Tube Amp connections : 0V vs EARTH vs HEAT vs CURRENT

[systemtruck]

Tomorrow I will try a test from the opposite perspective, which is to just test the isolated performed of the output stage…. I’ll connect a balanced audio output from an interface to some caps and then to the grid inputs of the EL84’s. I’ll leave the grid stoppers in place too, which are at 82k right now.
Will this work ok with impedance relationships? The audio source would probably only have like 50ohms, but those grid stoppers add 82k. Is there some impedance interaction with previous stage that the EL84’s depend on to function?
Maybe I can successfully get the output to perform as it should, and if it does and the audio sounds as one would expect, then I think further reinforces the idea that the preamp and PI sections are being distorted simply because they’re losing their rails quite dramatically.

 

[Cqwet Dbdfte]

82k "grid stoppers"?
That is not a grid stopper. Try a typical value.

 

[systemtruck]

82k "grid stoppers"?
That is not a grid stopper. Try a typical value.

I’ll bump them down.

 

[systemtruck]

I ran a test of just the output EL84/XFO stage. I was able to provide an additional 42dB of gain after the +4dBu audio interface output, using two audio transformers in a somewhat weird fashion. The audio passed clean through them so i know it functioned enough for this basic test.

I think it was just enough volume to get the EL84/XFO to clip in the ugly fashion. It was actually close, i think my levels were just on the verge of being as loud as the PI output level required to clip the output stage. So, comparing the output of interface, which I patched back in to check, the EL84/XFO stage was clipping when the interface output was not. There was only like 5dB of room to play though, before the interface itself clipped at its output.

But, i was able to get the EL84’s/XFO to clip. I switched up the EL84’s since I have 4 of them, and it clips just the same.

Anyways, something is up. The output stage clips, and it draws too much current I believe. So i disconnected the EL84’s again, and measured impedance from the top and bottom legs of XFO to the center tap. Each side is almost exactly the same, about 164 ohms.

Also, in the recent past i already confirmed that the HV coming out of each leg is identical, both with the EL84 connected and without it connected. Without the EL84’s, it’s much higher of course, up in the 400’s instead of mid 300’s.

I could try buying yet another set of EL84’s. Or i could try hooking up a different transformer. But if i need a different transformer it’s getting to the point where I’ll have built an entirely custom amp with no original iron haha.

Oh well, sometimes you just don’t win! Maybe it’s time to give up and continue working on builds that have gone perfectly recently..

 

[CJ]

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

 

[ccaudle]

The output stage clips, and it draws too much current I believe.

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?