first time build, everything went pretty clean but... [resolved]

[xaanix]

status update:

I attempted to prod nearly all of the terminals with a plastic pen and could find no real correlations between which terminals I poked and the effect on the A3 LED. (all of them, even on socket B, seemed to affect it equally)

I did put my phone under the amp and tapped very lightly on the tip of the 12AU7.  This seemed like a strong correlation because of how softly i was touching it and the fact that it did reliably cause the A3 LED to go on and off.   I'm thinking either a problem with the 12AU7 or something with the socket.

I did monitor the voltage of the headphone jack during the problematic behavior.  After 'boot up' of the amp, the voltage did increase but stayed below 9 volts (think it peaked around 6, and then declined).  Since normally the startup of the amp never experienced the bad behavior (it seemed only possible once the tubes heated up) I never got to see any abnormal fluctuations during the startup period.  Voltages went down to 0 on the jack after about 4-5 seconds after the peak.   Then after a while i began to tap the amp and got the the A3 LED to flash, Once this happened I saw wildly fluctuating voltages on the headphone jack (tip), 24v was certainly hit several times while the problem was occurring.  Maybe this provides a clue?


Since the volume control was brought up as possible cause, and because i was not terribly happy with the tightness of my braid (after seeing doc's photos of one of his builds), and because I now have a well tinned iron and some fresh solder, I decided to re-braid and re-do all the connections on the volume control.  During this process i must have damaged the volume control as I am no longer able to measure any resistance across the 2 red terminals.   I can still measure resistance across the 2 white terminals (which is up around 106 ohms, as it was before).  So, sadly, I'm going to need a replacement for this part.

I had resoldered T1L, T2L, T3L, A-Center, and A1, and A2, and I had soldered the input termals (all 3 wires, and all of the connections to the volume control (after i broke it apparently). 

To proceed, and so multiple parts orders aren't needed (I'd like to minimize shipping delays) I'd like to order the following parts:
1 12AU7
1 socket A
1 socket B
4 LED's
1 volume control
24" more of each color of wire (black/red/white)

I don't mind paying for this stuff, my priority is to get this completed as soon as I can.




 

[xaanix]

This shows the behavior of tapping on tip the 12AU7 https://www.dropbox.com/s/8mz7mrq5ik377v6/20160416_191357.mp4?dl=0

[Natural Sound]

Have you trued another 12AU7?

[xaanix]

First time builder and this is my foray into tubes, so unfortunately I have no spare parts to test with.  My intuition tells me it's the tube but since I'm noob at this was looking for others opinions.

[Natural Sound]

Its either a bad tube or a bad solder joint somewhere. If you have a music store nearby you might be able to pick up spare tube there. The 12AU7 is a common tube used in guitar amplifiers. The place down the street from me sells 12AU7 JJ's for less than $15. FWIW I had a 6SN7 do the same thing. One of the internal wires (inside the glass envelope) wasn't terminated at the pin properly causing an intermittent connection. I replaced the tube and all was fine.

You may want to post your location. There may be a fellow bottlehead nearby that can help. We are a pretty friendly group. Alternatively you could give me your shipping address in a private message. I could send you a known good tube or two. I have more than I'll ever use in my lifetime.

[xaanix]

I'm in central Jersey.  I'll give the local music store a call (there's one pretty closeby, near trenton).

I cooked my volume pot while troubleshooting the issue (while desoldering).

2 questions:
1. can i stick the new tube in and power up the amp even while the volume control is busted (i'd imagine so)   BTW i'm reading 106 ohms on the white jacks and the red jacks i get no reading anymore.
2. Is there a place i can get a compatible volume pot replacement?

Thanks for the advice btw

I also did order the repair service but i've been waiting for a box (from bottlehead to make its way across the entire nation which is taking days and days heh)

-Jason

[xaanix]

Got a replacement 12AU7W (tung sol) from the music store and viola!  Damn thing now works like a charm.  Passed all my voltage checks and tapping/jostling the amp (or tapping the tube) now has no effect whatsoever.

I feel vindicated.  The GE tube that came with my kit is defective.  I'm not clear if this unit was new old stock or simply used.  It looked used.  I spent like 20 hours messing around with this damn thing and it wasn't even my fault :/

Now i just need to get this volume pot replaced and i'll be good to go.  Hopefully bottlehead will send me a replacement GE tube, assuming that sounds better than the cheapish (russian made) tungsol.

(12au7 matters more than the power tube right?)

[Natural Sound]

Jason,

So glad you figured out the problem. I was staring to sense your frustration. Your problem was a little unusual but you persevered. Good job! I'm sure bottlehead will make good on the tube. email [email protected] and they will hook you up.

When you are dealing with 50 year old parts (more or less) its hard to tell sometimes if they are new, old stock or used. Remember, GE got out out of the tube making business a long time ago. Sometimes getting a used, tested good tube is better than one that is new and more or less untested. That said I have some used 6AS7's from the 40's that sound better than they a right to.   

My offer still stands. If you want to do a bit of tube rolling I can send you a couple of 12AU7's to try out, no charge. I'll even pay for shipping. And yes, the 12AU7 seems to have more effect on the sound than the the 6AS7/6080. That is only my opinion and others may disagree. Only you can make that judgment for yourself. Happy listening and welcome to the addiction. I hope you enjoy your amplifier as much as I enjoy mine.

As I type this I'm listening to  Anthony Phillips - The Geese & The Ghost on my Crack / Speedball / Sennheiser HD600 setup.

Best regards,
Tom

[xaanix]

Thanks very much for your help Tom (and the others who chimed in on this thread).

My experiences with this issue lead me to ask a few follow up questions:

Is this typical behavior of a tube which worked properly and eventually went dead?

With the old (broken) tube, I noticed the voltage on the jack fluctuating up above 20V.  Since we were specifically told to test this voltage in the manual (not to exceed 9V during startup) I'm assuming >9V can damage headphones.  If so -- is there a risk of a tube dying and at some point exhibiting these same characteristics -- possibly damaging your hearing or headphones out of nowhere?

I've heard tubes may require burn in -- 20 hours? 

Since one of my channels is currently busted -- I'm pulling the headphone out of the jack a bit to get a mono signal in both ears.  (saw this on another forum post and figured it would allow me to at least hear the SQ of the amp for now).  Any downsides in doing this?

Regarding tube rolling, I will take you up on your very generous offer, thanks so much!  Would love to experiment with a few and find out what sounds good.   I'm sure you are serious when you call it an addiction

Is there some distilled data which shows the community's impressions of the various tubes?  (are some more clean and others more warm?  Which?)

Personally, i'm looking for warm (my purpose for getting the crack)-- i have HD700's and i want to warm them up a bit.  I purchased them as my primary set because of their comfort. I've seen the crack's output impedence is 120 ohms, and HD700s are 150 ohm.  Since it exceeds, i figure i'm ok but i have also seen people say you want 200ohm  or above on headphones to work well with the crack.   Wondering if i'm not deriving the real benefit from the crack at this point -- especially since i'm on a replacement tube.

I have no issues with hiss or anything.

How much of a tube amp's sound comes from the tubes themselves vs the wiring/capacitors/etc.  (I've seen some pictures of crazy crack mods, with GIANT capacitors, and probably many other changes).

[xaanix]

Volume pot arrived today and i promptly replaced it.  Finally i can listen to this thing for real

So far i'm enjoying it!

Dream Theater - Enigma Machine  -- AWESOME!