Re: B-15-N Build Help

For those building and repairing amps on their own. Learna and share ideas.

Biasing Input & Help

Postby Kid on Tue Jul 16, 2013 12:59 am

Jack Hester wrote:On a new build, I solder a 1 Ohm resistor between the cathode of the power tubes and the next component(s) (or ground on a fixed bias). I have a tube socket adapter, but it's only for the standard, run of the mill tubes. I tend to try out non-standard at times, in some of my experiments. You may be able to find a suitable 1 Ohm resistor, with higher wattage, at a Radio Shack. Here in the small town near where I live, they don't restock like they should. The stuff is setting in boxes in the back room. I believe I found some at 5 watts, a year or so ago. You can check online to see what they carry, and if it's in the stores.Assuming the 6L6GC voltages are plate voltages (Pin 3), with a max plate dissipation of 30 watts, 30/455 = 66mA, 66ma x .70 = 46mA safe operating bias. By your schematic, you have a fixed bias, but no adjustment that I can tell. Should be fairly easy to set yourself up an adjustable bias. I don't like relying on bias voltages as a point if reference, though it's a good start to compare against the schematic. 70% is a safe starting bias, so the experts say. And tweaking the current up and down changes the tone. I'm not a musician, so I have to depend on my friends to tell me if this is so. I play just well enough to know that my restorations are working fine. I shoot for 70%, but feel I'm ok if I'm below the max.My B-12-XY bias is very much like yours, except I have 7027A tubes. I have not done so, but I plan to install an adjustable bias in mine. That will be the last thing that I do, when I finish. It still has a subtle high frequency hum in the first gain stage of Channel 1, and a hiss in Channel 2. My B-12-N restoration is the standard for all amps. Quiet is an understatement. I had no idea that a tube amp could be so quiet, until I cleaned this one up (electronics, that is). I've got my work cut out for me on the XY. My day job at the power plant keeps me out of my shop as much as I'd like to be, so my projects have been on hold for the better part of this year. Anyway, I had a moment to read your thread, and ramble a bit about it. I didn't say anything to contribute. Mostly just thinking out loud. But, I'll follow your progress as time permits, as I enjoy a build such as this. Have a good one.


Guess I'm the one rambling - just have no experience with amps. Very limited and basic knowledge of electricity - Have been learning a hell of a lot more though :). Almost intimate knowledge of transformers now :lol:

Ran for 3 more hours yesterday after sorting the LV secondary on the P.T.

Removed Ground From Pin 8 of power tubes.

Put 1 ohm 1/2 watt resistors cathodes to ground.

Now at (/w resistors in):

Bias Supply -50vdc

V1
Pin2 182vdc +6v
Pin5 163vdc +7v

V2
Pin2 238vdc +5
Pin5 232vdc +6

V3
Plates 467vdc +5
Plate Voltage -465vdc
Cathode Current 57.8ma

V4
Plates 467vdc +5
Plate Voltage -465vdc
Cathode Current 66.3ma

467 Plate Voltage 0.063 Cathode Current = Plate Dissipation of 31 watts (Weber Bias Calculator puts it at 29 watts).

Any input appreciated.....

Damn near ready to clean her up, put on OT permanently, close the cover..... :thumbsup:
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Re: B-15-N Build Help

Postby gusfinley on Tue Jul 16, 2013 4:43 am

Keep in mind that the cathode current is conservative when using it to bias an amplifier. It contains the plate current and the screen current. Even at zero signal, you will likely have 5mA or so of screen current. Don't kill yourself trying to trim down that extra 5mA.
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Re: B-15-N Build Help

Postby Baron Von Machinenmann on Tue Jul 16, 2013 1:50 pm

Biased pretty freaking hot at no signal IMHO.....
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Bias

Postby Kid on Tue Jul 16, 2013 10:31 pm

Baron Von Machinenmann wrote:Biased pretty freaking hot at no signal IMHO.....


Looks that way to me too...
Took out the 100k pot I had in there, put back in the original 56k resistor, took readings and I used the Weber Bias Calculator, not absolutely sure I entered everything right, it puts the plate current in each tube at 62ma and has me shooting for 45ma. Have a 39K Carbon Comp. Ordered this 25K Trim pot.
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Guess it will be 2-3 weeks.....

After Cabinet and Guitar.....
Don't know if anyone remembers that old jukebox power amp..... Valves in Stereo :D
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Re: B-15-N Build Help

Postby jaywalker on Sun Aug 18, 2013 1:05 pm

Kid wrote:Is there a solution, alternate to toting a variac, to resolve the difference between the wall outlet and the P.T. rating of 115?


Do a search for the use of "zener diodes" on the PT secondary center tap for knocking down voltages.
Just be aware that sometimes the method has to be altered if there is a bias tap on the PT secondary in which case you have to create a bias feed off one leg of the secondary iirc. Of course the one time I tried this I read some very goofy numbers for the bias voltage. A little more digging turned up a solution.

p.s. I need to do this to my B15N. I've been dragging a home made bucking transformer around for this amp and my tweed deluxe, which is better than toting a variac but still another piece of junk to carry around.
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Re: B-15-N Build Help

Postby Kid on Tue Aug 20, 2013 4:02 pm

jaywalker wrote:
Kid wrote:Is there a solution, alternate to toting a variac, to resolve the difference between the wall outlet and the P.T. rating of 115?


Do a search for the use of "zener diodes" on the PT secondary center tap for knocking down voltages.
Just be aware that sometimes the method has to be altered if there is a bias tap on the PT secondary in which case you have to create a bias feed off one leg of the secondary iirc. Of course the one time I tried this I read some very goofy numbers for the bias voltage. A little more digging turned up a solution.

p.s. I need to do this to my B15N. I've been dragging a home made bucking transformer around for this amp and my tweed deluxe, which is better than toting a variac but still another piece of junk to carry around.


Thanks for the info! Sounds like the way to go - will certainly look into it! Had thought of a variable tranny, or small UPS - as you said another piece of junk to tote. Still have her hooked up to that 60 lb. variac on the counter. I did get in the bias trim pot. last week. I might actually get around to soldering that in, coolin' 'em down a little, removing the zizters, re-attaching the grounds and cranking it up again :). Also still have to bolt on the OT permanently and sort the outputs into the back panel.....
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