series filament amp design question?

[ALBATROS1234]

i got a box of tubes that are oddball when it comes to guitar amp design. i have a bunch of 50c5 types and have heard guitar amps that sounded pretty low/fi good and unique. i have a few isolation transformers and was thinking a parallel single ended or push pull 50c5. my voltage runs 121-122 consistantly so i was wondering why cant i use a 5y3 as i have a box of those as well. that gives me 105v giving me room to add 15v spare. thinking 6volt pre tubes like a pentode/triode combo like this slightly used 7199 or 6u8s with 6sl7 or 6sn7 i have as phase inverter. this gives me 117v. in old radios they usually used 1 50v power type with a 35v rectifier and string the 35 first in the series string. does the rectifier have to be the first filament in the string like most aa5 radios? can i even use a directly heated rectifier in a series string? if not i have 6 volt inderctly heated rectifiers as well. my sense of logic says i should go to the power tube filament 1st then goto the rectifier and pre tubes.?.?.? i know some of you guys are probably like whats the point bonehead just build something from a schematic. but i like experimenting, i have the stuff here, i am curious of whats possible and where the boundries are in design, plus it should yield a unique sound. as per usual thank you in advance for any responses to my noob questions.

[sluckey]

One big requirement for operating tubes with filaments wired for series operation is...

     ALL TUBE FILAMENTS MUST OPERATE AT THE SAME CURRENT.

Your 50C5s operate at 0.15A filament current. Therefore, any other tubes you decide to put in series must also operate with 0.15A filament current. The 5Y3 needs 2A so no way to use it. Also, the filament of a 5Y3 is also the cathode and that means that B+ will be present on the filament string.

So, your project must begin with pairing up tubes that operate at the same filament current. Then pick and choose until the voltages all add up to 120V. Many times you can only get close and must use a "ballast" resistor to drop the additional voltage. For example, say you have a bunch of tubes that operate at .15A but the total voltage only adds up to 105V. You still need to drop 15V @ .15A so the ballast resistor would need to be 15/0.15=100Ω. Power would be 15*0.15=2.25W, so use a 5W for safety.

I suggest taking your box of oddballs and use a tube data book to gather info about the tubes. Put this info into an Excel spread sheet. Then sort the table by filament current to group the tubes using the same currents together. Now try to pick enough tubes to build your amp.

[PRR]

> can i even use a directly heated rectifier in a series string?

NO.

(Sluckey's answer implied this, but it bears repeating.)

You put the 50C5 or two in series with 12AX7 types wired 12V 0.15A.

Any "unique sound" is from cheap-cheap transformers and speakers, or low B+; tubes is tubes.

[ALBATROS1234]

thanks gents. forgot about ohms law series means voltage drops current is constant. i guess because normally i use 6.3 for all the heaters so the currents can be whatever and votage is constant in parallel branches. i'm thinking 2 12ax7 types and the 50c5 types in parallel se or push pull. so i think since my load resistance is 2500 per 50c5 single ended should be approx half that ideally. luckily i have a se with 1500k to 8ohm. cheers