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Scott LK-150 rebuild/modification thread

Sam Cogley

Sam Cogley

Last of the Time Lords
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This project sat around for a while after the last thread went so far off the rails. Since then I've been poking around the LK-150 a bit, and it's time to get busy with it.

Some ground rules to start: if you have negative, pointless commentary about modifying a holy piece of equipment blessed by the sainted Herman Hosner Scott, take it somewhere else. I will report anything of the sort to moderation as soon as I see it.

Now that that's out of the way, the first step in the project was getting a legible schematic to work with. Since legible schematics of the LK-150 don't exist, I redrew one from the final, Nov. 1962 revision. Please feel free to use it for non-commercial purposes. If you're going to use it for commercial repairs or to clone the LK-150 for sale, please contact me first.

That shrunk horribly. I'll submit it to the AK database.
 
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Plans for the Scott originally included regulating the HV and bias supplies. After reviewing the work that Dave Gillespie has done with his EFB II system, I think I'm going to try to implement that as both an easier and more sophisticated method of controlling voltages in the amplifier. I will also be installing a toggle switch in each channel allowing the preamp tube to be easily switched between the stock 7199 and the much easier to locate 6U8/6GH8. Finally, the bias adjustment circuit will be modified for individual tube bias, using a modified version of the H/K Citation II circuit for inspiration.

Now that I have a clean schematic to work with, I'll begin work on the revised schematic.
 
Please add a pic here and there! I have a hard time "imagining" what this would sound like from just a schematic!
 
Schematic modified with individual bias adjustments for each output tube. My math for the grid resistors was checked by Kegger in the insane mess of the last thread on this amp.

Edit: OK, that's better.
 
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I was just wondering if you have done a basic test of the amp before making the mods? Do you have any info on the output transformers?,I have a 272 I'm going to change into an amp and I think the transformers are the same as a 290, but if they are close to the 150,I would think about that route instead.
 
Sam -- couple of quick observations:

1. Changing the bias arrangement of the output tubes does not require changing the original value of the grid stopper resistors. To maintain the original HF characteristics of the design, I would maintain those at the original value of 1K each.

2. Consider going to a Bias/Balance arrangement, which allows for the quiescent current of the entire output stage to be raised or lowered with the Bias control. This then allows for easy setting of the minimum distortion operating point if equipment is available for setting that.

3. If still going the individual bias control route, then consider a resistor between the low side of the controls in each channel and ground -- this to prevent the possibility of setting the controls for no bias at all on the output tubes.

4. Is the value of R204 correct? It seems very strange in value and wattage for the place it occupies in the design.

5. C103 got lost in the translation. Somewhere along the line, its designation remains, but the actual cap is shown as a short circuit.

C202 and C203 should have balancing resistors across them to equalize the DC voltage drop between them. 220K 1W resistors will do the trick.

7. EFB(tm) will require some tweaking, but the basic topology is generally correct. Along the way, I'll need you to run a quick experiment, and we'll also be able to dispense with R205 and R206 which are real space heaters! I should be able to work on that later today or tomorrow.

You've got a very neat project going here!

Dave
 
dcgillespie said:
Sam -- couple of quick observations:

1. Changing the bias arrangement of the output tubes does not require changing the original value of the grid stopper resistors. To maintain the original HF characteristics of the design, I would maintain those at the original value of 1K each.
Click to expand...

Dave, that was done to maintain the overall grid resistance of the circuit, something left over from the old thread that went off the rails and drove the project back into the dark for a while. Not hard to change back, but I'll have to do a bit of math to figure out what to use for the resistors going into the bias adjustment.

2. Consider going to a Bias/Balance arrangement, which allows for the quiescent current of the entire output stage to be raised or lowered with the Bias control. This then allows for easy setting of the minimum distortion operating point if equipment is available for setting that.
Click to expand...

I don't have a distortion test set, but how would the Bias/Balance arrangement be implemented?

3. If still going the individual bias control route, then consider a resistor between the low side of the controls in each channel and ground -- this to prevent the possibility of setting the controls for no bias at all on the output tubes.
Click to expand...

That seems reasonable enough.

4. Is the value of R204 correct? It seems very strange in value and wattage for the place it occupies in the design.
Click to expand...

R204 is a 220 ohm, 1W resistor in every version of the LK-150 schematic that I've seen. I'll double-check the assembly manual pages, but that seems to be what Scott put there.

5. C103 got lost in the translation. Somewhere along the line, its designation remains, but the actual cap is shown as a short circuit.
Click to expand...

Whoops. That cap was missing on the badly degraded schematic I started with. I'll fix it right now.

C202 and C203 should have balancing resistors across them to equalize the DC voltage drop between them. 220K 1W resistors will do the trick.
Click to expand...

Seems reasonable enough. Those can be easily added.

7. EFB(tm) will require some tweaking, but the basic topology is generally correct. Along the way, I'll need you to run a quick experiment, and we'll also be able to dispense with R205 and R206 which are real space heaters! I should be able to work on that later today or tomorrow.

You've got a very neat project going here!

Dave
Click to expand...

I hope this experiment doesn't involve actually operating the amplifier. If so, we might have to find someone else with one to help out - this amp was a hacked-up mess when I got it, and I currently have it completely apart to do some chassis repairs and to test the transformers.

Will a 500V MOSFET be sufficient to survive in the EFB(tm) circuit?
 
Hi Sam .

I would suggest a power supply on a separate chassis and a regulated supply for each channel built were the power transformer and rectifier tube had been .
 
Just checked the assembly manual. I don't have the diagrams that accompany it, so I can't be 100% sure, but it appears that R204 is correct, per Scott. Of course, I'm not wedded to doing everything the way Scott did, so I'd be happy to change that value to something more appropriate.

Mark, I kind of want to keep it looking mostly stock. This is more or less an experiment in making this old beast to be the most of what it could be if Scott had had some modern gizmos in 1962.
 
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Added some pre-rectifier diodes as Dave suggested on another project of mine a while back, added a safety cap to the mains side of the power supply, and tinkered with the bias adjustment section and grid resistors. I'm not sure what the resistor on the ground side of the bias adjustments should be, H/K used a 5.6K there, but they also used 7.5K bias adjustment pots.

If I install a meter instead of just putting a pair of test points in the hole, that resistor will go away. It's just there to provide proper loading for the meter. Haven't had a good reason to re-do that section, though. The switches located there will be mini push-button switches.

Also, if you click on the schematics above, they'll take you to the Photobucket page - where you can zoom to a much larger and more legible size.
 
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Sam -- what is the AC voltage level supplied to the bias rectifier from the tap on the HV winding in the power transformer?

Dave
 
Still haven't had a chance to check voltages, though I have gone through all of the transformers and verified that I don't have any blown windings. Relief.

Someone emailed me and asked what program I used to re-draw the schematic, but I accidentally deleted the email. I have an ancient copy of Adobe Photoshop CS2, I just used that. Photoshop Elements or any other simple image editing/drafting software would work fine. I opened my previously stitched-together and touched-up schematic in there, opened a blank, transparent layer on top of it, and started drawing bits. For repeated stuff, I just copied and pasted over and over, lined everything up, and drew lines between them. The transformers were a bit fiddly, I ended up using two overlapped circles (like a Venn diagram), then erased the unnecessary bits. Merged those layers together, copied, merged the two new layers together. Kept going until I had what I needed to depict the transformers. Likewise, the fuse symbol was just two circles with a bunch erased and a little bit of straight line in the middle. Not too hard, and not super time consuming once you get going. Once it was done, I added a plain white layer to the back (on top of the original schematic), saved an editable ,PSD copy, then flattened it into one layer to save as a .JPG.

The eagle-eyed might notice a bit of sepia tone in the documentation box at lower right. The block-lettered bits I just re-typed, but I preserved the original engineers' signatures by copying and pasting.
 
Sam Cogley said:
Anyone know of a replacement meter that will fit the front panel of the LK-150?
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You could try something like this:

p1010023kc_zps62e7d092.jpg
 
s-petersen said:
You could try something like this:

p1010023kc_zps62e7d092.jpg
Click to expand...

I actually have a very similar meter here as one of the options.

battradio said:
HI Sam ,

I have a couple of meters that should work , will give them to when we meet up next time
Click to expand...

Mark, I'll send you the manual page with the illustration of the original meter. It's a weird beastie to give the front pair of capacitors clearance. Considering the tight clearance up front, I'm considering cutting a new hole at the bottom of the faceplate, as I have to make an overlay for it anyway - the previous idiot drilled holes for new bias pots right through the "HH Scott Stereomaster" logo. I can flip the logo to the top on the overlay, and put the meter at the bottom so it gives room for the cap connections. I'll have to add a couple of angle braces to reinforce the weakened area of the chassis, though.
 
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dcgillespie said:
Sam -- what is the AC voltage level supplied to the bias rectifier from the tap on the HV winding in the power transformer?

Dave
Click to expand...

OK, I did some testing today. At around 115V, the outside HV taps are putting out 665VAC (about 332 on either side of the center tap), and from the ground tap to the bias tap it's 106VAC. Bumping voltage up to 124VAC (what my house usually gets), 738VAC on the outside taps and 115VAC from the ground tap to the bias tap.
 
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