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I'm troubleshooting an SVT and i'm wondering about the plate voltages on the preamp tubes. The service schematic shows 300V but i'm getting 450V. After the rectifier in the power section, i'm getting around 700V which agrees with the schematic but after the next resistor R49 I get 600V instead of 485 on the schematic. My resistor is 4.7K 15W where it show 8.2K 15W on the schematic. I figure i'll pop in an 8.2k 15W and I should get lower plate voltages, but I'm just wondering if this resistor may have been lowered for a reason. More headroom? Is 450V too much for these preamp tubes? Thanks
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Well if I did my math right, applying Ohms Law V=IR with a voltage drop of 100 across a 4.7K resistor, means 213 mA is being drawn through the resistor. Changing that to a 8.2K with the same current will yield a voltage drop of around 175 V. That yields 525V which is indeed much closer to the schematic...me, I'd go with the 8.2K but I'm no SVT expert. Maybe a previous tech had read some Internet advise espousing a change in voltage...
It's also possible some of the plate resistors and cathode resistors have drifted up in value significantly and somebody was trying to Band-Aid the problem by boosting the voltage. -
I agree with nashvillebill about changing R49 back to the stock 8.2kΩ value. I'd also check R47,(the 8.2kΩ, 2W) resistor in the preamp section of the power supply. Then re-check the preamp tube plate voltages. You will see on the schematic that only three plates are at 300V. The others vary from 130V to 230V nominal.
Cheers,
TomLast edited by Tom Phillips; 07-14-2013, 07:47 PM.Comment
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100v/4700 ohms = +21ma .... 21ma^2 x 4700 ohms = 2.12 watts.
21ma across an 8200 ohm resistor = 172v drop ... 172v across an 8200 ohm resistor uses up 3.6 watts....Comment
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Yep, 21 mA not 213, I mistyped when I answered earlier... but the 175 volt drop across the 8.2K was right at least.
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I checked those above mentioned resistors and they all check out ok. The trouble with the amp is as follows. When you turn it on, everything plays fine. After a few minutes, give or take, the volume drops to nothing. The tubes check out ok. Has anyone experienced this or knows what could be causing this?Comment
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My first guess is that there is a bad connection somewhere that disconnects itself because of the thermal expansion caused by the amp heating up. If you literally mean that the volume drops to "nothing" that indicates that the fault is breaking the direct signal path or interrupting the heater connection to a tube section through which all the signal must pass. When the volume drops is it abrupt or a slow fad away? If abrupt it could be a bad connection or internal to a part in many places. Does thumping on the cabinet with your fist cause it to cut in and out? If it is a slow fad out then a heater problem such as a cracked solder joint where the tube socket is soldered to the PCB. Those are my thoughts for now.Originally posted by rhaigh View Post
More questions to help isolate the problem area:
When the volume cuts out are both preamp channels dead or just one? (If just one then which one?)
When the volume cuts out is the signal still present at the rear panel "Ext. Amp" jacks?
Cheers,
TomLast edited by Tom Phillips; 07-15-2013, 09:39 PM.Comment
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Thanks for all the suggestions. I think the cutting out problem was do to a bad, scratchy volume pot. The sound would just cut out and with a quick turn of the volume pot, the sound would return. It's real noisy when you turn it too. I'll have to order a new one. One other issue... With both amp volumes down, the is a fairly loud background hissing. Its as if the amp is cranked up, but its not. I tried removing all tubes in the preamp and its still there. I also tried plugging in a cable into the power amp in jack, and the problem still there. Is this normal for these amps (i'm new to them)? What would be the first thing to check in the power amp section? Thanks again.Comment
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I would check your bias, first. You could just be running extremely hot."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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I checked the bias and it looks good.Comment
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Well, the problem with the hissing was a bad 12DW7/12AX7 (V1 on the power amp schematic). Swapped it out and the hiss is completely gone. Now if I could only get rid of the loud hum. The filter caps look original and real old. Maybe start there...Comment
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Good to hear you solved the hiss problem, and......
Yep, almost every one of these I've rebuilt/repaired has at least some bad filter caps. I've yet to find the exact multi-cans but, as I recall, you can get at least a couple in a can at Mojotone and then you have to secure others in the chassis. Maybe somebody here knows of a better solution/source for the exact caps, but that's how I usually do it.
Edit: Here's a link to their page of multi-cap cans."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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Would a 8.2k 10W wirewound resistor get too hot? The 4.7k is 15W or is that a bit of overkill? The reason I ask is because I have access to a 10W but not a 15W.
Thanks.
Originally posted by Bruce / Mission Amps View PostComment
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It wont get too hot for the resistor itself since it's only dissipating 3.6 W. However, it will still put out lots of heat so you need to mount it so it's not right against the PCB (Use spacers) or other component that would be overheated by the heat coming off the resistor. Note that a 15W resistor will give off the same amount of total heat energy in that application. The energy would just be spread over more surface area and the body of the resistor would stay at a lower temperature.Originally posted by rhaigh View Post
One additional gotcha is that some modern resistors are made with materials that can withstand higher temperatures and can therefore be made in a smaller case size. Since the laws of physics still apply, those resistors can get much hotter than a larger bodied resistor in the same circuit.Comment
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