Is possible to have 2 bias circuit in a Fender 6g2 ? and 1ohm bias res. question

[kagliostro]

Is possible to mod a 6g2 as to use a double bias circuit ?

my problem is due to the vibrato circuit who is connected in the middle of the two 220k resistors

if I want to double the bias circuit I must double the little bias PS, disconnect the two 220k resistors at their junction and connect one bias PS for each branch

but I don't know if this thing is feasible due to the fact the vibrato is connected at the junction

there is a way to connect the vibrato separately at the two resistors ? an isolation cap for each branch ?

---

Other question if I want to measure the bias via the 1ohm resistor I can connect it between cathode and ground

is the same if I use:
. one resistor for two tubes, with only one bias PS
. two resistors, one for each tube, with two bias PS
. two resistors, one for each tube, with only one bias PS

here the 6g2 schematic

http://www.el34world.com/charts/Schematics/files/fender/princeton_6g2_schem.pdf

Thanks

K

[PRR]

Don't get tricky.

One 1r resistor per tube.

One bias pot.

If two tubes won't match at the same bias, find a better matched pair.

Matching the idle bias does NOT mean matched at Full Power Output.

Tubes that "need" very different bias will also give very different response to tremolo.

Pretend it is 1959 all over again. We didn't worry too much about bias.

[kagliostro]

OK PRR I'll follow your council

Thanks

K

[kagliostro]

OK, I've understand that if tubes are too different is not a good thing to have a double bias pot

But as I've think about a possible solution and I ended with something like the attached schematic

I would like to know if the basic idea was completely wrong

or if it will function if the tubes were not too different

Not that I want to build such architecture at any cost

only I would like to know if my thinking was completely off

In red part of the 6g2 original circuit

Thanks

K

[stratele52]

Kagliostro ,

Yes good idea, but use a Bias pot with a balance pot .

[kagliostro]

Hi Stratele52

My english is not as good as I would like, so, many times, some shades confuses me

please can you say what you mean using other words

Thanks

K

[sluckey]

Your circuit will basically work but I believe the bias balance network will load the trem signal too much.

Ed Chambley built a dual bias Princeton Reverb. He used a dual pot for the trem intensity and separate bias pots. Look at reply #15 of this thread for a schematic...

http://www.el34world.com/Forum/index.php?topic=13855.msg130260#msg130260

[kagliostro]

Thanks Steve I'll read the tread

Franco

[Ed_Chambley]

It is true what PRR said.  I have told others the same as they want me to build one for them.  It is easy to add another bias to have dual.  Split the 220k resistors, use a dual pot.  PEC pot preferred for accuracy.  Dual Alpha pots cause the tremolo to get a little out of phase.

What I wanted to do is to be able to match bias at playing volume.  I am not really concerned with idle bias as I never play idle.  As long as there are no red plates, I a fine.  I have found even new "matched tubes" are not matched as signal is increased.  Also, the tremolo is weak with new tubes.

I actually changed over to 7C5 tubes as NOS in this are very cheap.  I got 6 RCA NOS for $12 on ebay.  New in the box.  Since I have made some adapters and can use 6V6 and 7C5.  A 7C5 really is the same.  Here is the schematic, but it does not show an added cap in the bias that Sluckey suggested I put in.  It is an additional electrolytic, 22uf/50v just after the diode.

If you are using a flat faced chassis, the PEC is easy to install.  If you are going to use a later fender chassis with the with the bevel front, you will have to cut a hole in the bottom of the chassis to install the PEC and then make a cover for it.

[kagliostro]

Hi Ed Chambley

After the explanation from PRR I don't think a dual bias circuit will be tried in the TSA15H Ibanez modded to 6g2

However I was and I'm interested about how this kind of mod is implemented

In the circuit of the Ibanez there is a 32v AC bias winding, so the bias circuit is a bit different, but what is interesting to me is the way you connected the double bias voltage to the Tremolo circuit

In the schematic you kindly posted I see also one other cap missing (as from the 6g2 schematic), the 0.5 cap that is connected between the tremolo pot and ground, I don't know in which way this can affect the signal

PEC pots, I've some of those pots, some new and some old, dismanted from MIL apparatus, they are really very nice pot, I like them

About the 7C5 tubes I'm looking for those tube at all the radio amateur flea market I go, but till now I've find no one, only some MIL Surplus sockets

Many thanks for the info

K

[Ed_Chambley]

I compared the 2 schematics and there are a couple of differences in the tremolo circuit.  I do not know the reason for the .5 cap on the wiper of the intensity pot.  I am sure someone will say soon.  I would think it has to do with the tremolo depth.

I think Sluckey mentioned the schematic as a way of making a dual bias and not so much as an identical circuit.

I will say that having a dual bias allows me to balance the tremolo effect.  If you put you meter on hold and add a signal, then change to the other bias for the next tube and do the same thing, you will see how far out new tubes really are.  Then you can adjust for fairly equal bias at playing volume.  May sound like a lot of trouble, but it is really nice.  I ended up with a 3 knob tremolo because I like it so much.  Intensity, speed and depth.  I like it, but it is not necessary.  

Keep an eye on ebay for 7c5.  I get lucky sometimes and I will get some NOS cheap.

[kagliostro]

I think Sluckey mentioned the schematic as a way of making a dual bias and not so much as an identical circuit.

For sure, I would only say that I've seen that difference in the schematic

Thanks Again

K