H. H. Scott 99 series troubleshooting questions

[Istari]

I'd like to ask a couple of questions to the resident tube gurus.

For starters I'm using a Fluke 23 digital multimeter (autosensing) and a generic variac to adjust line voltages , I also have a digital meter hooked up to the variac to acurately measure its output voltage.

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I have a H.H. Scott 99A that Id like to rebuild. I have a bit of an electronics background but its somewhat rusty. I do have the schematics for the units (Sams Photofacts). One of my questions is this: On the 99A I have checked the voltage of V6 - 5Y3GT (output side of a new or original 5Y3GT, I tried both thinking that the old tube may have been defective) at pins 2 & 8 and it measures 396 V after a 10 minute warm up. The voltage measurement on the schematic reads 355 V. I have the 99A plugged into a variac that is set at 117 VAC, which the Photofact states in the nominal voltage. (The Photofact states: Nominal tolerances on component values makes possible a variance of + or 15% in voltage and resistance values.) The higher reading of 396 V throws off all of the other readings Im taking on tube V2 12AX7 (pin 1 actual 210.2 V, schematic 175 V; pin 6 actual 241.3 V, schematic 195 V and on tube V4 6V6GT (pin 3 actual 359 V, schematic 325 V; pin 4 actual 364, schematic 320 V.

If I adjust the variac to 106 volts, I then get the correct reading from V6 pins 2 & 8 of 355 V and the other readings are very close to the schematic values.

My question is, should I worry about the higher voltage readings since the schematic says that there could be a + or 15% in the readings of voltage and resistance? Should I insert a high wattage pot and reduce the output voltage on V6 pin 2 or 8? I have changed out the coupling caps but not any of the multisection cans.

I have a 99C that is exhibiting similar increases on the 54UGA output and 12AX7 and 6L6GB tubes. I have replaced the 4X20 MF multisection electrolytic capacitor and I have replaced the coupling caps with orange drop equivalents. Would replacing the multisections have an effect on the voltage readings of the rectifier tubes?

Both units power up and play well, I just want to be sure that the 99A can last another 53 years!

I intend on using either the 99A or the C as center channel amps, the C model looks better with the Scott 222A I have, but the 99A is a real looker with its brass faceplate all polished up. The Scott 222A was rebuilt by Craig Otsby (NOS Valves) about two years ago and works well. Id just like to see if I can bring these back up to equal the 222A.

I'll attempt to attach the 99A schematic to this post.

Thanks for the help!

[3dzapper]

You are on the high side of the tolerance for sure and at 120+ of modern line voltage would be higher still. You could try a 50 Ohm 5W resistor in place of R47's 33Ohms to see if that works. That is the first resistor off the pin 2 of rectifier if the voltage concerns you. It won't change the voltage at the rectifier though.

Rick

[Istari]

I thought that the voltages were a bit high. The nominal AC line is 117VAC on the schematicand the schematic gives the voltage readings on test points at that nominal line voltage. My main concern it the voltage readings on the rest of the components in the circuit since if they are too high it would shorten the life of the components. I guess I could buy a variac to step the AC line down to the 106VAC level but that might be going a bit too far.

i have notice that when the unit is running on the 122VAC from the wall i get a lot more volume from the 99A. As an experiment I set the variac at 106VAC and increased it to 122VAC, the volume increased as the AC line voltage increased. There didn't seem to be any audible distortion due to the increased AC voltage.

I will try the 50 Ohm 5W resistor bit tomorrow.

Thanks for the help!

[esl_57]

I'd like to ask a couple of questions to the resident tube gurus.

For starters I'm using a Fluke 23 digital multimeter (autosensing) and a generic variac to adjust line voltages , I also have a digital meter hooked up to the variac to acurately measure its output voltage.

<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

I have a H.H. Scott 99A that Id like to rebuild. I have a bit of an electronics background but its somewhat rusty. I do have the schematics for the units (Sams Photofacts). One of my questions is this: On the 99A I have checked the voltage of V6 - 5Y3GT (output side of a new or original 5Y3GT, I tried both thinking that the old tube may have been defective) at pins 2 & 8 and it measures 396 V after a 10 minute warm up. The voltage measurement on the schematic reads 355 V. I have the 99A plugged into a variac that is set at 117 VAC, which the Photofact states in the nominal voltage. (The Photofact states: Nominal tolerances on component values makes possible a variance of + or 15% in voltage and resistance values.) The higher reading of 396 V throws off all of the other readings Im taking on tube V2 12AX7 (pin 1 actual 210.2 V, schematic 175 V; pin 6 actual 241.3 V, schematic 195 V and on tube V4 6V6GT (pin 3 actual 359 V, schematic 325 V; pin 4 actual 364, schematic 320 V.

If I adjust the variac to 106 volts, I then get the correct reading from V6 pins 2 & 8 of 355 V and the other readings are very close to the schematic values.

My question is, should I worry about the higher voltage readings since the schematic says that there could be a + or 15% in the readings of voltage and resistance? Should I insert a high wattage pot and reduce the output voltage on V6 pin 2 or 8? I have changed out the coupling caps but not any of the multisection cans.

I have a 99C that is exhibiting similar increases on the 54UGA output and 12AX7 and 6L6GB tubes. I have replaced the 4X20 MF multisection electrolytic capacitor and I have replaced the coupling caps with orange drop equivalents. Would replacing the multisections have an effect on the voltage readings of the rectifier tubes?

Both units power up and play well, I just want to be sure that the 99A can last another 53 years!

I intend on using either the 99A or the C as center channel amps, the C model looks better with the Scott 222A I have, but the 99A is a real looker with its brass faceplate all polished up. The Scott 222A was rebuilt by Craig Otsby (NOS Valves) about two years ago and works well. Id just like to see if I can bring these back up to equal the 222A.

I'll attempt to attach the 99A schematic to this post.

Thanks for the help!

Im not familiar with that unit but one thing did occur to me. Check the bias on the out put tubes. The high voltage supplies on this vintage stuff is so mushy that you can get a higher voltage if you try and run the unit with no out put tubes installed or even if they are biased to the lean side.

***Watch the voltage rating on the main filter cap!!!***

This is very noticeable on a Dynaco st70.

[Erik Mandaville]

This might sound silly, but you do have the rest of the tube compliment installed along with the rectifier, don't you?

What are your heater voltage readings? This is important.

I haven't checked the schematic on this, so if you could post it, that would be helpful. There would likely be voltage dropping resistor/s between filter cap sections which may also have drifted in value. Are the values you replaced the same as what is shown on the schematic? Plate load resistors may have also changed in value, which can influence the B+ readings you're getting.

Is this a capacitor or choke input to filter power supply?

Your voltages are a little high, but you should be able to get things within a reasonable tolerance against the schematic.

Good luck (allow me to say be careful). I know what several hundred DC volts feels like!

Erik

[3dzapper]

Link to a similiar schematic Erik: http://hhscott.com/pdf/99B.pdf