PT sag with different power tubes

[birt]

I have a big PT designed for a cathode bias PP 6L6GC amp. because i want to try different power tubes i checked if the PT can handle the extra heater current.
i tried 6V6, 6L6, 6L6GC, EL34 and 6550 on standby so i just powered the heaters. the heaters are in series and went from 13,3V to 13V with the 6550's. so this seemed ok to me.
then i tried the power tubes and adjusted the cathode resistor value. i noticed the plate voltage went down with more powerful tubes.

since the PT was made for a couple of 6L6GC i'm not sure if i should try EL34 and 6550 because of the extra current draw. will the PT just sag and drop the plate voltage? or can i damage it if i try?
and if i bias it colder so there is less current draw, will it be worth trying or will it sound bad?

[TIMBO]

Hi birt, There is a CHEAT SHEET getting around that lists APPROX. current draws of tubes but i was unable to find it.So hopefully someone can find a copy

[navdave]

That's normal especially with 6550's from standby to on your b+ will drop by 30 to 35 vdc.
EL34's will drop the b+ the least out of the bunch. If you had a adjustable bias you could watch the B+ go down as you lowered the negative voltage on the grids turning up the current going through the tubes.
The only thing you have to worry about is the power transformers heater winding overheating since 6L6's draw about half an amp less than 6550s or EL34's.

[Ed_Chambley]

The PT will run any tube provided you do not exceed the maximum voltage of the tube.  The heaters will be fine as well unless you are running a lot of preamp tubes with added tremolo, reverb, extra gain stage etc......  The difference lie in the resistor values on the grid as each tube you mentioned would also need a change here as well.  I have found there is no set rule, but looking at old schematics will give you an idea.  If you are tube rectified you will have some control over the change in the B+.

No, it will not sound "bad."  But just changing the tubes without adjustments made to other components will probably not give you what you want.  You will need to make changes to the PI values.  Lowering the dropping resistors or raising them according to what you want.  If you add a different tube and do not like the result because the lows got flabby then a cap change may be needed.  Same thing is you lose your highs.

I am not too sure about your heater reading tho.  Most PT's have somewhere around 6.3vac under load, so I probably don't know what I am talking about in your specific case.  What are the specs of your PT?

[firemedic]

He's got the heaters wired in series, thus the 13v.

This thread interests me because I'm about to install a quad of 6550s in my homemade bass amp, where I had EL34s before. When I built it, I had 6550s in mind when assigning cathodyne PI & grid leak values. But I wasn't real sure the plate current draw will be OK with the Twin Reverb PT I'm using. I have an aux. filament tranny for the heaters.

[jjasilli]

I think you need to go to the tube charts.  Here's one for the 6550:  http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf  The same site has data for all the other tubes too.  The charts show heater current draw.  Also pinout, and plate current at different B+ voltages: at idle and under max signal conditions.  It also suggests a cathode resistor value for cathode bias.

Watch your pinout to make sure swapping tube types is OK - especially if you employ an "unused" pin as a handy soldering lug.

If you want to target different cathode R values for different tube types, you can put two R's in series, and use a SW to short-out one to get different values on the fly.

[birt]

I think you need to go to the tube charts.  Here's one for the 6550:  http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf  The same site has data for all the other tubes too.  The charts show heater current draw.  Also pinout, and plate current at different B+ voltages: at idle and under max signal conditions.  It also suggests a cathode resistor value for cathode bias.

Watch your pinout to make sure swapping tube types is OK - especially if you employ an "unused" pin as a handy soldering lug.

If you want to target different cathode R values for different tube types, you can put two R's in series, and use a SW to short-out one to get different values on the fly.

i'm well aware of that and i have read a lot of datasheets. my concern is if i can damage the PT by drawing too much B+ current or will it just stop delivering extra current at a certain point?

[John]

I think you can burn up a PT given enough time and too-heavy current demand. If your PT is rated at 150mA, and you're running a quad of power tubes that suck 90mA each tube, prolly not so good. Same thing with the heater current draw, which is why some guys install a separate 6.3v transformer just to run the heaters.

[jjasilli]

Ok. The PT will be damaged by excessive current draw.  Note that the tranny does not deliver current per se.  Rather the tubes produce a load which draws current through the tranny from the AC wall supply.  If too much current is drawn through the tranny it will overheat.  It does not have its own way to stop overheating.  This is up to the designer.  You could use fuse(s) to protect the PT. Good info on Merlin's site.