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Old 11th February 2013, 02:21 PM   #11
mepper is offline mepper  United Kingdom
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I recently had a Ming Da amp do something similar: transformer uncomfortably hot, loud hum, vibration. The problem was a faulty bridge rectifier (tested it with a multimeter). Replaced that and it's working great, so far.
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Old 11th February 2013, 03:39 PM   #12
rknize is offline rknize  United States
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How hard are you running those 6550s? I'm using the XPWR035 in mine, which is rated somewhat similarly to yours except the HT is 370-0-370. It is dead quiet in most cases, except when I lean on it really hard. With KT-88s on the hottest cathode resistor setting, it develops a bit of a buzz and gets toasty after a few hours. I am essentially overloading it: the KT88s draw nearly 100mA each.

Do you have SS diodes installed?

Do you have a clamp-on meter? If you can measure the current going into the amp, we can see if you are exceeding the traffo's VA rating.
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Old 11th February 2013, 03:58 PM   #13
rmyauck is offline rmyauck  Canada
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Could he pull one of the output tubes to check loading? If so try each output tube. Let it cool good between powering back up though. Is that the new board with IN4007 or UF4007 SS diodes added? Maybe as Russ mentioned a problem with those like polarity.

Randy

Last edited by rmyauck; 11th February 2013 at 04:03 PM.
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Old 11th February 2013, 11:23 PM   #14
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Looks to be well within spec for the transformer, about ~130mA on a 200mA tranny, 5AR4 draws 1.9mA good there, 3.2mA on the KT88 heaters, good to go... how about the OP taking some voltage measurements at the PSU and on the output tubes and report back. Wiring problem or faulty part. Something is drawing too much current, the power tranny should be loafing along and be barely warm under normal conditions.

Cheers,
Bob
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Old 12th February 2013, 03:41 AM   #15
kkcinc is offline kkcinc  United States
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So I gutted the entire amp today. Checked and rechecked all wiring. Reassembled and even left out the triode ul switch. Just wired it straight ul with cfb. No extra switches. Just on off. Then got rid of the auxiliary capacitor for a new one. Shortened up some wires. Mounted the transformer on felt pads (grounded it to the case of course). And for now used 5u4 and some 6l6gc's. Same 6201. Less hum. Less heat. After several hours I can touch the thing for ten seconds and not get burned. This could be an improvement or could just be the tubes change felt pad thing. Can a bad c1 cause transformer to hum? I think it is fine but it is a little bit big. 56uF. I forgot I had it in there. I'll change it tomorrow and then try the 6550/5ar4 combo. If all that doesn't change the heat/hum issue, then I'll call it a bad transformer.
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Old 13th February 2013, 01:03 AM   #16
kkcinc is offline kkcinc  United States
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Quote:
Originally Posted by rknize View Post
How hard are you running those 6550s? I'm using the XPWR035 in mine, which is rated somewhat similarly to yours except the HT is 370-0-370. It is dead quiet in most cases, except when I lean on it really hard. With KT-88s on the hottest cathode resistor setting, it develops a bit of a buzz and gets toasty after a few hours. I am essentially overloading it: the KT88s draw nearly 100mA each.

Do you have SS diodes installed?

Do you have a clamp-on meter? If you can measure the current going into the amp, we can see if you are exceeding the traffo's VA rating.
560ohm resistor/450ish B+
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Old 13th February 2013, 01:05 AM   #17
kkcinc is offline kkcinc  United States
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Quote:
Originally Posted by rmyauck View Post
Could he pull one of the output tubes to check loading? If so try each output tube. Let it cool good between powering back up though. Is that the new board with IN4007 or UF4007 SS diodes added? Maybe as Russ mentioned a problem with those like polarity.

Randy
I have the old board.
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Old 13th February 2013, 02:27 AM   #18
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Originally Posted by kkcinc View Post
560ohm resistor/450ish B+
If you'd tell us the voltage across the resistor it'd be a lot more helpful.
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Old 13th February 2013, 02:54 AM   #19
rknize is offline rknize  United States
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Quote:
Originally Posted by kkcinc View Post
So I gutted the entire amp today. Checked and rechecked all wiring. Reassembled and even left out the triode ul switch. Just wired it straight ul with cfb. No extra switches. Just on off. Then got rid of the auxiliary capacitor for a new one. Shortened up some wires. Mounted the transformer on felt pads (grounded it to the case of course). And for now used 5u4 and some 6l6gc's. Same 6201. Less hum. Less heat. After several hours I can touch the thing for ten seconds and not get burned. This could be an improvement or could just be the tubes change felt pad thing. Can a bad c1 cause transformer to hum? I think it is fine but it is a little bit big. 56uF. I forgot I had it in there. I'll change it tomorrow and then try the 6550/5ar4 combo. If all that doesn't change the heat/hum issue, then I'll call it a bad transformer.
The is a tad on the big side for C1, but it will be more stressful for the 5AR4 than the transformer. The voltage across the cathode resistors when in operation with the 6550s will help us determine the load. However unless something is going on with one or both of those tubes, they should be more on the cold side with those resistors.

When running the 6L6's this time, it sound like the transformer is operating fairly efficiently. What is your line voltage? Is the transformer buzzing or humming? Do you have the SS rectifier option installed?
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Old 13th February 2013, 01:14 PM   #20
kkcinc is offline kkcinc  United States
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Originally Posted by rknize View Post
The is a tad on the big side for C1, but it will be more stressful for the 5AR4 than the transformer. The voltage across the cathode resistors when in operation with the 6550s will help us determine the load. However unless something is going on with one or both of those tubes, they should be more on the cold side with those resistors.

When running the 6L6's this time, it sound like the transformer is operating fairly efficiently. What is your line voltage? Is the transformer buzzing or humming? Do you have the SS rectifier option installed?
GREAT QUESTION. My line voltage has been running quite high. I'm on the east coast and I think George had the same problem. I measured almost 127 the other day. I have no idea what the power company is doing. Must be all these windmills! Supposed to be 115. I'll try to do a full checkout with the meter today if I can get to it.
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