New PT , how to implement it?

[uki]

Hey guys !

Gonna share some experience here for new people, not like I'm that smart about amps but we all learn with our mistakes.

When I build my 1st amp I had to find out the specification for the PT, in the schematic for the amp there is a number, 67233, so I went searching the internet for that mysterious PT, and found a schematic somewhere with that number, and I thought that I had found the right one, but only for the number printed on it, the PT wasn't enough for the amp, it were well custom made and although it worked for a year and in a VERY VERY HOT way before it finally shorted  , gladly it didn't damaged the amp circuit, and only had the heater wires working in low voltage. After some thinking/reading about it, I realized that the PT only should feed about 5-6 tubes tops, 2x 6L6 and 3-4 12ax7s or something of the sort of amp,. NOT 10 tubes, 4x 6L6s and 6x 12ax7s.....

So now I got a "new" PT the old one were remade stronger, but it is slightly different and now have separated bias wires. The guy that made the PT was very kind suggesting separated bias wiring so the PT would work much cooler.
After some research I realized that I couldn't just plug and play it... and thanks to Sluckey and his scrapbook I found out the right way to use the separated bias taps, but I have no clue whatsoever what are the values for the components in the in that circuit,
could someone please help me with it?    The amp is the Fender 6G8, here is the schematic fot it, the diagram for new PT and the bias circuit for separated bias wires.

Thanks in advance !

[sluckey]

You don't need that circuit with the full wave bridge.

Just connect one of the bias winding wires to chassis at the same point your red/yellow center tap is connected. Then connect the remaining bias winding wire to the bias diode (like the red/blue wire on the layout you posted).

[PRR]

> one of the bias winding wires to chassis
> {other} bias winding wire to the bias diode

[uki]

Hey guys thanks!

Oh.... I thought that couldn't be done, that simple ?

How to find out which wire is zero and 50? I measure both wires with the PT chassis and got nothing, only when measuring across both wires it read 50v. Does it matter ?

[sluckey]

How to find out which wire is zero and 50?

This can be tricky, especially if both wires are the same color. Close your eyes and choose one. Call it zero and connect it to chassis. The other one will be called 50 and will connect to the diode. I hope you get it right.

[uki]

Thanks Sluckey ! 

[uki]

Just for the sake of understanding, this is one big question, could someone explain please.

Wouldn't I be putting 50VAC potential in the chassis GND?

[sluckey]

Wouldn't I be putting 50VAC potential in the chassis GND?

No. Remember, you will call one wire zero and connect that wire to chassis.

[pompeiisneaks]

Wouldn't I be putting 50VAC potential in the chassis GND?

No. Remember, you will call one wire zero and connect that wire to chassis.

Exactly, if you put BOTH wires to chassis, then you're right, the circuit closes out with a ground in AC it's like the 'neutral' vs 'hot' idea here, but the circuit just needs some way to 'return' the current back to where it started, closing the loop, the chassis and ground becomes that for pretty much everything in the amp.

Then the 'earth' comes from the three pronged cord's green wire connected to the chassis as well for safety in case somehow both wires of any voltage end up directly shorted to ground, that shoots to earth instead of shocking the crap outta the user.

~Phil

[uki]

Alright many thanks guys!!

In what situation I would need that full bridge rectifier/bias circuit?

I'm also taking it as an opportunity and I remade the rectifier board and added a fuse between the diodes and 1st filter cap. 250mA fuse will do ?

[sluckey]

The FWB will give about 70vdc voltage and it will have a 120Hz ripple (assuming 60Hz line frequency). Easier to filter. That's enough voltage for 6550s.

But you don't need that. The bias voltage is a critical circuit. A lot of expensive components depend on a reliable bias circuit. Keep it simple.

[uki]

PT is connected as you instructed Sluckey/PRR Thanks !!


About this FWB bias circuit, supposition if it were going to be used in my amp, It would then need only 30 volts coming out of the PT? Is that correct ? Or there are ways to adjust the voltage with 20 volts higher ?

[sluckey]

You would juggle the resistor values to give the voltage you want.

[uki]

I got the amp wires all soldered back, my readings are, without tubes, power on, and standby off,

B+ 438-440 vcd

bias -54 vdc

All pin 5 of power tubes are reading -44vdc is that the expected voltage ?

I got a mod on power tubes, got resistors on each tube of 1500R . look the pic.

Anything else to look at ?

[uki]

All good to go ?!

[sluckey]

All pin 5 of power tubes are reading -44vdc is that the expected voltage ?

That may bias your output tube on the hot side. The original schematic shows -55v.

Try it and see what you think. Watch for possible redplating.

[uki]

All pin 5 of power tubes are reading -44vdc is that the expected voltage ?

That may bias your output tube on the hot side. The original schematic shows -55v.

Try it and see what you think. Watch for possible redplating.

I did turn on the amp and played for about 5-10 mins tops, watching the power tubes, I didn't see any redplating, but the power tubes are working way hotter than before, I would say something around 20-30% hotter, amp is breaking up sooner at volume 6-7, before it was only breaking up at 8 and half, the power seen to be little smaller as well. What to do?

Oh I only have V1 and V6 plugged for the preamp tubes, all power tubes plugged.

[sluckey]

Decrease the value of that 10K resistor on the rectifier/bias board until you have about -50 to -55v on pin 5 of every output tube. I would not go any lower than about 2K.

[uki]

How about add to the bias circuit a pot to adjust it, would that be a good move ?

[sluckey]

Sure. But first you should get it working right.

[uki]

Sure. But first you should get it working right.

What type is the biasing in this amp? Grid ?

Reducing the bias resistor will decrease the voltage to compensate the drop after the resistors before the tubes? How many volts I will need, -65, to get -55 at pin 5(getting -10 drop) ? Something like that yes? I'm guessing between 7-9k resistor will do.


 

[sluckey]

JUST DO IT

[pompeiisneaks]

JUST DO IT

Like a dealer pushing his stuff on you :)
DO EET :)

yeah Sluckey's right, just try making sure the 'expected' behavior is working, then try modding it to a new possibly 'not working' method, so you can ensure 'I know the bias worked before, what did I break with this change' instead of 'I've changed two things, sheesh which is the 'wrong' one'

[2deaf]

All pin 5 of power tubes are reading -44vdc is that the expected voltage ?

I am curious about how you got -44Vdc from a 50Vac winding that is rated at 100mA.  The turns ratio is correct and the components/wiring looks correct in the pictures.  The first thing that comes to mind is that the winding is 40Vac instead of 50Vac.  If that is the case, you may only get -53Vdc half-wave with a single RC filter.

Another thought that comes to mind is that the winding is 50Vac, but the first filter after the diode is not functioning properly.  It takes a couple of uF's at that position to get the full potential of this circuit type.  Lessor value capacitors will result in lower voltages (absolute values) at the bias point.

It could be helpful if you could post the measured ac voltage of that bias winding.   

[pompeiisneaks]

The bias supply goes through a voltage divider resistor to get it to the 'right range' for the tubes.  Some also include a variable potentiometer to allow for tweaking it.  You could do the math pretty quickly to figure out the resistors used, but I think they were listed anyway. 

[uki]

JUST DO IT

Yes Sir !!   

I did read the bias winding suggested above by 2deaf and it is reading 47vac ...

Ok I did some tests, and here are the results

with 8.2k resistor the result is -57vdc at the bias resistor and at pin5 -46.6vdc

then 5.6k  -60vdc  -49.2vdc

then 4.15k -60.4vdc -49.3vdc

I don't see it getting any better, the amp now sounds awful, there is a bad overdrive at volume 4-5...

Too bad I don't have those reading before the problem with the PT to compare.

 

[sluckey]

I don't see it getting any better, the amp now sounds awful, there is a bad overdrive at volume 4-5...

Well, put it back like it was and live with it.

[2deaf]

with 8.2k resistor the result is -57vac at the bias resistor and at pin5 -46.6vac

Is the voltage at pin 5 the same as the voltage at the junction of the 8.2K and 56K resistors?

[uki]

The tubes are going way too hot.

What really change in the way the PT is now in comparison to the previous one? Could be the amperes in the new winding ?
Why the new PT change so much the way the amp work ?

with 8.2k resistor the result is -57vac at the bias resistor and at pin5 -46.6vac

Is the voltage at pin 5 the same as the voltage at the junction of the 8.2K and 56K resistors?

When I read from the that junction(orange dot in the picture) to ground I get a different reading than pin5(green dot in the picture) to ground.

[2deaf]

Should be the same.  Time to find out why they are not.

Pull the power tubes and see if the voltages are the same.

[uki]

Without the tubes the readings still different, at those same points.

[sluckey]

Without the tubes the readings still different, at those same points.

So that means that those coupling caps are leaking positive voltage from the PI plates ***OR*** something is not wired correctly. You did a mod to add grid stopper resistors. Check that for accuracy.

[uki]

Without the tubes the readings still different, at those same points.

So that means that those coupling caps are leaking positive voltage from the PI plates ***OR*** something is not wired correctly. You did a mod to add grid stopper resistors. Check that for accuracy.

The grid stoppers check good and connections are correct, those have been in the amp since the begin.
There are two things that changed is the PT, and the rectifier board, I've remake the board to better fit inside the chassis.
How do I check those coupling caps ?!

[sluckey]

How do I check those coupling caps ?!

Just disconnect one end of BOTH caps. Now is the voltage on pin 5 of each tube the same as at the orange dot?

[super&plexi]

What about some wheels? ....................I'm sorry! But I had to!. Hope it at least made you smile.

[uki]

What about some wheels? ....................I'm sorry! But I had to!. Hope it at least made you smile.

  Heh !! 

How do I check those coupling caps ?!

Just disconnect one end of BOTH caps. Now is the voltage on pin 5 of each tube the same as at the orange dot?

The readings still the same no changes, the bias resistor in place is the last one tested with same results as before 4.15k -60.4vac -49.3vac. It isn't the caps then. What else to check ?

[sluckey]

Turn the power off and measure resistance from pin 5 of each power tube to chassis. Should be about 270KΩ. What do you measure for each tube?

[uki]

V7 294k
V8 294k
V9 283k
V10 283k

A bit too high ?

[sluckey]

Resistances are fine.

-60.4vac -49.3vac

You keep saying your bias voltage is vac. Bias voltage is negative DC. Be sure your meter is set to measure DC voltage, not AC voltage.

[uki]

Sorry about that , that is correct it is indeed DC , that was a mistypo...

I put back the 10k resistor and gonna check how it sound again and try to get some feeling about how hot the tubes go... I had only V1 and V6 preamp tubes on when tried 1st time, Having all 10 tubes on makes difference?

 ok what next ?

[2deaf]

I don't think you should put the power tubes back in until the bias is functioning properly.

[sluckey]

When you put the 10K back in the tubes will run hot. I would put a 470Ω in and measure the voltage on pin 5 with no tubes plugged in. Once you get that voltage up to -55vdc or higher, we can tackle making it adjustable.

But I'm still concerned about the difference in voltage between the junction of those resistors and pin 5 of the tube. This difference may be caused by your meter. What is the name and model of your meter?

[uki]

I don't think you should put the power tubes back in until the bias is functioning properly.

Understood !

When you put the 10K back in the tubes will run hot. I would put a 470Ω in and measure the voltage on pin 5 with no tubes plugged in. Once you get that voltage up to -55vdc or higher, we can tackle making it adjustable.

But I'm still concerned about the difference in voltage between the junction of those resistors and pin 5 of the tube. This difference may be caused by your meter. What is the name and model of your meter?

Power tubes been off of the amp all this time.

My meter is china cheap stuff model is  DT830B brand TBlack here is a pic:

[sluckey]

That meter only has a 1MΩ input impedance. That explains the difference in voltage readings. So, forget about that and replace the 10K with a 1K. Then install a 1Ω resistor between pin 8 and chassis. Do this for all 4 tubes.

Now read this about biasing an amp.

    http://www.duncanamps.com/technical/lvbias.html

[EKDENTON]

LOL I  emailed Uki that exact link last friday.

[sluckey]

Hey, where you been hiding Ed?

[uki]

When you put the 10K back in the tubes will run hot. I would put a 470Ω in and measure the voltage on pin 5 with no tubes plugged in.

Do you mean in place of the 10k ?

Now read this about biasing an amp.

    http://www.duncanamps.com/technical/lvbias.html

I been reading that, did most of it, still need to complete the reading and read all over again to keep it in mind. I got most of the idea, and I see you're guiding me that way, the doc does help !!

That meter only has a 1MΩ input impedance. That explains the difference in voltage readings. So, forget about that and replace the 10K with a 1K. Then install a 1Ω resistor between pin 8 and chassis. Do this for all 4 tubes.

I need to get some 1Ω resistors to do that I don't think I have enough, I know I have some 1Ω-5W is that too much ?

About the meter could you please educate me, 1MΩ input impedance, what that means?
What kind of meter is recommended?

Thanks sir !!!

[2deaf]

When checking a dc voltage with a meter with a 1M ohm input impedance, it is the same as inserting a 1M resistor into the circuit at the points contacted by the probes.  The attached diagram shows what happens when your meter is used for voltage.

The meters that I use have 10M input impedances.  Even with that, they sometimes alter high impedance circuits enough to throw me.

If you measure the dc voltage between point A and point B in the top diagram with your meter (or any meter) you should get 0.00V.  This would have been the better test to suggest.

[uki]

When checking a dc voltage with a meter with a 1M ohm input impedance, it is the same as inserting a 1M resistor into the circuit at the points contacted by the probes.  The attached diagram shows what happens when your meter is used for voltage.

The meters that I use have 10M input impedances.  Even with that, they sometimes alter high impedance circuits enough to throw me.

If you measure the dc voltage between point A and point B in the top diagram with your meter (or any meter) you should get 0.00V.  This would have been the better test to suggest.

Oh I see, the 1M in the meter is doing that 10v drop ! The picture show it very clear! Thanks!!

Does the 1R(found some) resistors have to stay, or is it just for this reading ?

[2deaf]

Does the 1R(found some) resistors have to stay, or is it just for this reading ?

I would install them in the amp permanently.  They are too small to affect the performance of the amplifier and the occasion for their use is going to come around again.