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HH Scott Owners Thread

My latest project is this very nice condition LK-48 B I have had for a few years, mostly as eye candy because of leaky power supply capacitors. I decided this unit was worth a total rehab and a couple modifications to bring it into a long and happy second life. First up was some Hayseed Hamfest cans, then all the usual underneath capacitors that might have longevity issues. I installed a screen buss with stability resistors, a CL-80 inrush limiter, and modified the factory bias adjustment circuit so I can bias and balance each tube instead of in pairs. It's basically all finished up except for having to wait on a new rectifier tube since the one that came with it is on the way out. She's a beauty though!

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Looks great, Steve! are you keeping this one? or selling? I'd be curious to see how the EL-84's sound compared to the 7591's in my LK-72B. Our faceplates could be twins :)

looking forward to seeing the LK-48 in the case. don't think I've seen a mahogany case before on a Scott. mine is walnut
 
sbcrockett said:
Looks great, Steve! are you keeping this one? or selling? I'd be curious to see how the EL-84's sound compared to the 7591's in my LK-72B. Our faceplates could be twins :)

looking forward to seeing the LK-48 in the case. don't think I've seen a mahogany case before on a Scott. mine is walnut
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I have an earlier version 299B in line for restoration also, we will see which one I end up liking better. I might of used the mahogany wood case on my 299C already, I will have to dig into my stash and find out.

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spantou01 said:
Nice work! Could you elaborate on what this entails, please.
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I removed the stock screen circuit from the tube sockets, installed a piece of solid copper wire along the tube sockets, hooked up the screen supply resistor to the one end, and then 100 ohm 1/4 watt resistors from the new buss to each tube socket.

I changed the original bias and balance test circuit that tested the two pairs of output tubes combined into an individual tube bias and balance circuit where each tube bias is measured across a 10 ohm resistor. Notice the 4 new test point jacks installed between the pairs of tubes, and the 4 new sampling resistors.

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I have long been a fan of adding the 4 individual test points and 'converting' as necessary to a 4-pot / two-channel bias/balance system. The flexibility to use good tubes that aren't necessarily all that well matched as a quad is a real cost-saver while also being a nice performance-enhancer! Honestly, I don't know why a lot of other manufacturers didn't incorporate this level of adjustability into their products! A simple bias level control for all 4 output tubes is better than nothing, but it still relies heavily on a well matched quad that ages equally across all 4 tubes. How often does that happen?
 
I’ve had this 299a for a few weeks now, it needed some work and I am still waiting on a 33k resistor to bring the heater/bias voltage circuit down. I absolutely love this amp.
 

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I'm still testing my LK-48B after restoring it, everything seems to be working well although the bias has to be cranked all the way up to get close to recommended bias voltage. Tubes are very strong also. These call for 44MA per pair of 7189's across a 10 ohm resistor. So now having the bias test points for each tube separate, that would mean 22MA for each tube. Seems very low for 7189 tubes, I run most at 27MA - 30MA on my Fisher gear. I'm thinking that if I reduce R-214 20% and raise R-211 20% that I would have higher adjustment range. Anyone with a LK-48B, or that's more knowledgeable than I able to confirm this?
 
Do you still have the original bias bridge rectifier in there? If so, replace it and that may add some needed voltage to your bias network. If not, then fooling with the supply and bleeder resistors is not an uncommon need in order to get the correct voltage at the tube to hit the bias target.
 
Wharfcreek said:
Do you still have the original bias bridge rectifier in there? If so, replace it and that may add some needed voltage to your bias network. If not, then fooling with the supply and bleeder resistors is not an uncommon need in order to get the correct voltage at the tube to hit the bias target.
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I replaced it with a silicone one, but I'm so used to dealing with Fisher models that I always like to refer to those that have dealt with this model Scott.
 
Hey 299 folks. What 12AX7's are you using in your pre stage? I've tested RCA, Baldwins, Gold Lions and some somewhat tired telefunkens plus some unlabeled ones that sound great but are starting to hum. The phono stage seems the hardest to get that "just right" sound. I'm ready to pull the trigger on something perhaps a bit pricier. The Gold Lions were a bit too strident and bright and I found them disappointing in the phono stage though they sounded ok in the line stage. Opinions?
 
As mentioned, that bridge rectifier I generally find to be 'weak' in most cases. So, replacing it with a new one is usually a 'fix' enough to bring up the bias voltage to usable levels. But, I've also found that it's sometimes necessary to play with those 'feed' or 'bleed' resistors in the bias control network in order to get things where it needs to be to get the bias set properly.
 
Wharfcreek said:
As mentioned, that bridge rectifier I generally find to be 'weak' in most cases. So, replacing it with a new one is usually a 'fix' enough to bring up the bias voltage to usable levels. But, I've also found that it's sometimes necessary to play with those 'feed' or 'bleed' resistors in the bias control network in order to get things where it needs to be to get the bias set properly.
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Having worked on many HH Scott tube amplifiers, I have found this to be correct.

With todays higher line voltage, you "may" need to adjust the first resistor after the new silicon bridge. But if you are using a variable output autotransformer (often called a "Variac", which is a trade name) and adjust the input voltage to the Scott schematics of 117VAC, you may be right in the ballpark, leaving the original 18Ω 1-Watt resistor.

Also, if you look at the various Scott schematics for the voltage feeding the DC to the filaments of the preamplifier tubes, it might be at -48VDC or lower. Since you have four tube filaments in series, -48 VDC would be appropriate. Remember, this circuit is for the DC negative bias on the output tubes as well as the filament voltage for the preamplifier tubes.
 
kvflyer said:
Having worked on many HH Scott tube amplifiers, I have found this to be correct.

With todays higher line voltage, you "may" need to adjust the first resistor after the new silicon bridge. But if you are using a variable output autotransformer (often called a "Variac", which is a trade name) and adjust the input voltage to the Scott schematics of 117VAC, you may be right in the ballpark, leaving the original 18Ω 1-Watt resistor.

Also, if you look at the various Scott schematics for the voltage feeding the DC to the filaments of the preamplifier tubes, it might be at -48VDC or lower. Since you have four tube filaments in series, -48 VDC would be appropriate. Remember, this circuit is for the DC negative bias on the output tubes as well as the filament voltage for the preamplifier tubes.
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I need to check all voltages before changing anything obviously, but I'm trying to get this beauty dialed in and operating as correct as possible. I know Fisher tube gear well, and I'm trying to understand some of these Scott's a little better. I don't grasp running the 7189's at 22MA each as per what Scott specs show, seems low from what I'm used to. I'm able to get around .19V to .20V across a 10 ohm resistor running on 117V from my variac. The tubes check out strong on my tube tester, so I need to verify all other voltages and see where I need to tweak it. Thanks for your input!
 
Unfortunately my beloved 340B had a meltdown. Heard a loud buzz, looked over and a tube had redplated. Haven’t had a chance to investigate further. I hope I didn’t fry a transformer.
 
Destructor said:
Unfortunately my beloved 340B had a meltdown. Heard a loud buzz, looked over and a tube had redplated. Haven’t had a chance to investigate further. I hope I didn’t fry a transformer.
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That's why we try to install screen stability resistors that are metal film, they act like a fuse if a tube goes Chernobyl. Hopefully yours will be okay.
 
Sadly, I just noted that the extremely useful HHScott Web Site is currently 'down', and seemingly abandoned. I don't know how many of you are familiar with this web site, but it had a plethora of information, including everything from retail sales and marketing info, brochures, and photos, to 'tech' data including schematics, spec sheets, service bulletins, etc. Frankly, I'm somewhat devastated by this loss, as I frequented that website often, and relied heavily on the tech data and schematic info it contained. I believe there is a pretty good substitute for the loss of the schematics here on AK, within the AK DataBase archives. But, in total, the loss of that HHScott website is a real shame. There may be a chance that the holders of the URL reinstate it, and I hope they do. If anyone has any info on how to contact them to perhaps offer some kind of contribution to help them keep it going for a while longer, I'd like to know. As it stand now, I don't see any contact info for them, and if you attempt to go to the site you just get a generic page with a "...pending renewal." message. Sad........... a great loss to the H H Scott community!!
 
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