Stop thinking, get some books on electronics & spend time on more than Ohm's Law.
You can't solve everything here on DIY without some real knowledge & work done by yourself.😉
Since you don't at the moment have a pre/driver section loading the PT, you should get that online before you make a decision on increasing the (-) bias. As it stands now, you have the 6L6's running right at the maximum plate dissipation, considering 410v @ 80ish mA. at the cathode. So adding the pre/line section should pull the B+ down some...how much... have to wait to find out. Then you can plan to lower the idle current with better hard numbers on B+. But if you change the 15K to 12K it wouldn't hurt. I doubt it would give you twice as much bias, but if it was -30v that would be a good point to work with. 75mA of idle would be very safe place.
Honestly, if it was my amp, I'd remove the fixed bias circuit altogether and make it a K biased output. Less fuss and oportunity for failures.
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I'm going to take your advice and get the pre amp section built and see where everything is At then , I wasn't actually expecting to get Minus - 50 with just the resistor change, I was just trying to be safe about it with the 50/50 e can and shoot for mid or lower - 30's . I didn't do the math on paper with ohm's Law I just took a guess At where I should start at. I'm Sorry to The Guy railing on me about Ohms Law.
I did the ohm's Law thing and ran the formulas and it appears that my guess of going from 15 K to 12 K was right on the money. It says that it should bring the negative Bias voltage from Minus - 26 VDC to Minus - 40 VDC . That is about right to me for PP 6l6 GC . I'm still going to wait until the Preamp section is completely finished and up and running before I do anything else.
The bad news is that if the negative bias voltage increases, your voltage may also increase. If the current of the power tube decreases, the supply voltage will increase.
I don't have a problem with the voltage with the 6l6 GC tubes installed, It dropped down to around 420 Plates, if it goes up a little bit I should still be okay. I had the 6l6 tubes out because I didn't have a test speaker. I didn't want to hurt the OT. I got a speaker for it and it is okay now when the tubes are in. I'm not completely stupid about it either like Stewart thinks just because I have forgotten about a few things over the past few years that I used to know quite clearly. I understood that I was definitely going to be A little bit rusty when I started the project and that's why I got on here to start with. I'm not ashamed to ask an expert opinion if I don't remember what I did the last time. Plus Someone else might have a better idea about it. Things are changing every day and Nothing is written in stone with the thing. Today I changed my mind about the pre Amp section as a matter of fact, I decided to go with The Fender Bassman 50 instead of a using A Model T pre amp section. I think that both are good designs but they are definitely different designs. I play guitar and bass also sometimes so I feel that having the bass channel and the bright channel is a definite plus for me so I decided to go with it that way. I'm waiting on parts so I might as well study some of the different pre Amp designs for the best one for me. If I don't like it I can always change it later on to something else. I really appreciate all of your help with the amplifier. Will let you know how it comes out when I get it all together and running again. Thanks for Everything !
in my practice, if i calculated dissipation is 1/2 watt, then I use a 5 watt resistor, if calculated is 1 watt, I use 10 or 20 watt resistors, the choice is based on how much ventilation the resistor will get inside of the chassis..
if that is a stereo push pull amp, then you can assume 60ma per cathode, so 240ma is passed at your series resistor, so a 100 ohm resistor is a 24 volt drop....
Do yourself a huge favor and pick up a copy of Morgan Jones valve amplifiers vol. 4. It will be well worth the money, and you won’t be stabbing around in the dark anymore….
I used to be where you are, 25 years ago or so, that book is a Goldmine of information that will only help you on your way.👍🏻
I used to be where you are, 25 years ago or so, that book is a Goldmine of information that will only help you on your way.👍🏻
@Analog Gary - if you want to save your tubes and your speakers, this may be a good time to get some high power 8 ohms resistors ...
Jan
Jan