Jensen or Mundorf coupling capacitors?

[W9TR]

Timm,
I was discussing the situation with your preamp over lunch with another engineer. After I described what was happening, he said “Sounds like it is oscillating at a high frequency well above the audio band when it is connected to the amp. “

I still think this is the only thing that would cause the power supply current to increase enough to blow the mains fuse when connected to any power amplifier.

Again, get a kill-a-watt to see if indeed the mains current increases when the preamp is connected to a power amp.

Or if you have a friend or friendly service tech with an oscilloscope have them see if the amp is oscillating when connected to a power amp. That would tell you for sure.

To answer an earlier question, yes, a tube swap from at7 to ax7 could cause an issue as the ax7 has approx 2x the gain of the at7.

Tom

[timm]

Tom. Thanks I appreciate it. If this is happening. Do you know what a proposed fix might be? In other words -what might create this oscillation in the pre? Could it be a tube??

Sorry. Didn't see your last comment...

[W9TR]

I don't know what a proposed fix might be. At this point it might be best to send it back and see if the factory can duplicate the problem. Ultimately they are going to have to fix it anyway. The reason I thought measuring the current draw would be helpful is that you would then have some kind of proof there is an issue with it being connected to another piece of gear.

It could be a tube but it seems more than a coincidence that you are having this issue after the amp was modified at the factory.

Tom

[timm]

Unfortunately it went in for a fix. I did not realize it had a blown fuse when I sent it in - but it did. So this condition is the reason it went in - in the first place.

[timm]

So step XXX- you fill in the blank. I wanted to get my phono tubes out of the mix. Cary said it would be ok to pull them and just run the line stage. I think I am on my fifth fuse in 2 weeks. I have suspected these tubes to be an issue. Now we can see . If things run fine I'll get tubes for the phono stage and re- test w all 8 tubes glowing.

Maybe I could get a job at this!??

[jadedavid]

It seems that the factory was not able to duplicate the fuse failure in your piece while it was originally in for repair.
It is not uncommon with an intermittant problem such as this.
I would not spend any more time on it and would send it back in and describe in detail as to when it fails and under what circumstances.

[timm]

Yes. I have been in complete communication with them. I want to try this before I let it go.

[timm]

Well 20 hours in ... so far so good - without the phono tubes.

If we continue to get good results - either it was a bad tube (my guess) or the additional need of powering up the 4 phono tubes creates an additional load the ps can't handle for whatever reason.

Positive signs:
Power supply hum is lower
Chassis on ps seems a tad cooler
No hum coming thru my electrostats which pick up on this quickly

I'd like to assume it's a bad tube. I had a hunch it was the phono tubes - but those hunches are 50/50. . We will see. Assuming all goes well...I need to purchase phono tubes. Do these tubes have any impact on the sound of the linestage? Can they add noise to the linestage?

Saying all of this (my edit).... I said I would give some sort of review of this - once it settles in.... I have it on my 'old' system right now with ML Odysseys - and the stats are really lovin' it.... And again - I didn't want this to be a 'let's bash Cary' type of a discussion ... I'm glad I figured this out.... so far... - and I appreciate eeveryone's help (knock on wood) 23 hrs and going.... It is just sounding really really sweet right now.... bass, imaging, depth... smooth highs - not rolled off.... and this 'I'm home' sound... it is sounding pretty spectacular right now... I really like the direct coupled version.... I'll do an a/b on my other system once things seem 'good'.....

[decooney]

Ask them. I wonder if they upgraded it, then bench tested it with no tubes in it - initially, checked values, and tested tubes separately. Then, put tubes in it afterward, ran it again, all tubes lit up and worked for xx minutes, turned it off and boxed it up. Shipped it out.

Did you tell them it was broken or had a blown fuse when you sent it in for upgrades?

[timm]

Well getting info has been difficult to say the least. Yes - it went in for repair - blown fuse. I didn't realize it was blown as it didn't look blown - but tried a second one stored with it that apparently was also blown. This has never blown a fuse in 5 years.
Ive asked important questions like - did you test with my tubes with my umbilical .. no answer to that. Now I wonder if they tested with phono stage tubes in at all since changes were only made to linestage. The fact that I had 3 12ax7s and 1 12au7 as opposed to 2+2 - again makes me wonder a) why didn't they bring that to my attention b) did I get the right tube back because I had never noticed that before - after owning for 5 years. C) I was told measurements of tubes would be sent back - they were not. D) the remote volume no longer works (remote mute does) but nobody wants to talk about that.

So in the end -yah I've been holding back a bit. The unit just ran over 30 hrs straight without phono tubes. Once I get phono tubes - I can hope that it isnt just the load of the tubes creating the issue (another unanswered question )

The unit sounds great.