Are you converting this thing into a guitar amp?
If so why not build a VOX AC 15?
That is a wonderful transformer for guitar, it is what I use in a Princeton Reverb.
I think,
https://groupdiy.com/threads/fisher-x-100-3-output-transformer.73750/#post-1069456
This is where an oscope would tell you right away what is going on.
Presumably you have changed the gain structure of the amplifier compared to the original hi-fi configuration. Have you calculated or measured what the amplitude of the PI stage is, and checked what that level will do going in to the output stage? Is it just pushing the grids positive and getting into grid distortion?
All right folks, I have one piece of hard evidence that i didn’t have before, so i could use some expert detective work! …
I separated the cathode biases of the EL84’s. I used a fudged set of 340R high watts, and 50uF….
One of the tubes, an older one that did NOT red plate, reads about 0.04V DC on the cathode no matter what. When I swap the same two tubes from one side to the other, that same tube still reads about 0.04V DC at the other cathode now and the opposite is now healthy. It follows this tube. Again, this was a NON red plating old original Mullard.
All 3 of the other tubes read a healthy 9.5V-10V at its cathode no matter which way they’re installed.
Interestingly, when the cathode biases were unified as one segment before… i was getting heathy DC readings in the ballpark of 9V-12V. So, does this mean that one tube was constantly pulling voltage upwards or something like that? To compensate for the 0V tube constantly pulling down?
So i was able to then pick two tubes that provided a good cathode reading of 9.5V ish. But still, the distortion occurs. I had high hopes that when i got two tubes holding good cathode bias that it would solve the distortion.
Maybe this solves red plating, haven’t tested that yet.
And is it possible that maybe this situation has already damaged the other EL84’s so i just need a fresh set again? A new pair of JJ’s are 30 bucks right now, so maybe I’ll just order those to have a clean slate. I do worry about max voltage though, in the new tubes vs NOS.
Yes, I’m building a guitar amp. It would be done weeks ago if not for this. I’m starting to think that maybe the output stage just needs to be adjusted for this context instead of Hi-Fi. It already sounds fantastic in lower volumes, just a warm round rich clean tone. I don’t quite understand how i could be causing grid distortion if I’m just sending sound into the EL84’s at low to moderate volumes. It’s really not very loud in the room when it clips.
And yeah I would be super bummed to find out that the output transformer is fried, because i knew well that this generation of Fisher output transformers was going to be terrific which is why i started this project. But my rudimentary transformer DMM tests keep coming back fine. I tested the secondary too, which reads around 0.7 ohms DC and as far as i can tell that is pretty normal. And considering the fact that audio passes beautifully at lower volumes, it doesn’t seem that the OT is fried. Fingers crossed.
I wish i hadn’t just thrown out my old analog scope. It was just too faulty to deal with and i’ve been shopping around for a digital one. But even just listening to the audio in the same exact point in circuit, one time with the whole circuit connection in tact and another time with the PI outputs snipped and recorded from there, the difference is alarmingly clear. This is the audio test just posted yesterday.
Pardon my newbie level of tube stuff but here’s a stupid question… but what would you try changing about the EL84 design to see if gain/design of this stage is the issue? I’ve tried running the varic lower so that the B+ is only 300 not 340, but that doesn’t solve it either.
