Scott 299b Restoration
- Thread starter bberkom
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- scott 299b
Oh I see the heater string voltages now on the schematic. I could not find them for the life of me a minute ago. I'll take some readings tomorrow and compare them to the schematic and see if the look too high. I believe I had right around 46v last time I measured when I was done with the power supply.
How would I go about lowering the voltages for just the 46v heater string? Add a resistor before pin 5 of V7 or just before V6 and V1 since I am only getting noise from the phono section? Would changing the value of the last 18ohm resistor in that string also affect the -25v bias supply voltage?
kvflyer
Super Member
You might try changing resistor R126 (in the schematic below), the 10Ω, 2-Watt resistor to maybe 22 or 15Ω 5-Watt and see where the voltage goes. Of course, you will have to set the bias again to the output tubes because any change in voltage affects both the tube filaments and the negative bias. You could "tack in" the new resistor just to try it. You don't want to drop (go less negative) too much or you won't have enough bias for the 7189 outputs.
Did you replace that silver Siemens Selenium bridge (M1 in the schematic below)? If you did, you will most likely have a higher negative voltage because silicon diodes (bridge) is more efficient (less voltage drop) than selenium. I didn't see where you had replaced it.
http://hhscott.com/pdf/sd/299B_3_4s.JPG
Here is one of my 299B amplifiers (I think!) and you can see that the selenium bridge has been replaced with an 8-Amp silicon bridge. The orange wires feed the AC to it from the power transformer.It is mounted on the aluminum bracket which holds the power transformer. Also, if you look above and to the right of the power transformer bracket, you can see that the first resistor has been changed to a 5-watt white ceramic resistor.
That's exactly what I did. I replaced the rectifier and changed R126 to a 30ohm 5 watt wirewound resistor to bring voltages down. I also went ahead and swapped the 18ohm resistors to new wirewounds while I was rebuilding the bias supply caps. When I checked my final voltages, all were very close to the Schematic.
The line level inputs are all dead quiet. It's just the phono that is noisy. New tubes and plate resistors on the phono tube sockets had very little effect on the noise. I know it's always going to be noisier than other inputs due to the extra gain, but it is still enough to be distracting while I'm listening. Could any of the other carbon comp resistors in the phono section be the culprit? I didn't find any far out of tolerance, but can they introduce noise even if they measure correctly?
The line level inputs are all dead quiet. It's just the phono that is noisy. New tubes and plate resistors on the phono tube sockets had very little effect on the noise. I know it's always going to be noisier than other inputs due to the extra gain, but it is still enough to be distracting while I'm listening. Could any of the other carbon comp resistors in the phono section be the culprit? I didn't find any far out of tolerance, but can they introduce noise even if they measure correctly?
kvflyer
Super Member
I hate to admit it but I have not tried any of my Scott amplifiers with a magnetic input. Why? Well, I have been in turmoil for the last six years with going to Australia (My kid and family live there) for half of the year and moving. Most of my stuff is still in boxes... except for the LK-72 in my avatar... which is on a shelf, unpacked but NOT connected. Oh, how embarrassing...
kvflyer
Super Member
You could take say a .05ɥFD capacitor and ground one end. Touch the other end on the plate of each 12AX7, working backwards from the volume control. Make sure the volume control is turned down when you do this and only advance it when you make the connection. As you work backwards, you should be able to isolate which tube section is the offending one. Then, work around that tube. Kinda like a crude way but it works.
The capacitor will shunt the signal to ground and and you will only be hearing amplification from that point to the output.
Does that make sense? Sometimes I can think it but it is difficult to type it...
The capacitor will shunt the signal to ground and and you will only be hearing amplification from that point to the output.
Does that make sense? Sometimes I can think it but it is difficult to type it...
You could take say a .05ɥFD capacitor and ground one end. Touch the other end on the plate of each 12AX7, working backwards from the volume control. Make sure the volume control is turned down when you do this and only advance it when you make the connection. As you work backwards, you should be able to isolate which tube section is the offending one. Then, work around that tube. Kinda like a crude way but it works.
The capacitor will shunt the signal to ground and and you will only be hearing amplification from that point to the output.
Does that make sense? Sometimes I can think it but it is difficult to type it...
I think I get what you mean. I may give that a shot later and see what I can isolate. The noise is in both channels at equal level however, so would it make sense for only one of the phono sockets to have a bad component? I keep thinking it must be something common to both channels? I don't really know anything though!
Looking at the schematic, the nominal tube voltage into the string is -46 vdc, or 11.5 v per tube. Adjustment for this would usually be done by varying R126, the original 10 ohm 2w resistor which you now have at 30 ohms.
Another thought: I have read that you can also replace the 10k and 47k grid resistors in the preamp first section with metal films for noise reduction. Also make sure you replace both the 270k plate resistors.
Dave
Another thought: I have read that you can also replace the 10k and 47k grid resistors in the preamp first section with metal films for noise reduction. Also make sure you replace both the 270k plate resistors.
Dave
Thanks Dave. I thought it turned out well. I'm going to use the amp for a little longer before making any more changes I think, but I'll look at the 10k and 47k resistors when I've got it out again. Already changed all of the 270k plate resistors. May just end up ordering a bunch of resistors to have on hand when I start in on the preamp section again.
I ordered some 10k and 47k resistors with some parts for another project and took some time to install them today. The 10k resistors read about 11k and the 47k about 50k. So they weren't too far off value but I still was hoping that replacing them would reduce the rushing noise I get from my phono section. Well it didn't really have much effect on the background rushing noise, but the clarity of the phono section does seem improved, especially the highs. I used carbon films for these, and it really does sound better at least to my ears.
The noise is still a little frustrating. It is present with phono, tape, and eur78 inputs selected but dead silent for tuner and extra inputs. It's just loud enough that I can hear it in very quiet song passages and between tracks, but really only when I listen for it. I know the klipsch speakers are going to really bring it out too. My voltages measure correctly for the preamp heater string. I probably just need to relax and enjoy but if anyone has other suggestions I'd love to hear them.
The noise is still a little frustrating. It is present with phono, tape, and eur78 inputs selected but dead silent for tuner and extra inputs. It's just loud enough that I can hear it in very quiet song passages and between tracks, but really only when I listen for it. I know the klipsch speakers are going to really bring it out too. My voltages measure correctly for the preamp heater string. I probably just need to relax and enjoy but if anyone has other suggestions I'd love to hear them.
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trainbuftony
Electron Herder
Clean your tube pins and retention the sockets in the phonostage. If that doesn't help, try putting the original plate resistors back in... Is the turntable grounded securely. (stupid question probably)
Thanks for the suggestions. The turntable is grounded, and the rushing happens at the same level with or without anything connected to the phono inputs. I have cleaned the tube sockets and tried different tubes in the preamp section. The rushing was present before I changed the plate resistors to new metal films. Nothing I have done has really had an effect on the noise, and I can't figure out what could be causing it!
trainbuftony
Electron Herder
My next step would be to reflow each solder joint starting from the phono inut jacks then working your way through both phonostage tubes and all associated caps, resistors, and connections to power and ground. When you have reached the main selector hook it back up and have a listen! Don't be discouraged if nothing changes this is a necessary step in a complete diagnosis.
I will give that a try. At this point I have retouched most of them, but I will trace back through the entire phono section.
I went through all of the voltages on the schematic and checked them in the amp while it was hooked up to my variac at 117v, and what I noticed is that the 170v source for the phono section is a little high at 178v. This reads 184v when I run in on my 124v mains instead of 117 on the variac. The rest of the voltages fall in line with the schematic or are even a little low on 117v with the CL-80 I have installed. This causes my plate voltages to read a good bit higher than the specified 100v on pins 1 and 6 of the phono preamp tubes. I read 117v for V1 and 106v for V6. I also measured between 600-700mv on each pin 3 and 8 of these sockets instead of the 500mv specified on the schematic.
Do these voltages point to any potential cause for the increased noise I am getting? About to start tracing a reflowing.
trainbuftony
Electron Herder
There is a way to find out, find the dropping resistor which feeds the plate supply for that stage and bump the value up 10 to 20 %. Your intuition is probably correct. One of my favorite things about this old stuff is how easy it is to modify different things to get the desired result. Through process of elimination you can solve just about any problem!
Here is a shot of the schematic showing the power supply. The 170v comes straight off of can cap C3 that I replaced with a 32uf cap. I may order a 51k and 56k resistor to replace R116 and see if that has the desired effect on lowering the plate voltage on those preamp tubes.
I reflowed all of the joints I could find tracing from the inputs all the way through the phono section, and the noise remains!
I reflowed all of the joints I could find tracing from the inputs all the way through the phono section, and the noise remains!
trainbuftony
Electron Herder
I would say you are on the right track changing the value of resistor 116. I wouldn't hesitate to place that order. If I recall correctly you installed an inrush current limiter in this. I would check a few other voltages like the ac voltage to the power tube heaters, if they are towards the high end (6.2 to6.4 volts) the whole amp may still be running a bit over voltage. If I determine that the entire amp is running at a bit higher voltage I will add a second CL 80 thermistor, in serries to drop another volt or two from the primary, which tends to bring every other voltage in the amp down a bit. The data sheets all say 6.3 volts, for a filament, but some wise old dudes have noted that 6 volts works just as well for twice as long. A second CL80 will definitely not hurt things, and may just bring all the voltages closer to what they want to be! You can always put it back if you decide it isn't right! Best of luck, keep us posted
Took a little break from this project to work on another, but I am going to stick it out with this 299B. I changed the 47K R116 to a 56K resistor, and this brought my supply voltage down right in line with the others, just a couple volts over the 170v spec'd. This brought the plate voltages on the phono tubes down to around 92-98v, actually a little lower than the 100v on the schematic. However, this did not affect the rushing sound in the phono section. So I decided to do some more measurements, and I found that the cathode voltages are all reading very high, as in 60% high. These are all supposed to be 500mv.
V1: both pin 3 and pin 8 read 820 mv.
V101: pin 3 reads 777mv and pin 8 reads 790mv.
The 2.7K cathode resistors are in spec with the highest being 2.96K. The 2.2meg resistors are also in spec. Does it look like there is something wrong with how these tubes are being biased, causing excessive current draw? That's the only other thing I can think of in the phono section. I should probably just go ahead and replace all of the resistors and ceramics in the phono section at this point, but I want to learn what is causing these voltages to read so high.
V1: both pin 3 and pin 8 read 820 mv.
V101: pin 3 reads 777mv and pin 8 reads 790mv.
The 2.7K cathode resistors are in spec with the highest being 2.96K. The 2.2meg resistors are also in spec. Does it look like there is something wrong with how these tubes are being biased, causing excessive current draw? That's the only other thing I can think of in the phono section. I should probably just go ahead and replace all of the resistors and ceramics in the phono section at this point, but I want to learn what is causing these voltages to read so high.
@trainbuftony and @Dave451 do you guys have any thoughts on why I am reading such high cathode currents on the phono tubes compared to the schematic? Could this be related to the rushing noise I am hearing in phono, and what would I need to do to bring the cathode currents back in line? Thanks again for all of your help.