500 B 'On/Off' Again
- Thread starter pgerhardt
- Start date
audiodon
Addicted Member
The DC on the four resistors coming from the terminal strips (going to pin 6 (I think) on the 7591s should be between -17Vdc and -22Vdc. You're not pulling enough current at -15Vdc. There's a pull-down resistor you can find on the schematic that needs to have its value changed. On a 500-C, it's near the input AC. On a 500-B, I'm not sure where it is.
I don't have a 500-B schematic, but your unit will sound anemic if you don't have enough electron flow through the 7591s and with the "gate" that closed with a -15 value, you're starving the 7591s.
Put a closely matched quad of 10 ohm resistors between the cathode of the 7591 and ground. That might be pin 5, again, I'm going off memory here. That pin will be tied to ground, either individually or in pairs. When you've done that, you can measure the current draw of each 7591 by putting each lead of a multimeter on one side of that resistor. If it reads, say, 200mv, that's 20 milliamps. Voltage/Resistance=current.
Measure that value and you'll probably find that your current draw may be off and can be adjusted by changing that bias resistor mentioned in the first paragraph. The 10 ohm resistor on the cathode will also act as a fuse and pop, breaking the circuit if you have a red-plating runaway tube. Once it pops, your transformer will be safe.
I don't have a 500-B schematic, but your unit will sound anemic if you don't have enough electron flow through the 7591s and with the "gate" that closed with a -15 value, you're starving the 7591s.
Put a closely matched quad of 10 ohm resistors between the cathode of the 7591 and ground. That might be pin 5, again, I'm going off memory here. That pin will be tied to ground, either individually or in pairs. When you've done that, you can measure the current draw of each 7591 by putting each lead of a multimeter on one side of that resistor. If it reads, say, 200mv, that's 20 milliamps. Voltage/Resistance=current.
Measure that value and you'll probably find that your current draw may be off and can be adjusted by changing that bias resistor mentioned in the first paragraph. The 10 ohm resistor on the cathode will also act as a fuse and pop, breaking the circuit if you have a red-plating runaway tube. Once it pops, your transformer will be safe.
Last edited:
pgerhardt
Member
OK on the 15v not being enough to provide proper current. I attached the 500B service man.
The on off switch is now working after another shot of DeOxit and reheating the connections at the end of the 3 wire lead to the on/off switch. I removed the short and both contacts are now working. So one is probably on its last legs but is working NOW.
Got to use my new used Hako 808 desoldering gun for the 1st time. This will be a very useful tool.
Put the Old Heathkit Cap checker on all the filter caps and they seem at least OK for now.
I tested the smaller 50V electrolytics and they seemed to test ok except for the one in the MX circuit (maybe its bad or won't test in the circuit).
Am going to go ahead with replacing the one bad output tube and getting the idle current to be correct and see how it sounds. I may order some new .047's as well as they won't be too hard to get to.
The on off switch is now working after another shot of DeOxit and reheating the connections at the end of the 3 wire lead to the on/off switch. I removed the short and both contacts are now working. So one is probably on its last legs but is working NOW.
Got to use my new used Hako 808 desoldering gun for the 1st time. This will be a very useful tool.
Put the Old Heathkit Cap checker on all the filter caps and they seem at least OK for now.
I tested the smaller 50V electrolytics and they seemed to test ok except for the one in the MX circuit (maybe its bad or won't test in the circuit).
Am going to go ahead with replacing the one bad output tube and getting the idle current to be correct and see how it sounds. I may order some new .047's as well as they won't be too hard to get to.
pgerhardt
Member
After looking at the schematic Pin 6 is supposed to be 14.7 V on the 500B and I am getting going back and measuring it starts at about 15 but comes down a bit in a few min and stays pretty steady.
14.55V at pin 6 .0MV accross the 1k resistor
14.32 .4
14.50 .0
14.48 .0
This is with no signal no speakers and vol all the way down.
The interesting thing to me anyway, is the reading of .4mv across the resistor is the tube that fails the grid emission test on the B&K 707 tube tester so it is drawing excessive idle current through the grid circuit anyway.
14.55V at pin 6 .0MV accross the 1k resistor
14.32 .4
14.50 .0
14.48 .0
This is with no signal no speakers and vol all the way down.
The interesting thing to me anyway, is the reading of .4mv across the resistor is the tube that fails the grid emission test on the B&K 707 tube tester so it is drawing excessive idle current through the grid circuit anyway.
audiodon
Addicted Member
What are the measurements of your high voltage rails?
They're never exact and with today's AC being typically over 120Vac, they should run high but be proportionally similar. So I would expect to see more like 405 on the high voltage rail and not 385.
So, if the B+ rail on the schematic is supposed to run 395, the next one down is supposed to be 348, then 320, then 290, then 165, right?
How are yours running proportionally? If your B+ is 385, are the others proportionally lower?
They're never exact and with today's AC being typically over 120Vac, they should run high but be proportionally similar. So I would expect to see more like 405 on the high voltage rail and not 385.
So, if the B+ rail on the schematic is supposed to run 395, the next one down is supposed to be 348, then 320, then 290, then 165, right?
How are yours running proportionally? If your B+ is 385, are the others proportionally lower?
pgerhardt
Member
I studied the schmatic and made some measurments today
TP B reads 394 vs 395
TP C reads 356 vs 348
TP E reads 319 vs 320
TP D reads 169 vs 165
TP F reads 294 vs 290
When I measured it b4 the line voltage must have been different. Today it is at 120 and I think it was 117 b4 when I got 385.
So I feel confident that the correct DC Voltages are there.
I read some of the other threads you posted to about the 10 ohm resistor and measuring the plate current using the voltage drop accross the cathode resistors.
What is a good wattage rating for these and is ten ohms really an 'OK' value.
I did order an EH tube yesterday and a set of .047 caps.
TP B reads 394 vs 395
TP C reads 356 vs 348
TP E reads 319 vs 320
TP D reads 169 vs 165
TP F reads 294 vs 290
When I measured it b4 the line voltage must have been different. Today it is at 120 and I think it was 117 b4 when I got 385.
So I feel confident that the correct DC Voltages are there.
I read some of the other threads you posted to about the 10 ohm resistor and measuring the plate current using the voltage drop accross the cathode resistors.
What is a good wattage rating for these and is ten ohms really an 'OK' value.
I did order an EH tube yesterday and a set of .047 caps.
audiodon
Addicted Member
The 10 ohm resistor serves two functions.
1. Gives you a per-tube current measurement in this fixed bias configuration
2. Acts as a fuse and pops if a tube red-plates, therefore protecting your output transformer from overheating and shorting.
I use 1/2 watters, though I've used 1/4 watters also. Typically I use metal film and not carbon comp. They pop and you can hear 'em.
You're good to go on your DC voltages. They're much closer than mine typically are.
Regardless of what that schematic says, though, I still think you're starving those output tubes.
1. Gives you a per-tube current measurement in this fixed bias configuration
2. Acts as a fuse and pops if a tube red-plates, therefore protecting your output transformer from overheating and shorting.
I use 1/2 watters, though I've used 1/4 watters also. Typically I use metal film and not carbon comp. They pop and you can hear 'em.
You're good to go on your DC voltages. They're much closer than mine typically are.
Regardless of what that schematic says, though, I still think you're starving those output tubes.
pgerhardt
Member
OK on the resistors it looks like 'normal' full power dissapation of a 10 ohm Cathode resistor will be about .007 watt so x 300% would be .02 Watt so I put some 1/4 watt 10 ohm film resistors in. I found a nice set of 4 from same lot that were very close. After installing them here is the voltage drop accross them with vol turned all the way down and no input.
1 366mv
2 373 mv
3 349 mv
4 365 mv
#3 is the Bad tube that fails grid emission test on B&K 707. so it is drawing MORE screen current and Less Plate Current than the other three it seems.
Interesting....
EH tube will come in a few days
1 366mv
2 373 mv
3 349 mv
4 365 mv
#3 is the Bad tube that fails grid emission test on B&K 707. so it is drawing MORE screen current and Less Plate Current than the other three it seems.
Interesting....
EH tube will come in a few days
audiodon
Addicted Member
Yep, everything looks good.
If tube rolling in the preamp section and/or changing the output coupling caps doesn't produce sound the way you like it, this just may not be the amp for you. I can offer no other explanation.
What kind of coupling caps did you order?
One other question: You stated that you've brought this out again. Did you enjoy it's sound before? Has the sound changed somehow?
If tube rolling in the preamp section and/or changing the output coupling caps doesn't produce sound the way you like it, this just may not be the amp for you. I can offer no other explanation.
What kind of coupling caps did you order?
One other question: You stated that you've brought this out again. Did you enjoy it's sound before? Has the sound changed somehow?
audiodon
Addicted Member
pgerhardt,
I think that, if you're going to use EH7591s, you should change out all four. They require a change in the grid resistor for long-term use. Of course, if you're just out to test it, one EH should be fine for a short time.
The output coupling caps make a BIG difference in tone.
I think that, if you're going to use EH7591s, you should change out all four. They require a change in the grid resistor for long-term use. Of course, if you're just out to test it, one EH should be fine for a short time.
The output coupling caps make a BIG difference in tone.
pgerhardt
Member
Received the single EH7591 and it tested bad on my B&K 707 no meter deflection at all even though the filament is lit so it is going back. Just to check my tester I pulled all four of the GE's out of the Fisher 500 B and the marked one with the high grid emission (which I tested at least four times B4) is now testing good. Tested all the tubes four times to make sure. All the GE's are testing good and the new EH ZERO meter movement.
I really like the idea of the individual bias adj board that Drew Bolce posted. In the meantime I may turn it over, listen to it some more and start putting in the new coupling caps.
The On/Off switch is still working and switching a 60 watt bulb plugged into the rear as well.
I really like the idea of the individual bias adj board that Drew Bolce posted. In the meantime I may turn it over, listen to it some more and start putting in the new coupling caps.
The On/Off switch is still working and switching a 60 watt bulb plugged into the rear as well.
Paul, I'm just wondering if you don't have a problem with your B&K?
Do you have another tester to compare?
Larry
Do you have another tester to compare?
Larry
pgerhardt
Member
Larry
Have no other tester. It does seem strange but I got consistant results over 4 tests. Ran the 4 GE's and the one EH at least 4 different times. The GE's were tested a month or so ago and again this week. 4 times for all 5 tubes this week and 4 times for the 4 GE last month. One of the GE's changed! Maybe running it for a month partly upside down did something....
Have no other tester. It does seem strange but I got consistant results over 4 tests. Ran the 4 GE's and the one EH at least 4 different times. The GE's were tested a month or so ago and again this week. 4 times for all 5 tubes this week and 4 times for the 4 GE last month. One of the GE's changed! Maybe running it for a month partly upside down did something....
audiodon
Addicted Member
pgerhardt,
I moved the information on front panel cleaning to here . . . your thread. But I fat fingered it and lost it, so here we go again.
To clean your front panel and knobs:
1. Verify that your front panel is brushed aluminum.
Verify that it doesn't have the champagne lacquer on it. That'll make it kind of varnish brown if it has the lacquer. If it does, just clean with soapy water on a rag.
2. If your front panel is brushed aluminum, remove the knobs, unscrew the nuts at each end and remove the front panel.
3. Fill a flat bottomed sink or pail with 1/2 inch hot water. I use the house's slop sink in the basement next to the washer.
4. Add some ammonia, but the maximum you should use would be 1 part ammonia to 4 parts water.
5. Get a plastic container (like those ones that food comes in) and scoop up some of the mixed solution and throw the knobs in. Set them aside. Fill a second container with clear cold water for rinsing later.
6. Immerse the whole front panel in the solution for about two minutes. It should be immersed for long enough to soften the grime, and no longer.
7. Pull the front panel out. Wipe with a coarse towel piece or, at the most abrasive a white 3m Scotch scrubbie, but do not be aggressive and watch what you're doing. Concentrate on the worst areas, dipping the cleaning wipe in the solution, but move quickly and don't obsess or you'll wipe lettering or numbering. The whole scrubbing part shouldn't take more than two minutes. Use an old toothbrush dipped in solution for the worst areas if they need it. Be careful of the silkscreen.
8. Rinse the front panel in the sink and blot it dry.
9. With a used toothbrush, pull out the first of the knobs and brush back and forth in the same front-to-back direction as the grain in the plastic goes. Do them all and then flush with clean water. If any brights fall off, you can reattach them with rubber cement.
10. Wax front panel. I use a carnauba based car wax. There are as many opinions on what you should use as their are wipe-on preservative solutions. Pick one.
11. Clean the dial glass with glass cleaner. The front doesn't have the lettering, the back does. If you remove it and do the back, blot around the lettering and wipe elsewhere.
12. Put it all back together and enjoy the fruits of your labor.
I moved the information on front panel cleaning to here . . . your thread. But I fat fingered it and lost it, so here we go again.
To clean your front panel and knobs:
1. Verify that your front panel is brushed aluminum.
Verify that it doesn't have the champagne lacquer on it. That'll make it kind of varnish brown if it has the lacquer. If it does, just clean with soapy water on a rag.
2. If your front panel is brushed aluminum, remove the knobs, unscrew the nuts at each end and remove the front panel.
3. Fill a flat bottomed sink or pail with 1/2 inch hot water. I use the house's slop sink in the basement next to the washer.
4. Add some ammonia, but the maximum you should use would be 1 part ammonia to 4 parts water.
5. Get a plastic container (like those ones that food comes in) and scoop up some of the mixed solution and throw the knobs in. Set them aside. Fill a second container with clear cold water for rinsing later.
6. Immerse the whole front panel in the solution for about two minutes. It should be immersed for long enough to soften the grime, and no longer.
7. Pull the front panel out. Wipe with a coarse towel piece or, at the most abrasive a white 3m Scotch scrubbie, but do not be aggressive and watch what you're doing. Concentrate on the worst areas, dipping the cleaning wipe in the solution, but move quickly and don't obsess or you'll wipe lettering or numbering. The whole scrubbing part shouldn't take more than two minutes. Use an old toothbrush dipped in solution for the worst areas if they need it. Be careful of the silkscreen.
8. Rinse the front panel in the sink and blot it dry.
9. With a used toothbrush, pull out the first of the knobs and brush back and forth in the same front-to-back direction as the grain in the plastic goes. Do them all and then flush with clean water. If any brights fall off, you can reattach them with rubber cement.
10. Wax front panel. I use a carnauba based car wax. There are as many opinions on what you should use as their are wipe-on preservative solutions. Pick one.
11. Clean the dial glass with glass cleaner. The front doesn't have the lettering, the back does. If you remove it and do the back, blot around the lettering and wipe elsewhere.
12. Put it all back together and enjoy the fruits of your labor.
Last edited:
