Grounding point for bias circuit on an AB763 - 1 channel

[Mike_J]

I am in the final planning stage of this project.  I am going to separate V2 and V3 by another 1-3/4" so it is like the blackfaced Fender amps.  Then the reverb transformer can be placed between the preamp tubes.  I will put the same space between the bypass caps and resistors for V2b and V3a.  The board I have is about the same length as the original board for a one channel build.  Accordingly, I have to remove something from the board in order to have space for the change.


I have decided to place the bias circuit on its own board like Fender did.  The question I had was why wouldn't I tie all grounds associated with it to the buss wire that terminates at the input jack?  The grounds are now going to the transformer ground lug and the speed pot is tied to the buss wire through a 100K resistor.  The bias pot feeds the intensity pot which is also part of the vibrato circuit.


Thanks
Mike

[mresistor]

I think that since the bias circuit is associated with the power tubes... then therefor the grounding of such should be to the PT bolt. Someone else will clarify I'm sure.

[Mike_J]

I think that since the bias circuit is associated with the power tubes and then therefor the grounding of such should be to the PT bolt. Someone else will clarify I'm sure.

That may be the logic behind why the layout shows it going to the power section. 


Thanks
Mike

[mresistor]

Well, if it is associated with the first stages of PS filtering then it should go to the same grounding point.   That's my understanding.

[Mike_J]

Another question I had.  The Fender bias circuits all had the electrolytic cap tied to the diode associated with the bias circuit.  The junction of the two went to the entrance of the bias pot.  In the Hoffman 1-channel layout the diode goes to the entrance of the bias pot and the electrolytic cap goes to the wiper of the pot.  Should this be different?


Thanks
Mike

[mresistor]

the short answer is -- if you are building a Hoffman SC AB763 circuit. Then no. Because it works as designed. The long answer is; well I only have the Hoffman AB763 schematic in front of me so I can't say at the moment.

[Mike_J]

Well, if it is associated with the first stages of PS filtering then it should go to the same grounding point.   That's my understanding.

Thank you for the replies.  The intensity pot ties to the PI which is fed from a 20 uF power cap that is fed from the power supply after the Screen supply and a 1K power resistor.  However the entrance to the intensity pot is tied to V4 which is fed from the same source as the Screen supply.  It looks like it might have a foot on each side.


Thanks
Mike

[mresistor]

Mike  I'm thinking whether the bias filter cap is either before the voltage adjustment pot or after it makes no difference. The bias circuits work the same way.

[Mike_J]

Mike  I'm thinking whether the bias filter cap is either before the voltage adjustment pot or after it makes no difference. The bias circuits work the same way.

That it works is certainly the most important consideration.  Just noticed a difference and was curious as to why.


Thanks
Mike

[sluckey]

The intensity pot ties to the PI

No it doesn't. Look again.

Another question I had.  The Fender bias circuits all had the electrolytic cap tied to the diode associated with the bias circuit.  The junction of the two went to the entrance of the bias pot.  In the Hoffman 1-channel layout the diode goes to the entrance of the bias pot and the electrolytic cap goes to the wiper of the pot.  Should this be different?

Yes it should be different. Hoffman put the 6G16 trem circuit in his AB763 circuit. The 6G16 had NO BIAS POT. That's the difference.

[Mike_J]

The intensity pot ties to the PI

No it doesn't. Look again.

I am confused again.  Aren't the two 220K resistors part of the PI?  The junction of those two resistors goes to the wiper of the intensity pot.


Thanks
Mike

[Mike_J]

Another question I had.  The Fender bias circuits all had the electrolytic cap tied to the diode associated with the bias circuit.  The junction of the two went to the entrance of the bias pot.  In the Hoffman 1-channel layout the diode goes to the entrance of the bias pot and the electrolytic cap goes to the wiper of the pot.  Should this be different?

Yes it should be different. Hoffman put the 6G16 trem circuit in his AB763 circuit. The 6G16 had NO BIAS POT. That's the difference.

I agree that the 6G16 bias circuit has no bias pot.  It appears to use fixed resistors to me.  The 6G16 bias circuit had the electrolytic cap tied to the diode like the AB763 bias circuit does.  The 6G16 also has a resistor across the electrolytic cap as I recall from looking at it earlier.


I have no doubt the way Doug has this drawn is correct.  I just was hoping to find out why he did it differently.


Thanks
Mike

[sluckey]

Aren't the two 220K resistors part of the PI?

Those are the grid return resistors for the output tube. Ground the bias cap at the same point the output tubes are grounded, a PT bolt for Hoffman's AB763.

[sluckey]

The 6G16 bias circuit had the electrolytic cap tied to the diode like the AB763 bias circuit does.

When you crank Doug's bias pot all the way to the top, the cap will be connected to the diode. But you can put the cap either place. You can even use two caps.

[Mike_J]

Aren't the two 220K resistors part of the PI?

Those are the grid return resistors for the output tube. Ground the bias cap at the same point the output tubes are grounded, a PT bolt for Hoffman's AB763.

Thank you for clearing this up.  I will ground the bias circuit to the power side.


Thanks
Mike

[Mike_J]

The 6G16 bias circuit had the electrolytic cap tied to the diode like the AB763 bias circuit does.

When you crank Doug's bias pot all the way to the top, the cap will be connected to the diode. But you can put the cap either place. You can even use two caps.

Thanks for making this clear.  I will make the bias circuit the way Doug drew it.


Thanks
Mike

[sluckey]

I had forgotten that I just did an amp with 6G16 trem circuit just last month. Look at page 4 of this schematic...

     http://sluckeyamps.com/hammond_2/AO-63.pdf

Notice that I used two bias filter caps? OK, now here's my reasoning for having a filter cap connected to the Intensity pot like Hoffman's circuit. That filter cap smooths the rectified bias voltage but it also puts the end of the intensity pot at signal ground. This means when you turn the intensity pot to zero, the trem will be dead like you probably want. But if you only put a cap at the diode, the end of the intensity pot will not be at signal ground and when you turn the intensity to zero, you may still have some trem sneaking through to the output tubes.

I don't know if Doug used the same reasoning. If you send him a PM I bet he'll tell you why he did it the way he did.

[Fresh_Start]

I've used a 2 capacitor bias supply in 2 or 3 amps with bias vary tremolo. Did it for better filtering but Sluckey's reasoning is solid.

Always used a dedicated ground by the bias board. It's a dedicated power supply with negative voltage. Why tie it anything else?  Corrections - looking at my layouts, I always grounded the bias supply with the rest of the power amp. However, Fender just grounded bias boards on the chassis.

I came up with a really neat, tight bias board layout for an original Princeton Reverb.  Not much space there but this worked nicely.  (We wanted to keep the original bias board completely intact despite the leaking capacitor.)  The schematic is a snippet from a Super Reverb so the values are different, plus there's a 1 meg "protection resistor" in case the pot wiper ever let go.  Note that I added a 100uf filter cap on the second leg of the pi filter on the Princeton Reverb board.  Probably was overkill, but the two 10uf caps didn't smooth the bias voltage out enough to suit me.

Please note that the Princeton Reverb bias board layout is a more recent design than the Super Reverb schematic.

Hope this helps.

Chip


P.S. Sorry for any confusion - the layout is from a Princton Reverb but the Schematic snippet is from a Super Reverb.

[Mike_J]

I had forgotten that I just did an amp with 6G16 trem circuit just last month. Look at page 4 of this schematic...

     http://sluckeyamps.com/hammond_2/AO-63.pdf

Notice that I used two bias filter caps? OK, now here's my reasoning for having a filter cap connected to the Intensity pot like Hoffman's circuit. That filter cap smooths the rectified bias voltage but it also puts the end of the intensity pot at signal ground. This means when you turn the intensity pot to zero, the trem will be dead like you probably want. But if you only put a cap at the diode, the end of the intensity pot will not be at signal ground and when you turn the intensity to zero, you may still have some trem sneaking through to the output tubes.

I don't know if Doug used the same reasoning. If you send him a PM I bet he'll tell you why he did it the way he did.

What you are saying makes sense to me now.  I have attached the bias layout Fresh Start posted and made changes to reflect what your schematic showed.  The only difference is the entrance resistor which you don't have on your schematic and the other resistor values were different although they look to have about the same ratios.  On another schematic Fresh Start sent he had 1M "protection" resistors on the intensity and bias adjustment pots in case the pot wipers ever let go.  My question is, what does the large resistor before the diode do?  Also, does it make sense to add the 1M resistors for protection?


Also, thank you for the good information regarding modifying the speed and intensity of the tremolo.


Thanks
Mike

[sluckey]

DON'T USE MY RESISTOR VALUES! The resistor values are determined by the amount of AC voltage applied from the PT. My PT puts out 33VAC for the bias. I doubt your PT will do the same. Use the values Hoffman has on his schematic. Also notice that the resistor between the PT and the diode is called the bias range resistor? That resistor will usually be 470Ω if the AC voltage is coming from a approx. 50VAC bias tap on the PT. That range resistor will usually be 100K (or higher) if the AC comes from the HT winding of the PT. You can change the value of that resistor if you need to shift the range of bias voltage adjustment for your amp. My amp doesn't have a range resistor because the range of bias voltage adjustment is already right where it needs to be.

I highly recommend you build this circuit EXACTLY like the Hoffman schematic. And use his board layout. Then if you need to troubleshoot when you fire it up, you won't have to wonder if something you changed is the problem. You can concentrate on finding the wiring error or wrong component value. Adding the 1M (or 2.2M) resistors to the bias and INT pots is a good idea, but millions of Fender amps work just fine without them. Are you going to use Hoffman's pre-drilled board or will you roll your own?

[HotBluePlates]

... I have decided to place the bias circuit on its own board like Fender did.  The question I had was why wouldn't I tie all grounds associated with it to the buss wire that terminates at the input jack?  ...

First, listen to/follow all of what Sluckey said.

Ready for the simple, easy-to-remember reason to ground the bias board with the power transformer & output tube stuff?

• Your bias circuit takes a voltage from the power transformer via a tap. However the "circuit" (circle) of that tap is from the power transformer high voltage winding's center-tap through to your bias tap. You then process that bias tap a.c. voltage with the stuff on your bias board, which has a board-ground. That board-ground ideally connects directly to the PT high voltage winding center-tap as directly as possible to complete the "circuit" (circle).

So if you grounded the bias board somewhere other than the PT center-tap, you're just making an unnecessarily-long ground path for the bias circuit.

[Mike_J]

DON'T USE MY RESISTOR VALUES! The resistor values are determined by the amount of AC voltage applied from the PT. My PT puts out 33VAC for the bias. I doubt your PT will do the same. Use the values Hoffman has on his schematic. Also notice that the resistor between the PT and the diode is called the bias range resistor? That resistor will usually be 470Ω if the AC voltage is coming from a approx. 50VAC bias tap on the PT. That range resistor will usually be 100K (or higher) if the AC comes from the HT winding of the PT. You can change the value of that resistor if you need to shift the range of bias voltage adjustment for your amp. My amp doesn't have a range resistor because the range of bias voltage adjustment is already right where it needs to be.

I highly recommend you build this circuit EXACTLY like the Hoffman schematic. And use his board layout. Then if you need to troubleshoot when you fire it up, you won't have to wonder if something you changed is the problem. You can concentrate on finding the wiring error or wrong component value. Adding the 1M (or 2.2M) resistors to the bias and INT pots is a good idea, but millions of Fender amps work just fine without them. Are you going to use Hoffman's pre-drilled board or will you roll your own?

My PT has a 50 volt bias winding so the 470 ohm range resistor is correct.  Fresh Start must have been coming from the B+ and that is why he had to use the 82K value.  Are you recommending Doug's exact bias circuit and not using the two electrolytic caps?  Fender had better pots to work with versus what is made today in my opinion  Can't even get a decent pot today for $30.00.


I bought the G-10, turrets and eyelets from Doug as well as the aluminum spacer tool and the flaring tool.  So of course I am going to build my own. Wouldn't want to do it the easy way.


I built an AB763 amp probably 10 years ago using a Fender chassis that was scrapped.  Fender had a 3-3/8" center-to-center gap between V2 and V3 of the one channel amp.  This is where they installed the reverb driver.  Making extra space for the reverb driver has the additional advantage of shortening the wires between the reverb and vibrato pots and their corresponding circuit board connections.


Doug's layout, once it is blown up to scale, has 1-1/2" center-to-center spacing between V2 and V3.  What I thought I would do was separate the V2 and V3a cathode caps and resistors to accommodate the 1-7/8" difference between Fender's and Doug's spacing.  Another advantage to doing this is it allows space for larger tone caps.  I have some .047 and .1 blue molded caps Fender used in their blackfaced amps which I like quite a bit.  I could use a little of this new found board space to create more room for the tone caps and probably the PI caps tied to the grid return resistors, or in my case the PPIMV pot that I spent a small fortune for.


As you may recall I am also planning on putting in a passive effects loop.  You recommended against running the .001 cap feeding the grid of V5a (PI tube) and instead said it should be on the board.  It would be a lot more direct and the grid wire would be much shorter if the cap went from the tip of the power amp input jack of the FX loop to the V5a grid. 


I have learned it is a bad idea to disregard your recommendations.  What I think I will do is go to my computer guy.  He has the 2007 version of MS-Visio that he will install for $35.00.  Learn how to use it and then make a layout reflecting the changes mentioned above.  The hen scratched versions I have been making are impossible to read.


Thanks
Mike

[Mike_J]

... I have decided to place the bias circuit on its own board like Fender did.  The question I had was why wouldn't I tie all grounds associated with it to the buss wire that terminates at the input jack?  ...

First, listen to/follow all of what Sluckey said.

Ready for the simple, easy-to-remember reason to ground the bias board with the power transformer & output tube stuff?

• Your bias circuit takes a voltage from the power transformer via a tap. However the "circuit" (circle) of that tap is from the power transformer high voltage winding's center-tap through to your bias tap. You then process that bias tap a.c. voltage with the stuff on your bias board, which has a board-ground. That board-ground ideally connects directly to the PT high voltage winding center-tap as directly as possible to complete the "circuit" (circle).

So if you grounded the bias board somewhere other than the PT center-tap, you're just making an unnecessarily-long ground path for the bias circuit.

Great explanation.


Thanks
Mike

[Fresh_Start]

Here's a useful link on bias circuits in Hoffman's Library:

http://el34world.com/charts/Biascircuits.htm

Chip

[mresistor]

  Can't even get a decent pot today for $30.00.

There are plenty of very high quality potentiometers out there and available for less than $30/piece.   For instance ..  http://www.precisionelectronics.com/0-40a.html


and  http://www.ebay.com/sch/i.html?_nkw=pec+potentiometer


there are also bourns  and clarostat   and others   

[Mike_J]

Attached is my first effort at using MS-Visio.  A bias board layout for my AB763 - 1 Channel build.  I know the pot is backwards versus what Doug has on his layout but I check voltages on the power tubes and adjust the pot for the most negative voltage before I put tubes in so it doesn't matter to me which way I turn the pot to balance the bias.


The reason a separate bias board is needed is to make space for the reverb driver between the tubes.  My chassis is large so it is not an issue so there is room to make changes and I like the way Doug's main circuit board wiring interacts with the tubes so I wanted to maintain that relationship.  Still working out spacing for larger tone stack and PI caps.


Thanks
Mike