[S Clark]

I've got a Dodd battery phono pre that uses 6dj8 type tubes that I am very happy with.  But tonight while listening, it gave a medium volume pop then a descending stepped tone, but not staccato, from mid to very low frequency before I could shut it down... sort of a da da da da da da da almost a violin type tone.  I switched to a different pre and there was no problem.  After listening for about 30 minutes, I put the Dodd back in and it worked fine for about 20 minutes, then did the same musical motorboating.  I'd rather not bother Gary unless I have to, so I thought I'd ask here first.  Could a tube do this? There are 4  6N1P's in the unit.  Or do I have bigger problems? 
 

[S Clark]

Bumping this once...

[kingdeezie]

I am not the most technical guy on the forums, but it sounds like the tubes to me. I have had similar issues happen to me, and both times they were the tubes.

One time, I had the loudest pop while playing a record. This was years and years ago, when I was using a Mapletree Audio 2A with a built in phono stage. After the initial loud bang, that made me soil myself, it made popping noises sporadically. I replaced the tubes, and the issue went away.

Recently, my preamp tubes started to go, and they would let out this whistling sound (what I could see someone else saying sounds like a violin tone) that ranged from dog whistle frequency, and would spiral down to low midrage tones. Replaced the tubes, got rid of the problem.

I would try changing the tubes.

[S Clark]

Ok, changed two of the four tubes, played great for 25-30 minutes, then pop, and downward zzzzzzzzzzzzzzzum.  Then put the original two back in for the remaining original two.  Turned it back on, and 30 seconds later, pop and zzzzzzzzzzzum.  I don't think it's tubes, unless I have two bad ones that failed at the same time.  It does seem to be heat related, since the hot unit immediately misbehaved. 
Any other ideas? 

[HAL]

Scott,
The 6N1P uses more filament current than a 6DJ8.   Are the new tubes you put in 6N1P's?  The one thing might be the filament voltage regulator if it uses a L7806 like the tube buffers. 

It sounds like a power supply related issue from the description of the noise.



 

[S Clark]

I'm think that Gary used a Russian tube originally (but may have been 6N6 instead of 6n1), but I'll switch the all out to 6922's and see what happens.  As far as power, I'll also change batteries, although it shows to be charged.  And yes the new tubes were also 6N1P. 

[HAL]

Scott,
The 6DJ8 has a 0.36 Amp heater current and the 6N1P has a 0.6Amp heater current per tube.  The L7806 is a 1.5Amp max regulator.  If there are more than 2 6N1P, then it is overloaded.  You can have up to 4-6DJ8 style tubes on the same regulator for the filament.

The 6DJ8 will show if the filament regulator is the issue.  If it goes longer before oscillating, then it is the L7806 going bad.

[S Clark]

Thanks Rich.  I've got four 7308 playing as I type. We will see how it goes.

[S Clark]

Thanks Rich.  I've got four 7308 playing as I type. We will see how it goes.

  No luck.  Plays for about 30 minutes, then begins with the noise. 

[HAL]

Power supply issue.  Would get Gary to look at it.

My suggestion is that with 4 6N1P's, it will need a bigger filament regulator if it uses an L7806. 

[sfox7076]

I have the same issue with my recernly(ish) fixed EL84 amp that Gary fixed.  I think it could be a bad cap.  Either need to open the hood or send it to a friend that fixes things.

[S Clark]

Well, shoot.  I was hoping not to add to Gary's load right now.  I don't see the voltage regulator on the main board, so I guess it is the potted module- otherwise, I'd change it out myself. 

[HAL]

Scott,
It is a three terminal device normally installed on the chassis as a heatsink.  Usually goes to a switch if the filament voltage is selectable as 6/12VDC.  That is the way the tube buffers are wired.  Probably looks similar to what the original install looked like in your DAC.

Take an interior picture and I can probably pin point it.

[S Clark]

I recognize everything except the little clear plastic and silver things that look like miniature beer kegs. 
If you're looking for this, it's not there. 

[HAL]

Scott,
Then the power supply is different than the tube buffer.  You need to talk to Gary.

[Danny Richie]

Try unplugging the charger.

[S Clark]

Tried unplugging the charger.... still has problems.  I'll just have to drop it in Gary's lap and have him look at it. 
Thanks for all the suggestions, guys.