Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bluestone on September 22, 2010, 09:57:10 am
-
I have a Hoffman AB 763 DR build with 435 volts on the PT plates. Using the tube data sheet for the JJ 6v6S if I want to set the bias at 70%:
435/14 watts (max dissipation)= 32ma
32ma x .70= 22.4ma
So I can set the bias at 22ma ???
I know that if it doesn't red plate I am ok, I am just trying to see if I am figuring it out correctly.
Thanks,
Tom
-
Thats just fine. I go higher on the bias,keeping in mind that most people don't dime their Deluxes much and use pedals.If you don't crank the amp,a hotter bias setting will deliver fatter tones.
-
435/14 watts (max dissipation)= 32ma
That should be 14w/435v=32ma
Your idea is sound but you should be aware that when you decrease the current flowing thru the tube, the plate voltage will rise. This is normal but it means that everytime you adjust the current to a new value, you need to remeasure the plate voltage and do the math again.
For that particular tube with a max dissipation of 14 watts, 70% dissipation would be 9.8 watts. So, adjust the current to where you think it should be (25ma is a good starting point), measure your plate voltage, and do the math. If the result is not 9.8 watts, readjust the current, remeasure the plate voltage, and recalculate. You may have to repeat a couple more times to get the current exactly where you want it, ie, providing 70% max dissipation.
Another thing to remember if you are measuring current using the 1 ohm cathode resistors method, cathode current does not equal plate current. Cathode current equals plate current plus screen current. I usually ignore the screen current for a 6V6, but this causes my calculations to be off a bit. That's OK with me because the error is on the safe side, ie, if my calculations say 9.8 watts, the actual dissipation will be a little less.
If you just gotta be exact you will need to know what the exact plate current is. You can measure it directly (dangerous or more trouble than it's worth IMO) or you can measure the voltage drop ACROSS the screen resistor and calculate the screen current using Ohm's Law. Then subtract the screen current from the cathode current.
Lord Valve wrote a decent procedure... http://www.duncanamps.com/technical/lvbias.html
-
Thanks for the help...It's weird, you do something for years, i.e. set the bias at 25ma check for redplating etc. and go on your way. But when you finally want to "Do the math", you get a little unsure of yourself....
Thanks again for all the help!!!!
-
The math can be a pain in the ass.I turn it to where it sounds good and then check to make sure I'm not burning them up.
-
I do know what you mean....I am currently running a pair of GE 6v6GTA's at 29ma about 420volts. Thay are not red plating and sound oh so good but when I did the math I almost lost a load!!! :laugh:
They probably won't last too long.......But they sure sound sweet!!!!
I don't know why, but it seems like I always want to push the envelope....
-
I do know what you mean....I am currently running a pair of GE 6v6GTA's at 29ma about 420volts.
Assuming you are in fixed bias and have subtracted screen current, that is 87% - bit on the hot side but still fine for 6V6GTAs (which are a pentode, and are really a 14W tube - compared to 6V6GTs, which are a 12W beam tetrode)
I figure about 2mA for screen current in a 6V6 at idle
-
Out of curiosity, does anyone ever measure plate current/dissipation when the amp is cranked and someone's playing guitar?
Cheers,
Chip
-
Out of curiosity, does anyone ever measure plate current/dissipation when the amp is cranked and someone's playing guitar?
Cheers,
Chip
I've done something like that with a couple of meters while I whack barre chords. But its hard to get anything like an accurate reading. I guess the best way would be to scope it with a signal generator (and maybe some sort of attenuator) hooked up.
-
Got the JJ's installed,burned in. I biased them right at 25ma with 435v on the plates. Used it at practice last night. I like the sound of these tubes. Maybe they don't compare to NOS RCA's but for $25 bucks a set they do a nice job. Now we will see about how long they last with out drifting too much.
Thanks for the tips.....
-
Just for a differant note I was contemplating Making a 30 watt 59 marshall when I asked this question at a differant forum the consenses was to just make the standard 100 watt set up and put in jj 6v6,s because they could take the high plate current and bias them real hot. The problem was nobody would say what real hot meant. even thopugh i new my plate voltage came in at 455 volts. :rolleyes: Bill
-
Out of curiosity, does anyone ever measure plate current/dissipation when the amp is cranked and someone's playing guitar?
Cheers,
Chip
Sure do - just for fun though. Its not of a lot of use, as it really goes all over the map. You need to use an analog meter so you can catch the peaks.
-
Got the JJ's installed,burned in. I biased them right at 25ma with 435v on the plates. Used it at practice last night. I like the sound of these tubes. Maybe they don't compare to NOS RCA's but for $25 bucks a set they do a nice job. Now we will see about how long they last with out drifting too much.
Thanks for the tips.....
I have never had any problem with their 6V6S - they seem to last for ages. On the other hand their 6L6GC is much less reliable.