6550 grid characteristic help please

[jasondace2b]

i am running 4 6550 ultra linear w/ 40% tap, the primary impeadeance is 1922 ohm . after its all warmed up i have 415 volts on the plate and 420 on the grid. are the voltages in the ball park for this transformer ? i cant go up but i could go down to make it match up.

also how far out of whack can those #s be before i have a problem.
thanks

[kevinkr]

Originally Posted by jasondace2b

i am running 4 6550 ultra linear w/ 40% tap, the primary impeadeance is 1922 ohm . after its all warmed up i have 415 volts on the plate and 420 on the grid. are the voltages in the ball park for this transformer ? i cant go up but i could go down to make it match up.

also how far out of whack can those #s be before i have a problem.
thanks

Looks reasonable to me - you do realize that there is less winding DCR between the center tap and UL taps than there is between the center tap and plate taps? The voltage on the UL taps will always be higher for this reason.

I ran KT88 at up to 550V with relatively few problems as long as I stayed around Pd of 30W or so and was using tubes from a reasonably reputable manufacturer.

[jasondace2b]

thanks for a fast response Kevin. looking at some tube info it says for 2 6550 at 450 volt UL 4000 ohm plate load. i just didnt want it to be like 6000 ohms for 420 thats all. what the heck iam gonna run it what else can i do? drop another deuce on outputs. nah

[kevinkr]

If I understand you correctly you have a quartet of 6550 driving essentially a 2K transformer, this should be just fine at the operating voltage you stated.. Yeah I would just run it.

[jasondace2b]

Kevin bias is 43 volts. would say that be also reasonable, and wont present any problems?
thanks dude.

[kevinkr]

Originally Posted by jasondace2b

Kevin bias is 43 volts. would say that be also reasonable, and wont present any problems?
thanks dude.

Hi Jason,
Need to measure the cathode current, I suspect these are running pretty hard at -43V, but the supply voltage is lower than I am used to.

[jasondace2b]

hey Kevin, thanks for the help.
43 volt dropping accross a 150ohm resistor. .286 amps/4 = .0716 per tube. ip on the tube box was 79ma per tube. tungsol reissues.
Jason

[kevinkr]

Sounds just fine to me, but going forward you might consider an individual 600 resistor and 100uF cap per tube as it makes matching much less critical, and helps to assure that the tubes are sharing the plate current relatively evenly. Is the 150 ohm resistor bypassed by a cap? - If not there is some feedback between the tubes which slightly affects the harmonic signature of the amp.

[richwalters]

Originally Posted by kevinkr

......
I ran KT88 at up to 550V with relatively few problems as long as I stayed around Pd of 30W or so and was using tubes from a reasonably reputable manufacturer.

This was one of the reasons I walked over to the EH KT90 corner......600V with 100mA quies UL operation......no sweat, so long the heater can supply the stuff.

richy

[jasondace2b]

right on Kevin, that teaches me a lot about that part of it. i do have a cap bypassing the resistor a 50v 330mfd. so i should up that value, thats no problem. the tubes came matched up from tctubes where they burn them in. i always wanted to know why the bypass cap, and got some very confusing answers.
lets say i got 4 more 6550s and the ip on the box was at 55 instead of 80. would this be less of a draw on power transformer? therefore raising the available supply voltage from 420 to 450 or something like that?
thanks,
Jason