Adding Bias Adjustment Pot

[RightLurker]

I have just finished re-capping and re-tubing a '62 Fender 6G6 Bassman, and I would like to add a bias adjustment pot to the bias circuit. I know I need to replace either the 10K or 56K resistor on the bias circuit board with a resistor of half the value, and then wire a cermet pot of the original value in series with the new resistor. My question is, which of the two resistors - the 56K or the 10K - gets replaced? Thanks in advance.

[jjman]

Looking at the 6g6 schematic there are filters on each side of the 10k. So you don't want to be placing the pot in that vicinity since there would be less filtering before the bias tap point. So the 56k should be replaced with the pot/resistor string.

Since you're adding the pot you could do a better/safer approach. The Marshall way results in cold bias if the wiper's contact goes intermittent. The other/Fender way results in But few Fender pots ever go bad like that or we would all have been told to change the setup by now.

[martin manning]

I have just finished re-capping and re-tubing a '62 Fender 6G6 Bassman, and I would like to add a bias adjustment pot to the bias circuit. I know I need to replace either the 10K or 56K resistor on the bias circuit board with a resistor of half the value, and then wire a cermet pot of the original value in series with the new resistor. My question is, which of the two resistors - the 56K or the 10K - gets replaced? Thanks in advance.

The existing 10K and 56K resistors form a divider that puts ~85% of the rectified DC voltage at the junction of the 220K bias feed resistors.

If you replace the 10K with a 5K in series with the element of a 10K bias pot, and take the bias voltage from the wiper of the pot, you will have a variable divider with a range of about 79% to 93% of the rectified DC voltage. That will give +/- ~8% of the bias voltage you are getting now, and that sounds like what you are looking for.

Wire the #3 (CW) terminal of the pot to the 56K so you get the range quoted above and CW=more idle current. It would be best to wire the second filter cap from the wiper to ground, but it probably doesn't matter much. The change in filtering isn't an issue; the -3dB frequency actually moves down one or two Hz.

MPM

[tubeswell]

What the others said - you wire a 20k-50k pot (depending on how much bias range you want) as a variable resistor (with the wiper shorted to one on the other pot lugs to avoid bias catastrophy if the wiper contact fails), and you put this in series with another resistor (say 39k-10k respectively - depending on what sized pot you chose), and these two resistors collectively substitute for the 56K (which is the leg of the voltage divider in the stock bias supply)

[selloutrr]

you might want to try a bias pot like this substitute the 75K pot for a 50k.

[RightLurker]

I thank you all for your advice. It seems to me that jjman recommends replacing the 56K resistor with a ~28K resistor and a ~56K pot, while Mr. Manning suggests replacing the 10K resistor with a ~5K resistor and a ~10K pot. Am I missing something? Does it make a difference? Maybe a diagram would help me out. (I wouldn't know how to post a diagram if my life depended on it!) Thanks again.

[jjman]

I like tubeswell's recommendation the best. But I think all would function.

[martin manning]

There are several ways to skin it. If you replace the 56K with a 50K pot and a 27K resistor you probably will need to change the 10K to 7K5 as well to get the range you need. The other way (replacing the 10K resistor) requires a 10K bias pot, which is a typical Fender, and the 56K stays. The failure mode people worry about is the pot's wiper losing contact which, depending upon the circuit used, will result in either the hottest or coldest bias available. Taking the bias voltage off the pot's wiper (which is often done) will result in the worst failure case of zero voltage, unless a safety resistor is placed as shown.

MPM

[selloutrr]

i like the V4 bias mod ( just a name ) only because if the bias pot failed for any reason the tubes would still be protected.

[RightLurker]

For some reason I wasn't seeing the diagrams posted by selloutrr and Martin Manning before - now they're showing up, and they are a tremendous help. Thanks, guys.

I haven't modified the bias circuit yet, but I've checked the bias voltage (-56V, right where it's supposed to be), the plate voltage, and the plate current using the OPT shunt method.

With new manufacture (Russian) Tung Sol 5881s in the amp, on the first tube the plate voltage is 490V and the plate current is 16.5ma, and on the second tube the plate voltage is 489V and the plate current is 16.2ma.

With a very low-mileage set of old RCA 5881s, the plate voltage is 473 on both tubes, and the plate current is 18.3ma on both tubes.

That current draw seems awfully low to me (shouldn't it be up around 35ma?), and I'm not sure the Tung Sol tubes can handle 490V of plate current. As I understand it, raising the current should lower the plate voltage, and hopefully get me where I need to be. Any comments on this would be greatly appreciated.

Again, thanks for your help!

RightLurker

[martin manning]

If you are not signed in, attached images don't show in the postings... maybe that was it?

5881's are supposed to run 23W max plate dissipation, 70% of that is 16W... You are running 490*0.0165 = 8.1W on the TungSols, 473*0.0183 = 8.6W on the RCA's; so about half of what it could be. What are the screen voltages? Do they look good? What does it sound like?

A quick way to put in a variable bias in that amp is to pull the 56K resistor from the bias supply board and replace it with a trimmer pot wired as a variable resistor in series with a fixed resistor. A 50K pot and a 15-20K resistor should get you a pretty wide range of hotter bias voltages. This would be like the diagram in the lower right of the page I posted above.

MPM

[RightLurker]

Martin -

You're right - when I looked at the message board before, I wasn't logged in, hence no diagrams.

I came up with the same numbers as you did as to where the tubes were running.

I've never checked the screen voltages, but will do so.

There is no humming, sputtering, static, etc. when operating the amp, and it's quite loud, but it's a little brittle or cold sounding. I only run the with the volume on about 4, at the most. I don't want to use this amp for a distorted sound. The bass control in the bass channel has very little effect, and the presence knob has even less effect. On other Fender amps from this era that I've played,the presence control had a tremendous effect. (Interestingly, my brother has a brown Vibrasonic, and the presence control has no effect whatsoever.)

Last week I ordered a 50K ohm bias pot and a 27K ohm resistor to put in place of the 56K ohm resistor, just as in your lower right diagram.

Thanks for all your help. I'm really enjoying this.

[martin manning]

Checking the screen voltages and screen resistors would just confirm everything is ok, and that all you need is hotter bias. That's what it sounds like from your description.

One caveat on the part values for the adjustable bias: A 7K5 plus a 50K pot and a 27K resistor will result in a range plus-and-minus about the original 10K/56K fixed divider. Leave the 10K you have there alone, and you may still need to reduce the size of the 27K fixed resistor to get what you need.

MPM

[Andy Le Blanc]

I've always wondered about that 70% BS, it didn't surface until the last decade
and sounds like crock. The total dissipation from the tung sol data is 26w
its repeated over and over in the RCA data too.
Biasing to 70% will get you 18w dissipation per valve, that's less than a fart.
That's less power than you can get out of an old 6l6 in a metal can.
Isn't the point of a improved and ruggedized valve to get a higher plate dissipation.
Doing the 70% tango is only perpetuating a bad assumption, and will only make
the amp sound like a Fender
.
Who the hell wants to suck like that.

The design maximum plate dissipation for the tube type is 23w P, 3w G2, 26w total.

Use it well grasshopper.....

[RightLurker]

Martin -

The screen voltage on the first 5881 is 484v, and 483v on the second. The screen resistors (original carbon comps) read 478 and 494 ohms, respectively.

7K5 doesn't seem to be a commonly available resistor value. Would 8K2 do the job?
_____________________________________________________________

Andy -

I've read and heard things for and against the 70% theory. As for who wants the sound of a Fender, I'm afraid I'm one of them. With this amp, I'm after the sound of the lead guitar in the Venture's Walk Don't Run, or the Beach Boys' version of Honky Tonk. Not your thing, I take it, but different strokes, as they say.