scott 299c

[oldmanaudio]

Hi every one. I need some help with a scott 299c. I have the amp working with a variac but I can't apply full line voltage without blowing the fuse on the amp. I have replaced the power supply caps the rectifier for the bias and changed resistor R214 to bring voltage close that on schematic. Voltages at the plates of the output tubes is higher than expected. I know line voltages are higher than when the amp was made so how do I compensate for this. Doesn't this change how the output tubes are biased. Please note I have limited tube experience.

Harry

[planet10]

I'm probably not going to be much help, but i did wake up one morning with acrid smoke thru-out the house from a 299 power trafo that had shorted....

dave

[HollowState]

Originally Posted by oldmanaudio

I have the amp working with a variac but I can't apply full line voltage without blowing the fuse on the amp.

Harry, it's time for some step by step tests to determine where the problem is. I assume you've done the basics like testing all the tubes and are using the correct fuse of 2½ amp slo-blo. (even 3A is OK) And also made sure the new filter capacitors aren't getting warm. Here's where we start.

First, remove the 5AR4 rectifier tube and see if you can go to full line power. If not pull all the tubes and disconnect the bias rectifier AC leads and try again. If the fuse still blows, it's the power transformer. Also check the small .01 cap across the primary.

If the fuse stays with no 5AR4, the PT is OK. Something is drawing excess current and you must narrow it down. The output tubes may be pulling excessive current because of improper (low) negative bias voltage. The coupling capacitors feeding the output tubes (7591s) may be leaking positive DC to the grids and upsetting the bias voltage. It's always a good idea to replace these, especially if they're original. You'll need a voltmeter to check this. Excessivly higher screen voltage will also make the tubes run harder. Plate voltage not as much. The amplifier should tolerate today's line voltage.

[oldmanaudio]

Thanks for the reply. I will check the .01 cap at primary and all of the coupling caps have been replaced. If the output tubes are not installed I can go to full power and can trace a signal through all the stages. This makes me think the output tube bias may be my problem. I do get operation of the amp with the output tubes installed and it sounds fine but I can't go to full line voltage without the fuse blowing. This makes me think the output tubes may be fine. How do I adjust the bias and what voltage should I have there?

[HollowState]

Originally Posted by oldmanaudio

I do get operation of the amp with the output tubes installed and it sounds fine but I can't go to full line voltage without the fuse blowing. This makes me think the output tubes may be fine. How do I adjust the bias and what voltage should I have there?

According to the schematic (http://hhscott.com/pdf/299C.pdf) Scott uses the negative bias supply to power four of the small signal tube filaments. Apparently they are run a little under rated voltage as well. (the -40V point) You said you changed the value of R214. I will assume you increased it to provide the same -40 volts. This may be part of the problem since the plate and screen voltage is now higher, the negative bias voltage needs to be increased as well. I would first try putting the original value of R214 back and see how it runs. The tube filaments will probably run closer to rated voltage, but that won't hurt them.

If the amplifier now works ok, make note of the negative voltage present on the 7591s control grid (pin 5). The schematic originally called for -18.5V. With an increased B+ level, the negative bias voltage will need to be a little higher to keep quiescent current through the output tubes to around 60mA for class AB1 operation. (The RCA tube manual calls for -21V w/450 plate & 400 screen @ 66mA)

One handy modification I would make is to install 10 ohm resistors in the 7591A's cathode to ground. This will provide an easy way to measure tube current and 10 ohms makes the math simple.

Should you wish to keep the original "starved" filament condition, you will need to increase the bias voltage to the grids by other means. Since the circuit voltage is essentially fixed, increasing the value of R210 (?) 1.5K ohms should raise it up. Some experimenting will be necessary. Or you could keep the 1.5K and tap voltage off further back. Say from C208 or C209 if needed.

[Bob, W7ETA]

Dumb things I have done, installed electrolytics with the arrow as +; rebuilt a power supply and tested the voltage in it with no load on it. Assumed I could simply not install the two 12AX7s for the phono section in a Scott 299. Start measuring AC going into a power supply and then wonder what I could have done to get such a strange low reading out of the rectifier--I've still got my meter set to measure AC!

Best from Tucson
Bob

[oldmanaudio]

OK I have returned to the original resistor for R214 this has helped 21 volts. it was suggested I change it due to the fact that I changed the bridge rectifier and there is less loses with the new one. You also suggested a cathode resistor to check quiescent current do I need to install one for each tube.
Harry

[m6tt]

It's also entirely possible that once full cathode temperature is reached in the output tubes, one of the tubes is running away due to gas. The close grid cathode spacing of the 7591 would help that along a bit too.

[HollowState]

Originally Posted by oldmanaudio

You also suggested a cathode resistor to check quiescent current do I need to install one for each tube.
Harry

Yes, one for each tube is desireable. ½ or 1 watt carbon comps. Measuring the voltage across the resistor will give you current. (I=E÷R) Normally the advice to change the resistor you did is valid. However, you have extenuating circumstances. Putting it back was a test to see if the amp would take full line voltage. Perhaps a though check of the resistor values in the bias circuit is in order. And if you are using todays "modern" tubes (chicom garbage) performance and specifications can be way off base. Like mt66 says, one (or more) of the 7591As may be acting up when hot. Try looking closely at them in a darkened room for signs of redness on the plates or a glowing screen grid inside the plate structure.

[oldmanaudio]

Ok looked at tubes in the dark two of them had a blue arura seen around outer of the plates. one I could see blue inside the plate by the grids and one nothing at all. the tube that was clear the current was 48 mil amps the 3 others were @ 100mil amps. Sounds like I have some bad tubes. Would you agree? If so any suggestions about replacements? JJ's Harmonix