Autobias OTL....Any thoughts?

[florindo]

I build the cheap version 8.2 without the double cell capacitors and the choke.

I have the 4700Uf 200V capacitors with 2x toroidal trafos (One for the B+ and one for the driver circuit and the heathers

[florindo]

yes, the same as in post 7

[M Gregg]

It is very important to burn the heaters for at least 12 hrs with no B+

If they have flashed over then they may be damaged...

The other version has autobias on all OP tubes..and many other mods..

Regards
M. Gregg

[florindo]

Gregg.

I have no inrush suppression.

[florindo]

Wha t do you mean by overflashed, how can i check if they are Ok

[M Gregg]

Is the 1A that blows on the B+ or B- rail?

You say you have built this circuit before and have it working OK with no faults?

Regards
M. Gregg

[M Gregg]

Originally Posted by florindo

Wha t do you mean by overflashed, how can i check if they are Ok

This may not be the case if you have an fault on the PSU...

Its commom to have the tubes go faulty without a burn in period..
They apparently distort on early warm up and are not in correct running position..The tube seems to expand on first warm up and then you can apply B+ after cool down and restart..

Flash over means gone short circuit..test between all components on ohms scale for low ohms...short..

If you are sure the circuit is OK no problems then possibly bad tubes?

12 hours minimum..burn in heaters only no B+<<<<<<do this first then try again...If you are sure the circuit has no faults..

Regards
M. Gregg

[flatheadmurre]

Florindo did you check voltage at bias tap marked A in schematics.

[Alastair E]

Try this--

Wire an ammeter in series with the fuse that blows.

Turn on BOTH the B+_ and the Heaters At The Same Time...

Monitor the Current on the meter....

Also keep an eye on the 6C33C's to see if there's and Sparks or flashing inside--IF so, Then they are no-good.

What Should happen is the current should rise smoothly as the valves warm up and settle around 200-250mA,--assuming a Cathode-Resistor of around 150 ohm...

6C33C Absolutely MUST MUST be Burned in with Heaters ONLY for at Least 1 day.
(I use a heater supply Just a LITTLE over the Maximum recommended for this. 13.8V, both heaters in SERIES.)

Two or three days is best though, but if you have had a 6C33C valve arc/spark, then Throw It Away!
--It will Always be prone to arc/spark and blow fuses, and its possible it could damage a speaker.....

[M Gregg]

Alastair,

Thought you had gone south for the winter..LOL

Could the tubes be drawing enough current to stop the cap slow charge with the 10K so at full B+ on the surge takes the fuse... I think Mr Murre refered to this..

I guess the only way to know for sure is to put a voltmeter across the reistor and see if the caps reach full B+ before turning on the switch..
IE low volts across the resistor = close to same voltage each side..


Regards
M. Gregg