• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Autobias OTL....Any thoughts?

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#23
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
 
#27
florindo said:
Wha t do you mean by overflashed, how can i check if they are Ok
Click to expand...

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
 
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#29
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.....
 
#30
Alastair,

:wave: 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
 
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#31
M Gregg said:
Alastair,

:wave: 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..


Regards
M. Gregg
Click to expand...

Yes, Highly likely, The current drawn from the rails when the cathode is hot at the point of HT turn-on is huge, as the PSU will be fully charged and Very low impedance, the valve not biassed at all, as the caps in the cathode circuit will be discharged.

He Could try disconnecting the cathode-caps and just try it again to see if the fuse still blows on turn-on...
 
#33
One thing you could try..

The 10K after the rectifier drop the value..Try 5K or less.. The wattage will be higher

The rail needs to be up close to full voltage Before the switch is closed....If it isn't then the fuse will blow..Also put some inrush suppression in the mains to the power Tx primary...

What you have to find out is ..what is the inrush current to the caps with a 1A fuse in the line if they have only reached half supply or less?

IE your problem may be the soft start limiting resistor on the B+ line..with current drawn by the the tubes giving a volt drop across the 10K



Regards
M. Gregg
 
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#35
Thanks for the info
Yes, the problem is always the same: Switch on the heathers and the B+ true the resistor 10K
Voltage rise up until 30/40 V and remains stable even after 5 minutes (and will not increase)
By the switch on and by pass of the 10K the fuses blows.So i think you are right.I will burn for 373 days the valves which are brand new and check for sparks,glows etc.etc. after will check again by checking the inrush current on the B+
Could you all maybe suggest some inrush current limiters i can insert in the star up in order to have a soft start or something similar
Thanks
 
#36
Florindo here´s a pic of the Softstarts i use.

softstart.jpg


I have them in my OTL.
 
#37
Inrush protection..

Inrush Current FAQ - Ametherm

put in series with the primary..they get hot so don't let them touch anything..

I think after the burn in drop the 10K to 5K or less maybe even 1K..remember the power rating will need to be increased...your 10K is supplying the amp at the moment...so as you reduce the value the current through it will increase..IE it will get hot...its only to stop the surge into the caps..and then switch out..Its not a standby switch..You might be better with a higher value fuse into the caps and a 1A in the B+ rail before the tubes/valves..you could do this in both rails..
That way you can allow high current charge and still have protection for the tubes and speaker...

Regards
M. Gregg
 
#38
I think a better idea is to get rid of the switch/resistor and go for a inrush limiter on the primary side.

Changing the 10K resistor for a termistor is another way to go.

With the 10K resistor dropping voltage and tubes hot you get a pretty good current rush closing that switch.
 
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