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HH Scott LK-72 rebuild log


The LK-72 has a non-polarized 2 prong AC connector. So this means either line could be the "hot" line depending on how it's plugged in, right? Does this mean I should have a CL-80 on both lines before the power transformer?
 
Not having a cathode resistor to measure across is one of the things this vintage of LK72 suffers from. You can’t really know where the tubes are running unless you add one or have one of the socket adapters that taps into the circuit.
However they are still around sounding great anyway.
I keep thinking about adding grid stoppers and IBBA to mine but have just been listening to it for the last few years instead!
 
I experimented with the CL-80s, and two in series with hot seems to be the ticket. Here are the new voltages with the 2 CL-80's:

7591 plate (pin 3)
Expected: 450V
Actual: 448V

7591 grid 3/cathode (pin 5)
Expected: 0V
Actual: 0V

7591 grid 1 (pin 6)
Expected: -20V
Actual: -21.5V

7591 grid 2 (pin 8)
Expected: 430V
Actual 435V

Much better! I will have to re-measure the voltage on the 12AX7's and adjust the resistor accordingly, and I still have to measure the bias of the 7951s. Getting closer!
 
Just double checking here... to measure the bias of the 7591's, I should connect a 1 or 10 ohm resistor between the cathode (pin 5) and ground, measure the voltage across the resistor, and use ohms law to calculate the mA bias, correct? Doing that, here are the voltages across a 10 ohm resistor for the 4 output tubes (closest to the PT to furthest): 0.5mV, 1mV, 1.3mV, 1.5mV. Using ohms law, this means the bias on each tube is 0.05mA, 0.1mA, 0.13mA, 0.15mA. That seems way too low to be true. For sanity, I plugged in my new production JJ 7591's, and I got the exact same readings for each tube. That makes me think I'm either measuring them wrong, or something else in the amp isn't quite right... Any ideas or advice?
 
Some updates from the weekend:

1. I connected speakers to the amp just to see where I'm at. There's a really bad buzz in one of the channels... and a much less, but still noticeable buzz in the other channel. I tracked this down to a bad 7199. I flip-flopped the 7199 tubes, and the loud buzz switched channels. I have some 6U8A tubes on hand, and am waiting for 7199 -> 6U8A adapters to come. Hopefully the buzz will be better with the new phase inverters.

2. The left channel is still weaker than the right (made sure the channel balance pot was in the middle). Even with the new production JJ tubes the left channel is still weaker.

3. The left channel has much worse DC balance than the right no matter the output tube configuration (measured by connecting DMM to the plates of both output tubes in one channel). Maybe related to the left channel being weaker?

Original 7591's
Left DC balance: 875mV
Right DC balance: 275mV

JJ 7591's
Left DC balance: 990mV
Right DC balance: 35mV

4. Still not sure how to correctly measure the bias of the output tubes. See previous post.
 
For the bias measurement, you would place the red probe on the junction after the 10ohm resistor, and the black grounded to the chassis or the neg terminal on the first HV capacitor.
I can’t offer anything on the DC balance except to suggest the pot itself maybe be bad, which is not uncommon.
Any chance you could post a picture of the wiring around the bias sense resistor?
 
carlman14 said:
3. The left channel has much worse DC balance than the right no matter the output tube configuration (measured by connecting DMM to the plates of both output tubes in one channel). Maybe related to the left channel being weaker?
Click to expand...

You can't really check balance at the plates. Each side of the output transformer will measure a different resistance and so will have differing DC voltage drops. If you measure the resistance for each side of the OPT primary you can adjust for it in your measurement but it's a bit of hassle that way. You can also put a 1 or 10 ohm resistor in series with each plate but you have to measure separately. This will take screen current out of the measurement. You want current balance in the primary. A resistor in the cathode will include screen current.

Have fun,
John
 
LinderK said:
For the bias measurement, you would place the red probe on the junction after the 10ohm resistor, and the black grounded to the chassis or the neg terminal on the first HV capacitor.
I can’t offer anything on the DC balance except to suggest the pot itself maybe be bad, which is not uncommon.
Any chance you could post a picture of the wiring around the bias sense resistor?
Click to expand...

I feel like that's what I was doing. 10 ohm resistor on pin 5. One lead of the DMM goes to ground, the other lead goes to the side of the resistor connected to pin 5. Seems simple, but my bias measurements all seem bogus. I'll post a pic of that area later when I get home. As for DC balance, this amp doesn't have DC balance pots. I'd have to add them if I wanted DC balance.

nerdorama said:
You can't really check balance at the plates. Each side of the output transformer will measure a different resistance and so will have differing DC voltage drops. If you measure the resistance for each side of the OPT primary you can adjust for it in your measurement but it's a bit of hassle that way. You can also put a 1 or 10 ohm resistor in series with each plate but you have to measure separately. This will take screen current out of the measurement. You want current balance in the primary. A resistor in the cathode will include screen current.
Click to expand...

Thanks for the input. I saw someone on another Scott thread suggesting to measure balance that way. After what you said and more research, I'm finding there are better ways to measure DC balance.

Side note: I just discovered IBBA's. Being that this is the version of the LK-72 that doesn't have DC balance pots, does anyone have any opinions on adding an IBBA to this amp? Seems like a good idea, and would take care of both my balance and bias needs. I've found many folks here who have added an IBBA to fisher amps (and found schematics for those), but not much for adding an IBBA to this amp.
 
I figured out the bias measurements. You have to remove the existing connections from pin 5 first (ground wire and one lead of a 330k ohm resistor), then connect the 10 ohm cathode resistor. The one I measured so far is at 20mA. Not a good start. Will report back when I've measured all of them.
 
carlman14 said:
I figured out the bias measurements. You have to remove the existing connections from pin 5 first (ground wire and one lead of a 330k ohm resistor), then connect the 10 ohm cathode resistor. The one I measured so far is at 20mA. Not a good start. Will report back when I've measured all of them.
Click to expand...
Cold bias...
 
It appears that you have plenty of voltage available to spread between the tubes. IBBA or IBAM could easily accomplish that for you. My vote would go to the IBBA configuration.
 
LinderK said:
It appears that you have plenty of voltage available to spread between the tubes. IBBA or IBAM could easily accomplish that for you. My vote would go to the IBBA configuration.
Click to expand...

That's what I was thinking too. I'm not too familiar with these, so I have a couple questions:

1. IBBA is "Individual Bias Balance Adjustment", correct? Does this mean it handles both the DC balance as well as bias? That would be great!
2. I've seen a ton of people implement IBBA's and IBAM's for fisher amps, but not much in the way of LK-72's. Is there a post or guide you're aware of for assembling an IBBA for the LK-72?
3. Similar to above, once the IBBA is assembled, is there any guide on how to connect it to the rest of the amplifier circuit?

Never done this before, so I might need a little bit of hand-holding or resources for the IBBA.
 
It is the same assembly process... it is connected on the existing bias feed. The balance portion of the circuit balances the bias voltage between the two channels. The harder aspect is where you decide to put it in the chassis, and where you want to locate the test points.
 
Or like member FalconEddy did years ago: https://1drv.ms/u/s!AsCftN_jdRMFgbsgyJ-Ky8TRVzPsLQ

Personnally, I think I used 100k pots (could be 50 also, not sure, it’s been a while...) but no bias caps... Similar to what Scott did on the « D ».

**Edit: I don’t know why FalconEddy used 4.7 ohms@5w to measure bias? Much easier with 10 ohms 1/4 w.

In my case, no test points on the top added so I have to remove the cover to check it...
 
Last edited:
@Patrice B @LinderK Thanks for the info! I am ordering parts for an IBBA.

Some other updates:
- I moved everything connected to pin 5 to pin 1.
- Installed 10 ohm resistors on pin 5 to pin 1 (pin 1 is grounded now).
- Got a couple 7199 -> 6U8 adapters to see if the buzz I'm hearing is any better.

Still hearing a pretty prominent buzz/hum through the speakers, but it's not quite as bad after using the 6U8's. I'm hoping that maybe after I get the output tubes properly balanced it will be resolved. Also still have a slight channel imbalance, but this is remedied by rotating the channel balance pot a bit. I'm going to test the channel balance pot later to see if it's failing.
 
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