my SP14 build

Here is my second Dynaco build . A new SP14 preamp .
Last april I built my first ever electronic project an ST120 amp , and it has been working flawlessly and sounding great since . I've been trying to talk myself into doing a preamp since and finally bit the bullit and ordered the SP14 . The build went fairly easy with only one mishap ( I think ) of soldering the wrong resistors in one location . Which I luckily caught before getting too far .

I decided to upgrade the selector switch and attenuators to Goldpoints , and on seeing them they are very nicely built and have a nice crisp feel to them . I also upgraded the output caps to a pair of Russian PIO's in one spot and Mundorf silver and oil's ( which I also put in the ST120 ) in the second . And I had Roy do the chassis in black with white lettering , Which turned out very nice and is a real well made cabinet . I can't say enough about him , an A + guy and great customer support !

Well here are some pictures of the build .

Some of the parts that come in the kit . I went through and Ohm ed out all the resistors and stuck them to some tape and labled them first to make it easier to assemble the board .


I built the chassis and wired it up first before doing the board .




I marked everything to prevent mistakes.


After cutting the wire pairs to size I also Marked each pair of selector switch wires with a small piece of different color heat shrink tubing at each end to make shure they went in the right places after I heat shrinked up all the wire bundles .


Same with the tape in and outs .






On to the board , which is very sturdy . It comes with a nice board layout schematic .And went together fairly well except for the above mentioned resistor malfuntion.




Here it is all wired up finally . Hopefully , I checked it over a few times to make sure ?



I Also marked the output rca jacks so I know which pair of caps are which .


Well I plugged er in and crossed my fingers and turned it on without the tubes in and no sparks , smoke or blown fuses . So I then put the tubes in and it lites them all up .
I got a pair of NOS RCA's and Sylvania tubes from Roy , I also got a couple other NOS tube brands from ebay to try later too.


All buttoned up and ready to plug into the stereo which will have to wait till tomorrow to see how it sounds .



I'll hopefully get it set in the cabinet tomorrow and give it a listen and if I have everything together right it wil be nice and quiet . I'll give a post then and let you know how it sounds .

congrats! looks good!

Berry, berry nice looking! How long did your build take?

Great job, PigFace,

I can hardly wait to hear your listening report. I'm still getting acquainted with my SP14 (run to a Bob Latino VTA ST 70).

I expect you'll be thrilled. What preamp have you been using?

very nice posting, thanks, great photos and commentary, now hopefully no mistakes and it all works perfect!

Well unfortunatly something is not right . I have hum from both speakers which dosen't change as the volume is turned up , And I only get any music out only if both gain and volume attenuators are at full . And if I turn it off with signal going through it the hum goes away and the sound stays at the same volume level for a few seconds then slowly bleeds away . The right and left gain attenuators seem to work correctly , as does the selector switch .

I checked all the wireing against the diagram and didn't see anything wrong so far . All the solder joints looked good also . I tried different tubes and still the same problem . I notice the tubes don't seem to get as bright at the preamp tubes in the ST120 do . So I don't know if I have a voltage problem , a transformer problem or something on the board got too hot during soldering . I don't think that as I was carefull . But I'm going to have to go through and check the resistors and make sure and figure out how to check the transformer voltages . It says in the wireing to connect the yellow wire to the left hv tap which should be 240vac but it says on the transformer that the yellow wire is 0v ?? I guess it's time to do some research .

Roy if you see this , Since this transformer dosen't have a purple shield wire to go to the solid rca jack ground wire , shouldn't it be grounded to something else? Where is the board getting it's ground from then ? According to the diagram thats the only ground point to the board . Mark.



I guess I forgot to say that you can barely hear any music unless your ear is right up against the speaker and you don't hear any untill the knobs are at almost 90% full .

well you have all the wires to the back panel going under the circuit board, all bundled up. thats an attractor for perhaps AC on the heaters, if the heaters are AC. however those PSUs look like Roy's DC regulated heater/B+ supply. but they might be picking up hum from something else, even the AC in the PSUs. try lifting the board out while its on only if you are 100% sure you can do it without shocking/killing yourself (i will take no responsibility). i would wear rubber gloves if i did that. and see if the hum changes. it might be the wires as i mentioned. usually you create small bundles, if any, and route them around the board and any chassis mounted transformers etc. keeping them away from any AC. if not that check and re-check all your grounds. and solder connections, check all of them - there should be a cone of solder on the wires or pins on both sides of the board.

Pigface,

You may want to scan this forum for other pre-amp builds. I do remember the similar hum issues coming up in other pre-amp builds and a lot of discussions around how to fix. Some had success with this fix. These forum discussions were with the small tube preamps but I think the circuit is much the same with the big tube pre you just built.

You may want to re-step every step of the build you did if you have not yet done so. Check a recent forum post from Roy where he has a build of his. I bet in the photo you can get an idea of how he ran his wires. I do think running the wires under the board like you did could lead to issues.

Have you called Roy?

Right now I'm less concerned about the hum as I am about little or on signal coming out of the output rca's .The hum is low you can't hear it if your more than three feet from the speaker.
But the same volume of of sound ( music that is , when I have a source going through it ) comes out of the speakers when it's turned on as when its turned off , except the hum is gone when its turned off .

I went through and rechecked the wiring against the diagram again and it looks correct. Last night I went through and checked and re-did the solder joints on the board , switches and wires , and nothing made a difference .It's just the same as before.

I did notice though after I had it on on the bench for a while one of the heat sinks on one of the voltage regulators was getting much hotter than the other one so maybe the one of them is bad ? Maybe I did something to it while trying to spread the pins getting it to fit the board ?? Or one of the resistors or diodes on that side is bad and its not letting enough voltage get to it .
My multimeter went on the blink last night while trying to check so I'm going to bring my other home with me tonight and try some more checking . The hum might be related to this too .

I also think there is a ground problem too as the hum actually gets much louder when I tried a three to two adapter on the plug to see if that might be the hum problem .

The wires going under the board are the same as Roy has pictured on his build page for this preamp.

I haven't contacted him yet as he's going away and seems real busy at the moment .

Ok , After a little more reading this evening, I unsoldered the transformers just to be safe and to double check and see if they were working correctly , and they are . The correct voltage on both of them.

But I did find out why the left regulator heat sink was getting 100 degrees warmer than the other side . I had the two blue wires of the 9v transformer blue-green pairs switched around on the board . It was hard to tell just by looking at them .

Since my other volt meter was acting strange last evening and I was getting odd voltage readings I couldnt tell before , but now checking them with my other multimeter the voltages on both transformers to the board is correct now . But after correcting the wires , the same problem still exists . There might be a small increase more in volume now than before but not by very much .

I'm wondering now if the crossed wires ruined the regulators or something else on the board ? The left regulator was getting over 200 degrees and climing .

It does sound like the regulator is blown. you usually cant reverse polarity or run out of spec, silicon. try replacing that regulator if you can. i havent built it yet but on the schematic for the PH15 it shows a high voltage regulator for B+ that seems to have a part number implying its a propietary part. that would kind of suck if it was. id want to get some spares.

i think if you dont have B+ then a smaller amount of current will still flow because of the grid. the grid would still be creating a positive charge and maybe some stray electrons flowing toward the weaker charged grid. there is no B+ to amplify but still some current flows.

The sinked regs are your heater supplies. Are you getting the correct heater voltage when measuring across H- and H+ for each of the two rails on the board?

From the two pictures you posted of the board top and bottom before it was installed in the chassis, I don't see any traces on the board for B+. I do see two holes at the top by each regulator marked B+, and two holes at the bottom by the tubes also marked B+. Perhaps you need to install jumper wires to get the B+ from the supplies to the tubes?

If you haven't already, you do need to install jumpers on the bottom of the board for the B+. On your PCB layout you will see a point near Q1 (the high voltage regulator) which must be connected to a point near R13 (between the 6SN7's) on the left and right side of the board. The voltage between H+ and H- should be near 6.3 volts DC and your B+ to ground should measure about 265 volts DC if I recall. Carefull measuring the B+, you could hurt yourself or the B+ regulator if you are not carefull!

TMadden & jjones3318 , THANKSSSSS !! And also to everyone else's help , suggestions and input . I was about to ship this off to someone that knew what they were doing .


That's exactly what it was .

There was no mention about jumpers in the wireing diagram . But it probably obvious to someone with more electronics experience than me ( which is most people ). But I actually wondered about that myself seeing the empty B+ holes , but didn't want to just start soldering wires in just on a chance . But from the first it seemed to me that the tubes were not getting power .

The board wireing before



I was so eger to see if that worked I didn't take a picture but here is where I put the jumpers .



The hum seems to be completely gone now , and is dead quiet with my ear up against the speaker .

Too bad its too late to really listen to it , but from just a couple minutes listening it already sounds much better than my 20+ year old Carver C1 Preamp .

So I had about eight to ten hours in the build and thirty hours trouble shooting . But I got a better idea how it works now , so I guess the education was worth it .

I'll also have a couple spare regulators too , as I ordered a couple new ones to try because I thought I fried them .

After all this I was getting pretty discouraged and thought I might want to get the PH15 prebuilt from Roy that I wanted when I got the money , but now I might just give it a try .

I'll Let you know how it sounds after I spend some time listening to it soon .

Mark.

Hi Mark,
sorry I've been gone the past week and just now read this post in a few days.
Both Troy and myself have now built 3 or 4 each of these preamps, so glad he filled you in on that.
There is a photo of the B+ jumpers here (about 6th pic down) but it would have been easy to miss any links to this page:
SP14 build #3
BTW, the PH15 will be easy after doing the SP14!!
Roy

Roy , I literally stumbled on that part 3 build page less than an hour before you posted your reply ! It would have been better a few days ago . But I learned a few things that I wouldn't have otherwise . I didn't even try to contact you about it yet because I knew you were busy and I wanted to try to figure it out myself. As for th PH15 being easier , I don't want to sound like I know what I'm doing , but the SP14 build , except for the jumper problem went pretty easy . It actually took less time than the build of my ST120 . But I studied the first two build pages every day for a few weeks before I even ordered it .

I finally got to spend Sunday afternoon and evening to the SP14 .
First off my old system , I thought sounded good , a Carver M-1.0t amp and C1 preamp powering first a pair of Infinity Kappa 8's and now Kappa 6's .( I moved the 8's down to my main listening room ) I had to equalize them at lower volumes to get them to sound like I thought they should sound . Less so or not at all at higher listening volumes . These speakers seem to need some power going through them to sound at their best .
With the SP14 I don't need the eq in the system any more . This is dead quiet with no hum at all . First the bass is much tighter and solid sounding no "muddyness" ? The highs are cleaner and clearer not "bright" sounding at all . Like I think someone else mentioned with theirs I noticed the mids were a little harsh sounding at first on some of the vocals but after a few hours and CD's that smoothed out some already and started sounding better . I'll see how it sounds once it fully breaks in . I haven't had the chance to listen to any vinyl on it yet . But so far I'm very happy how it sounds . Much better than before .