacing. Now days everything is done by cable ties. I carefully laid out the audio wiring and laced them into a nice neat loom. I was a little worried that I might create an expensive audio oscillator by doing this but no fears. The amp is stable.



    This picture shows the amp part way through construction. Almost ready to test out once I had joined up the mains and HT leads. So moving on I decided to incorporate fuses into the HT feed prior to the rectifier, and a further two fuses into the feed to the main transformer and also the low current standby toroidal one. A suitable place for these fuses was found to be on a panel above the triode/pentode switches.



    Finally a control board was built up on veroboard. I fully intend to make a proper PCB once the summer has passed and winter is here. The control board layout is a little messy but at least is can't be seen when fitted into place.



    The firmware was written in C using the microchip XC16 free compiler.

    From start to finish it took me a week and a half to complete on and off. I did have a little hiccup in that one of the 12AU7's didn't work out of the box. It must have suffered from an impaired vacuum as the getter was still silver but the heater did not light and it just got hot. Another one was ordered and it was here in a couple of days. Glad it wasn't one of the output valves!!


    I suppose that I ought to stop typing and go and enjoy my ST-120 now. I'll review the amp and how it sounds another day.

    Steve.




    Three videos of the front panel LED's






    Bob Latino
    Admin

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    Location : Massachusetts

    Re: VTA ST-120 - Born in the UK...

    Post by Bob Latino on Wed Aug 24, 2016 7:20 pm

    Hi Steve,

    You did a great job with your custom version of the VTA ST-120. Your wiring is impeccable .. The bias led's and how they are wired into the circuit is an interesting concept. Your custom version of the VTA ST-120 shows what can be done with the basic kit if you are well versed in electronics ..

    Bob

    corndog71

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    Join date : 2013-03-19
    Location : It can get windy here

    Re: VTA ST-120 - Born in the UK...

    Post by corndog71 on Wed Aug 24, 2016 9:31 pm

    Great job! The LEDs look slick. I'm surprised you didn't use a different box considering all of the mods but it looks terrific.

    vtshopdog

    Posts : 69
    Join date : 2015-07-11
    Location : UT, USA

    Re: VTA ST-120 - Born in the UK...

    Post by vtshopdog on Wed Aug 24, 2016 9:56 pm

    Nice wiring can't imagine any cleaner routing

    Enjoy

    pedrocols

    Posts : 63
    Join date : 2014-11-24
    Location : Western MA

    Re: VTA ST-120 - Born in the UK...

    Post by pedrocols on Wed Aug 24, 2016 10:55 pm

    Nice beautiful work...

    pichacker

    Posts : 13
    Join date : 2016-08-01
    Age : 52

    Re: VTA ST-120 - Born in the UK...

    Post by pichacker on Thu Aug 25, 2016 2:44 am

    Thanks for all the kind comments. In respect to the case, I don't have metalworking facilities over and above a bench vice so the ST's chassis was an ideal starting place. It's well laid out and of a good heavy gauge. To me it looks just fine and is of a compact size. A good starting point but is very unkind on drill bits!

    Did anyone notice my little pieces of OCD? I made the triode/pentode switches a mirror image to preserve symmetry and fitted rubber grommets in the holes for the transformer wires. I know no one will see them but ......

    Steve.

    peterh

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    Re: VTA ST-120 - Born in the UK...

    Post by peterh on Thu Aug 25, 2016 3:37 am

    Excellent build.
    With the PIC there is a useful function ( that i don't see in your description ) : If power fails and
    comes back again, the PIC should prevent power off / on in rapid succession.
    Myself did this with an arduino by always delaying power on by 20s. ( see separete thread if you are
    interested).
    The PIC on a board would be a valuable asset to ampbuilders, maybe you can sell it to bob ?

    pichacker

    Posts : 13
    Join date : 2016-08-01
    Age : 52

    Re: VTA ST-120 - Born in the UK...

    Post by pichacker on Thu Aug 25, 2016 4:48 am

    If the power fails the PIC goes to power off. I didn't want any possibility of the amp coming on when unattended if there was a power outage.

    The beauty of Bobs kit is its simplicity and easy of construction with good instructions. (A complete circuit diagram with mods would have been useful).

    By adding any of our "upgrades" we stand the chance of making it too complicated and frightening off potential DIY buyers.

    Steve

    LeGrace

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    Join date : 2016-08-07

    Re: VTA ST-120 - Born in the UK...

    Post by LeGrace on Thu Aug 25, 2016 6:31 am

    Impressive build, talk about some mad skills! I'm gobsmacked by all the extra functionality and protections you've incorporated.

    pichacker

    Posts : 13
    Join date : 2016-08-01
    Age : 52

    Re: VTA ST-120 - Born in the UK...

    Post by pichacker on Thu Aug 25, 2016 6:59 am

    Call me paranoid!!! I'm now monitoring the temperature with the case on and wondering if a pair of small fans should be installed around the back to create air movement between the output and power transformers to force air out of the front past the output valves. (Will keep the smoothing cap cooler). I intend to replace the rectifier with an SS equivalent, plus side there will no heat associated with the valve and also less demand on the power transformer. Downside a slightly higher HT and possibly output valve dissipation.

    Using the "aux" socket on the back I have all the signals I require to control the fans including power. Already thought about that one Smile

    Tube Nube

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    Location : Calgary, AB

    Re: VTA ST-120 - Born in the UK...

    Post by Tube Nube on Thu Aug 25, 2016 2:40 pm

    I'll add my voice to the others congratulating you on your tidy build and innovative mod! Also, a big welcome to the forum.

    Further to the idea of making a commercial venture of this, I certainly agree with you about the idea of building it into the amp would scare off many prospects. But what about making it a small, stand alone unit that can be connected to the amp? You could supply it as a kit or ready-made, plus instructions for adding the wiring inside the st 120, or st 70, and "Bob's yer uncle." Roy, too.

    pichacker

    Posts : 13
    Join date : 2016-08-01
    Age : 52

    Re: VTA ST-120 - Born in the UK...

    Post by pichacker on Sat Aug 27, 2016 4:36 am

    If it was a simple overlay case then that would be a possible suggestion. But given the need to move the choke, the rewiring, and fitting the switch and LEDS then it may just be a liitle too much of a retrofit. However a simpler version that just monitors the bais and displays on LEDs with possibly a mains relay control to protect from overcurrent may be a possibility.

    The LEDs have allowed me to make a couple of observations. When the amplifier first comes on there is a brief period of over current, for about a second, in the output valves as the HT comes up and presumably the PIO caps charge and settle down.The second is that when I first put the amp together I noticed that one of the op valves had quite differing bais requirements to the others. After the initial "overcurrent" mentioned above the valves took a few seconds to come up and settle down. The bias was generally low. I could watch the LEDs flash and settle down. It was always the valve mentioned above that took the longest to come up to full emission. Anything up to 30 seconds more.

    Now that I have put a few hours on the amplifier the valves are a far closer match. The "lazy" one has caught up.

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