Scott 299b Restoration
- Thread starter bberkom
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- scott 299b
My VAC runs really high typically. It causes some intermittent transformer hum in a few of my vintage receivers/amps.
I did not put the CL-80 in it yet! That was next on my list, and I forgot to think how that would affect all of my voltages. Hopefully that helps some. It's about 4v high across the board with the present values. If I am still a little high after installing the CL-80, how close is close enough to the schematic values? I do still have the original 10ohm resistor that I can add in series with the 30ohm if needed.
Dave, thanks again for all of the help. This is the most complex restore I have done, and I think It'd be a mess without your guidance!
I am about to tackle C1, C2, and C3 for which I am using JJ's as you did. For the new ground points you added around these caps, these grounds need to be isolated from the chassis and not grounded to the chassis directly at the new lugs, correct? I see the one chassis grounding point close to the selector switch, and as long as all my grounds are connected and reach that point, I am good right? Just want to make sure I don't create a bad grounding scheme or loop!
I am about to tackle C1, C2, and C3 for which I am using JJ's as you did. For the new ground points you added around these caps, these grounds need to be isolated from the chassis and not grounded to the chassis directly at the new lugs, correct? I see the one chassis grounding point close to the selector switch, and as long as all my grounds are connected and reach that point, I am good right? Just want to make sure I don't create a bad grounding scheme or loop!
drew_t
Well-Known Member
Use a bucking transformer if it's still too high for your liking. http://www.geofex.com/article_folders/vintvolt/vintvolt.htm
I've considered this as an option, especially as I have more closely monitored the VAC in my house lately. It's pretty consistently up near 124 VAC, and I am sure my old gear would appreciate the step down.
OK, you're bringing back memories on how I hooked them up. Since the JJ caps were glued to the wafers, you do have to run a ground from a tab on each to the chassis. I added a ground point (use a screw and nut with a star washer) as I recall between C1 and C2 and grounded both to it. Keep the ground point as close to the cans as possible. As for other black ground wires that went to the other can case tabs, wire these as before. Check the pictures in my thread. I'm traveling, so my gallery is not fully accessible for a couple weeks. C3 would be done similarly. I didn't get any hum this way, so evidently no ground loops.
Makes perfect sense. I found those photos but couldn't tell if you grounded or isolated the terminals you put in that all of the grounds from each can connect to. I've got C1 and C2 wired up the same way you did, just have to add the new grounding point tomorrow.
I powered through and finished the recap today. Replaced a handful of resistors, but most had not drifted much if any. All voltages are within a couple volts from the schematic, and the new quad of sovtek output tubes biased nicely. I'm listening to it drive some Klipsch Heresy I's right now and it sounds excellent. All of the preamp tubes and rectifier are original I believe, and they all tested strong. I forgot to take some good photos before putting it back in the case, but I'll have it out again in a bit to recheck the bias and maybe add cathode to ground resistors.
Well I went ahead and got back in it already to replace the 270k resistors in the phono section like you did in your restoration Dave. The amp sounds great, but there is a pretty loud rushing noise with phono input only. All other inputs are dead quiet. I changed them to metal film and then rushing noise didn't seem to improve any. I also tried some new sovtek 12ax7's that I use in my Fisher 500c, but the rushing stayed effectively the same in the Scott. What other components in the phono section could cause this? I'm guessing it's one of the other carbon comp resistors, but didn't know if you were able to reduce the rushing in yours that you talked about without replacing all of them.
I also wasn't going to replace the caps on the selector switch if I didn't need to, but not sure these are contributing to the rushing.
I also wasn't going to replace the caps on the selector switch if I didn't need to, but not sure these are contributing to the rushing.
Others may want to weigh in here, but my experience is that you can always expect some tube rush from the high gain phono section, especially with the volume turned up high. If the rush is at 'normal' levels, you'll never hear it at listening levels. Too much rush is usually the product of tubes or bad carbon comp plate resistors and you have tried those things.
I don't think the early stage input caps will help and they are a bear to change anyway ( I did it once). I'm not where I can look at my notes or the schematic now, but I'll study it later and post again later.
Dave
I don't think the early stage input caps will help and they are a bear to change anyway ( I did it once). I'm not where I can look at my notes or the schematic now, but I'll study it later and post again later.
Dave
Yeah I know there's going to be some tube noise from the phono stage compared to the other inputs, but it just seemed a little too loud compared to what I am used to with other tube preamps. I think changing those resistors and leaving the sovtek 12AX7LPS tubes in the phono section have helped the noise a little, but the real test will be hooking it back up and doing some listening. Those Klipsch speakers aren't very forgiving with that kind of rushing noise, but it's not something that's noticeable outside of silent space between tracks. Just trying to squeeze every last bit out of this thing I guess.
I actually did go ahead and change the .047 and .022 caps on the selector just now since I had ordered them anyways. You're right, it was a bear! I managed to do it with no ill effects I think, so one more thing off of the list anyways. Just have to turn off my OCD and enjoy the music for now. Thanks again for all of your help. If you come up with any other notes on the phono section rush, let me know. With the other reading I've done, I can't find any other suggestions to lower the noise there.
I actually did go ahead and change the .047 and .022 caps on the selector just now since I had ordered them anyways. You're right, it was a bear! I managed to do it with no ill effects I think, so one more thing off of the list anyways. Just have to turn off my OCD and enjoy the music for now. Thanks again for all of your help. If you come up with any other notes on the phono section rush, let me know. With the other reading I've done, I can't find any other suggestions to lower the noise there.
BinaryMike
Pelagic EE
Heater voltage on the four-tube preamp string varies from a low of 40V to a high of 46V among versions of the 299. Operating these heaters well below spec (12.6V) reduces tube noise when anode current is low, so you may be able to reduce phono preamp noise a bit by dropping the total for the string down to about 42~44V.
Next time I am in it, I'll measure the voltage across the heater pins for each of the preamp tubes to see if it's running close to the 12.6V max. I'll probably need some guidance on which component value(s) to change in order to bring the voltage down for the heater string if it looks too high.