Poseidon trouble in River City

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Poseidon trouble in River City

Postby logos47 » Sat Jan 07, 2017 9:58 pm

I could sure use a bit of guidance. After eight years of happy use, I erred in leaving ECC83/12AX7s in both gain stage and driver positions of the Dynaco MKIII Poseidon boards for a month or so. I corrected this and things were fine for two days, but now there is a sputtering sound despite tube swappings. I run them in triode. Perhaps Shannon or members of the Council of Audio Elders could give me some badly needed guidance. Thank you! :(
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Re: Poseidon trouble in River City

Postby TomMcNally » Sun Jan 08, 2017 2:17 pm

I don't think that would cause a problem, the tubes are the same pinout and not high current level. Maybe check all of the resistors and see if something changed value, and the LM334s. Do both amps do the same thing ? How about with no input and thre RCA jacks shorted ?

... tom
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Re: Poseidon trouble in River City

Postby logos47 » Sun Jan 08, 2017 7:46 pm

Tom, as one who has read your posts for years, I appreciate your suggestions and will follow up, starting with the resistors. Interestingly, that ECC83 is a GL reissue and built lake a tank. However, it was very hot upon extraction. I just hope it isn't the little chip. Thanks again. I will follow up in a week or so.
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Re: Poseidon trouble in River City

Postby EWBrown » Mon Jan 09, 2017 9:11 am

The PI draws about 4 to 5 mA plate current, the 12AX7 is designed around 1 -2 mA plate current, it would not operate properly in the PI circuitry. bias voltage determined by the LM324 would set the grid voltage too close to zero volts, so it would cause clipping and excess distortion fairly easily.
Running a 12AX7 at 4-5 mA would cause the tube to become fairly hot. Using a 12AU7A or 12BH7A in the PI should set things right.

/ed B
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Re: Poseidon trouble in River City

Postby logos47 » Mon Jan 09, 2017 10:41 am

Thanks EW. With the correct tubes in, I am getting the sputtering which has the unit on the bench (I'll be doing this with the work of a good friend). I did send a charge through the system that fried some fuses when I plugged a live rca plug in with the two 12ax7 tubes in both positions. It generated a loud noise and two nearly NOS GE 5ar4 went with it. I've inadvertently done this before, but never with this result.
PS: once running, will a 12BH7 be plug and play, or will a resistor value need to be changed?
Thank you, EW!!
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Re: Poseidon trouble in River City

Postby EWBrown » Mon Jan 09, 2017 2:07 pm

I've changed out 12BH7 for 12AU7, they're pretty much plug 'n' play interchangeable.
Alaso try 5751 for the 12AX7. Also interchangeable.

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Re: Poseidon trouble in River City

Postby logos47 » Mon Jan 09, 2017 8:08 pm

EW and Tom, thank you so much. After the suggestion of unplugging signal input from the unit (which is now on my friend's bench across town), I took the "good" unit and swapped in the linestage IC from the faulty channel and voila--the nasty sputtering hash was now in the previously quiet MKIII amp. The problem was from the linestage (nicely done RCA New Orthophonic in a PAS chassis). Evidently, fader lube got into the function selector switch (long story)...so a good deoxit cleaning and pin tightening cleared it all up. Thanks to you, I used the tools at hand and am in business. I think I'll get Michael Geier's "How to Fix Everything Electronic" and get an oscilloscope if I'm to satisfy my addiction to the sound of my all-analog rig of 45 years (and keep my wife and friends). Cheers!
PS: I've got two 5751 Sylvania triple mica bp--a nice tube. I'll try the 12BH7 along with it.
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