Fisher 400 tube arc, cathode resistor popping after rebuild

dcgillespie said:
If there is no voltage at pin 5 on V16, then either the tube is dead, the heater is not lit, or there is no B+ voltage on the screen grid (pins 1&7).

Dave
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It was a dead tube Dave. I've had 3 or 4 EH 7868 tubes test bad. I bought some spares from a local auction site seller and they tested bad in my Sencore TC 162 right off the bat. I contacted J. McShane who told me he no longer carries them because of install problems. I thought it was install issues but now I think its plain old QC problems.
 
tonefoto said:
It was a dead tube Dave. I've had 3 or 4 EH 7868 tubes test bad. I bought some spares from a local auction site seller and they tested bad in my Sencore TC 162 right off the bat. I contacted J. McShane who told me he no longer carries them because of install problems. I thought it was install issues but now I think its plain old QC problems.
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I tried bringing that problem up here https://audiokarma.org/forums/index...g-to-make-a-7591a.876929/page-2#post-12904272

I was wondering the same thing.
 
tonyg88 said:
I tried bringing that problem up here https://audiokarma.org/forums/index...g-to-make-a-7591a.876929/page-2#post-12904272

I was wondering the same thing.
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After installing another EH 7868 replacement tube, I've stabilized the bias at 320 mV, 117 V on my variac. I've marked the dial on the variac where my meter reads 117. It holds steady. Can I run it up safely on my 115V bucking transformer at this point? I have a CL-80 soft start thermistor installed across the black transformer wire and the 7 lug terminal strip near the power cord and fuse. I didn't install the CL-80 across the fuse.
 
If you have a CL-80 installed in the receiver, generally you should be able to operate the receiver just fine directly from a 121-122 vac power line. Check the heater voltage at the terminals of the output tube socket that is farthest from the power transformer. If it is 6.30 vac +5/-0%, then you're good to go. If not, adjust the AC power to the receiver so that the heater voltage at those terminals are within this range.

Dave
 
dcgillespie said:
If you have a CL-80 installed in the receiver, generally you should be able to operate the receiver just fine directly from a 121-122 vac power line. Check the heater voltage at the terminals of the output tube socket that is farthest from the power transformer. If it is 6.30 vac +5/-0%, then you're good to go. If not, adjust the AC power to the receiver so that the heater voltage at those terminals are within this range.

Dave
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I have the CL-80A installed between the black transformer wire and the terminal the black wire was connected to. At 117 VAC on my variac I read no more than 3.1 VAC on pins 4 or 5 with variac set up to 122 VAC.
 
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tonefoto said:
I have the CL-80A installed between the black transformer wire and the terminal the black wire was connected to. At 117 VAC on my variac I read no more than 3.1 VAC on pins 4 or 5 with variac set up to 122 VAC.
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dcgillespie said:
If you have a CL-80 installed in the receiver, generally you should be able to operate the receiver just fine directly from a 121-122 vac power line. Check the heater voltage at the terminals of the output tube socket that is farthest from the power transformer. If it is 6.30 vac +5/-0%, then you're good to go. If not, adjust the AC power to the receiver so that the heater voltage at those terminals are within this range.

Dave
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Not sure if I understood correctly about connecting at the terminals of the output tube socket. Do you mean reading from the farthest socket's pin 4 or 5 terminal? If so, I'm only reading 3.1 VAC at 121-122 VAC.
 
tonefoto said:
Not sure if I understood correctly about connecting at the terminals of the output tube socket. Do you mean reading from the farthest socket's pin 4 or 5 terminal? If so, I'm only reading 3.1 VAC at 121-122 VAC.
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Here's where I installed the CL80A. Not sure if this is keeping the heater voltage down. I measured plate voltage at 413 VDC at 121 VAC. Plate voltage drops to 387 VCD at 117 VAC.CL80A.jpg
 
HTR AC VOLTAGES on OUTPUT TUBES are pin 4 (Blk)to pin 5 (RED). The Only DC HTR"S are V-7,V-8, V-9, V-10. These get checked with RED on pin 4 or 5, and Blk on Chassis. They should be in spec when running. Set your variac so you are getting 117AC on the terminal strip side of the CL-80. Then check the output tube AC HTR's (should be 6.3 +/-5%) and V7 to V10. These will have 12VDC on them. You;'l get one of the two scenario's as the tubes are in Series parallel. One tube on each pair will show 25.2V on one pin and 12.6V on the other Pin (for a 12.6V drop) The other tube will show 12.6vdc on one pin and 0.0vdc on the other pin. Allow +/-5%.

Vary the Wall/Variac voltage until both AC and DC Htr voltages are in spec. Note the Voltage. If you don't mind a varying voltage (which the bucking transformer will do based on wall voltage) then a bucking transformer will vary input voltage based on what the Wall voltage is at the moment. And it's fine. If you have gas with varying voltages keep the Variac set for the voltage needed to maintain the heaters at spec. Plate and Screen voltages can vary as much as 50-60 volts either way depending on Wall Voltage, Dynamics vs. Volume, etc. You plate voltages are fine considering the wall voltage on an early 400. Remember that the 400 was designed to operate on Voltages of 105VAC to 120VAC. About a 15% differential. Fisher spec'ed a 15% differential in Voltages on the schematics based on component tolerances. The voltages on the schematic are a guide, and are ideal test voltages based on a highly regulated 117VDC. I run my 400's, 800c's and consoles on CL-80's and wall voltges btwn 120 and 124. No problems over the last 10 years.

Your voltages "IDEALLY" @ 117V input should be PLATE 395VDC, Screen 345VDC, grid -15Vdc(based on OLD STOCK Tubes). Of the nearly a dozen 400's I've rebuilt, NONE of them have Plate, Screen or Grid voltages that matched the Schematic @ 117VAC Wall Votlage. Most have been less than 5% high or low. Close is Good enough in These, Hand grenades and Golf.
 
larryderouin said:
HTR AC VOLTAGES on OUTPUT TUBES are pin 4 (Blk)to pin 5 (RED). The Only DC HTR"S are V-7,V-8, V-9, V-10. These get checked with RED on pin 4 or 5, and Blk on Chassis. They should be in spec when running. Set your variac so you are getting 117AC on the terminal strip side of the CL-80. Then check the output tube AC HTR's (should be 6.3 +/-5%) and V7 to V10. These will have 12VDC on them. You;'l get one of the two scenario's as the tubes are in Series parallel. One tube on each pair will show 25.2V on one pin and 12.6V on the other Pin (for a 12.6V drop) The other tube will show 12.6vdc on one pin and 0.0vdc on the other pin. Allow +/-5%.

Vary the Wall/Variac voltage until both AC and DC Htr voltages are in spec. Note the Voltage. If you don't mind a varying voltage (which the bucking transformer will do based on wall voltage) then a bucking transformer will vary input voltage based on what the Wall voltage is at the moment. And it's fine. If you have gas with varying voltages keep the Variac set for the voltage needed to maintain the heaters at spec. Plate and Screen voltages can vary as much as 50-60 volts either way depending on Wall Voltage, Dynamics vs. Volume, etc. You plate voltages are fine considering the wall voltage on an early 400. Remember that the 400 was designed to operate on Voltages of 105VAC to 120VAC. About a 15% differential. Fisher spec'ed a 15% differential in Voltages on the schematics based on component tolerances. The voltages on the schematic are a guide, and are ideal test voltages based on a highly regulated 117VDC. I run my 400's, 800c's and consoles on CL-80's and wall voltges btwn 120 and 124. No problems over the last 10 years.

Your voltages "IDEALLY" @ 117V input should be PLATE 395VDC, Screen 345VDC, grid -15Vdc(based on OLD STOCK Tubes). Of the nearly a dozen 400's I've rebuilt, NONE of them have Plate, Screen or Grid voltages that matched the Schematic @ 117VAC Wall Votlage. Most have been less than 5% high or low. Close is Good enough in These, Hand grenades and Golf.
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OK, reading off the terminal side of the CL80A, I adjusted my variac to 117 VAC, (variac dial now reads 110). I read 6.6 VAC on the output tube's pin 4 and pin 5 off the tube farthest from transformer. The DC voltages on pins 4 or 5 on V7-V10 are about -13/-14VDC and -27VDC. Is something the matter here?
 
tonefoto said:
OK, reading off the terminal side of the CL80A, I adjusted my variac to 117 VAC, (variac dial now reads 110). I read 6.6 VAC on the output tube's pin 4 and pin 5 off the tube farthest from transformer. The DC voltages on pins 4 or 5 on V7-V10 are about -13/-14VDC and -27VDC. Is something the matter here?
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I should mention, my selector switch is set on Tape/Aux.
 
You just need to install a dropping resistor (with a filtering cap) in the htr feed wire to the V-7-V10 tubes. This feed is one of the wires coming off the Hayseed cap underneath. When you installed the bridge rectifier, it's output voltage is a bit higher than what it was with a selenium. As you have an IBAM/IBBA board (and possibly New issue Russian tubes) the bias buss votlage is about right for the adjustment. However that voltage (-27vdc) 3volts higher (away from 0.0VDC) than needed for the heaters. try a 27ohm 2W with a 100uf 50V (the cap after the resistor with the + lead of the cap to chassis ground). This will drop the voltage and add some additional filtering. You may have to adjust the resistance to get the DC htr value close to spec.

Heater voltage drop.jpg
 
larryderouin said:
You just need to install a dropping resistor (with a filtering cap) in the htr feed wire to the V-7-V10 tubes. This feed is one of the wires coming off the Hayseed cap underneath. When you installed the bridge rectifier, it's output voltage is a bit higher than what it was with a selenium. As you have an IBAM/IBBA board (and possibly New issue Russian tubes) the bias buss votlage is about right for the adjustment. However that voltage (-27vdc) 3volts higher (away from 0.0VDC) than needed for the heaters. try a 27ohm 2W with a 100uf 50V (the cap after the resistor with the + lead of the cap to chassis ground). This will drop the voltage and add some additional filtering. You may have to adjust the resistance to get the DC htr value close to spec.

View attachment 1570225
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heaterfeedV9V10.jpg The only wire I could see coming off the Hayseed Bias can underneath is the yellow lead going to pin 5 of V9. Would the resistor go in line with this lead and the cap's negative side attached to the resistor lead attaching to pin 5 of V9?
 
tonefoto said:
View attachment 1570240 The only wire I could see coming off the Hayseed Bias can underneath is the yellow lead going to pin 5 of V9. Would the resistor go in line with this lead and the cap's negative side attached to the resistor lead attaching to pin 5 of V9?
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I'm going to try a range between 27-33Ω dropping resistor and cap to ground and post a photo of the connection tomorrow. When I adjust voltage on my variac to 117 VAC at the CL80 and terminal, the variac is actually putting out 127 VAC when I read it with my meter at the variac outlet. That looks like the CL80 is dropping it 10 VAC.
I'm confused about where I should be reading the voltage prior to adjusting my bias. My wall current measured with my meter at the outlet is about 122 VAC. When I adjusted voltage to 117 VAC at the CL80 my bias was too high. I adjusted the bias with the voltage set at approximately wall current.
 
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Take it(the 400) off the Variac. Take a reading at the wall (ACV) and again at the terminal the CL-80 (oneside) and at the other lead from the wall on the fuse(again ACV). I want to see what your voltage drop is. CL-80's typically drop no more than 2-3 volts at most.
 
72C01B34-963A-4A60-B73F-5B5C3FAE777B.jpeg
larryderouin said:
Take it(the 400) off the Variac. Take a reading at the wall (ACV) and again at the terminal the CL-80 (oneside) and at the other lead from the wall on the fuse(again ACV). I want to see what your voltage drop is. CL-80's typically drop no more than 2-3 volts at most.
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From the wall I read 122 VAC. At the point shown in photo I read 118 VAC turned off. At the terminal side of CL80A I read 111 VAC turned on. I only read about 8 VAC at the fuse itself turned on. It looks like the CL80A is dropping at least 10 VAC I connected the CL80A the same as another AKr posted here.
 
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tonefoto said:
View attachment 1570765
From the wall I read 122 VAC. At the point shown in photo I read 118 VAC turned off. At the terminal side of CL80A I read 111 VAC turned on. I only read about 8 VAC at the fuse itself turned on. It looks like the CL80A is dropping at least 10 VAC I connected the CL80A the same as another AKr posted here.
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I am now reading about -25 VDC at the heater feed to V9 with V7-V10 tubes installed. I'm not sure what location I should be adding a dropping resistor to bring that voltage in the other direction. I've read a post saying the -25VDC at the heater to V9/V10 is normal with tubes installed on late model Fisher 400. Mine is early model though. Wish I could get this sorted out if it is a problem
 
tonefoto said:
I am now reading about -25 VDC at the heater feed to V9 with V7-V10 tubes installed. I'm not sure what location I should be adding a dropping resistor to bring that voltage in the other direction. I've read a post saying the -25VDC at the heater to V9/V10 is normal with tubes installed on late model Fisher 400. Mine is early model though. Wish I could get this sorted out if it is a problem
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Going to try a 20Ω 5W resistor and 100uf cap before the heater of V9 to see if this lowers the negative to voltage closer to spec.
 
larryderouin said:
Take it(the 400) off the Variac. Take a reading at the wall (ACV) and again at the terminal the CL-80 (oneside) and at the other lead from the wall on the fuse(again ACV). I want to see what your voltage drop is. CL-80's typically drop no more than 2-3 volts at most.
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Should I be reading the voltage drop with meter lead at terminal side of CL 80 and other meter lead at the fuse? I originally read the terminal side of the CL80 with one lead to chassis.
 
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