[ultraman31]

I checked the tube by swapping, but still the same left channel.

 

[tomb]

  I checked the tube by swapping, but still the same left channel.

Unless you noticed a burning smell or an unusual amount of heat coming from the amp, my guess is a dodgy solder connection somewhere that came loose.

 

[schubert]

I don't know if anyone else has experienced this, but I've had long-term reliability problems with the Mosfet Max.  It's a great sounding amp but it has petered out on me twice now, both times because QB1R/L failed.  I biased it at the minimum recommended setting but it still got quite hot, so I thought maybe I might solve the problem by re-casing it and maybe adding a fan.  I couldn't find a case I liked, however, so instead I replaced the top of the existing case with a bit of expanded metal cut from an old file holder and made heat sinks for QB1R/L out of sheet aluminum, and glued them on using JBWeld.  It still gets pretty warm in there (50-55º, and the QB1 sinks approach 60º) but now the heat can escape and it looks cool, too.
Original version:

 
New version:
 

 

 

[Marzie]

I am having an issue with my Max, it's probably something dumb. It was working a few weeks ago. Since then, I have finished my enclosure and made a few wiring changes. I removed the hard wires for power and added terminal screws for power and output. I made a couple changes to the input wiring, but other than that, it should be the same. I plugged it in, when I threw the switch it blew a fuse. I thought I might have a short but + to - is like 470 k Ohm. Pictures attached.

EDITED: Formatting.

 

[Marzie]

Nevermind, somehow a fast blow fuse found it's way into the amp.

After almost a year and a half, I have my pi-max project working as I wanted. I have the ability to play my pi straight to my max, in the same enclosure. I can also play the pi to my stereo system un-amped, or the pi through the max to my stereo. Lastly, I can play content from anything else hooked up to my stereo either to the max with headphones, or feed it back to the stereo to my speakers.

I have some casework left to do, and I'm planning to add a small screen to the enclosure for the pi along with some buttons for playback.

Thanks to everyone for all your help. When it's done I'll post some pictures here and in the other thread I started for my build.

 

[PeteyZ]

I know I'm way late to this project but I built Pete M's more simple "low-voltage hybrid headphone amp" to use as a tube preamp in my system first and became interested in this higher level project (hybrid mosfet-maxed) because of his mentioning it on his project page. I intended to adapt it to use as a preamp as well since I don't listen through headphones these days. I've built more complex things like Nelson's BA-3 and Wayne's BA2018 preamps so thought this project should present no hurdles...everyone else go it to work just fine so I should have no issue, right?

Here was the problem(s) that plagued me after the build was complete. Once powered up to 27V the regulator would shut down, I could never get both tubes biased when in circuit (maybe one but not both), and the tubes would glow brightly. All signs of an high current flow, right? Painstakingly rechecked all the components previously mentioned in this forum. Removed the mosfets and retested them them put them back in. Checked the K170s & J74s I bought from the diyAudio store (they were fine). Checked the whole PCB for anything touching or causing a short. Could find nothing suspect for high current flow. Finally I was left looking at the OnSemi LM317 regulator. I happened to have some TI LM338s around and dropped one of those in place. Low and behold everything stabilized and I'm now currently letting the unit warm up for 30min or so to finally set all the biases. I can't blame the use of LM317 in the circuit since all of you used it successfully so must have been that the batch mine came from was not as hardy as the ones you folks used many years ago. Now to keep checking everything for a few more days before making SE input/output connections and see what it sounds like.

Cheers,

Pete

 

[tomb]

I know I'm way late to this project but I built Pete M's more simple "low-voltage hybrid headphone amp" to use as a tube preamp in my system first and became interested in this higher level project (hybrid mosfet-maxed) because of his mentioning it on his project page. I intended to adapt it to use as a preamp as well since I don't listen through headphones these days. I've built more complex things like Nelson's BA-3 and Wayne's BA2018 preamps so thought this project should present no hurdles...everyone else go it to work just fine so I should have no issue, right?

Here was the problem(s) that plagued me after the build was complete. Once powered up to 27V the regulator would shut down, I could never get both tubes biased when in circuit (maybe one but not both), and the tubes would glow brightly. All signs of an high current flow, right? Painstakingly rechecked all the components previously mentioned in this forum. Removed the mosfets and retested them them put them back in. Checked the K170s & J74s I bought from the diyAudio store (they were fine). Checked the whole PCB for anything touching or causing a short. Could find nothing suspect for high current flow. Finally I was left looking at the OnSemi LM317 regulator. I happened to have some TI LM338s around and dropped one of those in place. Low and behold everything stabilized and I'm now currently letting the unit warm up for 30min or so to finally set all the biases. I can't blame the use of LM317 in the circuit since all of you used it successfully so must have been that the batch mine came from was not as hardy as the ones you folks used many years ago. Now to keep checking everything for a few more days before making SE input/output connections and see what it sounds like.

Cheers,

Pete

Glad you got it working!

It's actually a wonder that our world of IC chips (and transistors) works as well as it does. Actual failures of something like the LM317 are almost unheard of. It's why they're the last thing one would check. Yet, as you found - sometimes a failure occurs.

Let us know of your progress and listening results!