Scott LK-72 Rehab
- Thread starter Dave451
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ab2forever
Active Member
I have a pair of those good Scott OPT.
The plan is to repurpose them from the original 7591 set up, to a couple of sweep valves.
One of them is almost identical to the original 7591 but cost me 1USD for 5.
Those things can manage nearly 1/2 amp at 400V between a pair.
I just got a load of 5 damper diodes for the same money, which can handle the current, much better than the weak little GZ34 (with its 90V drop), and a pair of compactrons to drive the OPVs for 10USD.
How to build a rebuilt a Scott stereo amp for less than 100USD with much higher spec than original?
The plan is to repurpose them from the original 7591 set up, to a couple of sweep valves.
One of them is almost identical to the original 7591 but cost me 1USD for 5.
Those things can manage nearly 1/2 amp at 400V between a pair.
I just got a load of 5 damper diodes for the same money, which can handle the current, much better than the weak little GZ34 (with its 90V drop), and a pair of compactrons to drive the OPVs for 10USD.
How to build a rebuilt a Scott stereo amp for less than 100USD with much higher spec than original?
Hi everyone. Back in town now and about to dip back into the LK-72. Recall that I need to 1) get higher negative bias voltage and will do that by going "up the string" in the bias supply R-C per Gadget's suggesion and 2) get the 330K grid resistors properly grounded as advised by Dave G. After those things, I'll also power test the amp and check distortion levels, voltages, and general operation and report back next day or so. I want to see whether the Sylvania 7591A's installed in the amp are still as good as I hope they are.
Dave
Dave
Took care of the 330K resistors today (wired directly to nearest ground point) and re-wired the bias supply to post R209 take off in the R-C string to get the negative voltage up to the new network. This combined with a 22K "tail" resistor got the bias where it needed to be to adjust the tubes properly with about 3.5VDC range from the new bias level pots. Biased the tubes to 36 mA cathode current (around 360 mV at the measuring pins across the new 10 ohm cathode resistors). The bias/balance responded quite well to adjust each tube. With the tubes conducting properly, got the following voltages (schematic voltages in parens): Line 120 VAC; 7591A plates 450 VDC (420); Screens 410 VDC (380); Cathode voltages 360 mV (--);
V7 grid 19.6 VDC (-20); V8 grid -18.7 VDC (-20); V9 grid -18.2 VDC (-20); V10 grid -20.4 VDC (-20); B+ 455 VDC (425); Bias supply take off -50.7 VDC (-45); 12AX7 filament -45.8 VDC (-45).
Power test (1kHz single channel driven): Left (A) 34 WRMS; Right (B) 32.4 WRMS
Power test (1 kHz both channels driven); Left (A) 24.5 WRMS; Right (B) 23.5 WRMS
So, right channel is a little weaker than the left, but the 7591A's seem to be in pretty good shape. Both channel power seems about right compared to single channel driven results. At 36 mA cathode current, we are seeing 33 mA plate current (after subtracting screen current) and with 450 VDC on the plates, this gives a quiescent dissipation of 14.85 W, or about 78% of the 19W maximum.
So, buttoned her up, cleaned up the knobs and the front panel and moved the LK-72 to the listening room just under the Dynaco SCA-35 and the Scott 350B tuner. Sounds very good into the Klipsch KG4's. Very quiet. A little tube rush in PHONO mode, so I may want to change out the plate resistors at some point, but I'm not listening to a turntable these days (gave it and all my vinyl to my daughter along with a Scott 299B--trying to get the next generation started on the right foot!).
Quite happy at this point with the new addition and will keep it in rotation for a while to play in and check out. Listening to "Circus Money" and love the deep, tight bass (RIP Walter)...
Dave
V7 grid 19.6 VDC (-20); V8 grid -18.7 VDC (-20); V9 grid -18.2 VDC (-20); V10 grid -20.4 VDC (-20); B+ 455 VDC (425); Bias supply take off -50.7 VDC (-45); 12AX7 filament -45.8 VDC (-45).
Power test (1kHz single channel driven): Left (A) 34 WRMS; Right (B) 32.4 WRMS
Power test (1 kHz both channels driven); Left (A) 24.5 WRMS; Right (B) 23.5 WRMS
So, right channel is a little weaker than the left, but the 7591A's seem to be in pretty good shape. Both channel power seems about right compared to single channel driven results. At 36 mA cathode current, we are seeing 33 mA plate current (after subtracting screen current) and with 450 VDC on the plates, this gives a quiescent dissipation of 14.85 W, or about 78% of the 19W maximum.
So, buttoned her up, cleaned up the knobs and the front panel and moved the LK-72 to the listening room just under the Dynaco SCA-35 and the Scott 350B tuner. Sounds very good into the Klipsch KG4's. Very quiet. A little tube rush in PHONO mode, so I may want to change out the plate resistors at some point, but I'm not listening to a turntable these days (gave it and all my vinyl to my daughter along with a Scott 299B--trying to get the next generation started on the right foot!).
Quite happy at this point with the new addition and will keep it in rotation for a while to play in and check out. Listening to "Circus Money" and love the deep, tight bass (RIP Walter)...
Dave
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I'd call that power output difference nothing to worry about.
Any idea what the 6.3 vac heater line runs at? That might provide some perspective on the rest of the voltages. Its showing the B+ at 7% high. Sort of curious if the heater line is high by the same amount.
Any idea what the 6.3 vac heater line runs at? That might provide some perspective on the rest of the voltages. Its showing the B+ at 7% high. Sort of curious if the heater line is high by the same amount.
Didn't check it. I'll have a look next time I'm in. I haven't put in a CL-80 yet, but I will and that will cut the line voltage about 2 VAC and help the voltages a bit. Interesting that the voltages are much closer to those on the LK-72 schematic that doesn't have the DC balance control. (Plates 450V, screens 430V, and B+ 455 V). So, I wasn't too worried about the overage. The power supply is the same, so I'm not quite sure why the voltages are different between these series.
Gadget, I was thinking more about the voltages in my LK-72 (always dangerous--the thinking, not necessarily the voltages). When I had raised the amount of negative voltage available to the bias network, the current draw of the tubes, of course, went down and I saw a corresponding rise in the plate and screen voltages of the tubes. I recall the plates were running about 465VDC when the cathode current was in the 20's of mA. This makes sense; as the tubes draw less current from the power supply, the voltages should swing up. With the tubes biased to about 35-36 mA, the voltages are as I reported: around 450 V on the plate at 120 VAC line, which matches the LK-72 models without the DC balance control. With the same power supply set up, for plate voltages to run down to 420V (with corresponding drop in screen voltages), it would seem that the tubes would have to be drawing a lot more current and the amp running much hotter. Probably in the 40's of mA cathode current.
Point is, by setting up our 7591A's to run at lower current draw than original design (from the 'power war' era) for extended tube life, can we not always expect the B+, plate, and screen voltages to run higher, line voltage notwithstanding? If so, I wouldn't therefore expect that the filament voltages would be much above the nominal 6.3 VAC. Make sense? I see a lot of folks scratching their heads about B+ voltages being higher than the schematics beyond what might be accounted for just by line voltage increases.
Dave
Point is, by setting up our 7591A's to run at lower current draw than original design (from the 'power war' era) for extended tube life, can we not always expect the B+, plate, and screen voltages to run higher, line voltage notwithstanding? If so, I wouldn't therefore expect that the filament voltages would be much above the nominal 6.3 VAC. Make sense? I see a lot of folks scratching their heads about B+ voltages being higher than the schematics beyond what might be accounted for just by line voltage increases.
Dave
One other 'dangerous' thought--where does running the tubes at 75-80% of max plate dissipation put us on the operating curve (load line) of the tubes? I probably should take a look at the 7591 data sheet and study it. Wondering if reducing the quiescent bias/current level makes us more prone to non-linear operating, i.e. more distortion than the original designs? I seem to recall reading opinions that this wasn't enough to worry about, but just wondering.
Dave
Dave
Dave -- Since these power supplies are not regulated, there will absolutely be a increase/decrease in B+ voltage with a corresponding decrease/increase in output tube current draw as you state. On top of that however, Scott did in fact somehow manage to allow some power transformers for their 7591 based products to be installed that clearly produce too much juice from the HV winding, based not only on their own schematic voltages, but also on any concept of reliability. Probably the worst case of this that I've seen is some examples of their model 208, which I plan to offer a thread on soon. This is one of their standard 7591 offerings with only DC Balance controls provided. It is also rather unique in that the Scott schematic I have for it exactly matches my unit. Wonders will never cease!
Now the schematic indicated B+ voltages start from a lofty level to begin with in these models -- 490 vdc at the rectifier cathode, 470 vdc after the 80Ω rectifier dropping resistor (supplying the OPTs) -- as measured from a line voltage of 117 vac, and based on an understood quiescent cathode current of 35 mA per tube. This is also based on using output tubes of bogey characteristics. Working backwards, this would require a HV winding of 410-0-410 vac. Except that at the required line voltage and tube current draw, the transformer in my unit has a HV winding that produces 425-0-425 vac under these conditions. This produces 510 vdc at the rectifier and corresponding higher voltages to the output tubes -- in spite of the higher current they are now drawing (remember, no adjustable bias). But this is all at the stipulated 117 vac. Plug it into a typical line voltage today of 122 vac and now the HV winding produces 440-0-440 vac, 528 vdc at the rectifier output, and this again despite correspondingly even greater output tube current draw. This original Scott power transformer, in the stock Scott design, has over 500 vdc appearing at the plates of the output tubes when operating from a 122 vac line, with Scott indicating that it is acceptable to operate the unit on a line voltage of from 105 to 125 vac. The output tubes show color in the plates, and the screen grid bleeder resistor can now cook two turkeys instead of one. And the final straw, is that operating as indicated from a 122 vac line, the 6.3 volt heater winding produces just 6.45 vac, and the rectifier is supporting over 200 mA of continuous (DC) current flow, or just over 400 mA peak when the ripple current is accounted for.
This is just nuts. Safe operation for the stock design of this unit requires a line voltage of less than 117 vac, but that of course compromises heater voltage, which for output tubes is not good. And backing off output tube current draw to protect their health only makes matters worse. With a 122 vac line, decreasing output tube current draw from an established 35 ma per tube at 117 vac so that an 80% plate dissipation level is maintained (versus rating) at the higher line voltage produces an operating B+ voltage at the rectifier cathode that soars to 540 vdc! All of this is to say then that besides the normal B+ variations produced by tweaking output stage current draw, some HH Scott 7591 based units really have seriously high B+ voltage issues to contend with, stemming from what could only be considered on some (or likely multiple) levels within the company as a major power transformer "oops".
Dave
Now the schematic indicated B+ voltages start from a lofty level to begin with in these models -- 490 vdc at the rectifier cathode, 470 vdc after the 80Ω rectifier dropping resistor (supplying the OPTs) -- as measured from a line voltage of 117 vac, and based on an understood quiescent cathode current of 35 mA per tube. This is also based on using output tubes of bogey characteristics. Working backwards, this would require a HV winding of 410-0-410 vac. Except that at the required line voltage and tube current draw, the transformer in my unit has a HV winding that produces 425-0-425 vac under these conditions. This produces 510 vdc at the rectifier and corresponding higher voltages to the output tubes -- in spite of the higher current they are now drawing (remember, no adjustable bias). But this is all at the stipulated 117 vac. Plug it into a typical line voltage today of 122 vac and now the HV winding produces 440-0-440 vac, 528 vdc at the rectifier output, and this again despite correspondingly even greater output tube current draw. This original Scott power transformer, in the stock Scott design, has over 500 vdc appearing at the plates of the output tubes when operating from a 122 vac line, with Scott indicating that it is acceptable to operate the unit on a line voltage of from 105 to 125 vac. The output tubes show color in the plates, and the screen grid bleeder resistor can now cook two turkeys instead of one. And the final straw, is that operating as indicated from a 122 vac line, the 6.3 volt heater winding produces just 6.45 vac, and the rectifier is supporting over 200 mA of continuous (DC) current flow, or just over 400 mA peak when the ripple current is accounted for.
This is just nuts. Safe operation for the stock design of this unit requires a line voltage of less than 117 vac, but that of course compromises heater voltage, which for output tubes is not good. And backing off output tube current draw to protect their health only makes matters worse. With a 122 vac line, decreasing output tube current draw from an established 35 ma per tube at 117 vac so that an 80% plate dissipation level is maintained (versus rating) at the higher line voltage produces an operating B+ voltage at the rectifier cathode that soars to 540 vdc! All of this is to say then that besides the normal B+ variations produced by tweaking output stage current draw, some HH Scott 7591 based units really have seriously high B+ voltage issues to contend with, stemming from what could only be considered on some (or likely multiple) levels within the company as a major power transformer "oops".
Dave
Probably so on the plate voltages, but its worth a check of the heater voltage just to see if its all up by the same amount as a sanity measure. Reducing the current would definitely bump the B+ though, not regulated as Dave G explained.
The other thing to keep in mind is that the 36ma is plate + screen current. Using that to figure dissipation actually comes out with a number that is a little higher than reality. Datasheet conditions for a 7591 with 450 on the plate and 400 on the screen say 41ma of plate current plus almost 6 ma for the screen, so 47ma total at the cathode.
Thanks for the responses, guys. Dave G: Yikes! Doesn't appear like my LK-72 is one of the red hot units you are describing. Glad to hear that 35 mA is the target. Now that you mention it, I recall the recommendation for my Scott 340B was 35 mA per 7591A. I was assuming 3 mA for screen current, so a net 33 mA from a 36 mA cathode current gives 14.85W quiescent plate dissipation at 450V on the plates, or 78% of the 19W max. If the manufacturer recommendation is 35 mA, then I have to presume the linearity will be good in this part of the envelope, right?
Dave
Dave
Absolutely.
Dave
Dave
ab2forever
Active Member
All the more reason to drop in some of those xHxx TV valves isn't it??
They will stand up to as much abuse as you can throw at them, and they're nigh on identical to the 7591....
kvflyer
Super Member
I have never owned an HH Scott 208. So, does that amplifier have the same power transformer as say the 299C/D etc.? If so, could the excess B+ possibly be due to the lack of other tubes drawing current, namely the preamplifier tubes, 12AX7? I realize that preamps don't draw much B+ current.
Just a thought?
Just a thought?
jacksonwalke
New Member
Dave, thanks for the info and the pictures. My landlord (!) just gave me a mint Scott Stereomaster LK-72 because he knows that I have been recapping some vintage electronics over the past few years, and thought I might like the challenge of working on some tube gear for once. The unit seems like it has not been touched in years (if not decades). It still has the original Scott branded tubes, the dark brown faceplate found on the early models and the original Siemens selenium bridge rectifier. Who ever put this together did a great job because the soldering and dressing looks to me a lot like the early seventies Pioneer and Kenwood units I have worked on.
After reading on line about some basic repairs for this unit, the Siemens selenium bridge rectifier replacement seemed like a real challenge. But thanks to your photos I have a better idea how to approach this. I am still a little nervous about some of the grounding warnings I have read about the cap replacements, not to mention I am more familiar with non-tube gear, but I am not in a hurry and plan to take my time. Thanks again.
After reading on line about some basic repairs for this unit, the Siemens selenium bridge rectifier replacement seemed like a real challenge. But thanks to your photos I have a better idea how to approach this. I am still a little nervous about some of the grounding warnings I have read about the cap replacements, not to mention I am more familiar with non-tube gear, but I am not in a hurry and plan to take my time. Thanks again.
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A LK-72, along with the matching tuner and multiplex decoder, landed on my bench a few days ago for evaluation for restoration.
The new owner brought it in so I could bring it up on my variac, monitor the current drawn, and do a distortion and output check with my Sound Tech 1701.
The unit has all Scott tubes, seems to be completely orginal, and does have some issues.
It matches none of the 299 or 72 schematics I can find in the AK database, there are no balance or bias controls that I can see. Tube complement is 12ax7, 7199, and 7591s.
The owner does have the schematic and constuction manual which he forgot to bring along with the equipment but he did send me a couple of photos which I will try to post tomorrow.
Most interesting to me is he has the first owner's McIntosh Amp clinic performance graph from Nov. 1962. This must be from my friend Dave O'Brien's second or third amp clinic. According to Roger Russell's site DOB did his first Mac clinic in Oct. of that year.
In 1978 I hosted my first Mac of a dozen or so amp clinics with Dave so there certainly is a good feeling seeing those initials again......
The new owner brought it in so I could bring it up on my variac, monitor the current drawn, and do a distortion and output check with my Sound Tech 1701.
The unit has all Scott tubes, seems to be completely orginal, and does have some issues.
It matches none of the 299 or 72 schematics I can find in the AK database, there are no balance or bias controls that I can see. Tube complement is 12ax7, 7199, and 7591s.
The owner does have the schematic and constuction manual which he forgot to bring along with the equipment but he did send me a couple of photos which I will try to post tomorrow.
Most interesting to me is he has the first owner's McIntosh Amp clinic performance graph from Nov. 1962. This must be from my friend Dave O'Brien's second or third amp clinic. According to Roger Russell's site DOB did his first Mac clinic in Oct. of that year.
In 1978 I hosted my first Mac of a dozen or so amp clinics with Dave so there certainly is a good feeling seeing those initials again......
I can find no potentiometers to adjust balance or bias level and it does appear that a pair of .05 ceramic disc caps are used for coupling (circled).....as I said I am waiting for the build manual to see what was actually used.
I also have a picture of the amps test from 1962......
I would suspect the owner might have a been a bit upset to discover his new XX watt amp actually came up a bit short....especially in the bass department.
I also have a picture of the amps test from 1962......
I would suspect the owner might have a been a bit upset to discover his new XX watt amp actually came up a bit short....especially in the bass department.
Often times the test gear that David would bring in was far superior to what many dealers repair shops had....and Mac had a tradition of making the units "right" at no charge if they needed any service.
Of coarse since many manufacturers used specs that seemed to come out of the ether the clinics did have tendency to show up these euphoric claims......
As a dealer back in the day it was good will for Mac owners and a store traffic builder along with thing like turntable calibration clinics during the slow time for walk in trade.
Old school marketing in the days before the net......where your customers were your neighbors, bleacher mates from your kids baseball games etc.
Of coarse since many manufacturers used specs that seemed to come out of the ether the clinics did have tendency to show up these euphoric claims......
As a dealer back in the day it was good will for Mac owners and a store traffic builder along with thing like turntable calibration clinics during the slow time for walk in trade.
Old school marketing in the days before the net......where your customers were your neighbors, bleacher mates from your kids baseball games etc.
