Autobias OTL....Any thoughts?

[SemperFi]

As with all cathode biased amps you will get a high current surge at turn-on if the cathode is warmed up. This b/c the bypass caps are a virtual short untill theyre charged up. Either slow down the heater and have b+ on as they heat up, or have some current limiting at turn-on.
Also, I've come to the habit of oversizing the fuse a tad to give some headroom. And of course use slo-blo fuses.

[flatheadmurre]

Have you let your 6C33C have a burn in period with heater only no B+

If you turn it on brand new you can be unlucky and have a short in the tube.

The 6c33c starts perfoming well after a week of use sounding very boring and flat in the beginning.

Originally Posted by florindo

Hello.

My name is Florindo.
I just set up a channel of this amp.

But i am facing some problems.At start up my fuses blows (1A)

All seems to work Ok without the 6c33 valves.Anodic is 160V witout tubes.

When i am inserting the tubes the voltage rise up only until 35V DC and when i switch on the full anodic tension the fuses blows.

Could you maybe provide me a list of the right values of tension in different parts of the circuit.
It could be a help

Thanks in advance

[florindo]

Thanks gys fir this hint.

But explain the following: With tubes inserted and the +B inserted with the resistor 10K mounted after the bridge the voltage rises up only to 35V (Heathers on) even after a wile the tension remains the same.

When i by pass the resistor and opensthe full voltage the fuse blows

[florindo]

Hi from Flo.

Thanks for your replay.What are you meaning by this: If i have a short in the tube is the tube faulty and not usable anymore?
I made this test (Fuses blown down) with 2 set's of 6c33 always with the same problem
What about the 35V not raising to 150 with tube's inserted but with the 10K resistor inserted??

thanks
If i made some burn in (1 week) would the tube respond better?

[flatheadmurre]

If tubes are conducting more current than your 10k will let through voltage will not rise.

Your bias tap marked A is in the right place so you have negatig grid voltage?

Burn in is kind of mandatory for 6C33C so put them on heater and enjoy the glow for a couple of days.

[florindo]

Ok

Thanks
I am doing some breadboard test now but will burn the tubes as you recommended for 273 days and after check again

Will replay later on this forum

thanks for now

[M Gregg]

Originally Posted by florindo

Thanks gys fir this hint.

But explain the following: With tubes inserted and the +B inserted with the resistor 10K mounted after the bridge the voltage rises up only to 35V (Heathers on) even after a wile the tension remains the same.

When i by pass the resistor and opensthe full voltage the fuse blows

10K after the bridge?

Do you power up heaters and B+ all togeather no delay on the B+..<<<slow current increase..

OK...if you run the driver stage on its own no 6c33c's in the socket is everything OK?
Then run the 6c33c's on their own..do you get correct bias on the cathode resistors..?
Do you have inrush suppression?

Regards
M. Gregg

[M Gregg]

Is the circuit you built the one in post 5#?

Or in post 7?


Regards
M. Gregg

[florindo]

Hello Gregg

Yes, without any 6c33 tubes all the voltages are OK and no SHORT CIRCUIT at all (Fuses OK)
Voltages drop to 150V plus and minus in aprox 1/2 minutes.When i inserted the 6c33
I power up heathers with +B together.The voltage rise up until 60 V then go down to 35 even after a 5 minutes (All this with the 10K resistor after the bridge)
I wait 5 minutes and by pass the resistor.My fuse of 1 A blows in this case.
I received suggestions to burn for a couple of days the 6c33 which are brand new.

What you think about testing only the 6c33 circuit after the 22uf capacitor (No driver ecc82 attached)
What voltages you suppose i need to have and what you suggest as start up procedure?

Thank

[M Gregg]

OK,

Just looking at what I think you have....

There has been a lot of work done on this circuit in another post..
However is this what you have as in post 7# ?


Regards
M. Gregg