stromberg carlson 1121 bias question?

[ALBATROS1234]

i got a 1946 sc 1121 console with am/fm/shortwave and 3 speed record changer a couple weeks ago which was d.0.A. after fiddling with it i realized it needed some parts and i ordered filter cans and a few other odd size caps i didnt have on hand. goodies came in and i changed filter caps. all paper and wax caps and all the resistors that looked darkened or funky. one resistor that was pretty dark was the cathode resistor which was supposed to be 330 ohm (its a 6v6 pp) but it read around 176 ohms. i also had to sub a choke for a field coil because the speaker was missing from the console. i brought it up on my variac and it starts playing i am picking up stations then after a few minutes the volume dropped a bit and it started making crackling noises and distorting. so i noticed the 6v6s were beginning to red plate. i checked plate diss and it avg at 18w per tube. i re biased it with a 500ohm resistor and it got my plate diss down to 11.75w ea. so my question is why did i have to put a 500ohm to make the power tubes happy when the schematic clearly states it used a 330 ohm? does this indicate i have more problems to track down? after i changed it to the 500ohm cathode resistor i played the set for a few hours and it isnt red plating anymore and seems to be working well. i left all of the micamold caps in place as well as 3/4 of the resistors. there are also a handful of small value cylindrical (ceramic?) caps that are still in place. should i leave well enough alone or start hunting for other possible problems? thanks guys

now i just gotta rebuild the record changer and do my most hated part of radio work......restring the godamn dial cord...ugghhh

[DummyLoad]

likely that you didn't compensate enough for the field coil and B+ is much higher now than when stock.

is it possible that the DCR of the field coil is probably more than you estimated? did you measure B+ with the field coil in place before you replaced the filters?


--pete

[sluckey]

Usually when replacing a field coil you would use a high wattage resistor with resistance equal to the DCR of the field coil. Chokes are rarely used to replace a field coil. The increased B+ resulting from using a choke may be enough to cause the output tubes to run hot.

[ALBATROS1234]

thats the thing the speaker was just not there, i have no idea what value to use. i had on hand a magnavox ps choke that i used , theres not to much hum but i am sure its not identicl in value to what would be a 12" field coil. i didnt think about the higher voltage thing that could be it. the reason i used a choke was because i saw a youtube where thats what a guy did to replace his field coil and it worked out. i used a 10watt resistor  to replace a field coil in a 1939 sentinel radio i fixed 6 months ago but it still hums a bit more than i would like, so i assumed a coil/choke or whatever may work better. i just found it odd that i put the correct value resistor and it red plated but thats why i am a newbie and thats why i ask the pros when i end up scratching my head. thanks guys.

[sluckey]

Replace that choke with a 1K 10W resistor and I'll bet the output tubes will be a lot happier.

[PRR]

Clues, actual volts, would be good.

Meanwhile, here is what SC/Ryder says the voltages should be. What is different?

[ALBATROS1234]

i did find that section in riders 15 but confess i did not check all of the voltages. i just got all the parts installed yesterday afternoon and after i tweaked the bias resistor i posted here. just now i checked everything with the 500ohm(reads499.7) 5watt bias resistor my plate voltage is 370vdc and cathode voltage drop is 32.5vdc. this puts current at 65ma times 337.5(370-32.5) =21.95 which divided by 2=10.97w per 6v6. dont know why it read a 11.75w per last night maybe line voltage was running different? anyway before i changed the cathode resistor from 330 the voltage was around 346vdc if i remember right and my plate diss was close to 18watts per tube. the rider reference prr posted says 340vdc but it also says 230vdc on screens my screens were almost as high as plate voltage, screens are reading 333vdc right now though. also on the riders it says 15.5vdc on cathode i had close to 36 last night and 32.5 now . i can only assume i have to multiply the riders stated value by 2 to get 31vdc for the pair right ? they must mean per tube when you divide by 2 to average. i am going by uncle dougs biasing instructions in which he says when you calculate pp bias you first find avg current which is cathode voltage drop divided by bias resistor actual value. then you subtract the cathode voltage drop from plate voltage and multipy by current. the result is the average dissipation of both tubes to get per tube you divide by 2 and get avg dissipation of each tube. if 15.5vdc is for the pair then something is hugely off because that gives me 7.6 watts p.d. per tube. would stromberg carlson run 6v6s at 50% max? is there something else i should be checking. i have had it playing for a couple hours today with no problem except i need a better antenna because sometimes fm signal will drift off and i will have to slightly adjust the tuner. am is rock solid though. i have only repaired a dozen or so tube radios so i am by no means a pro. i mostly build and design guitar amps. i just dont know how i should proceed , since it seems fine with the rebias does that mean it is? or should i try to see if i can figure out why my values are a bit off. one thing that has been changed in value is, the schematics call for 40uf 1st cap 20uf 2nd cap and 40 uf 3rd. i put a 32uf/32uf can cap in 1st and second position and a lone 33uf in the 3rd position, i dont think that could cause problems though. in the rca tube manual it states max 40uf for 5u4gt but i read somewhere it was safer to go a little less than max 1st filter so i went 32uf with a 32uf in 2nd filter instead of 20. i just dont know if riders means 15.5vdc cathode drop per tube which gives 31 total in that case i am not far off ,or if they mean 15.5vdc for the pair it means i am really off.

[sluckey]

also on the riders it says 15.5vdc on cathode i had close to 36 last night and 32.5 now . i can only assume i have to multiply the riders stated value by 2 to get 31vdc for the pair right ?

NO! That 15.5v on the cathodes is due to the current flowing through two tubes. It's fine. Look at the Ryders screen voltage. It's only 230V. That's why the Ryder's cathode voltage is so much lower than yours. And the reason Ryders screen voltage is so much lower than yours is because the voltages were measured with a high resistance field coil. The field coil acts like a dropping resistor between the plate node and the screen node. Plate node was 340v and screen was 230, so that means the field coil was dropping 110V. I bet your 'choke' is only dropping a couple volts.

This ain't a guitar amp that you are trying to squeeze every watt you can get out of it. It's a refined radio that's operating calmly at about 7-8 watts per tube. If you will pull that choke and put a 1K dropping resistor in it's place, that amp will be much happier and sound just as good, maybe even better. Heck, I'd experiment with the value of the dropping resistor. I'd keep changing the value until the screen voltage was 230v, like it used to be.

[ALBATROS1234]

Wow, that all makes sense and I kinda came to that conclusion but I guess I am so used to guitar amps where you are trying to get your plate diss 80 to 90 percent of mac value it seems like such a waste of tube power. But this was made in 1946 I suppose they weren't jamming to Glen Miller like we jam to led zeppelin guess i need to order a few values if 10 watt resistor. The chock is only dropping 37volts. I don't think it would hurt to get the screens down. This thing is louder than I need it to be as it is.oh and ps dial strings suck ass

[PRR]

> put a 1K dropping resistor in it's place

Maybe larger. G2 current of just 6V6es may be 4mA, which is hardly a drop. As long as he keeps the radio tubes going it may be 10mA-30mA, but he wants 100V drop, so 3k-10k resistor. A 10pack of 5k 2W will allow series/paralleling until happy.

[ALBATROS1234]

I do have some 2 and 3 watt resistors, perhaps I can make a series parallel array as you suggest in order to get the voltage drop on the screens.  Then I suppose I should put back the schematics 330ohm cathode resistor if I am gonna drop the screen. Perhaps I even have to swap back the 500ohm for 330 before I can get a proper value for the screen dropping resistor,?

[ALBATROS1234]

you were right again prr. 3k wasnt enough to drop screens to 230v .i found a 3k sand resistor prob at least 5 watts that i clipped in to see what it would do. at 500ohm cath resistor plates went to 387 screens dropped to 280 and cathode voltage drop was close to 28 volts. this gets me down to 10w diss per 6v6 but not near the riders manual which i think will be impossible to match. as you see my plates are now up close to 390vdc. i feel if i continue to increase the resistor size to get screens close to 230 plate voltage will be over 400vdc. i dont understand how i can achieve much better than 10watt dissipation per tube without frying my plates. should i just order a 3k 10 watt and leave it like this?

[shooter]

i dont understand

most of us have asked the same ?
do you understand how & why you get what you get, when you make the changes?
6V6 - new skool, run 400 plate, the 10W is the key #'s here

[PRR]

I believe it worked for S/C and their conditions will work for you. And the OT impedance was designed for those conditions.

> how i can achieve much better than 10watt dissipation

What is "better"? Hotter? This will never be a stadium blaster.

It does look like there should be some plate-drop, maybe in a missing field-coil (the schematic is not quite clear here). The Screen drop will depend on a FC/resistor *and* all the tuner tubes sucking design current.

[ALBATROS1234]

By better than 10 watt dissipation I meant lower. S.c. had it close to 7 watts per 6v6. What I meant was when I put the 3k resistor in place if the field cool it lowered my screens to 280vdc and raised my plates to 387vdc my cathode was still up at 28vdc . The original spec say 340 plates 230 screens and 15.5 on kathode which divided by 2 =7.25vdc per 6v6.  This gives me 7.63w pd. But if I raise the value if the resistor to the point where it drops screens to 230 will my plates not rise well over 400vdc. I do t think these are the original 6v6s but they are late 1950s he my reason. For wondering if 10w per pd was adequate was because I think the 6v6s would like 10w pd as is with the 3k resistor better than a 425vdc plate voltage which I think will be inevitable if get screens down another 50v when my plates are right below 390 as is. I just want the radio to work well it's plenty loud as is for my purposes .I was trying to get to the original spec which is what you guys suggested. Guitar amps are do much simpler lol

[sluckey]

and 15.5 on kathode which divided by 2 =7.25vdc per 6v6.  This gives me 7.63w pd.

You don't divide the voltage by 2! Doing so will produce an error in your power calculations. The voltage is the same for each tube. You divide the ***CURRENT*** by 2 because the current splits through the tubes and we assume that split is 50/50 (which it rarely is).

[ALBATROS1234]

how i calculate is by taking the volt drop on the kathode in my case presently 26v divide by measured ohms of kathode resistor which is 499.7 this gives me .052a or 52ma. i take my plate voltage which i just tested at 394vdc and 280 screen .subtract the 26v kathode drop from 394 and get 371 x .052=19.29 divide by is 9.65w per tube. which is different than yesterday. i dont know why. now s.c. says 340 plate 15.5 k 230 screen at 330ohm which gave me 7.63watt per tube. should i just go with the 3k 10watt to replace the field coil, i am 2w per tube off?

[PRR]

If you want it colder, you probably have to drop the plate supply voltage. Not sure why unless you have lost some significant drop from the as-built case. Penciling ~~80mA as typical for a two-6V6 team, and 80V as a significant drop (and eacy math!), try a 1K resistor. The heat is 80V*0.080A or 6.4 Watts- a 10W will work for a year+, and may not be the final answer, so worth a try.

Aside from trimming-down a high supply voltage, the other values WERE good. When you get the screens down, you can put the cathode resistor back to stock value, and get a happy total cathode current. Now drop plate voltage (which may mean un-dropping screen voltage) until all values are very near factory notes.

[ALBATROS1234]

sounds good prr, this is the most complicated radio/amp ive restore to date, i have built mostly se guitar amps with 2 exceptions, and most radios have been aa5 types except the 1939 sentinel 8 tubes and a 1936 airline which uses 2 digit types 80,75,78,41,6a7 so this guy with 12 tubes am/fm/sw with a pp amp has been alot of work. its been fun but i want it to be right so she can continue on into the future , after this is strait i am going to rebuild the record changer and its going to be a wrap. i will post pics when its done, still gotta get the black staining bleached out of the top with oxalic acid and refinish the top only but shes gonna be a beauty.