I also noticed hadn't paid attention to it before some of my EL84 have the screen grids visible and they seem sagged downwards. Can this be normal or have they been subject to abuse? (i got them from a junk electronics fair for like $3). This might account for instability yes? But oscillation there was in my previous attempt but they don't go to thermal runaway.
And it works! A bit low on gain as i think i used too much FB however it's stable. Tested with various frequencies of signal and tweaked the capacitor in the FB until it killed any overshoot. Interesting is while i could not detect even a trace of oscillation on the scope up to 1.something MHz that is it's range, the tubes do squak ever so slightly at the signal frequency. So i'll consider this a normal part of operation. Other than that there's the filter choke that's not impregnated in resin and hums horribly at 100Hz but i'll take care of it. As for the schematic it's inspired by the Masco MA10-HF, except with EL84 not 6V6 and a bit more FB. Will post the schematic later when i get home.
How it sounds? Quite hard to describe. Neither too soft neither too punchy what i believe some call penthode sound yet very chimy and detailed. Sadly i have no way to properly measure the output power but a simulation (that i find to be accurate even in the infamous FB component values) says i should get by with up to 15 clean watts.
How it sounds? Quite hard to describe. Neither too soft neither too punchy what i believe some call penthode sound yet very chimy and detailed. Sadly i have no way to properly measure the output power but a simulation (that i find to be accurate even in the infamous FB component values) says i should get by with up to 15 clean watts.
Thanks!
And i did say i'd post the finally final schematic so here it is. I'm not likely to change it any time soon anyway since i was beginning to dread seeing this amp on the table here not working
But all the trouble was not for nothing.
http://s30.postimg.org/mohnrk2o1/ampul.jpg
And i did say i'd post the finally final schematic so here it is. I'm not likely to change it any time soon anyway since i was beginning to dread seeing this amp on the table here not working
But all the trouble was not for nothing. http://s30.postimg.org/mohnrk2o1/ampul.jpg
Well, i was starting to wonder if i got lucky too easily, and voila! It messes up of course apparently it's triggered by specific vocals but it stops before i can put the scope probe on the output to check. Perchance i may have swapped the "up" and "down" tubes after i secured the choke to stop humming. I don't see how that could do anything but... I did however notice there was distortion this time at low power, so yes i may have swapped the tubes... (they are not matched not by far) Here's an interesting thought: how do i tell if it's phase error or the OPT primaries forming tuned circuits with the Miller capacitance of tubes?
First of all Merry Christmas, or Solstice or whatever it is that you observe
For the holiday i did not wish to bother anyone so i stayed put, plotting, planning. As it is efforts to tame the amp have failed (or not) since it still does it's "thing" every now and then but i did find out something. HAving reached my wits end i did:
- shield all the signal cables (where the Z was low enough to allow it)
- test with various signals succesfully both channels
- wait a while until the incessant noise appears
- measured the outputs - absolutely nothing
- set sensitivity and frequency to maximum (risky) on scope in AC mode and checked every plate & screen for trouble
On the 2 tubes that appear the most blackened could not find any oscillation waveform of any kind, BUT the baseline plate reading dances up and down, sometimes it settles by itself. At all settings of the scope it's just a flat line no hint of a form except it doesn't stay in one place (consequently the bias voltage does not stay in one place either).
Does this make any sense at all???
I did swap the tubes randomly and the problem SEEMS to follow those 2 tubes, one is only slightly misbehaving, one a little more. I tested what comes out from the phase splitter, there is nothing bad coming from there, no DC leaking, nothing.
Aside from resistors and perhaps some cathode bypass cap do i risk burning anything if i let it run as it is? (there are fuses for transformers)
For the holiday i did not wish to bother anyone so i stayed put, plotting, planning. As it is efforts to tame the amp have failed (or not) since it still does it's "thing" every now and then but i did find out something. HAving reached my wits end i did:
- shield all the signal cables (where the Z was low enough to allow it)
- test with various signals succesfully both channels
- wait a while until the incessant noise appears
- measured the outputs - absolutely nothing
- set sensitivity and frequency to maximum (risky) on scope in AC mode and checked every plate & screen for trouble
On the 2 tubes that appear the most blackened could not find any oscillation waveform of any kind, BUT the baseline plate reading dances up and down, sometimes it settles by itself. At all settings of the scope it's just a flat line no hint of a form except it doesn't stay in one place (consequently the bias voltage does not stay in one place either).
Does this make any sense at all???
I did swap the tubes randomly and the problem SEEMS to follow those 2 tubes, one is only slightly misbehaving, one a little more. I tested what comes out from the phase splitter, there is nothing bad coming from there, no DC leaking, nothing.
Aside from resistors and perhaps some cathode bypass cap do i risk burning anything if i let it run as it is? (there are fuses for transformers)
Last post ( i sincerely hope).
Changed to a simple cathodyne design supposedly more stable and still tended to oscillate... then i found a video on youtube of the issue... i couldn't believe it! I had cramped together the tubes too close and there was feedback from EL84s to the phase splitters. Placed a screen between them and they quieted down instantly...
Now i have to redo the paraphase i originally intended. Hope this helps whoever encounters this horrible problem, if anyone reads at all.
If it is desirable i will post the final schematic when i am satisfied that it's stable.
Changed to a simple cathodyne design supposedly more stable and still tended to oscillate... then i found a video on youtube of the issue... i couldn't believe it! I had cramped together the tubes too close and there was feedback from EL84s to the phase splitters. Placed a screen between them and they quieted down instantly...
Now i have to redo the paraphase i originally intended. Hope this helps whoever encounters this horrible problem, if anyone reads at all.
If it is desirable i will post the final schematic when i am satisfied that it's stable.
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