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HH Scott LK-72 rebuild log

kvflyer said:
Wiggle tubes, the 9-pin tubes. If anything changes, please clean the tube sockets. That had been the most troublesome issue on the Scott amplifiers that I have brought back to life. Easy to fix.
Click to expand...

I wiggled them all and cleaned out the sockets with some deox-it. The channel imbalance is a little bit better. I'll clean them out some more and see if that helps.

That buzz is a different problem though. Here are the symptoms: It sounds like it's 60 hz. Turning up and down the volume pot has no effect on it. The buzz starts 15-20 seconds after turning the amp on (basically after the slow start of the 5AR4 warms up and starts rectifying AC). Turning the channel balance pot all the way to the left channel causes the buzz to stop in the right channel. Similarly, turning the channel balance pot all the way to the right channel causes the buzz to stop in the left channel. All of the other front panel controls (treble, bass, input selector, phono EQ, etc) has no effect on the buzz.

Here's what I've tried so far:
- different input, output, phase inverter, and rectifier tubes.
- checked all solder joints and made sure nothing is shorted.
- wiggled tubes in their sockets and cleaned out the sockets.
- removed the AC wiring from the front panel switch (there were reports of AC hum bleeding into the sound from this un-shielded wire).

Here's a link to the sound if that helps anyone (turn the volume way up to hear it!): https://drive.google.com/drive/folders/1DCOg25xAYlfRVj39IkjTesuiaBps4ai7?usp=sharing

Honestly, I can live with the channel imbalance for now since I can balance them out with the channel balance pot... but this buzz is really annoying and I'd like to get it fixed. Anyone have any ideas?
 
I suspect it may be heater noise from the ouputs. There’s a few ways to approach remedying it. On my amp, I ran a .1mf 630v axial to ground from one of the heater leads where the lead attaches to the first tube in the series. Like I said though, there are a few ways to approach it. A search should bear fruit
 
LinderK said:
I suspect it may be heater noise from the ouputs. There’s a few ways to approach remedying it. On my amp, I ran a .1mf 630v axial to ground from one of the heater leads where the lead attaches to the first tube in the series. Like I said though, there are a few ways to approach it. A search should bear fruit
Click to expand...

Thanks, I'll try that out. I happen to have an extra 0.1mF cap lying around.

I have two more updates after some experimenting:

1. If I pull out the phase inverters, the buzz goes away. This leads me to believe the output section is ok, and that the problem lies somewhere in the input or phase inverter stage.

2. I've been using 6U8A's with adapters this whole time. I plugged in my original 7199's to compare. Like before (and the original reason why I got 6U8A tubes), one channel has the buzz and one channel is almost silent. I flip-flopped the 7199 tubes and the buzz followed the tube to the other channel, with the other channel becoming silent. With the 6U8A's, the buzz is equally present in both channels. Based on this... is it possible that I just got unlucky and I have two bad 6U8A's and one bad 7199?
 
carlman14 said:
Thanks, I'll try that out. I happen to have an extra 0.1mF cap lying around.

I have two more updates after some experimenting:

1. If I pull out the phase inverters, the buzz goes away. This leads me to believe the output section is ok, and that the problem lies somewhere in the input or phase inverter stage.

2. I've been using 6U8A's with adapters this whole time. I plugged in my original 7199's to compare. Like before (and the original reason why I got 6U8A tubes), one channel has the buzz and one channel is almost silent. I flip-flopped the 7199 tubes and the buzz followed the tube to the other channel, with the other channel becoming silent. With the 6U8A's, the buzz is equally present in both channels. Based on this... is it possible that I just got unlucky and I have two bad 6U8A's and one bad 7199?
Click to expand...
Very possible. It was suggested to me that these are often a source of hum, and I've replaced the 6GH8as in mine because of hum, and it faded a fair amount with the replacement tubes. Currently have a set of NOS Sylvania ECG/Philips ECGs in there. Still have a bit of hum, but not an issue when listening, and you have to put your ear right against the speaker to hear it.
 
carlman14 said:
Based on this... is it possible that I just got unlucky and I have two bad 6U8A's and one bad 7199?
Click to expand...

Maybe the adapters you’re using for the 6U8As are faulty? kvflyer says above that it’s easy to just change the socket wiring . . .
 
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drew_t said:
Maybe the adapters you’re using for the 6U8As are faulty?
Click to expand...

Could be. I ordered several 6U8A and 6GH8 tubes (luckily they're cheap), and one pair of 7199s. If it looks like the adapters are bad, I'll try different adapters or rewiring the socket. We'll see what happens.
 
6U8A substitution for the 7199
Since all worldwide 7199 tube production has ceased and is not expected to return, the
availability of this tube has begun to dwindle and subsequently the cost is quickly becoming
prohibitive.

Thankfully, there is a solution by implementing a simple wiring change and substituting the
plentiful (for now) and otherwise identical sounding NOS 6U8A tube in place of the 7199.
Note: The below descriptions apply to chassis mounted tube sockets. For printed circuit board (PCB) mounted tube
sockets, the pin to pin changes would be the same however, the actual traces on the board would have to be cut
and re-routed which is beyond the scope of this discussion.

7199 (before) 6U8A (after)

Step 1. Remove wire from pin 2 and relocate it to pin 6
Step 2. Remove wire from pin 6 and relocate it to pin 7
Step 3. Remove wire from pin 7 and relocate it to pin 2
 
I have exciting news! I received the new pair of 7199's and some new 6U8A's the other day. I plugged the 6U8A's in, and the buzz was the same. BUT, I plugged in the new 7199's, and the buzz is basically gone! So the old 7199's and the adapters for the 6U8A's seem to be the problem. There is still a small amount of low-level hum, but it's not audible after you get more than a foot or two away from the amp. That's fine with me! All I can say is wow... This sounds amazing! Eventually, I might rewire the 7199 sockets to take 6U8A's just to confirm that the adapters are bad... but I'm perfectly happy with the 7199 replacements for now.

Also, on the channel imbalance: I plugged in the original 12AX7's again, and the massive channel imbalance came back, so at least one of those 12AX7's is bad. Using the JJ replacements for now, and the imbalance is much better. It's small enough that it doesn't bother me, and with a slight adjustment to the channel balance pot, everything is good!

Now I really want to get the original tubes tested... it's looking (and sounding) like some of them are pretty bad.

I think my restoration ends here for now. Thank you all for your help. This was a lot of fun and a great learning experience.

For the record, here's a complete list of things I did for the restoration:
- Replace busted fuse holder and fuse.
- Replace the canister capacitors with Hayseed Hamfest caps.
- Replace all disc and ceracaps with orange drops (except the 4mF caps on the 7199's, I used IC yellow caps there).
- Replace the Selenium bridge rectifier with a new silicon rectifier.
- Replace first 18 ohm resistor in the filter chain to maintain about -45V to the 12AX7's.
- Replace all 330k resistors in the output stage (they were all measuring at least 100k ohms higher than they should)
- Add current sensing resistors on the cathodes of the output tubes (resistors go from pin 5 to pin 1).
- Rewire 330k resistors from pin 5 on output tubes to pin 1.
- Ground pin 1 of the output tubes.
- Add an IBAM for the output tubes.
- Add two CL-80 thermistors in series to AC hot to drop the effective AC voltage (might back off to 1 CL-80)
- New tubes all around (at least until I can get the originals tested)

I'm off to enjoy the music!
 
Your remaining channel imbalance could be wear in the volume control.
Mine improves when I turn the knob about 40% of its travel.
I indulged myself for Christmas and bought the $90 replacement available on ebay.
I saw a youtube video from xraytonyb, where he restores a Scott 399 receiver using the same volume control, and he says the ebay replacement tracks very accurately, although he mentions there is more rotation required in the control to actuate the on/off switch. This takes away some of the overall travel of the knob to adjust the volume.
I'll be installing the new control in the next few days, and will be able to form my own opinion on whether it's a worthwhile investment.
 
Well done! I’ve been playing all the mandatory Christmas Music for the family on mine all week.
You might want to also add grid stopper resistors. They are on my to-do list as well, along with many of the mods you nicely summarized here.
 
@LinderK I took your advice and put a 0.1mF cap on one of the heaters for the output tubes to ground, and the remaining buzz I was hearing is gone! The amp is completely silent now. Odd side effect though... After adding that cap, my plate voltage is about 10V lower (and resulting bias current is lower too). Not sure why this caused that, but I can just remove one of the CL-80's and be back to where it was.
 
Scratch that... I took out the cap, and my plate voltage is still 10V lower than it was yesterday. Same outlet. Same source AC voltage. Not sure if that's anything to be concerned about...

Also, I put the cap on pin 2 to ground on the first output tube in the heater chain. Before I inserted the cap, there was 2.1V on pin 2 and 5.4V on pin 7. After I installed the cap, there is 0V on pin 2, and 6.1V on pin 7. Is that shift in voltage on the 2 heater pins going to affect anything negatively?
 
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I wouldn’t think so. Essentially you are providing a reference to ground for that leg of the PT, which is providing AC to the heaters. Its fairly common to do so at various points in a circuit and AC circuits in general. I’m sure there are others that can explain it clearer than I, but I’ve had it in place on mine for years to no calamity. I would not give it a second thought personally.
 
LinderK said:
I wouldn’t think so. Essentially you are providing a reference to ground for that leg of the PT, which is providing AC to the heaters. Its fairly common to do so at various points in a circuit and AC circuits in general. I’m sure there are others that can explain it clearer than I, but I’ve had it in place on mine for years to no calamity. I would not give it a second thought personally.
Click to expand...

Perfect. I won't worry about it then. I found the cause of my sudden drop in plate voltage. Turns out the rectifier I swapped in is a bit weaker than the current production rectifier I was using. With the current production rectifier in and the cap on the heater installed... Everything is looking great and the amp is completely silent!
 
A little late to this discussion but are the heaters elevated to help with the hum. About +30v on the heater string center tap reference might make the 6u8/6GH8 tubes and the old 7199's quiet down. Just wondering.

Edit: just looked at an LK72 schematic and it doesn't have a center tap or derived center point. That may have quieted the noisy tubes as well.
John
 
nerdorama said:
A little late to this discussion but are the heaters elevated to help with the hum. About +30v on the heater string center tap reference might make the 6u8/6GH8 tubes and the old 7199's quiet down. Just wondering.

Edit: just looked at an LK72 schematic and it doesn't have a center tap or derived center point. That may have quieted the noisy tubes as well.
John
Click to expand...

Correct, the LK-72 doesn't have a center tap on the heater winding. Ultimately what quieted down the hum was replacing the tubes (that got me half-way there) and creating a reference to ground on the heater winding (that got rid of the remaining hum).
 
I understand. But having to replace tubes to find a quiet one vs. having a basic design that minimizes noise are different issues.
 
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Iirc the -72 has a pot on the back with the heaters tied to each end and the center tap, (probably) grounded.
My Scott 208 us set up that way, as are several other amps.
Simple, cheap and dirty way to null the hum for best performance on the fly.
 
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