K&K Audio / Lundahl Transformers

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K&K Audio / Lundahl Transformers

need help for ciuffoli kt88pp

216.211.180.11

se trans coupled to kt88pp
stock parts as called for with 5842 driver input on grid pin 4, and with 170 ohm cathode resistor -
amp is sweet on start up but becomes more harsh sounding over 20minutes of use - still sounds ok but not glorious as when first turned on-
both channels have same behavior = ???
also, filiments on turn on for 5842 flash white for few seconds with the turn on surge voltage - can't be good for tube life.

any suggestions?

Follow Ups:

What is your configuration on the SE couple transfomer ? - akltam 06:31:45 05/31/11 (0)
Have you measure the anode currrent of the tube at the beginning and what is the anode current of the tube when you think the sound went not so good. Any other parameters changed ?

I tried this with 6C45 using

2.25 : 2 + 2 and have a gain of around 17. It sounds not bad (actually quite good) but I want some higher gain. I don't have the problem you mentioned. Maybe as Kelvin suggest you need to have stopper resistors.

I also tried 2 : 2.25 + 2.25 and get a gain of around 22 and find I loose some bass.

This experient is still laying around. Maybe I will revert back to
2.25 : 2+ 2 and use V3a instead.

This is an interesting and easy to built amp.
The flashup is normal for some tubes. I think the 5842 - Bas Horneman 15:14:28 05/27/11 (0)
is one of those. Not 100% sure though.
To infinity and beyond!!!
RE: need help for ciuffoli kt88pp - KevinC 11:38:30 05/27/11 (1)
I wouldn't be surprised if the 5842 is oscillating at very high frequencies once it warms up. I have some experience with them and they require pretty scrupulous care to keep them under control. I've used and seen used grid stoppers (~100 ohms) on all four grid pins rather than leaving three unconnected and I used big ferrite beads on the filament leads. If you are using Amperex 5842s from a particular era, the bright filament at turn on is fairly normal. I don't know what influence the phenomenom has on tube life, but you will have to use a higher voltage and a dropping resistor or a current source filament supply to suppress the current inrush.
My experience was in a phono preamp, so it was the toughest test, really. I never was particularly impressed with the sound of that preamp, but then there could have been several reasons for that result. But I didn't hear what you are hearing with the measures described above.
Good luck!

Kevin Carter
K&K Audio
www.kandkaudio.com

Edits: 05/27/11 05/28/11
- RE: need help for ciuffoli kt88pp - andrewbee 12:10:08 06/06/11 (0)
  Or do what I used to... use only one of the grids and cut the others off, they act like antennae. Yes, drastic I know but that is how I tamed the beast. On the remaining grid as Kevin suggests, grid stopper (mandatory) and ferrite beads.

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