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Scott 299D late, help and guidance needed.

K

km5881

New Member
First time posting but a long time viewer. I have seen some great info on Scott gear here at AK and now I am in need of some advice/ assistance. A friend gave me the piece last summer and I have just started the refurb process. Here's where I am at with it.

All new electrolytic's including the can com positive. Sprague Orange Drops to replace the Ceracaps. Resistors are all close and will selectively replace. Everything is close to spec except the B+ which has tipped the scales at over 520VDC at Pin 8 on the 5AR4. Ive had it running and no red plates or excessive heat but the B+ worries me for the new caps and tubes. AC line average here is 120VAC. If any of the kind Scott Folk here at AK have any advice it would be greatly appreciated.
 
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You may want to use a bucking transformer in series with the Mains line. You can use a 6.3VAC or 12.6VAC filament transformer wired to reduce the voltage input to your amplifier. I will try to find an article that shows the wiring. That is the most efficient way to do it short of a variac...
 
I don't think your B+ would normally be that high on a 120 vac line -- unless your output tubes are not drawing the proper quiescent current, in which case, your B+ would be that high. If possible, check or set the bias on the output tubes, and see if that doesn't correct the issue.

Dave
 
I will certainly try the Bucking Transformer, but am wondering if dropping the Mains will get it down enough. Regarding Bias,it seems like I've got the necessary -58 VDC at the supply but can't seem to get lower than than 37mv at the test points. Any advice is greatly appreciated. Btw, with the main filters disconnected I get about 385VDC at pin 8 on the 5AR4.
 
What type of 5AR4 are you using? I have a new Ruby that makes higher output than the Vintage GE I ended up using. Also, I use a variac to drop the line voltage to 117v.
 
km -- I'm not sure what the value is of the resistor at your test point, but if it is the classic 10 ohm value so often used (or installed by diyers), then 37 mv only equates to a current flow of 3.7 ma -- much too low for any one tube or group of tubes.

For 37 mv to be anywhere near the ball park , it would have to be developed across a 1-2 ohm resistor. At 1 ohm, this would indicate 37 ma of current draw, but at 520 volts of B+, your tubes would be dissipating over 18 watts each, which you would surely notice! A 2 ohm resistor would cut this current draw and dissipation in half, but I've never heard of Scott using a 2 ohm resistor before.

Since you have indicated that there is no red plating, I would again suggest that the high B+ you are seeing is due to a lack of sufficient current draw in the output stage. If that is the case, it will also lead to greatly increased distortion in the output as well.

If you can verify what the value is of the resistance you are measuring the 37 mv across, then we can put the possibility of this concern to bed real quick!

Dave
 
Thanks' for the info. How about replacing the 3.3 ohm resistors at the test points.
I've seen other posts mentioning using 10 ohms for easier readings.
 
My mistake Dave, they are the 3.3 Ohms I mentioned earlier. I will replace with the 10 Ohm as most seem to suggest. The 5AR4 is a Sylvania which reads "made in Great Britain, tested in USA". AC voltage at pins 4,6 to ground is 370VAC without the rectifier.
 
KM -- If your Scott uses 3.3 ohm resistors for a pair of output tubes (something very common for Scott), then 37 mv equates to only 11 ma of quiescent current draw per pair of tubes. This is much too low, and almost certainly the reason your B+ is so high.

A much more appropriate voltage at test points with 3.3 ohm resistors for a pair of 7591 output tubes would be between .21 and .22 vdc, or between 210 and 220 mv.

I believe you will find that with this voltage developed by two 7591 tubes across a 3.3 ohm resistor, your B+ will fall in line, and the dissipation will be fine for your tubes as well.

I hope this helps!

Dave
 
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