Scott LK-150 Low Level 60Hz Hum at Speakers: Transformer-Induced?

Even if the output tubes are set to ostensibly the same bias- slight differences between idle current draw of one vs. the other, can induce hum.

If you VERY SLIGHTLY change the output bias balance of either channel when the amp is idling, does the hum increase or decrease in that channel? Might be worth a shot...

Regards,
Gordon.
 
This video discusses an LK-150 that had two chokes added near the OPT's, which I found informative:
The one double 50 cap on mine is in series to give 25uF at 1000V.
I've been marking up a copy of the power supply section schematic in order to place my cans in the context of the original setup, using the dropping resistors and ground points as references.
One of the things to do to bring it up to the 1962 TSB is to remove the 22K resistors at the bias pots, but I don't see that affecting the hum issue. I presume that would only affect the range of voltage the bias pot will adjust for.
Then it mentions placing different coloured wires going from cap terminals to other terminals which the TSB says will improve the sound and increase the power, though it already sounds terrific.
 
GordonW said:
Even if the output tubes are set to ostensibly the same bias- slight differences between idle current draw of one vs. the other, can induce hum.

If you VERY SLIGHTLY change the output bias balance of either channel when the amp is idling, does the hum increase or decrease in that channel? Might be worth a shot...

Regards,
Gordon.
Worth a go. Thanks for suggesting it as I'd not have considered it.
Click to expand...
 
GordonW said:
Even if the output tubes are set to ostensibly the same bias- slight differences between idle current draw of one vs. the other, can induce hum.

If you VERY SLIGHTLY change the output bias balance of either channel when the amp is idling, does the hum increase or decrease in that channel? Might be worth a shot...

Regards,
Gordon.
Click to expand...
Is that what a "hum pot" does in some amps? Sounds like it acts more of a balance pot, which would be nice to have though I know one can install them as dcgillespie talks of the bias-balance arrangement.
 
"Hum pots" act on the heaters.

I'm referring to the bias balance pot that adjusts the DC plate currents in the tubes of a channel, relative to each other. If they're slightly imbalanced (where one tube in a pair is drawing slightly more than the other), it can induce hum...

Regards,
Gordon.
 
Did we ever establish if there is a hum pot or some means of referencing the heaters to ground in this? If there isn't, there really needs to be.
 
gadget73 said:
Did we ever establish if there is a hum pot or some means of referencing the heaters to ground in this? If there isn't, there really needs to be.
Click to expand...

Yes the heaters are grounded at a terminal strip for each side.

I see the heater circuit for V2, 7 and 8 also supplies a light bulb (no. 12) which must have been for the bias meter. Since I got my meter freed up (needle rubbing against the dial) I will try to re-add that.

I just finished adding rubber grommets between the transformers and chassis to I hope absorb some mechanical vibration(The trannies are still electrically connected to it via nuts and bolts).
 
The light bulb is pictured in the original manual that's available to download from several locations. I think it mounts on the retainer bracket for the meter.
 
Well with or without the tubes there's some hum. Ditto with input shorting plugs.
I have a hard time reconciling the way the four JJ 50-50/500v caps are versus the circuit diagram.
Without detailing it (my crib sheet is elsewhere) I think there's a resistor missing that parallels the 150uF/75V (I don't know what value is actually there now given the aforementioned JJ cap confusion) which comes right after the 1.5 K resistor that comes after the silicon diode in the power supply.

Wonky voltage readings:
Where is says should be 67 volts it's actually 96.
Where it says 35 volts at another place it's really 100.
The B plus seems about right, 475 on one,of the output tube pins.
Bias voltage seemed about right. idle current seemed about right for the 6550s, measured for each pair.
Playing with the bias pots had no bearing on hum.

It's more complicated than my Fishers though it looks simple enough, and its been redone by someone else and is rarer and thus fewer rebuild threads. I'm out of my depth I'll say at this point. I've had it amlmost 8 years and this problem hasn't gone away. I've even had a tech, who knows vacuum tubes - he repairs several high end brands as well as vintage and designs his own - look at it twice and its come back the same. Don't get me wrong he did great things for my Dyna Mark iii's and CJ preamp etc...but his test gear didn't pick it up, or his speakers were less sensitive and so didn't reveal hum at a distance like mine do.

I know its potential so don't want to give up on it, but I'm stuck up here in Canada where there's little interest or expertise locally. My next bet will be to send it somewhere: anyone in Canada know a place? I mean that won't end up not returning emails and dishing excuses that is...
there's a man in Quebec City I recall who was very good and fast, though not cheap (one can NEVER have all three in our universe: pick any two as the saying goes). Mario someone...I'll research it. Shipping won't be cheap, maybe send it on a pallet!

Thanks for any help so far everyone.
 
I don't know which version you have and how far it has been modded but during production of the LK-150 Scott issued a Technical Bulletin about a recommended power supply modification/upgrade. Maybe you should check it and compare with your amp.
 

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Tubologic said:
I don't know which version you have and how far it has been modded but during production of the LK-150 Scott issued a Technical Bulletin about a recommended power supply modification/upgrade. Maybe you should check it and compare with your amp.
Click to expand...

That's the Nov. 1962 TSB/schematic that we were discussing earlier. :beerchug:
 
Hi Sam:

Here are some voltages, which I'll describe by placement on the schematic (1961, which is the one I already started to mark up)

After the diode (replaced, was I think the Siemens originally) coming FROM the bias tap on the PT and to the right of the diode (newer-original schem. called for 1.5K 2W) 3.3K resistor - minus 97.6V (schem said minus 67V).

To the right on the terminal strip there's the 220 1W resitor whereafter it drops only slightly to -97.2V.

I note R 210 (parallel to the left-more150uF/75V (on schem)), 6.8K 2W, is absent.

Between R205 and the two 80 ohm resistors, at the node with the series-aligned 60/500 caps (on mine it's hard to tell what caps are used for that, since it involves the 50+50/500 (in series would be 25/1000) plus a 100/150V electrolytic somehow tied in - its negative lead comes directly off the terminal strip on the schem. above or downstream of R203 and the positive lead ties to what on the schematic is C202- ill show it with a picture later, or there may be one earlier in the thread), it is +481V (490 on schem). Sorry for the muddled-ness but this is where it confuses me most.

At the node connecting C205(ref.40/450 but 50/500 on mine), R206 (15K 15W) and R207 (4K 7W) it is 407V (415V on schem).

Between R207 and R208 (not 10K but 8900K on mine) where meets positive of C206 (ref. 20/450 but 50/500 on mine): 338V (360 ref.)

To the right of R208 where it meets C207 (20/450 on schem-50/500 on mine): 315V (ref. is 300V).

Since I'm at work I'll put the other voltages in a separate post, lest what I wrote get somehow erased between breaks.
 
More voltages, and thanks to Tubologic for posting the TSB's:

Pin 1 of V2B (1/2 7199) on each side shows +237 volts, but the schem shows ".75V" which really dumbfounds me.

Pin 8 on each 7199 shows 100V, which is listed as 35V. Again I'm at a loss.

The 7199 pin diagrams are I think accurate (on an earlier redrawn pdf, pin 9 was shown in error as pin 4 which I corrected on my copy).

On my specimen there is a 22K 1W resistor between the bias supply and bias pot, but I THINK some LK-150's had it and others not.

Voltage at the grid of each 6550 is -41.5.

There is a 3.3 ohm 5W resistor off the cathode (pin 8) off one tube of each pair of 6550's which shows drops to ground of 0.431V and 0.449V, which when I calculate cathode current for each tube is about 70 milliamps.

Plate voltages on the 6550's is 470 each, give or take, same on the centre taps.

I wish I knew how to explain the above 7199 voltage discrepancies though, so maybe Mr. Inman in 2008 did the TSB's to it but I have yet to confirm until I can tell which terminals the changes are supposed to be made at, given the JJ double 50/500V filtering cap situation in relation to spec's. on the schemo.
 
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Another thing I noticed is when I moved the 0.047uF caps around in space at the "Lab-Normal" rolloff switches the hum varied in loudness but didn't go away. I read elsewhere that even nonpolarized caps can pick up hum if not oriented with correct polarity.
Ditto when I manipulated the 4uF/250V (C3 and C103) cap between pins 3 and 6 of the 7199's.
The coupling caps are less than spec (C4 and 5, C104 and 105) at 0.22uF/400V or 450V (I'm not at home as I write) versus 0.25uF/600V, but the voltage going into them is not greater than 400 or 450.
That in three posts is what I know so far.
Pictures to follow, using good light and a tripod.
 
Without working out a schematic of whatever power supply modifications were done to your unit, it's going to be difficult to sort out what's going wrong. Are you getting the same voltages on both 7199s, or just V2?

That missing resistor in the negative rail is probably causing the greater voltage there.

Most of those voltages are within a reasonable tolerance of the specified values. The ones that are off are way off, though. Are R103 and R104 in place and of the correct values? I think they'd be the first things I'd check considering the weird voltages on both sides of the V2 triode section. If one of them is an incorrect value or damaged, or there is a bad solder joint somewhere, that could be causing some of the weirdness. I'd check R101, C102, and R106 as well, as they're also acting directly on that triode section.

The 22k resistor between the bias pot and the negative rail was eliminated in the Nov. 62 schematic with the redesigned power supply. The TSB mentions removing it and installing a jumper wire.

.22uF should be fine for the coupling caps. One of my long-running questions that I keep going back and forth on regarding my slow rebuild is whether to use .22uF NOS Soviet K40Y PIO coupling caps, or paired .15uF and .10uF Sprague Vitamin Q PIO caps to get right on the specified .25uF value. Most of the gurus seem to think that either is fine. I'm considering trying one version in one channel, and the other on the other side in order to compare results. 400V or 450V parts might be a bit low, that doesn't leave you with much tolerance if there is a spike. .22uF 1000V caps are pretty readily available and not terribly expensive, both in film and PIO forms. .22uF 500V are very common. There are usually lots of NOS Soviet K40Y and K42Y PIO caps on eBay, and they make very nice coupling caps for tube amps.

I fixed that pin 4/9 thing on the schematic and sent it to the AK database moderator.

Another thought: you might look at the original assembly/operation manual and pictures of the underside of working amps to figure out if there is an issue with the cap positioning that's adding to the hum.
 
Pin 1 of a 7199 is the plate of the triode. There is NO chance it is supposed to be .75V. It sounds like your schematic is unreliable, which is making your challenges that much greater.
 
The voltages you quoted on the 7199 look fine. Because the pentode section is direct coupled to the triode section you can get quite varied readings. From what you quoted, the plate of the triode is dissipating about 0.9 Watts (might be running a little hot by my standards, but easily within spec), and has about 6.6 mA of idle current. From the characteristic curves, that *should* put the grid of the triode somewhere about 5 volts lower than its cathode. That all seems fine.

Would be good to measure the voltage on pin 2 (plate of the pentode which is also the grid of the triode) and verify that it is indeed about 5 volts lower than the voltage reading on pin 8 (cathode of the triode). If that verifies, then I'd say things are fine.
 
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