6G9 Tremolux bias questions

[Gone_Fishing]

I recently acquired a complete brownface 6G9-B Tremolux chassis with 3 burnt transformers.  I have rewired it to  6G9 specs which uses EL 84's and have bought the correct 6G9 transformers.  I plan to make it a combo amp (two 10's, or one 12 or one 10 speaker configuration,,,don't know yet). Here are my questions (keep in mind that I am not an Amp Tech,,,just a hobbyist,,,I would have introduced myself on the Introduction Board but my computer skills are severly lacking,,,I'm ole school I guess,,,still have dial-up but I do have an indoor toilet).

1) I want to install an adjustable bias pot but don't know how to wire it.  The bias circuit is different than most black and silver face amps than I am used to working on.  How would one wire it?

2) Does the EL84's in this amp even need to be biased?

Thanks,

Gone Fishing

[tubeswell]

Link for bias circuit conversions in Fender amps

http://www.el34world.com/charts/bias_conversions.htm

[Gone_Fishing]

Thanks but I have been to Doug's Bias info sites,,,,please look at the 6G9 Tremolux layout on his Schematics page.  There are two caps (10 @ 50's) and two resistors (39K andd 56K) in the bias circuit where there are only one cap and one bias range resistor in the example drawings. 

Thanks again,

Gone Fishing

[sluckey]

Replace the 39K with a 22K. Replace the 56K with a 50K pot and 22K. See pic...

[Gone_Fishing]

Thanks so very much Steven

Sincerely,

Eual

[Gone_Fishing]

Steve or others who are not as ignorant as me,

1) Does your example above have a range of 22K to 94K?  If not, what would it be?

2) If I used the original resistor values of 39K and 56K,,,used a 25K-L bias pot and wired the 56K across the pot,
    would the range be 39K to 120K?  If not, what would it be?

I appreciate all of y'alls help!

[sluckey]

Don't make it complicated. Just do it just like the drawing. If you ain't got a 50K pot, use your 25K pot.

You're only gonna need about -15v for those EL84s. If your pot doesn't give you enough range, then juggle values with the 22Ks.

[Gone_Fishing]

Hello Steve,

     With all due respect to you, I am not trying to make this complicated.  I'm trying to understand this stuff.  If you or someone else would have answered my questions, I would have understood how these resistors added up on this particular circuit.  I understand Doug's Princeton bias pot mod and how it works, but the ole Tremolux, just like Doug's example of his 5F6-A bias pot mod has two caps and two resistors in the bias circuit.  If anyone won't answer my previous questions then please answer these two questions.  If I know these, I will understand.

1).  In the original 6G9 Tremolux schematic, is the total bias resistance 39k + 56k = 95k?

2)  If it's not 95k, what is it?

I know that I'm new to this Forum but since I was a child, I never did like a "just do it because I said so" answer.  Before I joined up here, I think Doug said in an e-mail to me that there was a lot of nice, knowledgable people who could answer my amp bias questions.  I didn't know that this was an answer only Forum and an understanding of the answer was not relevant.  If so, I'll just go elsewhere to get my answers and won't bother you guys with my questions..

Sincerely and Respectfully,

Gone Fishing

[sluckey]

1).  In the original 6G9 Tremolux schematic, is the total bias resistance 39k + 56k = 95k?

2)  If it's not 95k, what is it?

Yes, it is 95K. The circuit I offered has a total resistance of 94K. And it's adjustable. It's the simplest, most straightforward circuit I can think of. Didn't mean to ruffle any feathers. Just trying to help.

[tubeswell]

The 6G9 bias voltage divider output voltage is 56k/(39k+56k) = 59% of the input voltage.

The values you are going to get from your voltage divider will depend on the VAC you get from the PT's bias supply tap.  On the 6G9 schematic it shows a 30VAC bias tap.  When 30VAC is 1/2-wave rectified and filtered, you get (30VAC x 1.4 =) -42VDC as the input voltage. 59% of 42 = 25. (The voltages on the schematic are slightly different but its no biggie - that could be accounted for by resistor tolerances back in the day).

The critical things are knowing what your bias supply voltage (from the PT tap) is, and what mid-point you want to aim for for the bias voltage.

There values suggested by Steve will get you sufficient range with a 50k pot to hit the approximate desired bias voltage.

If you want to use a 25k pot you'll have less adjustment range, but you can aim so that when the middle resistance of the pot (12k5) is dialed in, you have -25V at the voltage divider output (if you want).

So lets experiment a little. If you take a 22k resistor for the series resistor below the knee of the divider and add 12k5 that's 34k5. And if you also have a 22k resistor above the knee, then when the pot is at 1/2 resistance the output voltage is going to be 34k5/(34k5+22k) = 39% of the input voltage or -16.3V (is this too hot for a EL84? Depends on how high the EL84 plate voltage is etc).

So if you decide to (say) further increase the value of the series resistor below the knee to (say) 27k, this gives you (27K+12k5 =) 39k5 below the knee. Now the voltage divider output is 39k5/(22k+39k5) = 64% of the output voltage, i.e.: -27V (if the input voltage is -42V). With the pot rotation cut completely you'd have a 50% output i.e. -21V. This gives you some sort of range around the 6G9 schematic bias voltage. Get the idea?

[6G6]

You do know that the EL84 6G9 is very rare, right?
Off the top of my head, I think there were something like 2-300 of them, in early 1960.
I'd do some research on that, before deciding to make a combo out of it.
You might be about to mod away a couple of grand in value.

[sluckey]

His amp did not use 6BQ5s originally. He modified a 6G9B to 6G9 specs. It's not the rare bird. All he has is the chassis.

[sluckey]

OK, got a little more time this morning. Looking at the original bias circuit with the 39K and the 56K, you see 22.5v (bias output) across the 56K. That means there is 15.7v across the 39K. The total voltage available from the diode is 38.2v.

Now replace the voltage divider with a 22K, 50K pot, and 22K. That same 38.2v will divide across the new divider. With the pot set to 0Ω, the voltage will be divided equally across the two 22Ks. The output will be 19.1v. Now with the pot set to 50K, the output will be 29.3v. The adjustable bias output can swing from 19v to 29v, which allows you to go up or down from the original fixed value of 22.5v. IMO, this bias range is not optimum for EL84s, unless you want them to run cool.

And using a voltage divider consisting of a 39K, 50K pot, and 22K yields a bias swing of 13.8v to 24.8V. Better.

Or using a voltage divider consisting of a 39K, 50K pot, and 15K yields a bias swing of 10.6v to 23.9V. This is the bias range I recommend.

The above examples are all based on that -22.5v bias voltage number on the orig. 6G9 schematic. You will likely still need to juggle the resistor values depending on how much AC voltage your PT really puts out, but these samples should give you an idea how to achieve your desired bias range. I just used Ohm's Law for all these calculations. It's probably faster to just change a resistor value and check the new range. And that's where I was headed with my previous "just do it" statement.

BTW, remove the EL84s until you get the adjustable bias circuit roughed in.