NOSValves Posted April 21, 2004 Share Posted April 21, 2004 I'm going to do this in parts and keep posting them to this thread as I get each section done so if you find this interesting keep checking back as I update this thread. First up is the replacement of the always bad Selenium rectifier I have never seen one of these that wasn't putting out at least 25% less then required voltage to the 12AX7 heaters and more important the bias for the output tubes. Here is the pesky bugger before replacement its the 1" x 1" device bolted to the side of the chassis. Before replacement There are 4 connections to the original and the new part they both have 2 connections marked ~ and one marked + and one - pretty simple replacement except the new part has the markings on the rear so its easier to solder the connections before you mount the new bridge rectifier. In most Scott amps you also have to adjust the value of the first dropping resistor on the filter network because the new SS part is more efficient in this amp its not required since Scott had the heaters supplied with only -47 volts for 6 12AX7's in series that is less then 8V for a 12V heater I now have 10.5V much better. The other concern normally would be you would have to much bias voltage but I am reworking the bias circuit for full bias control so I will tackle that later. Bridge Rectifier Installed 3 High Voltage Can Replacement The cans to be replaced are the left one in the bottom row and both cans in the top row. The other can is for the bias & heater supply and out of hundreds of amps I have only seen 2 of these bad this one proved to be fine. Before Replacement With this being such a gorgeous amp Jordan and I decided on using original type twist lock cans for the power supply. Of coarse knowing me the way you all do I found a way to do some upgrades while were at it. The original first filter can was a 20/20/20/20uF @ 500V I ordered a 80/40/30/20uF @ 525V for its replacement. I kept the same 20uF on the 5AR4 rectifiers output and installed 80uF where it would do the most good right at the center tap of the output transformers. The remaining 2 sections of this first can are for filtering the dropping voltages before it heads to the preamp section. Now the 2 top cans is where we have to get creative they are both 4 section cans 2 -20uF @ 450V and 2 -25uF @ 25V what we do here is just replace the 2 450V sections with cans and use small axial separates for the 25V section and mount them else where in the amp. We used 2 40/20/20uF @ 500V cans strapped the 2 20uF sectioned together giving us 2 40uF section to replace the 20uF nice upgrade in filtering again . You will notice that I also rerouted the wiring to clear the area above the cans this is where the bias controls will be added in the next step. New Cans Installed Here is how I keep track of the 4 sections that I'm going to move to separates. This way I don't have to get out that nasty magnifying glass to read them schematics Next up full bias control installed stay tuned. Quote Link to comment Share on other sites More sharing options...
lynnm Posted April 21, 2004 Share Posted April 21, 2004 Nice Pics ! What kind of a camera are you using ? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 21, 2004 Author Share Posted April 21, 2004 Its one I just picked up for a whopping $154 to my door from http://www.digitalfotoclub.com. Cannon A60 for 2 mega pixels this thing really takes some pictures. Just think these are cropped and shrunk to 640 x 4XX they really look awesome in original form. My old trusty Fugi 1.2 MP was finally wearing out after 4 years of use. the camera actually works good but it has trouble writing to the smart media cards. I know its the camera because I tried a brand new card with the same results. Craig Quote Link to comment Share on other sites More sharing options...
fini Posted April 21, 2004 Share Posted April 21, 2004 NICE documentation, Craig! Keep it up! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 21, 2004 Author Share Posted April 21, 2004 Thanks Fini but you have to insert your closure on this thread when I get done Quote Link to comment Share on other sites More sharing options...
Deang Posted April 21, 2004 Share Posted April 21, 2004 Craig, that looks like pretty complicated stuff. Are you sure you know what your doing? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 21, 2004 Author Share Posted April 21, 2004 Heck no I have to make a call a few states away the low down Quote Link to comment Share on other sites More sharing options...
T_Shomaker Posted April 21, 2004 Share Posted April 21, 2004 Makes the Foreplay that I'm doing look like a preschooler could build it. I only have one more step until it is finished by the way. I will post some pics when I am done with it. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 21, 2004 Author Share Posted April 21, 2004 Taylor, Please do post some pictures of your handy work ! Craig Quote Link to comment Share on other sites More sharing options...
Audio Flynn Posted April 22, 2004 Share Posted April 22, 2004 i AM IN MEXICO STILL VIDEO RESOLUTION IS AWESOME. iF YOU MAKE THIS AMP SOUND BETERE THAN MY lk48, YOU GONNA HAVE TO MAKE ME A NEW ONE. SHOULDA BUGHT THAT 296 TOOTH FAIRY MAY BRING ME ENOUGH MONEY FOR VRD MONO BLOCK PP. I JUST DO NOT LIKE PLIERS THAT MUCH. RICK Quote Link to comment Share on other sites More sharing options...
T_Shomaker Posted April 22, 2004 Share Posted April 22, 2004 I am now finished with the foreplay except for checking the voltage and everything. I think I have put about 24 hours into this thing. This was my first time doing anything like this and also my first time soldering. I did practice soldering before I started though. I'm going to call my Grandpa today to see if he can come over this weekend to help me test all the stuff. When I talked to him earlier he said that he was going to bring over his Simpson 260 (I think that is what he said). Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 22, 2004 Author Share Posted April 22, 2004 Taylor, The Simson 260 is a nice meter ! It will be a good thing to have your Grandfather help you along with the voltage tests. By the way did Dave teach you how to bias your scott amp ? Its important this be done every so often especially when running used NOS tubes but really with any tubes. Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 22, 2004 Author Share Posted April 22, 2004 ---------------- On 4/22/2004 12:16:24 AM Audio Flynn wrote: i AM IN MEXICO STILL VIDEO RESOLUTION IS AWESOME. iF YOU MAKE THIS AMP SOUND BETERE THAN MY lk48, YOU GONNA HAVE TO MAKE ME A NEW ONE. SHOULDA BUGHT THAT 296 TOOTH FAIRY MAY BRING ME ENOUGH MONEY FOR VRD MONO BLOCK PP. I JUST DO NOT LIKE PLIERS THAT MUCH. RICK---------------- Rick, How many shots of Ta-Kill-ya had you drink when you wrote the above post or were you using a Spanish keyboard? Craig Quote Link to comment Share on other sites More sharing options...
garymd Posted April 22, 2004 Share Posted April 22, 2004 Taylor, Did you ever get any of those lps or is Dave still holding out on you? Quote Link to comment Share on other sites More sharing options...
mark1101 Posted April 22, 2004 Share Posted April 22, 2004 Craig, Maybe I'm getting a little ahead of the game, but I see all those original Ceracaps in there. I'm interested in what replacement capacitors were selected for that 296 ? Thanks Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 22, 2004 Author Share Posted April 22, 2004 Okay here is the next steps Bias Controls and Cathode sensing resistors Here is the 2 new bias voltage controls installed in the spot I cleared in the previous step. This is a pretty straight forward operation although it require a good bunch of wiring changes. Scott's existing setup was fix bias with balance controls per channel not exactly precise to say the least this setup is Identical to a 299C and 222C. What I did was tap into the bias voltage supply for the balance controls and install a 25k potentiometer to feed each balance control. You feed one leg the voltage and hook the center wiper to the center wiper of the balance pot. The other leg of the bias voltage control is run to ground through the resistor this resistor value is how you adjust the range of voltage you will be able to apply to the grids of the output tubes. I also added another filter stage on the wiper of the bias voltage pots. This amp will now easily be able to run a wide range of output tubes from EL34's to KT-66 and everything in-between with its now wide range of bias voltage sorry KT-88 would be a huge stretch for this amp. Here they are on top of the chassis Adding Bias Cathode Sensing resistors This next step takes some serious rewiring Scott has the Cathodes directly connected to ground and also used these same pins to ground many other items. So the first step is to disconnect everything from pin 1-8 of the output tubes and determine what has to be taken back to ground. What were accomplishing in this step is to lift the cathodes from ground with a 1% 10 Ohm sensing resistor to allow us to actually measure the bias with a ordinary multimeter. This takes some serious moving and rewiring to accomplish. I install 2- 4- lug terminal strips to make all the new connections. I strapped 3 of the lugs together on each terminal strip then run a wire from both to the main B+ filter cans ground connection The output stage before changes Cathode sensing resistors installed Having full bias control is great but not if you have to take the amp out of your system and get under the chassis to measure it. So I install test points on the rear apron of this amp. This is simple you drill and install the tip jacks then run a wire to each 10 Ohm resistor where it attaches to the tubes cathode. (Jordan relax those tube are not yours I'm not firing this amp up after all this work with $600 worth of tubes until proper operation is confirmed) Test points Well there is full bias control installed. Next I will recap the amp, replace the plate resistors and cathode resistors on all the preamp sections and this amp should be ready to roll stay tuned ! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 22, 2004 Author Share Posted April 22, 2004 I just spent way to much time trying to get this darn BB software to get rid of the bold text on the one paragraph I give up ! Geezz got it....man you have to be able to write HTML to beat this flaky BB Software Grrrrrrr Quote Link to comment Share on other sites More sharing options...
NOSValves Posted April 22, 2004 Author Share Posted April 22, 2004 ---------------- On 4/22/2004 10:25:15 AM mark1101 wrote: Craig, Maybe I'm getting a little ahead of the game, but I see all those original Ceracaps in there. I'm interested in what replacement capacitors were selected for that 296 ? Thanks ---------------- You are !! All in good time my friend Quote Link to comment Share on other sites More sharing options...
T_Shomaker Posted April 22, 2004 Share Posted April 22, 2004 ---------------- On 4/22/2004 9:21:58 AM NOSValves wrote: Taylor, The Simson 260 is a nice meter ! It will be a good thing to have your Grandfather help you along with the voltage tests. By the way did Dave teach you how to bias your scott amp ? Its important this be done every so often especially when running used NOS tubes but really with any tubes. Craig ---------------- I looked on the piece of paper that you gave me and I have biased the amp probably 5 times since I got it. How is your dad doing Craig (I remember when I was looking on your page everyday to check the progress of my amp)? Quote Link to comment Share on other sites More sharing options...
T_Shomaker Posted April 22, 2004 Share Posted April 22, 2004 I haven't talked to him about the lps yet but I guess I will go over there and pick them up sometime when I give him the rest of the money I owe him and his case back. If he's not looking I may have to sneak out with his McIntosh or Blueberry Quote Link to comment Share on other sites More sharing options...
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