can i use 6550's?

[black_labb]

a higher value would probably be what you need (larger value meaning more voltage drop). you may not need a larger value though, as the extra current across the resistor from the 6550 will drop the voltage a bit more. best to test it and see how it is.

[daz]

According to webers calculator they were at 30 watts and 70 something MA per tube. So i obviously had to drop it a good bit. I ended up with adding another 270R to each tube ! At that point i had around 700R per tube and was at 21w/55MA for each tube. Weber's chart stops at 400v for the 6550, so i assume i am running them lower than i should at 388v per plate. But it sounds fine so WTF. This is what puzzles me......after adding the 270R per tube, the plate voltage was still at 388....it didn't change. Can someone explain why?

[tubeswell]

Calculator over at AX84

http://ax84.com/contrib/biascalc.php...42&Action=calc

On this calculator 6550s do good biased at around 76mA for fixed bias AB1 and 108mA for cathode bias at 388V plate voltage. (Maybe the voltage isn't dropping much more at that point because you're not riding them hard enough????)

[daz]

Well, i can only go by these calculators because i don't know enough to be confident in my own estimation of what is too much. I also only had that chart at weber to go by and it only goes down to 400v. But then 388 is close so i figured i'd use that as a reference and it shows 61 MA as hot. But i gotta admit it sounded better with 70 something. It did however seem to be running hot, as i could smell the tubes. No red plating tho. What do you think i should do? maybe just add another 100R to the 450R that i was using per tube for the el34's and leave it at that? I just didn't think the smell and heat seemed normal. I could be wrong tho, and like i said no red plating.

[tubeswell]

I say go to between 70 (or even 100) and to heck with the smell (I reckon the hotter they go, the better they sound) - They are cathode biased are they not? - Just make sure you've got decent power resistor(s) at the cathode(s) and the screens. (Hopefully your OT should take the higher currents).

[daz]

Ok, well i trust you more than myself LOL! I'll just remove the extra resistors i added and leave it as it was which gave me something like 77ma if i recall. Saves me the trouble of adding a switch ! The tubes by the way are good stuff....old GE out of an early JCM800. One of the online stores is selling the NOS ones just like them for up to $400 a pair ! So i don't wanna fry em. The OT is a hammond JCM replacement so it should be fine.

And yes, i have 20 or 30 watt aluminum power resistors as cathodes and 5 watt screens.

[ETR]

Cathode bias works/sounds best in class A or close to it. Otherwise you can get into serious crossover distortion with the bias shift. Class A for that amp is probably close to double the current youre pulling now at idle. If you set it to pull 100 ma at idle your plate voltage will probably be down around 350 which is 35 watts. It's normal for a class A (especially 6550) amp to smell a bit and get hot. But boy do they sound great.

[daz]

Geez....you guys really want me to fry those expensive bottles don'tcha?! Well, i'll run it at almost 80ma as it is now and maybe look at trying 100 eventually, tho it will freak me out. This glass is good stuff and if i kill em i can't afford to replace them with the same. Don't know if there are any good replacements for 6550's today, as these are the only ones i ever owned and the amp they came out of was the only 6550 amp i ever owned.

[Chris / CMW amps]

wasnt thinking about the preamp tubes. good point.

i have an amp with fixed non adjustable bias running 4 6550's at 700v. the bias on that was 37v from memory. thats probably pretty hot though, as i replaced the old tubes in it with 6550's. 2 of the tubes in the apparently matched quad redplated, while the other 2 were fine.

the amp in question is cathode bias, so this isn't that helpful.

and it also depends on the G2 (screengrid) voltage...

[daz]

By the way, i don't even know what the OT primary should be for 6550's....what is optimum? Wondering if i should be using a different secondary tap.

[MWJB]

Same as for El34

[daz]

Wow, perfect ! I was worried that with a non optimal OT primary would prohibit going to 100%, but this is good news. Is 3.2k optimal for 6550 or just close, and if just close what IS considered optimal?

[tubeswell]

Wow, perfect ! I was worried that with a non optimal OT primary would prohibit going to 100%, but this is good news. Is 3.2k optimal for 6550 or just close, and if just close what IS considered optimal?

The datasheet section on 6550 type tubes here suggests various load resistances from 3k - 5k or so depending on voltage, class of operation, SE or PP, (method of bias??) etc

http://www.drtube.com/tubedata.htm

Check them out

[daz]

Thanks. Class A operation at 3k (my OT is 3.2 so thats close enough i assume) shows max current as 105MA. So i suppose i will try that just to see whether the tone is really notably better. if it is i'll leave it there barring any red plating. If not then i'd see no reason to change from the current 75MA or thereabouts. As good as the amp is sounding tho i'm really curious as to whether running at in class A would be orgasmic. If the amp sounds any better than it does now it WILL be.

[daz]

Someone elsewhere suggested i'm pushing the PT (which is a hammond 272JX 250MA) by running the 6550's at max. Opinions? would that be reason to pass on vbiasing them at max and leaving it at around 75? Or what would you say is the max i should bias them with 3 12ax7s and 2 6550's? Are there other current considerations than that?