50W OTS bias setting

[sixstringer]

I've just finished a Ceriatone OTS (50w). I would appreciate any advice on a proper bias setting. The 6l6s are a matched pair. I've attached a couple of photos of the amp. The amp is assembled from the Ceriatone kit, I scratch built the cabinet from pine to match the Dumble style.
Thanks for your help

[talbany]

ODS Style amps sound best biased on the cold side.. 450V plate voltage= 34-37ma

Tony

[ChrisM]

+1

Amp looks great btw. Might consider adding an aluminum strip along the front, just under the control panel. Then it would look even more like the real deal.

[Structo]

You did an excellent job on the cab!
I can recommend the use of finish washers under the chassis mounting screws to prevent them from digging into the top of the cab.
[img:130:130]http://www.victorybolt.com/images/victo ... ashers.jpg[/img]

How about some pics of the insides?

For what it's worth I bias my 50w at 34ma.

Remember that the bias test points do not take screen current into consideration.
They vary but 4-5ma is the usual for 6L6.

I just set it and consider the screen current a safety buffer.

If I were you I would try it at 32ma, then work your way up to 40ma.
Play the amp at the loudest level you will play it at and listen to the way it performs.
When you find a setting as long as you are under 70% of the max you are good to go.

When you set the bias, play the amp for a while then double check to make sure it hasn't drifted. New tubes some times take a little while to settle in.

Here is the fixed bias formula.

(W * .7) / V = I
Where W is the max wattage of the tube in question.
The .7 is 70% and V is plate voltage and I is the current.

To measure your plate voltage you will need to measure it at pin 3 of the power tubes.
Be very careful there as it is between 430 and 450v!
Just plug your black lead into the black test point and with the meter set to read DC volts, touch the probe to pin 3 where the output transformer primary wire is soldered.
Be careful to not short pin 3 to the heater pin. (don't ask me)
Right those numbers down and plug them into the formula.

Now you are ready to test the bias
The 1 ohm resistors on the test points convert current to voltage in millivolts, so set your meter to read DC voltage.

Also when you set the bias, be sure of course to have a speaker load connected anytime you power up the amp with power tubes installed.
Turn the volume and master all the way down, center the tone controls at noon, and presence at noon.
The bias pot can be touchy so small adjustments are needed.

Good luck!

[sixstringer]

Sorry for being so thick. I've always used the OT shunt method of biasing and it has worked well for me. I'm not sure what the bias points are telling me, they seem to have an adjustment range of about 5mA to 25mA.
Thanks for your help

[talbany]

Nothing wrong with the shunt method as this takes into account screen current and therefore very accurate..(Be careful with those probes on pin 3 and CT) If your range is on the low side try lowering the tail resistor on the bias pot like 10K .. This will lower the bias voltage raising the current draw on the tube.. Keep in mind by raising the resistor you limit the range on the pot but looks like you will still have sufficient range by your post.. Good Luck!!
Nice Build.. For a Ceriatone

Tony

[M Fowler]

Robin,

You did a great job on the amp and I am totally impressed with your cabinet too.

Mark

[Structo]

My bad, I assumed you had tip jacks on the back of the chassis.

[sixstringer]

I do have bias test points. I'm just not sure how to use them.
Is this correct:

(W * .7) / V = I

W = 25 (25 * .7 = 17.5)
V = 425

17.5 / 425 = .04117

I = 41mA

Using the bias points, I should adjust the bias to what? 41mA? That sounds high compared to the recommanded 34 - 38 range.

The 10k trim pot will adjust to a max of 31mA.

I checked the plate voltages with 2 different DMMs to be sure it was accurate. (they were with in a volt of each other). But when I set them
to the mA setting, I get a wildly different reading, one to the other.
I'm probably setting them incorrectly. I appreciate you time and patience in advising me. BTW, I added the aluminum trim to the front
as suggested, I post another photo when I get it back together.
Thanks again

[talbany]

If your going to use the math Tom gave you need to sub the .7 to a .6 or 60% max plate..
For now don't worry about the math.. Set it at 34 ma play it then raise it a few ma play it you can go on up to 46.6 that's 70% max plate dissipation..you can find the sweet spot you like within that range.. Most here me included set it at 34ma and let it eat..(Keep in mind that's if your plate voltage is around 450v)..Like I said in my last post your going to have to lower the tail resistor on the bias pot..I would start w/ 10K this will raise the bias..
If you don't have 1 ohm resistors attached to pin 8 on the output tubes( or a set of bias probes) then you don't have the test points to measure cathode current..then your going to need to shunt bias it..
Tony

[sixstringer]

I swapped out the 15k resistor for a 6.8k and now it trims perfectly.
The plate voltage measures 425v. So 60% = 35mA
Thanks for taking the time to help me with this.
Robin

[oldpicker]

Congratulations on your build Sixstringer. I just finished my Ceriatone OTs a few weeks ago and your question answered a lot of mine. I have been biasing correctly but my plate voltage at pin 3 is 462 volts. Do any of you think this is too high? If so, how would I lower this. I am using Mercury transformers. I love the sound but was wondering about tube life. My power tubes are the TAD 6l6 short bottles. Bias setting at 35.7 and 37.2 volts.

Thanks in advance

Dennis

[Structo]

Interesting that the Mercury PT is a bit higher voltage.
Do you remember what the secondary B+ voltage was spec'd at?

Tony the reason I gave him the 70% formula is because 6L6GC tubes are 30 watt tubes, but I usually figure them at 25 watts for a buffer.

Remember to double check your plate voltage after you bias it as the voltage will dip a bit when biasing higher.

I have 560R screen resistors and I measured the voltage drop across them yesterday.
I was surprised that there was only 1.1v on one and 1.2v on the other.

So that is only 1.9 or 2ma on the screens.

I noticed since I installed the choke that my screen voltages are just about the same as the plates. I may install some 1K 5W screen resistors to make the tubes more happy.

@ oldpicker keep an eye on those tubes. I had a pair of TAD's where one redplated after about 8-10 hours of use.
Luckily I had the rear facing me so I caught it in time before any damage could occur.
I think they were the tall bottles though.
They did sound good, but they kind of made me gun shy.

[oldpicker]

Tom,
I believe the secondary B+ was 345v. I have about 12 hours on my TADs. They replaced a pair of SED's that redplated at about 8 hours. This is why I was concernd about my plates being 462v. What power tubes are you using?

Thanks
Dennis

[Structo]

Currently I have Winged C 6L6GC in mine.

Which Mercury trannys did you buy?

You are probably OK for your power tube voltage.

What do you have for voltages at V1, V2 and V3?

Measure DC voltage on pins 1 & 6 on each tube.