813 Amplfiier Project and Filament Transformer

[K1JJ]

A Variac can be hard on filaments during on turn on, due to the very low impedance offering high current into a very low resistance filament (until it heats up).  Many BC transmitters use rheostats for that function as they essentially offer a 'soft-start' for the tube.

Jus' sayin.

73DG

Hola Dennis!  How's things out west?

Question: If we feed a filament transformer with the Variac and turn the Variac up slowly each time we turn it on, wouldn't this solve the cold surge problem?  Or are you referring to something else?


BTW, speaking of series filaments, a horror story:  I blew out three 6146 filaments, one at a time, not knowing any better.  I once had four 6146s in parallel - and their 6.3V filaments were in series using a 25.2 fil transformer.   I was testing a group that were hot and took out one or two tubes at a time to replace them with cold tubes. The cold tube filaments resistance was low and dropped most of their the fil voltage across the other tubes - and popped one of the other filaments right away. I saw a bright flash in the hot tubes and then nothing. I didn't realize why this was happening until I popped three in a row...  yikes!  Moral of the story - always heat up series tube filaments together.   Parallel filaments do not have this problem.

The other solution would be to use a Variac on the transformer to SLOWLY bring the hot and cold series tubes up to the same temp.  Still, risky biscuits.

T

[W1RKW]

Remember if you use a Variac to ensure that your filament voltage is at 'rated voltage' under key, not at idle.

--Shane
KD6VXI

I'm puzzled by this, if filament V is adjusted under load, wouldn't filament voltage climb when unloaded?  Why not set V at unloaded condition instead?

[KK4YY]

An Antek AS-1206 has dual 6v secondaries. Wire those in series, run a few turns 'backwards' through the core (or unwind a few turns, if you dare) and you'll have 10vct. With more or fewer turns, you can 'tune' the voltage. On Epay now for $29 shipped.

Or, your 20volt@10amp filament transformer would work nicely for a pair of Ruskie GK-71's... or three. Just sayin'.
http://www.mif.pg.gda.pl/homepages/frank/sheets/018/g/GK71.pdf

[WZ1M]

Another idea. Find a 24 volt secondary transformer and series up the 24 volt winding with the primary of the transformer you have. Measure the output voltage on the transformer you started with. See if it comes close. If not, try  another transformer to buck the primary. Dont use the secondary of output transformers as there are not enough turns. As for the variac, I never had any problems with soft or hard starts. Just be sure the variac is grounded to the amp. If its not, expect all kinds of weird stuff. Be sure the variac is rated at least twice the rated current that you need. Three times is even better.
Regards,
Gary

[Opcom]

There would be no real sin in putting a big resistor in the primary of a 20VCT transformer to make it 10VCT@ the desired current. waste of 50W per tube though. hams have done weirder things and will continue to do so in the future.

[KD6VXI]

Remember if you use a Variac to ensure that your filament voltage is at 'rated voltage' under key, not at idle.

--Shane
KD6VXI

I'm puzzled by this, if filament V is adjusted under load, wouldn't filament voltage climb when unloaded?  Why not set V at unloaded condition instead?

Yes it will rise between key up and key down.

However, you want to set your tube operating conditions up so that in a directly heated (filament) tube you are at the point of necessary emissions.  Running the tube colder will cause distortion and you won't be able to attain cathode current levels needed.  Finding the point of necessary heat to attain the emissions necessary keeps the tube cooler under operation as well.

If the tube is operated at 'best conditions', key down, and the filament climbs a half volt on key up, who cares (not Eimac!)....  If the tube is cut off, you aren't extracting electrons from the cathode, so you don't need to worry about it.  Note, I'm not saying make sure a 5 volt filament is at 5 volts key down and climbs to 8 volts key up!

If you run it higher voltage levels during key DOWN, then yes, you can see more power out.....  Lots in some cases.  But you will run out of emissions earlier.

Hope that made some sense 🤔

--Shane
KD6VXI

[W1RKW]

Thanks Shane.  I makes sense.

[n4joy]

I just want to thank EVERYONE for their input.  I have much to digest and will continue work!  I hope to have some updated photos and descriptions posted shortly.  MANY thanks to all!

Chris, N4JOY