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Sovtek 6C45pi-E bias

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#82
Yes, that will do as wel. As long as the tube is not totally in the open field it's okay. The main problem I have had without the shield was that there started a hum every time I came close with my hand (to turn the volume) This only stopped by touching the tube for and quickly move my hand away. Very strange effect. With the shielding this problem was solved. I did not need a complete shield, only a short circuit ring was enough.
 
#89
Sjef said:
The main problem I have had without the shield was that there started a hum every time I came close with my hand (to turn the volume) This only stopped by touching the tube for and quickly move my hand away. Very strange effect. With the shielding this problem was solved. I did not need a complete shield, only a short circuit ring was enough.
Click to expand...

That is a sure-fire indication that the valve was oscillating. 6C45P oscillates very easily, and you really need a good oscilloscope handy.
 
#90
6c45p grid stopper and cathode stopper

Hi Sjef:


Thanks for the good advices on how to stop oscillation.

I am a just a diy beginner. Can you explain to me a bit more about :

- use as grid stopper 1 k or should I use lower values, because my grid bias resistor for 6c45p is only 1 k !
- you mentioned for the cathode stoppers :"connect 4 x 20ohm resistors to 4 pins of cathode and then OTHER END comes together". Do you mean, just soldering the other ends together and leave it there OR should the other ends be soldered together and then go to the ground ?

Thanks for your help !

DoeiDoei
NG
 
#91
Hi NG,

1K for gridstopper should be sufficient in most cases.
The 4 cathode resistors are just some extra resistors soldered as close as possible to the tube pins. The 6C45 has four pin connection for the cathode so in this way you will use all four of them.

Thet are in fact in parralell with each other, yes soldered to each other at the end wich isn't on the tube pins. They do not go to ground but to the cahthode bias resistor. Just see the point where the four resistors come together as your new cathode connection of the tube, that's it.

good luck
 
#92
What is the need to use all four pins if they are all at the same potential by virtue of being directly connected together inside the tube ? If one is used the others automatically get tied to the same voltage. If not , why ?
Don't understand this at all.
 
#93
yes they are connected at the inside of the tube but only after a short lead wich is long enough to act like an antenna. Sovtek recomments using all four connection with eventually four stoppers for thei 6C45's because they are prone to oscillation at very high frequencies, yes in the hundreds of Mhz area. That's an area with very short wavelenghts and even a few mm is enoughto pick it up. This is also the reason why the tube has four cathode connections instead of one, this way you make the path a fourth of the inductance hence a fourth chance to pick up rfi.
 
#94
6c45p grid stopper cathode stopper

Hi Sjef:


Thanks for the explanation. I finally understand what the cathode stopper is all about .

For the grid stopper, do I need to put :
- 1 kOhm on pin 2 and another 1 kOhm on pin 8
- OR should I simply cut one of the grid pin short ?

Again, thanks for helping me out.

Best regards,
NG
 
#95
After post #70 by Sjef, this discussion focuses on cathode and grid stoppers for the 6C45pi. Based on the points raised in posts 70, 73, 75, 76, 77, 91, and 93, does anyone have experience with the cathode follower line stage (figure 7.41 on page 576 of Valve Amplifiers 3rd Edition) described by Morgan Jones? In this figure he connects pins 1,3,6,and 9 of the 6C45pi together rather than using small resistors.
 
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