Nice video. I use to work for Knight Kit back in 1966-1967 in Maywood Park, IL. Was a technician and worked on anything they threw at me. Havent seen one of these amps in YEARS. Still have some Knight Kit equipment and Heath Kit. Built the Heath Kits while in the Navy in the early 1970's. Now I am working on a Pioneer RT707 reel to reel. It keeps blowing the 3.15amp fuse. What fun. Have quite a few tubes left over and an tube tester. Tuber were easy to work on.
Regarding the 1 ohm cathode sense resistors; I had trouble with some Vishay 2W 10r resistors going high in the same application, up to several k. I used Welwyn 3W Vitreous resistors in the end to be 100% sure. Just a thought. Nice series of video's, wish you'd made these years ago when I was starting out, very useful for beginners. Keep up the good work, DA
About a month ago I found a rat turd covered HH Scott 340a in a family members garage, ever since I’ve been subscribed to your channel and trying to sponge up as much info as I can. I’ve found 4 ceracaps with cracks so far, I’ll be making a list of my own pretty soon and will replace all the electrolytics and ceracaps since they’re cracking.
Thanks Mark, I use paper for keeping notes as well. Just got into the habit of keeping engineering logs in my career and prefer to draw and write on paper. I have been like that since grade school. Good luck on the amp... and those can caps.. ouch that price. Cheers,
Great stuff as always. BTW, Mark, I recommend the book "The Revenge of Analog" to you. Yeah, I like writing things down, as opposed to powering something up, waiting a few minutes, then searching, then printing. Xerox (of all people) tried to create the "paperless office" and see where that got us!
Mark, it looks like the brown caps one the first board may have been changed, I noticed a hook and eye solder joint on the ends of both of them. Welcome back!
17th LIKE & 4th Comment !! Much Love Mark & Keep Those Video's Rolling ! #KeepThoseTubesGlowing ! Hey Mark, It's Chuck aka Miltowntubes414 aka mudchicken187 ! You Always give a helping hand out, everytime you can ! Got to be more great people like you out here some where !! @Miltowntubes
Also, you could just put the “cathode current” test jacks in parallel with the existing 1k cathode bias resistors; then you don’t need to buy extra resistors, though you will have to rewire it slightly! HTH! 😎
Math P = V2 / R P = I2 * R P = V * I A useful inversion of the first two formulas is to calculate what voltage across or current through a resistance of R gets you to power P. So V = SQRT( P*R) and I = SQRT (P/R). If you have a 1K resistor rated at 1/4W, what is the maximum current and voltage that it can withstand? V = SQRT(0.25*1000) = 15.8V. I = SQRT(0.25/1000) = 0.0158A. And not surprisingly, as Georg Ohm would have told us, 15.8V/0.0158 = 1000 ohms. What a surprise! Beyond that, there is derating. Resistors are usually specified to dissipate their specified wattage at a surface temperature of 100-200C. That is the boiling point of water to fry-an-egg temps. It is common (and good) practice to use a resistor of at least twice the power rating that the resistor will actually be dissipating. That keeps the resistor surface temp down to not being a burn hazard. And there is the fact that there is no advantage to trying to get micropower resistors. If the actual dissipation is 0.5W, you need a 1W resistor. If it's 1/4W, you need a 1/2W resistor. If it's 1/8W or less, use a 1/4W resistor, as that's about as cheap as through-hole resistors get.
if the amp has its own power transformer, can you connect the chassis to the household ground, but keep the circuit ground isolated from the chassis? so if the circuit accidently made the chassis live, you are protected ?
Good video Mark. One thing I think we should all really take away from this, aside from the restoration, when dealing with purchases on eBay, insist vehemently that you have all the photos that you request. Otherwise you might end up spending way more than what the item is worth. I'm not saying that the amp is overpriced, because those were coveted back in the day. However I'm sure you would have been able to knock a few dollars off of it when you saw that it was not restored properly.
Nice video. I use to work for Knight Kit back in 1966-1967 in Maywood Park, IL. Was a technician and worked on anything they threw at me. Havent seen one of these amps in YEARS. Still have some Knight Kit equipment and Heath Kit. Built the Heath Kits while in the Navy in the early 1970's. Now I am working on a Pioneer RT707 reel to reel. It keeps blowing the 3.15amp fuse. What fun. Have quite a few tubes left over and an tube tester. Tuber were easy to work on.
Regarding the 1 ohm cathode sense resistors; I had trouble with some Vishay 2W 10r resistors going high in the same application, up to several k. I used Welwyn 3W Vitreous resistors in the end to be 100% sure. Just a thought. Nice series of video's, wish you'd made these years ago when I was starting out, very useful for beginners. Keep up the good work, DA
About a month ago I found a rat turd covered HH Scott 340a in a family members garage, ever since I’ve been subscribed to your channel and trying to sponge up as much info as I can. I’ve found 4 ceracaps with cracks so far, I’ll be making a list of my own pretty soon and will replace all the electrolytics and ceracaps since they’re cracking.
Awesome, was looking forward to this restoration.
Thanks Mark,
I use paper for keeping notes as well. Just got into the habit of keeping engineering logs in my career and prefer to draw and write on paper. I have been like that since grade school.
Good luck on the amp... and those can caps.. ouch that price.
Cheers,
Great stuff as always. BTW, Mark, I recommend the book "The Revenge of Analog" to you. Yeah, I like writing things down, as opposed to powering something up, waiting a few minutes, then searching, then printing. Xerox (of all people) tried to create the "paperless office" and see where that got us!
Another one is “The New Analog”, writtten by the drummer of Galaxie 500.. I haven’t read it though.
Mark, it looks like the brown caps one the first board may have been changed, I noticed a hook and eye solder joint on the ends of both of them. Welcome back!
Christopher Villanti I saw that but if so the caps they used are 50 years old... figured that was done a long time ago.
Nice video. I think I'd replace those ceramic caps as well on the boards. You're rebuilding the boards anyway. Wouldn't take much to replace them.
17th LIKE & 4th Comment !! Much Love Mark & Keep Those Video's Rolling ! #KeepThoseTubesGlowing ! Hey Mark, It's Chuck aka Miltowntubes414 aka mudchicken187 ! You Always give a helping hand out, everytime you can ! Got to be more great people like you out here some where !! @Miltowntubes
$uper $exy Amp & Tubes !!! #Bugleboys #GZ34 #12ax7/ecc83 sorry just a little bored before i got pick up the kids from school !!!
Those light-blue electrolytics on the boards look newer than 1962.
Also, you could just put the “cathode current” test jacks in parallel with the existing 1k cathode bias resistors; then you don’t need to buy extra resistors, though you will have to rewire it slightly! HTH! 😎
Would love to see a comparison between this and the Mac240!
That was a just a quick fix, kind of work usually done on shops with whatever they have to keep cost and time bench low.
How do you determine the wattage on resistors if you don't have a diagram? Size?
Great vids, glad your back in action! Thanks for all your effort.
Math
P = V2 / R
P = I2 * R
P = V * I
A useful inversion of the first two formulas is to calculate what voltage across or current through a resistance of R gets you to power P.
So V = SQRT( P*R) and I = SQRT (P/R).
If you have a 1K resistor rated at 1/4W, what is the maximum current and voltage that it can withstand?
V = SQRT(0.25*1000) = 15.8V. I = SQRT(0.25/1000) = 0.0158A.
And not surprisingly, as Georg Ohm would have told us, 15.8V/0.0158 = 1000 ohms. What a surprise!
Beyond that, there is derating. Resistors are usually specified to dissipate their specified wattage at a surface temperature of 100-200C. That is the boiling point of water to fry-an-egg temps. It is common (and good) practice to use a resistor of at least twice the power rating that the resistor will actually be dissipating. That keeps the resistor surface temp down to not being a burn hazard.
And there is the fact that there is no advantage to trying to get micropower resistors. If the actual dissipation is 0.5W, you need a 1W resistor. If it's 1/4W, you need a 1/2W resistor. If it's 1/8W or less, use a 1/4W resistor, as that's about as cheap as through-hole resistors get.
@@The0nionKnight Thank you, I went to college just after slide rulers were repurposed(modified with a bowl)😁
if the amp has its own power transformer, can you connect the chassis to the household ground, but keep the circuit ground isolated from the chassis? so if the circuit accidently made the chassis live, you are protected ?
Has anyone found a schematic for the Knight KB-85? Thanks.
Hi, I wonder if there’s really an advantage to putting the two rectifier tubes in parallel, vs. just using one per channel?
Please replace those old ceramic feedback caps with silver mica or polystyrene..
Those are nice pictorials... :-)
Good video Mark. One thing I think we should all really take away from this, aside from the restoration, when dealing with purchases on eBay, insist vehemently that you have all the photos that you request. Otherwise you might end up spending way more than what the item is worth. I'm not saying that the amp is overpriced, because those were coveted back in the day. However I'm sure you would have been able to knock a few dollars off of it when you saw that it was not restored properly.
Good video
I have a working overhauled pair each one bridged to 60W Mono and adjusted to EL34 tubes: ua-cam.com/video/w2ytQcwWS6o/v-deo.html