Scott LK-72A: hum left channel disappearing after a while...

[Patrice B]

You may have a dirty tube socket.

I made the test again: swap of Sylvania right 6GH8 to left and GE to right: "scrtchhhhhh!!!" that won't end by any means. The tubes are fine in their original positions. I'll have to check the soldering on the left 6GH8 socket I think.

I'd spray just a tad of DeOxit or RidOx cleaner into each pin receptacle of each tube socket, and push in/pull out the tubes a few times. Make sure you give ample time for the solvents to dry (5 minutes should do it) before turning the amp back on.

Otherwise, it could also be a bad solder joint. Did your tech re-solder any of the tube socket pins? If not, one or more of the pins may need re-soldering.

As for bias- it would be a good idea to re-bias the amp since the tubes have been moved. However, the fact that they are ostensibly a matched quad, and it's working with little hum, means that it probably is pretty close...

Regards,
Gordon.

Deoxit already done in the left 6GH8 socket: no change (for distortion... Hum is gone with the 7591's swap). Maybe I'll have to do it for each socket.

I do not think he resoldered any socket pins but he soldered a new capacitor on the left socket (6GH8). Bad solder joint here maybe...

Another question, could a dirty socket or a bad solder joint (on 6GH8's sockets or 7591's sockets) be the cause of distortion?

Something related to 6GH8 for sure... but what?

**I just revert back v7 and v10 (7591's) because there was a difference of balance between left and right now. I'll also try to revert back v8 and v9 and if the hum doesn't reappear, a socket problem could be the cause of all of this.

Thanks for your help.

 

[FalconEddy]

I think you have given a very noble effort to start tracking down these issues on your LK72.

However, based on your photo inside the chassis, it's apparent to me that you really need to rebuild this amp if you intend to actually use it.

I spotted MANY original components that should be replaced to get this amp back into a 'two thumbs up' running condition.

AK'er kvflyer has already given you a minimal list of items to replace, so I will not attempt to duplicate his efforts. I will add the power supply can caps (over 45 years old!!!) above the chassis to that list, as well as changing out the four 330K grid resistors with 200K's (since you're using new production tubes).

You'll need to exercise EXTREME CAUTION inside that chassis since caps can store a LOT of energy, and there's 475V floating around in there in certain locations when the unit is turned on.

Part of troubleshooting is being able to take both AC and DC voltages, resistance, and other basic measurements. Otherwise, merely changing out components without having the ability to make fine adjustments will generally prove to be futile.

My .02

. . Falcon

 

[GordonW]

Actually, the LK72 does NOT need the 330K grid resistors changed.

It's funny to think about at first... but through Thevenin's and Ohm's laws, you can determine that, at DC, the grid actually sees the PARALLEL combination of the voltage divider created by the 330K resistor to ground and the roughly 520K combination of half the bias pot (500K divided by two) and the 270K resistor from the bias supply, as its DC feed resistance. That combination (330K || 520K) gives roughly 202K ohms. Well within spec for the 7591.

I've never seen a Scott with this setup have problems maintaining grid bias...

Regards,
Gordon.

 

[Dave C]

caps, caps, caps, caps, caps, Stop moving your tubes around until you get the caps replaced, it's not that big of a deal.

 

[Patrice B]

caps, caps, caps, caps, caps, Stop moving your tubes around until you get the caps replaced, it's not that big of a deal.

Yes, you're right. I checked today for caps suppliers located in Canada and I've found at least one: http://www.justradios.com/capacitors.html

I must now take my time and do it step by step. I will surely need some help from this forum during the process (how to check for voltages particularly and where to measure BIAS when the time will come).

Thanks.

 

[Patrice B]

Already a first question:

Is it ok to replace the paper caps with 630V Metalized Polypropylene Film (Tubular Axial Capacitors) as shown here: http://www.justradios.com/capacitors.html#product

Thanks.

 

[kvflyer]

...

AK'er kvflyer has already given you a minimal list of items to replace, so I will not attempt to duplicate his efforts. I will add the power supply can caps (over 45 years old!!!) above the chassis to that list...

. . Falcon

Absolutely correct. My advice is to only get the amplifier operating in "Safe Mode" (like Windoz!). Many more things should be done as are mentioned above. Good luck. The Scott will reward you with an excellent sound...

 

[Patrice B]

1) Is to replace the Siemens selenium bridge with silicon
2) Replace the first resistor after the bridge with the appropriate larger resistor
3) Measure the bias/filament supply for proper voltage
4) Replace the .25µFD (or whatever other value it is) on the phase inverter/AF amp tube
5) Replace the coupling capacitors to the output tubes

1) already done by the tech;
2) dunno if it's done, I'll verify. And what's the appropriate "larger resistor", what value?;
3) how do I do this, need help or howto (newbe, sorry)?;
4) and 5) I've found another place where I can have Solen Fast caps: 2x0.33 to replace the .25 ones and 4x.1 to replace the coupling caps (I assume these caps are the ones connected to the 7591's sockets in the picture I attached previously, correct?). Is it a good choice for first step caps replacement?

Edit: they also have Orange Drops, so which ones are better?

Thanks.

 

[FalconEddy]

Actually, the LK72 does NOT need the 330K grid resistors changed.

It's funny to think about at first... but through Thevenin's and Ohm's laws, you can determine that, at DC, the grid actually sees the PARALLEL combination of the voltage divider created by the 330K resistor to ground and the roughly 520K combination of half the bias pot (500K divided by two) and the 270K resistor from the bias supply, as its DC feed resistance. That combination (330K || 520K) gives roughly 202K ohms. Well within spec for the 7591.

I've never seen a Scott with this setup have problems maintaining grid bias...

Regards,
Gordon.

Actually. . . you're 100% correct!

Even though I've been somewhat of a supporter of the grid resistor value change, I've actually never made the change on my LK-72A.

One of the two reasons for that is because I'm running vintage 7591's and not new production 7591's. The other reason is because I only measured 194K ohms at pin 6 on my power tube sockets.

Since that's well within the allowable maximum grid resistance of 300K in a fixed bias circuit, I figured I was all set.

This should also hold true on the 299C. But, as always. . . .YMMV.

. . Falcon

 

[FalconEddy]

1) already done by the tech;
2) dunno if it's done, I'll verify. And what's the appropriate "larger resistor", what value?;
3) how do I do this, need help or howto (newbe, sorry)?;
4) and 5) I've found another place where I can have Solen Fast caps: 2x0.33 to replace the .25 ones and 4x.1 to replace the coupling caps (I assume these caps are the ones connected to the 7591's sockets in the picture I attached previously, correct?). Is it a good choice for first step caps replacement?

Edit: they also have Orange Drops, so which ones are better?

Thanks.

1) excellent!

4)-5) For coupling caps I'd use CDE WMF series, Mallory PVC series, Illinois Capacitor MWR series, or Sprague 716P's (orange drops) before I'd even think about the Solen PPE fast caps. But, that's because I wouldn't want to make that vintage amp sound too bright.

. . Falcon

 

[Patrice B]

1) excellent!

4)-5) For coupling caps I'd use CDE WMF series, Mallory PVC series, Illinois Capacitor MWR series, or Sprague 716P's (orange drops) before I'd even think about the Solen PPE fast caps. But, that's because I wouldn't want to make that vintage amp sound too bright.

. . Falcon

Thanks.

I'll go for Orange Drops as they're the least expensive. I'll look under the amp and take note of all the values I need to replace all paper caps first.

 

[Patrice B]

Oops... They carry only 715 Orange Drops. Is it ok?

Thanks.

 

[FalconEddy]

Oops... They carry only 715 Orange Drops. Is it ok?

Thanks.

I thought that 'Just Radios' only carried 'orange dips' made by ETR (not a Vishay/Sprague 715P or 716P 'Orange Drop'). At only 49¢ a pop I'd be skeptical about putting them into MY vintage tube amp. Especially in the coupling cap position. Plus, they appear to be a pressed cap, which I'm not always keen on using. (so are the orange drops)

Have you looked at Mouser or Allied Electronics; or one of the AK vendors?

But, it's your amp. . . and your call.

. . Falcon

 

[Patrice B]

I thought that 'Just Radios' only carried 'orange dips' made by ETR (not a Vishay/Sprague 715P or 716P 'Orange Drop'). At only 49¢ a pop I'd be skeptical about putting them into MY vintage tube amp. Especially in the coupling cap position. Plus, they appear to be a pressed cap, which I'm not always keen on using. (so are the orange drops)

Have you looked at Mouser or Allied Electronics; or one of the AK vendors?

But, it's your amp. . . and your call.

. . Falcon

My mistake, I linked first "Just Radios" but found also "TubeStore" in Ontario.

TubeStore carry Sprague Orange Drops 715P... Better? Or not...?

 

[GordonW]

715s are OK. The main difference is that the 715 has a plated steel wire, while the 716s have plated copper wires leading out of the cap.

The 715 will definitely NOT make your amplifier suck. Many of them have been used, with good results.

As a note: Personally, I'm becoming a big fan of the Illinois poly and mylar caps (the yellow cylindrical caps). Just seem to work great. Used them almost exclusively in my restoration and update of a Heathkit AA100, and couldn't be happier with the way it turned out. Seems to be a good match to the tonal balance of the original paper/foil caps. I'm about to restore a pair of Heathkit W5s with the same caps...

Regards,
Gordon.

 

[FalconEddy]

715s are OK. The main difference is that the 715 has a plated steel wire, while the 716s have plated copper wires leading out of the cap.

The 715 will definitely NOT make your amplifier suck. Many of them have been used, with good results.

As a note: Personally, I'm becoming a big fan of the Illinois poly and mylar caps (the yellow cylindrical caps). Just seem to work great. Used them almost exclusively in my restoration and update of a Heathkit AA100, and couldn't be happier with the way it turned out. Seems to be a good match to the tonal balance of the original paper/foil caps. I'm about to restore a pair of Heathkit W5s with the same caps...

Regards,
Gordon.

Yes. MANY people have used the 715P's in their restorations.

I believe one of the main reasons the Illinois Capacitor MWR series and CDE WMF series caps sound so good in these vintage tube amps is because of the cap materials themselves. Metallized polyester or polyester (Mylar) and foil film caps are pretty basic, and this closely replaces the very basic vintage cap 'sound'.

Manufacturers had to get the best bang for the buck to be competitive and move product. So, one of the designer's principle jobs was to build and tune the circuit around these low cost passive components.

A lot of people forget this fact when they decide to install certain $$$ boutique caps when restoring these units. If you're looking to keep those vintage sound qualities, some high-end polypropylene caps available these days just won't fit the bill. Sometimes they can be too bright and even boarder on a harsh presentation.

If I had big bucks to spend, I'd go with certain Teflon and tin foil caps, V-Caps, silver/gold foil in oil, or double-wound induction-free. Probably just so I could brag about it. But, in the end, practicality would prevail; and I'd get the caps I had great results with, without going into debt.

My .02

. . Falcon

 

[kvflyer]

I have said for years that spending a large sum of money on a coupling capacitor is money that could have been used on vintage, NOS output tubes. Never any problems with them... You will only find Orange Drops and yellow Lemon Drops in my parts drawers. It took me a while to get around to the yellow jobs. I still like the Orange Drops but they are sometimes difficult to use because of being larger and having radial leads...

 

[Patrice B]

Back to a shop... again!

Hi,

Just to report back, I finally took the amp to another tech (not the same one, do not worry). This guy repaired successfully (back in october) my beloved Heathkit AA-100 and it worked wonderfully since then. The reason I've gone with the second tech for the Scott is that it's a few streets near my house, but as I've found, it's not necessarily better!

I just want him (the first good tech) to take a look at what's wrong before I do (patiently) a recap job myself.

I just hope that's not a blown OPT. With the symptoms enumerated (distortion on high level, past 7 or 8, on the left channel and hum disappearing after a while or completely disappearing if the output tubes are swapped), do you think it could be the case?

What could be the symptoms of a bad OPT? Just asking to be reassured because I already invested a lot of money on this Scott (with practically no results so far!).

Hoping it's just something simple like bad adjusted BIAS or bad caps on the BIAS circuit or the 6GH8 circuit.

**When I received the amp first, there was no distortion (as I recall) before it goes for the first verification...

**I can expect to have some news in about two weeks.

What do you think?

Thanks.

 

[kvflyer]

Hard to say if the capacitors are original. Please don't be offended but I think that we are just beating around the bush instead of addressing the major problems with these amplifiers and that is coupling and bypass capacitors. (Read, American Radionic Ceracap!) I would not want to begin to try to find a problem until those items are replaced. I believe that you mentioned that the bias bridge is replaced. You may also have a problem with the bias/filament supply capacitor. It is usually a 75-75-75-75 µFD capacitor. But be advised that this capacitor has a common positive, not a common negative like most electrolytic capacitors. I don't know of a current manufacturer that makes a replacement today. I have found that for the most part, they are still good. But that is not a guarantee. Some have gone out. Many technicians will just put a terminal strip under the chassis and install 4 discrete capacitors so that it looks original from the top.

So, I think that maybe why we are not getting anywhere. Just my 2¢...

 

[Patrice B]

Finally back to home...

Hi,

For those that already read the story and helped, here are the developments so far:

The second technician repaired efficiently the Scott (back to working state that is, not rebuilt to original specs). Some resistors were shot in the left 6GH8 circuit, thus causing hum and distortion... The guy saw it instantly plugging his oscilloscope (and the other one said, all is ok on the O'scope, oh well!)! He also replaced (at my request) the first dropping resistor after the silicon bridge from 18 ohms 1 watt to 33 ohms 5 watts.

I also noticed that the Scott doesn't like to be plugged in a powerbar. Maybe it is my imagination but when I received it back, at startup there was again the "scrtchhhh" on left side but since I put it on his own extension cord on another wall outlet, no more scratching at startup/shutdown... Coincidence maybe (a cap is now completely reformed?)!!! Dunno but anyway.

Another thing: the amp is really sensitive to the different 6GH8's I put in. I found two that are quiet but it looks that some are more prone to emit noise. Is it a normal situation concerning the 6GH8's in general? I'm glad I bought a lot of 6GH8's lastly!

Now, it's time for me to do a proper rebuilt. So far, I'll order this from Tubestore in Ontario to replace the paper caps:

- 4 x 0.22 uf caps
- 4 x 0.15 uf caps
- 10 x 0.1 uf caps

(all these Orange drops 715P)

- 2 x 0.33 uf Auricap caps. Why? To replace the two 0.25 uf caps that seems to be shielded (I don't know if it's the correct term). The original paper caps having a black stripe on one end. Do you think it is ok or should I go with an Orange drop too without regards to the shield thing?

For the cans:

- to replace the two 30/30 uf @ 500v, I'll go with JJ's 50/50 @ 500v. Do you think I'm correct?

- to replace the 20/20 @ 450v - 25/25 @ 25v, I found a 40/40/20/20 @ 525v at tubesandmore ( http://www.tubesandmore.com/scripts...nts/cemirror/ELEVATOR.FXP?item=C-EC4020X2-525 ). Again, correct?

Lastly, I'm not sure I'll replace the 75/75/75/75 @ 75v because I understand that it must be done only if really necessary because a replacement isn't available anymore. Again, if my understanding is correct, the only way is to go with four discrete capacitors underside. It seems this one is positive common as kvflyer noted.

I ask kindly the advices of the tube experts here...

**BTW, I imagine that after all this work I have to BIAS again (sorry to ask maybe an obvious question but I'm not in any mean an electronic experts. If it's the case, I'll need a detailed procedure of how to do it properly (the kind of where to plug the VM, to what resistor or pin and so on...).

Oh and for those wondering, even if not back to specs, it now sounds wonderful... Much better without distortion!

Thanks.