Could not elevating heaters on a KT120 SE cause it to howl like it’s oscillating? I just built the amp and it was amazing for about an hour then it began oscillating and the only thing I can think of is I didn’t elevate the heaters. I’ve checked all connections and voltages and I am at Witt’s end. Ideas appreciated.
If it worked beautifully for an hour without elevating heaters, it won't make a difference when elevating them.Sonmething's gone bad during that hour.
most likely your amp is just marginally stable and was on the verge of oscillation in the first place; if you started out with brand new tubes they tend to gain emission during the first hours which may have increased stage gain just enough to kick it over the edge; or mains voltage rose since first turn on or ...
post a schematic ...
post a schematic ...
Unless the heater elevation has been improperly implemented there is no reason it would make the amp unstable. DOes the design include NFB?
Cheers
Ian
Cheers
Ian
No fb at all. It is the Abdella KT88 SE with LED bias on the input. Slightly higher B+ running KT120s in triode. Transcendar 5k output transformers I had on the shelf. I’ve built it before with no issues. Input tube is 6dj8 or 6n23p-ev.
In which case it must be something to do with the layout. Look for potential common impedances between the input and output stages. Check the 0V wiring and the power supply. The 6dJ8 is a hi gm tube so it might benefit from a 1K grid stopper.
Cheers
Ian
Cheers
Ian
I did the gridstoppers and no love. I’m going to just rip it all apart and check everything as I reassemble it. I think it’s a ps issue but we will see.
I once had a paraphase driver stage, my point-to-point layout was very prone to oscillation with air-born local feedback, it did 5-20 MHz. It did sound dynamic though . . I shuffled wire around a bit. It was there at some moments, gone another.
And Philips used coax cable in their radios from the output tube to the transformer. That connectioon with a plain wire is an antenna of course.
And Philips used coax cable in their radios from the output tube to the transformer. That connectioon with a plain wire is an antenna of course.
Just last week I had an Epiphone Valve Junior, (single ended EL84 output) that oscillated after half an hour or so. Turned out to be a power supply issue. The main smoothing capacitor went high ESR after it warmed up.
A new 22uF 450v capacitor cured the issue.
A new 22uF 450v capacitor cured the issue.
Well, as many have said before, I am an idiot. It wasn’t the amp it was the source! The RCA cable was bad. I flipped left and right and the problem followed. Changed the cable with another one of the same and all is good again. Don’t you hate it when you forget the basics? Lesson learned.
I had a loose cable on a preamp, line stage connection. The scope did not see it, it spoilt listening both channels, I even bought a new CD, then later I noticed a loose part after poking ariound. So indeed, I hate it when the basics go sour, and you can't find out. ..
Do i need to elevate the heaters on ecc8xx type tubes ? have you got measurements on noise improvements ?
Doesn't it add its own ac noise into the dc node you use to elevate the heaters ?
Doesn't it add its own ac noise into the dc node you use to elevate the heaters ?