• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

wiring a transformer on a Scott 299

I am in the process of getting a scott 299 up and running some more info can be read here

http://www.diyaudio.com/forums/tubes-valves/221393-scott-299-a.html

My question is since I have been having problems with high voltage I have been trying to figure out if everything is wired right. I figure start at the beginning,on the schematic it just shows 380 volt on pin 8 of the rectifier is that coming off one of the leads on the power transformer, if so how do I know which one, as there are 2 that have 380 volts on them, and yes this is with the transformer disconnected from the socket.

I only continuity between 2 of the leads, not the two i have voltage on but between one of those and another.

Now from ground to one lead I have 380, and from ground to another lead I have 380 and those 2 together I have 760. The last 2 have nothing to ground but have 5 volts in between them.

Anybody have any idea where these 4 wires should be landed, in the mean time I will keep searching.

Thanks
 
In your case the power transformer will have the high voltage taps that show on schematics as 380 - 0 - 380 where the "0" is center tap which is also chassis ground, and both 380 volt taps are both plates (pins 4 & 6) of the 5AR4 tube.

Yes, "0" or center tap to each 380 volt tap will be 380 volts, and both 380 volt taps together will be 760 volts, but the cathode connection of the 5AR4 which is also pin 8, should never have 760 volts coming from it.

According to your wall current, you might see as much as 430 volts at pin 8. Your 299 is like my LK-48 and the 430 volts is what my LK-48 schematic shows what it should be at pin 8.

Pins 2 & 8 are not DC voltage but are the 5 volt AC filiment voltage for the 5AR4 tube. Everything coming out of the power transformer is AC until it goes through the rectifier tube. Pin 8 will now be DC 430 volts and is filtered by the first can filter capacitor.

How high is your DC voltage at pin 8 of the 5AR4 tube?
 
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It took me about 6 beers or so to figure this out last night, however I don't recommend by all means drinking and playing with high voltage EVER!!!!!

Anyways


How high is your DC voltage at pin 8 of the 5AR4 tube?

The schematic shows it should be 380 volts dc on pin 8 but I have 460 which seems quite a bit higher. Now as I look at the tube data for the 5ar4 that seems to be about right give or take. So my next thought was to try a different tube as someone recommended in a different post. But then I got to thinking that putting a different tube in is only a bandaide and not fixing the problem. So maybe there is something wrong with the transformer maybe not, maybe this is why I need a oscillascope and set my ac balance to where needs to be. Since I am using the Sovtek el84m that candle more voltage this shouldn't be much of a issue.
 
The voltages shown are usually with a load (all tubes in), as you probably know so maybe your line voltage is way too high.

Make sure you have a load on the speaker terminals (speakers or resistors/ 16 ohm 25W+)

Funny you should say that, at times I would have either just the power tubes in or a couple of the line stage tubes in. Once I put all the tubes in and rechecked I had about 390 which is probably ok. Now when I hooked up the speakers to a 8 ohm load my voltage went back up to the 400's I will have to double check this evening. But I was able to set my bias adjustment to either -20 or -24 which ever, I couldn't get there earlier. Along with my dc offset voltage was kind of high but know I can get it to between -12 and -13 volts. Slowly but surely this is coming along, Which I wouldn't have if it wasn't for this wonderful forum and doing some intense research.

I was listening to music but only getting it out of channel a, channel b would only produce static at times, and with no music I would get some quick burst of static through both speaker. This all may be do to some dirty pots, and or switches so tonight I will stop off at the shack and pick up some deoxit.

What the heck are people using for RCA connections on these as all the ones I have are to close and push against each other. Looks like I will have to find the right ones that fit and mod everything to fit.
 
Check to see the tubes are seated well.

A couple do seem to be loose, I will either try and tighten up all the sockets or replace whats needed since there also seems to be some corrosion as well. I hope its something simple, the left channel wasn't working but going back and re-soldering some stuff it seems to be working fine now. Just the scratch in the right channel.
 
Trouble shooting a Scott 299

So now I have it wired right, and have gave it a try. I did try cleaning and tightening up the tube sockets. At first that did not help, then I found one socket that needed some more help. So I was listening for a while when the static came back, and I noticed one of the power tubes was hot plating. So I shut her down. I am going to order some new sockets and other stuff, is there anything else I should look at.
 
Bias pot is good, grid resistor off of pin 2 is good. This is the socket that has that .001 2500volt drain cap on pin 7 which should be plate voltage(would this have anything to do with the hot plating. I will double check the voltage.

In the mean time I have read here and there that people are adding a 10 ohm resistor on pin 3 cathode/suppressor grid and running a test spot with a pot somewhere to set bias more appropriately. Since I am in replacing sockets maybe its a good time to do so. Any idea how to do this.
 
So since today was the first I had to get back to this project, I had the tube that was red platting tested which it did test bad. Luckily the guy who tested it just so happened to have a matched set of the same tubes.

So now I have all the tube sockets replaced and most of the caps replaced. When I have the stereo switch set to stereo I only get audio out of one channel, when I switch to just channel b I have music out of both.

The burst of static is still there now there seems to be a hum as well, all in the same channel which I think is b. One thing I did notice when the burst of static comes the rectifier tube arcs inside. The tube test fine, but will source a new one in the mean time just to be safe.

One last thing how do I figure which phono input to use.

Thanks
 
On the phono there are a straight in and second one has a 100k res. in series try them both and see if one too load and one is to soft or on about right and one off the volume you want . the selector switch try in channel a and stereo reversed if he signal swaps in reversed then the problem is be fore the switch. The 10 ohm cathode res. is an easy way to measure current in tube. ( voltage drop across the res. ie 100ma is 1 volt across 10 ohm)
 
Ok I found a broken ground wire on the input from the phono. As I type and look back at the amp, I noticed I must have broken the ground wire on the other channel. Which makes some since, I will fix it now.

But my main concern is what was a loud burst of static in channel a, as I compare my schematic with my receiver. My receiver has some sort of ceramic cap going from high position on the speaker terminal strip or I should say where you hook up your speakers, to a ground. My schematic doesn't show this, so I removed it. Since I fixed the broken ground the hum has gone away but the weird burst of static comes out of the speaker, only now it is low tone not high like a scratchy record. I still see the burst of arc in the rectifier tube when this happens.

I did noticed the 1.8k ohm resistor going from the 16 ohm tap to the output on the center channel output was missing, but I would only think that would just not allow the center to get signal from that channel.

I all most feel like the output maybe bad, or maybe the test Triodethom told may to do would find the problem. I don't really understand it but will go over it again and see what I come up with.
 
Well, as I was going through the transformer, I am not certain that this is the problem. I got to thinking about it last night, and I tried to remember when this started. As when I first replaced the cans in the rear of the unit, which I think are part of the power supply. This was not happening, this problem did not start happening until after i replaced the rectifier tube socket. So I will be picking it over, over the weekend. I need to source some of the power resistors, I have the 3 tab terminal strip to replace the broken one that these resistors are connected to. I also found some corroded wires which will get replaced.