I was just gifted a David Eden VT.40 amp

Lightyear · July 23, 2011, 10:39:33 AM

But it's broken. One my of oldest friends just called and said that this amp powers up just fine and at about three to four minutes the LEDs for the EQ section fade out and the sound dies. He tells me that the mains switch stays on. This amp is from Eden's early run, dated '94, and my buddy tells me that it was a very pricey beast when he bought it. He's not interested in spending any more money to fix it as he already spent some dough on new EQ tubes thinking that was the problem. To me it sounds like the power supply for the EQ section is failing under load but that's just my off the wall guess.

PBG do you have any thoughts on this one? While I'm a qualified elctronics tech I'm not an amp guy - I can spot cold solder joints, swollen caps, burnt components, check voltages and the like but I'm not much of a tube guy - even though I covered them in detail in tech school.

Psycho Bass Guy · July 23, 2011, 05:07:36 PM

Yeah, I'm betting you need to replace some supply filter caps and/or possibly a voltage doubler circuit or two for the tubes coming off the PT. The nice thing about the amp is that its main power section is low voltage; only the tube preamp will have high voltage caps. The VT40's had the Demeter preamp. Finding a schematic with operational voltages listed will tell you exactly where to look.

dadagoboi · July 23, 2011, 05:47:27 PM

Quote from: Psycho Bass Guy on July 23, 2011, 05:07:36 PM
Yeah, I'm betting you need to replace some supply filter caps and/or possibly a voltage doubler circuit or two for the tubes coming off the PT. The nice thing about the amp is that its main power section is low voltage; only the tube preamp will have high voltage caps. The VT40's had the Demeter preamp. Finding a schematic with operational voltages listed will tell you exactly where to look.

I don't know much about amps but I've got a Demeter pre that will be the last piece of amp gear I sell. This seems a major score to me, Buzz. Congrats!

Lightyear · July 23, 2011, 07:02:41 PM

Quote from: Psycho Bass Guy on July 23, 2011, 05:07:36 PM
Yeah, I'm betting you need to replace some supply filter caps and/or possibly a voltage doubler circuit or two for the tubes coming off the PT. The nice thing about the amp is that its main power section is low voltage; only the tube preamp will have high voltage caps. The VT40's had the Demeter preamp. Finding a schematic with operational voltages listed will tell you exactly where to look.

I've poked around some today and read through a three year old thread at the Eden site and it appears that the schematic is lost to history - according to Eden anyway. There was a replay in the thread about someone who had one repaired and that it was indeed caps in the power supply. Like mentioned I can find puffed up caps and such but I'm not an amp tech by any means. There was also talk of adding fans to cool the power supply as well.

PBG, if I don't find something obvious are you up to some side work? Providing two way shipping won't bankrupt me.

Lightyear · July 23, 2011, 07:10:50 PM

Quote from: dadagoboi on July 23, 2011, 05:47:27 PM
I don't know much about amps but I've got a Demeter pre that will be the last piece of amp gear I sell. This seems a major score to me, Buzz. Congrats!

Thanks, Carlo. If I can fix his thing cheap I'll try my best to give it back to him. It's not that he needs the money or anything but we go way back, he started off on guitar and me bass, we used to torture his parents by playing in the front room of their house and I do mean torture. It's hard to believe it's been almost 35 years we've stayed in touch and been friends.

Psycho Bass Guy · July 23, 2011, 07:49:39 PM

I have zero time right now. Hopefully, things will shake down to a much better situation for me in a month or so, but right now, there's no way. Given the amp's age and the quality of parts, I'm betting you just have a few bad caps. Eden has always used ultra-cheap parts in the wrong places and 15 year-old cheap caps would be hitting the end of their life cycle. The fuse isn't blowing (make SURE it's the proper value), so there's probably nothing wrong in the power section like a shorted transistor, and the power light stays on, which is generally a separate low voltage tap off the power tranny, so I'm betting the problem is in the high voltage section for the tube itself, either in one or more voltage doubler circuits required to run the tube and/or in the filter caps on that line. Right now it sounds like at worst case you'll be replacing some diodes and resistors and caps. That's all just me guessing about how the amp is set up based on what I know about it though. I'm at work right now. When I get home in the morning, I'll see if I have the schematic somewhere.

dadagoboi · July 23, 2011, 07:54:40 PM

Quote from: Lightyear on July 23, 2011, 07:10:50 PM
Thanks, Carlo. If I can fix his thing cheap I'll try my best to give it back to him. It's not that he needs the money or anything but we go way back, he started off on guitar and me bass, we used to torture his parents by playing in the front room of their house and I do mean torture. It's hard to believe it's been almost 35 years we've stayed in touch and been friends.

Nothing like old friends!

Lightyear · July 23, 2011, 09:19:06 PM

Quote from: dadagoboi on July 23, 2011, 07:54:40 PM
Nothing like old friends!

Yep, amen to old friends!

"I have zero time right now. Hopefully, things will shake down to a much better situation for me in a month or so, but right now, there's no way. Given the amp's age and the quality of parts, I'm betting you just have a few bad caps. Eden has always used ultra-cheap parts in the wrong places and 15 year-old cheap caps would be hitting the end of their life cycle. The fuse isn't blowing (make SURE it's the proper value), so there's probably nothing wrong in the power section like a shorted transistor, and the power light stays on, which is generally a separate low voltage tap off the power tranny, so I'm betting the problem is in the high voltage section for the tube itself, either in one or more voltage doubler circuits required to run the tube and/or in the filter caps on that line. Right now it sounds like at worst case you'll be replacing some diodes and resistors and caps. That's all just me guessing about how the amp is set up based on what I know about it though. I'm at work right now. When I get home in the morning, I'll see if I have the schematic somewhere."

No rush here at all - seems like I work 10 hours a day, minimum, and have a lot of irons in the fire myself. I'm thinking it'll be fall before I can crack the cover and I'm going to give it hell to get this going myself. It's not like I'm gigging and this is my only amp - if I can't beat into submission on my own I can wait. But soldering is something I'm pretty good at so I'm hoping for the best.