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Old 11th October 2012, 11:51 PM   #1
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Default Scott 299

Ok so I just picked up a scott 299, and I believe it is the first generation because of the OPT placement. It came with no tubes, and sold as is but I got it pretty cheap for a project. I plan on pairing with some old Klipsch Heresy's when done.

I am pretty sure the PT is fine, I fired it up with no tubes and I cut the two orange wires for the bias/heater circuit and left them floating due to the hack job I saw when someone "upgraded" the selenium rectifier. All the voltages look good on the secondary and the fuse didn't pop

I have an 8A 400v silicon bridge rectifier to put in but was curious on the value change for resistor 106, stock is 10R but from what I read on the web most people find that 33R is used to compensate for the more efficient silicon rectifier.

Another question I had with the bias/heater circuit is do the 100-100-10 can capacitors have a common positive connection? It must because of the way they are wired but the schematic shows that the - side of the 100uf caps go to ground. This doesn't make sense to me? Anyway I think the new caps I install will have the - lead at the resistors and the + going to ground.

Ok so output tubes I plan on getting the russian military version of the EL84, I believe they are EL84M and are suppose to be a 7189 replacement.

I am also going to replace all of the black pyramid caps with some orange drops.

I am not sure if anyone else has encountered this but when comparing my amp with the resistance chart they give you my amp was pretty spot on except for R42 and R86. They are suppose to be 680R but measured 680k for both, to boot the color bands on the resistor are correct for 680R but I think they might have been mislabeled.
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Old 12th October 2012, 12:46 AM   #2
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Well, you are correct about the filter capacitors. The drawing is wrong! And the resistors in question should be 680 ohms, not 680K. Part of your problem is that you are looking at a Sams schematic which are prone to errors. Too many stoned heads working there in the 60's. Always use H.H.Scott Schematic Library.

I caution you to not replace any capacitors in the RIAA (or other) equalization circuits with modern fast dielectric types (polyproplyene, polycarbonate, polystyrene, etc) because they will affect the frequency characteristics of the network even if the same values are used. They tend to accentuate higher frequencies. All the other paper capacitors can be replaced with Mylar (polyethylene) orange drops with good results.

An eight amp bridge is overkill, but ok if it fits. And a 33 ohm resistor seems about right although the 10K bias pot would probably take up the excess voltage. And I would put 10 ohm resistors in each 7189 cathode to measure quiescent current across. Scott cheaped-out there.
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Old 13th October 2012, 08:10 PM   #3
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"Well, you are correct about the filter capacitors. The drawing is wrong! And the resistors in question should be 680 ohms, not 680K."

Thanks that is what I Thought.

"Too many stoned heads working there in the 60's"

LOL!

"I caution you to not replace any capacitors in the RIAA (or other) equalization circuits with modern fast dielectric types (polyproplyene, polycarbonate, polystyrene, etc) because they will affect the frequency characteristics of the network even if the same values are used. They tend to accentuate higher frequencies. All the other paper capacitors can be replaced with Mylar (polyethylene) orange drops with good results."


Thanks for the tip, and that is less soldering that I have to do so that works for me.

"An eight amp bridge is overkill, but ok if it fits"

I know it is overkill but I like how the 8A has it's leads as appose to all the others I saw, and actually it was roughly the same size L x W but just a little fatter.

" And a 33 ohm resistor seems about right although the 10K bias pot would probably take up the excess voltage"

I was more worried about exsessive voltage for the heaters going to valves 1, 2, 6, and 7. I guess I will start with 33R and go from there.
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Old 23rd October 2012, 11:11 PM   #4
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Well I finished up the cap job including all electrolytics and coupling caps (I left a lot of caps in the front/control section of the amp alone including thee RIAA section). I replaced the two 680k resistors on the ECF80's with correct 680R. All plate load resistors were spot on so I left them in, after about a week of listening everythings seems to be good. The amp is dead quiet and sounds fantastic! I can't be happier with the results and I want to thank Hollowstate for the knowledge :-)

The amp is bias to where I read 20v across the big 20w 160R power resistor, but the power transformer is still pretty darn warm after a few hours of operation. Not as hot as some of my guitar amp transformers get but still pretty warm. My question is do people run the amp without the enclosure to dissipate heat? Or does it not really matter?

Last edited by famousmockingbird; 23rd October 2012 at 11:14 PM.
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Old 24th October 2012, 12:01 AM   #5
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Quote:
Originally Posted by famousmockingbird View Post
The amp is bias to where I read 20v across the big 20w 160R power resistor,
What resistor is that? I see no such resistor in a Scott 299.
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Old 24th October 2012, 12:53 AM   #6
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I think most schematics show two 360R resistors from pin 8 of 5AR4. I guess I have the less common 299 where Craig from Nosvalves found a schematic where there is just one 20w 160R instead of two 360R 10w. The voltage drop across the resistor in the schematic was 20v so that's what I set it to.

It's funny I just ran across someone else with the same amp in this thread Scott 299 biasing [Archive] - AudioKarma.org Home Audio Stereo Discussion Forums

Craig works on these things all the time and he had to look through his folder to find the schematic so I guess not a lot of these came with one 20w 160R.
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Old 24th October 2012, 02:51 AM   #7
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Quote:
Originally Posted by famousmockingbird View Post
I think most schematics show two 360R resistors from pin 8 of 5AR4. I guess I have the less common 299 where Craig from Nosvalves found a schematic where there is just one 20w 160R instead of two 360R 10w. The voltage drop across the resistor in the schematic was 20v so that's what I set it to.
I'm sorry but that is not a good or proper way to bias the output tubes at all. Doing it as you said includes current draw from all of the other stages in both channels combined. And it does nothing to measure balance between the two output tubes in either channel. Please go back and read my earlier post about installing a small (10 ohm) 1 watt resistor in each output tube cathode to ground circuit. That's pin 3. Measuring across each resistor will give you each tubes current in terms of voltage. (I=EśR) Using 10 ohms makes the math super simple.

7189s have a plate dissipation of 12 watts. With 360 volts on the plate, you want quiescent tube current to be around 30 mA max. or less. 30 mA is almost 11 watts. Less is better for tube life. So 0.3 volts across 10 ohms is 30mA. 0.25 volts is 25mA and so on. And 25 mA is probably the lower limit without becoming too lean for class AB1. Adjust balance so that both tube pairs have equal current. Then adjust level for desired current. Do this with no signal applied. Any other way is just guess work and improper.

As I remember, most of the amplifiers from this period ran pretty warm. Running them without the cabinet will help them dissipate heat, but they were designed to run in the cabinet too. Choice is yours I reckon.
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Old 24th October 2012, 06:29 PM   #8
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I know the 10 ohm on cathode trick but unless I add some tip jacks to the top of amp I think it's useless. I am running the Russian military EL84M which has a max dissipation of 14 watts and higher voltage handling capabilities. I kind of used a couple different methods to check bias. One was I measured DC resistance from plates of EL84's to the center tap of output transformer and wrote them down, next I measured the voltage drop from center tap to the plates and did some math like 4/152=.026mA .026x400=10.4 watts 10/14=.74 or %74. Before I did this I balanced the bias voltage so each output tube gets the same negative voltage. The tubes I got were a matched quad and they were all pretty close without having to adjust anything so I just left them where they were. The 20v across 160R method isn't the best but it was just under twenty when I was done bias the output tubes. I had a rough idea of what all the other stages were pulling so deduct that from the total current across the 160R and it actually came really close to the same plate current readings I got from the reading voltage drop across output transformer.
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Old 24th October 2012, 07:17 PM   #9
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Ok, it's a roundabout way but gets you to the same place. My thinking was assuming you had things exposed you could measure at the cathods to set it up and then close it up when done. I've never used the Russky tubes mostly because they weren't available when I was actively working on these things. But they seem like a good replacement for the originals. Good work and enjoy.

Victor
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Old 24th October 2012, 07:44 PM   #10
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I found my notes when I bias the amp:

DC resistance / Voltage drop / mA in current

159 / 3.5 / .022

143 / 3.0 / .021

158 / 3.8 / .024

149 / 3.45 / .023

The .023 and .024 are a pair and the .022 and .021 are a pair.

My plate voltage is a little high at 415 but I think that is because my line voltage is 123vac lately. It usually is between 117 and 120. But still at 415vdc plate voltage and .024mA I get 9.96 watts which is only 71% plate dissipation. When my line voltage drops back down it will be running even cooler :-)

Thanks for the help again and I originally was going to do the whole tip jack sensing resistor thing at cathodes but for $60 a quad for the russky's I won't lose any sleep over it,
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