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Old 2nd May 2012, 09:55 AM   #1
Mihan is offline Mihan  New Zealand
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Default Vacuum State RTP Preamp Problem

I have one of these preamps which is over 20 years old now and have noticed that a 62K resistor combination (made up of two 30K9 resistors in series) from the 340V HT supply to the plate of the phono input 7DJ8 tube in both channels are probably going to fail soon. They are showing signs of overheating, discolored and the surface is peeling away on all four. All the other stages of the amp that use this same resistor combination from the HT supply are fine.

I have to hand some 11K, 51K and 62K Kiwames, all 2 watts. The question is would it be better to use a 11K/51K combination to make up 62K and therefore give a higher wattage, or would the one 62K be enough? The originals appear to be either half or one watt.

Last edited by Mihan; 2nd May 2012 at 10:02 AM.
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Old 2nd May 2012, 10:02 AM   #2
DF96 is online now DF96  England
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It might be useful to consider whether the original resistors were underspecified or whether a fault has developed which has increased the current through these valves.
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Old 2nd May 2012, 10:58 AM   #3
Mihan is offline Mihan  New Zealand
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Quote:
Originally Posted by DF96 View Post
It might be useful to consider whether the original resistors were underspecified or whether a fault has developed which has increased the current through these valves.
Good points DF96. I just noticed this problem while changing some tubes and haven't had it on the bench yet. The amp has two phono and one output stage that are all identical and it's only the first phono stage in both channels that are exhibiting this problem.
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Old 2nd May 2012, 11:29 AM   #4
DF96 is online now DF96  England
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"Identical"? I doubt it. Maybe uses the same valve, but it won't be the same circuit.
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Old 2nd May 2012, 01:55 PM   #5
Mihan is offline Mihan  New Zealand
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Each stage shares pretty much the same topography. There are differences of course, such as the RIAA section in the phono stage and elsewhere but I was thinking in terms of how the tube plates in each stage are feed the HT via 62K resistors.
If all the voltages are right and I can't find any source of extra current draw , which of those two options I mentioned at the beginning do you think is better? It's interesting that the problem is identical in the same phono section of each channel.
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Last edited by Mihan; 2nd May 2012 at 02:13 PM. Reason: add extra information
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Old 2nd May 2012, 04:12 PM   #6
DF96 is online now DF96  England
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OK, I see what you mean. If the original was 62k 1 watt then 62k 2 watt may be OK. Rough calculation puts the resistor dissipation at over 700mW, which is too high for a 1W component under a hot chassis. You need at least a 2:1 derating.
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Old 3rd May 2012, 10:15 AM   #7
Mihan is offline Mihan  New Zealand
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Thanks for your help DF96.
The original 64k are made up of 2 X 30.9K in series, presumably 1 watt each. I think I will use the 11+51K=62K 2 watt combination in series giving 4 watts power rating which should be enough.
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Old 3rd May 2012, 10:21 AM   #8
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Sorry, but no, the combination of a 51k and an 11k 2Watt resistor will not be able to dissipate 4Watts, but only 2.2Watts, so you don't gain much by doing this...

Use either two resistors of the same value or stay with one bigger resistor.
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Old 3rd May 2012, 12:25 PM   #9
Mihan is offline Mihan  New Zealand
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Thanks for putting me right Atarashi.
I new that two resistors of the same value would work but wasn't sure about mixed values. I was trying to work with what I had on hand.
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Old 3rd May 2012, 12:44 PM   #10
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Well, I try to do the same all the time, but sometimes it is better to buy a few resistors (even if the Kiwame are not really cheap...)
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