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tiept 6th September 2006 07:46 AM

Need help from Aikido builder
 
Hello,

I have been lurking on and off for a few years but haven't built anything. I did search on this but couldn't find anything related to what I am experiencing.

Anyway, I recently bought John B's 9-pin Aikido PC board just to have a decent preamp since all my other preamps are vintage types. I got my preamp up and running and it sounds very good. However, every 10 minutes or so, I would get a little distortion or oscillation sound in the form of a split second chirp sound. At first, I thought it might be due to the 4uF output cap charging and discharging, but I am getting the same random chirp using .47uF cap.

I am using a salvaged PT and there's a chance that it is not robust enough. I used the PSUD designer software to help with the PS design. Anyway, here are some notes:

- B+ @242 from a CLCLC supply
- 12au7 input tubes running at 4.5 mA (810 ohms or so cathode resistors)
- 6922 output tubes running at 8.5 mA (320 ohms cathode R's)
- Changed the output from 6DJ8 to 6922 as I suspected it might have been the source of the oscillation.
- 12.6 VAC heater
- R16 is about 87.4K (from JB's table for 6dj8 @250v)
- PC board is Rev A

Anyone encountering the same problem? I appreciate any helps.

Thanks,
T.

Bas Horneman 6th September 2006 08:52 AM

It sounds like some earthing wire is not soldered properly. Check all solders again. Or try fiddling with the rca plugs...if you hear the chirp when fiddling with the input rca plugs...then I think you might have found your problem.

ErikdeBest 6th September 2006 09:46 AM

Hi Tiept

Not specifically related to your question, but anyway I will post it. I see you are using 12VAC and 6DJ8...well, you are actually using 2 6DJ8's (1 for each channel), so I imagine you have the heaters wired in series, or a big resistor that drops 6,3V, so the 6DJ8 gets his 6,3V on the filament. Happened with me that I had a ECC88 running on 12V...well, all other ECC8X's needed those 12V applied to pin 4 and 5, why would the ECC88 be different? :S

Erik

Burnedfingers 6th September 2006 11:08 AM

I'm with Bas on this also.

I think it is either a grounding problem or cold solder joint on the board.

No problem with the board I bought.

tiept 6th September 2006 08:38 PM

Thanks everyone. I do remember that the connections to my vintage Grommes EL84 amp were pretty loose. I changed to another set of interconnects and that seem to hold the jack a little tighter. Since I am still in the breadboarding stage, I used cheap RCA input jacks at the Aikido end and that maybe a problem too. I am gonna check all the connections including the ground connections tonight.

Also, I am currently not using earth/chassis ground at all. When I do put this whole setup into a proper chassis, do I run a wire from the chassis ground to the earth ground symbol eyelet on the PC board? And do I then put in a jumper from that eyelet to the signal ground (eyelet right next to it)?

I must say that I am impressed with the sound I am getting so far. And like most of you, it doesn't sound like any preamps I have heard before. It is neither sterile nor overly warm, but just right. I am surprised to get this kind of sound out of the 6922 at the output.

Regards,
T.

tiept 23rd September 2006 06:53 AM

Just want to update fellow Aikido builders on my problem. You gentlemen were right, it was a grounding issue. Turned out one of the 1M resistors had one side not soldered. I guess it just made enough contact to be intermitten.

Anyway, I have another issue. This time it has to do with the attenuator that JB sells. Just wondering if the orientation of the switches important. I didn't pay attention to the tab location on the silkscreen; thus my center switch is on the wrong side. The attenuator works, except that I get a pop everytime I use the center switch. I have not asked JB about the orientation but did ask him about the popping issue. He could only come up with maybe DC from my source (and there are none out of my cd player). In my mind, the orientation of this switch shouldn't be important since it is a make-before-switch type.

Thanks again,
T.


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