Hi gang,
I've been a lurker around here in the past and now have a need for some specific help.
I recently purchased a mid-90s Conrad Johnson Premier 14 tube preamp. I am getting a static-like noise in one channel, then some frying egg type noise that usually ends with a mild pop. Then the pre settles into the static noise, repeating the egg frying pop again. I don't perceive a change in loudness of the noise with volume on the preamp and input source or no source have no effect. The previous owner had noted this noise as well and stated it went away after a few minutes. I have listened to it for 30 min or so and still didn't hear it calm down, though it seems to get less frequent with the pops as time goes on. Last night I decided to let it stay on for an hour or two before listening and then it was quiet. No static, nothing for the next couple hours I listened to music.
Diagnostics done so far: swapped tubes around L to R, and mixing the tubes front to back (there are 2 per channel). Noise seems to always stay on the left channel. I pulled the tubes in and out about 20x and gave then a brushing with the dental flossers and I will pick up some Deoxit this weekend. I retensioned a couple of the socket connections as one socket (on the left) seemed looser than the other 3. I have used the chopstick method and nothing changes the noise. I did feel like one time when the frying was actively happening wiggling at tube might have changed the sound, but that could have been coincidental.
Some info on the pre: It uses two 6GK5 tubes per channel, which mine are currently a mismatched pair of mullards and another brand. The volume control is a series of resistors and relays, effectively making a step attenuator. Vishay metal foil resistors and CJ poly caps in most everything short of the power supply.
What do you guys think I should test? I do need to get a complete new set of tubes just in case I have 3 bad of the 4. I am a little knowledgeable on working with electronics, enough to know how not to get shocked and I am handy with a soldering iron. I have read a ton on this and plate resistors come up a lot. Is there a way to test without unsoldering? Can I pull the tubes and check resistance or power up to check the voltage to see if L and R match?
Thanks
ben
I've been a lurker around here in the past and now have a need for some specific help.
I recently purchased a mid-90s Conrad Johnson Premier 14 tube preamp. I am getting a static-like noise in one channel, then some frying egg type noise that usually ends with a mild pop. Then the pre settles into the static noise, repeating the egg frying pop again. I don't perceive a change in loudness of the noise with volume on the preamp and input source or no source have no effect. The previous owner had noted this noise as well and stated it went away after a few minutes. I have listened to it for 30 min or so and still didn't hear it calm down, though it seems to get less frequent with the pops as time goes on. Last night I decided to let it stay on for an hour or two before listening and then it was quiet. No static, nothing for the next couple hours I listened to music.
Diagnostics done so far: swapped tubes around L to R, and mixing the tubes front to back (there are 2 per channel). Noise seems to always stay on the left channel. I pulled the tubes in and out about 20x and gave then a brushing with the dental flossers and I will pick up some Deoxit this weekend. I retensioned a couple of the socket connections as one socket (on the left) seemed looser than the other 3. I have used the chopstick method and nothing changes the noise. I did feel like one time when the frying was actively happening wiggling at tube might have changed the sound, but that could have been coincidental.
Some info on the pre: It uses two 6GK5 tubes per channel, which mine are currently a mismatched pair of mullards and another brand. The volume control is a series of resistors and relays, effectively making a step attenuator. Vishay metal foil resistors and CJ poly caps in most everything short of the power supply.
What do you guys think I should test? I do need to get a complete new set of tubes just in case I have 3 bad of the 4. I am a little knowledgeable on working with electronics, enough to know how not to get shocked and I am handy with a soldering iron. I have read a ton on this and plate resistors come up a lot. Is there a way to test without unsoldering? Can I pull the tubes and check resistance or power up to check the voltage to see if L and R match?
Thanks
ben
Seems to be fussy about tubes.
" Weakness:
"Noisy with most 6gk5 tubes. Need the best NOS-tubes there is."
http://www.audioreview.com/product/amplification/preamplifiers/conrad-johnson/premier-14.html
6GK7 has nominal Gm of 15,000. That's an astonishing number and suggests the grid is wound mighty tight to cathode, so the least flaw will be loud.
If with the new tubes and all solder joints checked, it still produces noise, you may have to replace
some resistors, one at a time. Do not replace them all at once. Start with the first plate resistor, test,
then the first cathode resistor, test, then the first grid resistor, test. Hopefully one of those in the first stage
will be the bad one. But if not, then go on to the next stage.
some resistors, one at a time. Do not replace them all at once. Start with the first plate resistor, test,
then the first cathode resistor, test, then the first grid resistor, test. Hopefully one of those in the first stage
will be the bad one. But if not, then go on to the next stage.
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I had such noises in a preamp when what I thought were 2W anode resistors turned out to be 1/4W. (Don't ask)
My guess is the anode resistors, could be a bad solder joint.
My guess is the anode resistors, could be a bad solder joint.
In my case, a number of years ago, it was a factory defective Dale metal film RN55 resistor.
Actually a whole bag of them. The factory said they knew about the problem and sent me some more.
Actually a whole bag of them. The factory said they knew about the problem and sent me some more.
A lot of manufacturers seem to have a touching but misplaced trust in metal film resistor voltage ratings (McIntosh, I'm looking at you). Parts are cheap; replace all the plate loads with the biggest you can find. If available in the right value, the Dale RS 5 watters, and up, wirewounds work well.
All good fortune,
Chris
All good fortune,
Chris
Thanks for all the great tips, gang!
New tubes are on the way. I'll hold tight until they are in and see.
Sounds like resistors are the next logical thing to try after tubes. Chris, thanks for the suggestion on a type to use. I found an old blog post of a guy using Yamamura and Shinkoh resistors to replace the factory ones and claimed an improvement in sound. Any reason to use one resistor type over another? This was a very expensive unit in its day and I don't mind spending an extra few bucks on premium parts if they are worth it. I searched high and low for what is currently in the preamp, I think Vishay VTA53. Either they are discontinued or I don't know where to look. Can you guys suggest a good place to buy from? I know of the usual Mouser and Digi-key, but if there is a more audio friendly place to avoid searching through 10,000 parts to find what I need, that would be great.
Thanks again,
ben
New tubes are on the way. I'll hold tight until they are in and see.
Sounds like resistors are the next logical thing to try after tubes. Chris, thanks for the suggestion on a type to use. I found an old blog post of a guy using Yamamura and Shinkoh resistors to replace the factory ones and claimed an improvement in sound. Any reason to use one resistor type over another? This was a very expensive unit in its day and I don't mind spending an extra few bucks on premium parts if they are worth it. I searched high and low for what is currently in the preamp, I think Vishay VTA53. Either they are discontinued or I don't know where to look. Can you guys suggest a good place to buy from? I know of the usual Mouser and Digi-key, but if there is a more audio friendly place to avoid searching through 10,000 parts to find what I need, that would be great.
Thanks again,
ben
Oh, one more question: how much variance from the factory resistor values is acceptable? The plate resistor is 13.7K for example. If I can't find pretty close to a 1:1 I suppose I need to replace both channels to match.
I f you find the trouble in a resistor you must change on both channel.
on Digikey
ROX2SJ10K 2w 10k TE ROX ( just an example)
PR02000201002JA100 Vishay PR02
Check the space between ends
I just wanted to thank those who offered help; it was a plate resistor that was bad. The offending resistor even had a dullish spot on it, but interestingly measured perfect ohms. Swapping them out with some new resistors fixed it up.
In the same deal I picked up a Conrad Johnson PV10B that has a hum, so now onto the next project!
In the same deal I picked up a Conrad Johnson PV10B that has a hum, so now onto the next project!