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Old 21st July 2011, 12:37 AM   #1
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Default Dynaco Mark IV Power Resistor

I have a question about the 6800ohm resistor in the power supply, the schematic doesn't specify the wattage for this resistor,so I installed one with a 5 watt rating,The voltages look good when I first turn it on,but then smoke starts to come from the resistor, do I need a larger resistor?
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Old 21st July 2011, 01:22 AM   #2
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Five watts should be plenty. Estimating 10mA through 6800 ohms gives a 68 volt drop and 0.68 watts. Perhaps a leaky 20ufd filter section or maybe the power take off socket is pulling current. Even if these numbers were doubled (1.5W) 5 watts should be enough. Try removing the 7199 as see what happens.
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Old 21st July 2011, 11:36 AM   #3
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HollowState is correct. Even a 2 watt resistor here is plenty. The fact that this resistor burned out on you (IMHO) is because of excessive current draw from some component on your Mark IV's driver board. If you have the original Mark IV driver board, you should probably replace the board and all parts on the board.

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Old 21st July 2011, 12:44 PM   #4
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hey-Hey!!!,
the question I have is what happens to the voltage across that resistor as it overheats? IIRC that same value was used in the Mk.iii as well and it was a 1W carbon comp. At full saturation the rest of the front end can't draw that much current; 96k total for the PI and a 270k for the input pentode...or something a bit less than 10 mA...so the current must be going somewhere else.
cheers,
Douglas
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Old 21st July 2011, 01:11 PM   #5
DF96 is offline DF96  England
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Must be a short somewhere. PCB track fault, power takeoff socket or 20uF cap (or the wiring connecting them). Most likely is the cap.
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Old 21st July 2011, 06:00 PM   #6
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Quote:
Originally Posted by Bob01605 View Post
If you have the original Mark IV driver board, you should probably replace the board and all parts on the board.
Bob Latino
Spoken like a true PCB vendor. Actually, the electrolytic cap (in the tall can) and can with plus on the end under the deck, are ten year life parts. One section (20 uf) is connected directly to your burning 6800 resistor. I've replaced the electrolytic caps on my 1961 ST70 three times since I bought it in 1970. The McIntosh salesman (1970) who tested the amp pointed out the outdated cap was keeping my wattage out down to 7w/channel. I put a new can cap in 1984, then didn't use it much because the 6CA7s were tired and replacements were sole sourced from the USSR at the time. I bought some Slovak tubes this year, tried to run them with the old practically unused cap, and it ran one night, before leaking the fluid out and blowing the fuse. Oxygen never sleeps, the rubber seals had gone bad. I get my can caps from tubesandmore.com or triodeelectronics.com, but for this time put radial lead 3000 hour electrolytic caps under the deck with solder terminal strips and 600 v wire. I plugged the can hole with a galvanized metal mesh.
The advantage to new high value (100k and up) metal film resistors instead of carbon comp, is less hiss. The advantage of metal film dropping resistors (3 W or up) is they are more stable than carbon comp.I get my metal film resistors from newark.com, $.07 to $.33 for the house brand or Vishay from India. I replaced the paper .1 uf caps with plastic film caps, but didn't hear any difference. I'm not fooling with the mica cap.
There are driver boards that use commonly available dual triode tubes instead of the unobtainable 7199, that people say have less distortion. . There is a more complete discussion under the ST70 topic, same circuit diagram almost, but an amp that sold a lot more units.
Have fun. My St70 sounds great with 1970 7199's and new JJ 6CA7's. Not quite up to the Peavey CS800s, but that is a $1000 amp, not a $100 amp like the ST70 was.
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Last edited by indianajo; 21st July 2011 at 06:19 PM.
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Old 21st July 2011, 06:31 PM   #7
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Bob spoke like a true PCB vendor. Getting your amp to stop setting parts on fire should be primary, improving the sound with the pcb should be secondary. Actually, the electrolytic cap (in the tall can) and can with plus on the end under the deck, are ten year life parts. One section (20 uf) is connected directly to your burning 6800 resistor. I've replaced the electrolytic caps on my 1961 ST70 three times since I bought it in 1970. The McIntosh salesman (1970) who tested the amp pointed out the outdated cap was keeping my wattage out down to 7w/channel. I put a new can cap in 1984, then didn't use it much because the 6CA7s were tired and replacements were sole sourced from the USSR at the time. I bought some Slovak tubes this year, tried to run them with the old practically unused cap, and it ran one night, before leaking the fluid out and blowing the fuse. Oxygen never sleeps, the rubber seals had gone bad. I get my can caps from tubesandmore.com or triodeelectronics.com, but for this time put radial lead 3000 hour electrolytic caps under the deck with solder terminal strips and 600 v wire. I plugged the can hole with a galvanized metal mesh.
The advantage to new high value (100k and up) metal film resistors instead of carbon comp, is less hiss. The advantage of metal film dropping resistors (3 W or up) is they are more stable than carbon comp.I get my metal film resistors from newark.com, $.07 to $.33 for the house brand or Vishay from India. I replaced the paper .1 uf caps with plastic film caps, but didn't hear any difference. I'm not fooling with the mica cap.
There are driver boards that use commonly available dual triode tubes instead of the unobtainable 7199, that people say have less distortion. . There is a more complete discussion under the ST70 topic, same circuit diagram almost, but an amp that sold a lot more units.
Have fun. My St70 sounds great with 1970 7199's and new JJ 6CA7's. Not quite up to the Peavey CS800s, but that is a $1000 amp, not a $100 amp like the ST70 was.[/QUOTE]
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Old 21st July 2011, 07:33 PM   #8
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I believe it is a faulty quad cap,I used my multimeter to measure the #4 pin,after I disconnected it,and got a measurement of 35 ohms. It's definately leaky.
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Old 21st July 2011, 08:07 PM   #9
DF96 is offline DF96  England
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35 ohms is not a leak, it is a dirty great hole! I think you have your culprit.
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Old 21st July 2011, 09:27 PM   #10
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I apologize folks,I was leading you astray. It was a screw-up on my part. By the way in case your wondering this is a NEW build. Theres nothing wrong with the cap,I mistakenly mis-wired the take-off socket. the wire was supposed to be soldered to pin #4 and the 22K resistor,was soldered to pin #3 by mistake,which would take the current through the 10ohm resistor straight to ground.
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