Help with Biasing my converted amp

[Dagan8]

Hello.

I have just finished converting my Fender Red Knob Twin 100w amp to a 50W with a AB763 Reverb and 5F6A Tweed channel.

It works great but I need help with setting the Bias. The original amp documents set the bias at only 20mA of Cathode current per tube. The original amp hardly had any power tube distortion, so I imagine the colder bias may not have had a negative effect on tone so much.

Here is where the bias is set at right now:
-Tube=Tung-Sol 6L6GC-STR rated at 500V max plate and 30W max
-Voltage at pin 4 = 463v
-Cathode current = 24mA

According to a chart I found of a 6L6GC-30W I can bias to 38-44mA.

I have attached a schematic showing the power tube section of my amp. Keep in mind I am only running 2 power tubes and my OT has been changed to a 50w Bassman type.

2 Questions
- How do you recommend I bias?
- R208 & R211 are still at their stock values. Should I leave these alone or should I change the to the 200K value used in vintage Fenders. I assume this wont have a large effect on tone and will just chage the swing of the bias pots?

[Dagan8]

After some reading on this forum I made a decision and changed out the 100K Grid feed bias resistors to 220K. I don't know if I'm delusional but the amp seems to have a bit more break up. I like it. Some of the harshness seems to be gone from the distortion as well. Again. I could be hearing things or wanting to hear things.

I also played with the bias adj a bit more. Here is where I am set to now:
- Pin 3 to 8 = 467V
- Cathode current = 34mA

This is actually as high as my bias adj will go.

What do you think?

[Bruce / Mission Amps]

I'd reduce the values of R210 and R213 by 20% and see what happens.

[ThermionicScott]

Raising the value of the grid-leak/bias-feed resistors allows a little more gain from the PI.

So, does the amp sound good yet? It may or may not improve much as you keep raising the idle current.

- Scott

[Dagan8]

I'd reduce the values of R210 and R213 by 20% and see what happens.

Yes. Good idea. This should give me a bit more bias voltage to play with. I would like to be able to try a hotter bias. Since I am only running 2 of the 4 6L6's the bias PS should have no issues with this. Raising the values of R210 and R213 by 20% makes more sense than my original idea of lowering R214 and R217 by 20%

Raising the value of the grid-leak/bias-feed resistors allows a little more gain from the PI.

So, I'm not hearing things. Good.

So, does the amp sound good yet? It may or may not improve much as you keep raising the idle current.

I won't really know until I try Bruce's idea. Once I have more bias voltage on tap I will do some comparisons.



Oh, one cool thing about this whole thing is the low power switch still works great!!!

I still want to try adding a 75ohm 20watt sag resistor between the diode rectifier and ground. The problem is finding room in the chassis where the heat wont be an issue. Do you guys think a can get away with using one of those aluminum heat sink encased WW resistors and mounting to the outside(tube side) of the chassis with some thermal gel.

Thanks again for all the info so far.

[Bruce / Mission Amps]

Yes. Good idea. This should give me a bit more bias voltage to play with. I would like to be able to try a hotter bias. Since I am only running 2 of the 4 6L6's the bias PS should have no issues with this. Raising the values of R210 and R213 by 20% makes more sense than my original idea of lowering R214 and R217 by 20%

So, I'm not hearing things. Good.

I won't really know until I try Bruce's idea. Once I have more bias voltage on tap I will do some comparisons.

Oh, one cool thing about this whole thing is the low power switch still works great!!!

I still want to try adding a 75ohm 20watt sag resistor between the diode rectifier and ground. The problem is finding room in the chassis where the heat wont be an issue. Do you guys think a can get away with using one of those aluminum heat sink encased WW resistors and mounting to the outside(tube side) of the chassis with some thermal gel.

Thanks again for all the info so far.

Just to be clear... lowering the values of R210 and R213 by about 20% (ie, knock 20K off their value... and use an 82K resistor) will bring your bias voltage closer to zero, not give you a deeper negative voltage. You want less negative bias voltage.
The closer to zero vdc bias voltage, the more current your power tubes will draw at idle and the hotter your tubes will run.
Also, with two power tubes you could use a 10 watt sag resistor.... to be honest, it might get hot but I'm not so sure you couldn't use a 10 watt there with 4 power tubes unless you run the amp flat out at 100 watts for hours at a time.

[Dagan8]

Thanks for clearing that up Bruce. I will try the 82K resistors.

I found a 70ohm 25watt WW aluminum heat sink resistor in my stash. Overkill I know. If I mount this to the chassis it should stay fairly cool. I don't know if I have room inside the chassis but I'll try to fit it in. If not I may try mounting it to the outside.

[ThermionicScott]

I still want to try adding a 75ohm 20watt sag resistor between the diode rectifier and ground.

Whoa! The sag resistor needs to be in series (go from your rectifier to the first filter cap) with your B+. If you put it to ground, it'll get really hot and you might blow something up!

- Scott

[Bruce / Mission Amps]

Whoa! The sag resistor needs to be in series (go from your rectifier to the first filter cap) with your B+. If you put it to ground, it'll get really hot and you might blow something up!

- Scott

Right!
Unless the rectifier block is a SS FW bridge (which it is in his amp) and in that case you can install the resistor on the grounded anodes of the FWB by holding the whole thing (FWB) up from ground with the 70 ohm resistor and then ground.

[Dagan8]

Right!
Unless the rectifier block is a SS FW bridge (which it is in his amp) and in that case you can install the resistor on the grounded anodes of the FWB by holding the whole thing (FWB) up from ground with the 70 ohm resistor and then ground.

That is the plan. Replace the wire from the anodes to ground with the resistor.

I read in another post that putting another 1N4006 diode in series with the sag resistor, in my case between ground and the anodes, helps keep the resistor cooler. Something about the sag resistor seeing pulsing DC instead of constant DC. Does this make sense?

[Bruce / Mission Amps]

That is the plan. Replace the wire from the anodes to ground with the resistor.

I read in another post that putting another 1N4006 diode in series with the sag resistor, in my case between ground and the anodes, helps keep the resistor cooler. Something about the sag resistor seeing pulsing DC instead of constant DC. Does this make sense?

Well, off the top of my head that doesn't make any sense since it will be sitting below a pair of diodes, the smoothing caps are on the cathode end and DC current is DC current anyhow.
It could be a misunderstanding of RMS, AC peak to peak voltage vs DC, but, I've been proved wrong here more then once.

[Dagan8]

Maybe the extra diode only makes sense if I put the sag resistor between the diode cathodes and the first filter cap?

This is where I read it:
Re: Tube Rectifiers vs Sag Resistors - AMPAGE Archive

I could always try it since I don't think the diode would have any negative effect on the amp.