My pleasure,
There was always slightly hum with volume pot set up on max almost not audible on normal listening level with all my input tubes except EF80 and EF800TFK they humed in my DK339. The source of interference could have been of course, the adapter also large carbon resistors across the tube socket in close proximity (above them) of parallel lying heater wiring and the (most suspicious) electromagnetic field due to the current flowing in the heater wiring. I rid off the grid leak carbon resistor and put in small film resistor soldered directly to the socket pins and replaced the short thin signal wires coming from volume pot to the grid, for DH Labs BL-Ag shield solid-core silver cables and I moved them away from the filament circuit. To minimise the external electromagnetic field, the heater wire should be tightly twisted what I've done very precisely in my own 339, not leaving any loop area as you see on my previous photo. To my surprise, issue with the hum/noise permanently gone, allows me to fully enjoy the cheap but very beautifully sounding tubes, RCA6AS7G & EF800TFK.
Is not show on picture I made it (solder) directly to the battery the positive and negative pole are copper, nickel plated, scrape off the top layer and can be soldered or you can buy, solder tab rechargeable batteries much easy to do. Very important: battery need to be isolated from the chassis, can not touch the metal enclosure I used this: self-adhesive cable-tie bases.
Originally Posted by
telecaster:
"If you fix cathode bias the 6J5 at 1.82V while the bias runs about 2.2V in auto, I would guess you a running less current on the plate is it right? That way it is safer for the tubes."
This is a compromise if you want rolling tubes u need to find the right bias for all your tubes.
I didn't see why the FR may depend on the bias method...ooh pardon me, it did only when the cathode resistor is bypassed with too small capacitor acting high pass filtering. I did compare the self biased to the LED bias and the main difference is the higher gain about 3dB ARTA and low noise and THD for LED biased cathode.
short about my modified dk339,
there is couple of things I did or I'll do later to improve the amp even more to bring it to another level.