5881 s.e.

[ALBATROS1234]

i got my hands on a used 5881 sounds good another amp which i designed for my 6f6 . want to give it a home of her own(are tubes she? like a ship or aircraft?) but i cant find what size resistor to use on the cathode. i have looked around and there doesnt seem to be too many guitar amps in this config. i have found some 6l6 se but values vary wildly from 160ohm to 750ohm. not sure how the rest of the amp is going to evolve,i have only done bridge rectifiers and have several tranformers ranging from 230vac to 460vac. was thinking of using one that puts out 300vac with the bridge x 1.41 =423vdc havent got the ot yet . not sure if i am going to go classic dual triode preamp or play with this westinghouse 6sj7 ,rca 6sq7 or 6j7 pair i have coming in on this reel to reel i bought for cheap. hopefully that tranny works well, it powers on is all i know,it was $20 so if i only get used tubes and sockets etc its worth that.

probably stupid question. but has anyone ever done a push pull with 2 different but similar power tubes that require similar voltage/amps. wondering if it would expand tonal range or sound like a trainwreck(obviously not ken fischers type) i know its just crap that crosses my mind and i want to try and see if it catch on fire or sounds like something.

[TIMBO]

Hey mate, 5881's are similar to 6l6's.
And your 300v transformer (not centre tapped) is good.
Cathode resistor should be somewhere in the middle of your observation.

Possible not a great idea to have to mismatch that way as different tubes react differently when pushed.

[ALBATROS1234]

thanks for your reply. thats what i figured, on the 2x different tubes together.i often play hypothetical scenarios in my head and have just been wondering if i were to put one tube which has more bite on one side like an el84 with something more smooth like a 6aq5 or 6v6 type would that give me a broader range and add tonal texture, i kinda thought electrically it may cause issues though. doesnt hurt to ask though right.

on hand i was thinking of trying a 330ohm or a 500ohm on the 5881 just wanted opinions before i soldered it in my circuit. maybe i can make jumpers with alligator clips and try each.

[John]

I run a 6SL7 into a 5881 SE and it sounds really nice IMO. I use a 300 ohm resistor on the 5881 cathode.

[pdf64]

5881 is just a ruggedised member of 6L6 evolution http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5881.pdf
As such, it was great for guitar combos, which are subject to intense vibration from the close proximity of the speaker.
Many tweed era amps used 6L6 / 5881 in cathode bias, eg http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5881.pdf
The shared 250 ohm cathode resistor and ~400Vdc HT indicates that your 500 ohm resistor would be a good starting point for SE, ie with only half the cathode current, twice the resistance is required to create the same voltage drop (=bias voltage).

Re the use of dissimilar tube type, note that even when using the same tube type, there's a benefit in having them matched, ie so that they both have pretty similar idle plate currents for a given bias voltage.
With dissimilar tube types, it will be a bit tricky to get them both working in their proper ranges of dc operating point and ac signal handling.
It's not super complex, as that's what the Mesa simu-class arrangement does, but for the time being it may be best to your head around more straight forward stuff.

[ALBATROS1234]

thanks guys, that will give me a start. will prob pop in one of these 500ohm 5watt cement types i have. waiting on new bridge units to come in the mail , the last one i have seems to have one diode burnt. it puts out less dc voltage than the amount of ac put in. when i run my 300 volt transformer i get 230vdc when i tested it with my meter on leg gave a weird reading so its burnt. maybe i killed it when i soldered it in.

[PRR]

Do you have a capacitor attached?

A rect without a cap WILL read ACV lower than DCV.

[ALBATROS1234]

No cap i just used alligator clips to attach a.c. and another pair to test dc output. So that must be the case then. Didn't think about that but it makes sense the dmm will see something odd and not a smooth peak dc line. Thanks