The VAS is a ECC81, also fed from regulated supplies, for both the heater and the B+. I also operated the 4P1L without the VAS stage (by disconnecting the ECC81 and grounding the 4P1L grids).
Of course, I can't fully exclude it is a problem in my building, OTOH I have quite some building and troubleshooting experience in getting things quiet, but it does not seem to work with this unit (but I hope to solve it eventually).
I think it is hum indeed, it is present all the time as a background hiss...
Of course, I can't fully exclude it is a problem in my building, OTOH I have quite some building and troubleshooting experience in getting things quiet, but it does not seem to work with this unit (but I hope to solve it eventually).
I think it is hum indeed, it is present all the time as a background hiss...
"I think it is hum indeed, it is present all the time as a background hiss..."
Hiss isn't hum, rather oscillation or bad contact of pin.
Which type of tube socket used?
Is it (4P1L) fix or cathode biased?
Hiss isn't hum, rather oscillation or bad contact of pin.
Which type of tube socket used?
Is it (4P1L) fix or cathode biased?
In two (and multi-) stage amplifiers, the noise is mostly generated by the first amplifying stage.
As said, I don't have a problem with my 4P1L outputs. But I do use 4mm aluminium for my casing. Very solid.
The important thing isn't the aluminum thick, the important thing is the decoupling so mandatory the silent blocks or similar.
I haven't used any kind of decoupling. Just the 4mm aluminium chassis. That works. It's non-resonant.
A word of caution here: In my experience, older Hammond chokes made in Canada can sometimes be used for choke input duty but the newer ones made in China buzzes loudly under the exact same circumstances.
Quite strange, as the build quality seems just about equal. Perhaps the Chinese are using lower grade iron or something?
The story in short: I was designing raw DC supplies for my 6S4S PP amp and decided to try LCLC-filters with Hammonds 28mH 3A chokesin the first position. I had one on the shelf which was absolutely quiet during the tests (14VAC in, 10VDV 1A out).
I decided to use the one I already had in a DC supply for the input tubes and ordered four more for the outputs, plus four small 14-0-14V EI transformers (185C28 IIRC).
Once in place both the transformers and the chokes turned out way too noisy to be used in a hifi amp. I ended up replacing the transformers with some industrial 400V/24V ones and and payed an insulting amount of money for a set of chokes from Lundahl...
Thanks for the heads-up, I will take a look. Didn't want to spend the LL money for a test at this point. Ordering some power transformers from toroidy soon, they might be able to make some dc chokes as well.
BTW: 20-25 years ago I got fed up with all the hum issues, and I just switched to batteries for the filament and B+ (500V stack for my 10Y amps, no fuses! lol). Add some caps for a lower Z, and it is to sound like what OLED is to LCD. The hassle just shifts to charging it all 😀
BTW: 20-25 years ago I got fed up with all the hum issues, and I just switched to batteries for the filament and B+ (500V stack for my 10Y amps, no fuses! lol). Add some caps for a lower Z, and it is to sound like what OLED is to LCD. The hassle just shifts to charging it all 😀
I read somewhere that chokes used in choke input filters should be rated for 4x the current draw. This corresponds well with what I've noticed during my own experiments.
The 4H 225mA Hammond choke in front of me works fine in a choke input filters up to 60mA or so, with a 230 VAC transformer.
Next to it there is a danish Induperm 15mH 3,8A choke that buzzed loudly when I tested it for a 300B filament supply but it is dead quiet at 650mA.
The 4H 225mA Hammond choke in front of me works fine in a choke input filters up to 60mA or so, with a 230 VAC transformer.
Next to it there is a danish Induperm 15mH 3,8A choke that buzzed loudly when I tested it for a 300B filament supply but it is dead quiet at 650mA.
Those all seem to be candidates for encasing in epoxy under vacuum. The more current, the greater the magnetic field. Those windings sound like they are moving with the 100Hz and not held in place properly. I am guessing the Lundahls, with their layered windings don't suffer from this problem. More reasons for me to ask toroidy for a pair to test.