Where to place the humdinger pot?

[Mike_J]

Looks like the best place to install the humdinger pot for ease of access is between the power and preamp tubes. Will be floating DC on top of the heaters and using a humdinger pot. Can anyone confirm that the humdinger can be inserted anywhere along the heater string.


Thanks
Mike

[PRR]

Don't go NEXT to the preamp tubes. Amongst the driver tubes is fine.

[Mike_J]

Don't go NEXT to the preamp tubes. Amongst the driver tubes is fine.

Thanks PRR.

Have created the attached AB763 Power Supply Layout. The purpose of the layout is to place components using the MS-Visio format before cutting holes in a chassis or power supply board.


Objective is to make a studio quality amp that is quiet but at the same time maintains the Deluxe Reverb tone. Accordingly, two resistors and a cap were added before the power supply caps to introduce DC into the heater supply thereby floating voltage above ground potential. I have heard that can quiet an amp. Also heard using a humdinger pot instead of two 100 ohm resistors or center tap for the heater supply can at times be useful so I added one of those too in the form of a 250R 5W wire wound pot.


Making the layout resulted in a few questions. First, the amp uses a 5AR4 rectifier tube. As I recall a 47uF electrolytic cap is pretty much max for a 5AR4. Does the cap added for the DC in the heater supply create a problem because it is 47uF as well? Secondly, have seen schematics that include diodes (IN4007) in front of the rectifier tube. I presume the red wires would go to pins 5 and 7 instead of 4 and 6 and IN4007s would be used to bridge to pins 4 and 6. Does anyone do this? Seems like it could prevent problems if the rectifier tube died.


Lastly, can anyone see a problem with the layout?


Thanks
Mike

[Willabe]

Accordingly, two resistors and a cap were added before the power supply caps to introduce DC into the heater......

I would not take the dcv stand off from where you propose. That's the nosiest, has the most acv ripple, B+ node in the amp's B+ dcv string.

I'd take it from the purple or maybe as a 2nd choice the blue. And you don't need a 450 cap for it. Most guys elevate the heaters ~30dcv to ~40dcv. I like to go to ~70dcv to ~80dcv. The voltage divider (2xR's) knock down the B+  to your stand off dcv, so 100v E-cap is all you need. I've been using a 180v E-cap cause it's easy to source.   

Also I see 1 small mistake on your drawing, the red B+ wire is shorting out your choke, see drawing below.)

See hi-lighted drawing below. (Edit; I took down the drawing, had 2 mistakes, too tired to fix it right now. Not feeling great went to an old friends wake today, sorry guy's.)

[Mike_J]

Accordingly, two resistors and a cap were added before the power supply caps to introduce DC into the heater......

I would not take the dcv stand off from where you propose. That's the nosiest, has the most acv ripple, B+ node in the amp's B+ dcv string.

I'd take it from the purple or maybe as a 2nd choice the blue. And you don't need a 450 cap for it. Most guys elevate the heaters ~30dcv to ~40dcv. I like to go to ~70dcv to ~80dcv. The voltage divider (2xR's) knock down the B+  to your stand off dcv, so 100v E-cap is all you need. I've been using a 180v E-cap cause it's easy to source.   

See hi-lighted drawing below.



(Also I see 1 small mistake on your drawing, the red B+ wire is shorting out your choke, see drawing below.)

Thank you for your comments Willabe. Good catch on the wire bridging the choke. Don't know how I missed that. Still confused about what you are saying otherwise though. Shouldn't the red wire from the 180K resistor to the B+ cap get eliminated? Then wouldn't I run the wire from either before or after the 1k resistor to where the red wire is currently connected to the 180K resistor? Sorry for my ignorance. Appreciate the tip on using a lower value cap. Have a 47uF at 160V cap on hand that would work well there.


Thanks
Mike

[Willabe]

I'm really Not trying to be a jerk but I have no idea what your talking about. 

It's sooooo much easier for me to see a schematic than a layout. Yes if your trying to finalize a layout you need to post that but for me I still need to see the schemo.

Sorry but I have to go get ready to go to a wake.   

[Mike_J]

I'm really Not trying to be a jerk but I have no idea what your talking about. 

It's sooooo much easier for me to see a schematic than a layout. Yes if your trying to finalize a layout you need to post that but for me I still need to see the schemo.

Sorry but I have to go get ready to go to a wake.   

You aren't the only person that prefers a schematic. Updated the layout to reflect what I was speaking of. Will prepare a schematic and forward it separately.


Thanks
Mike

[Mike_J]

Attached is the schematic for the Humdinger and DC to the heater supply. It is taken from after the 1K power resistor.


Thanks
Mike

[PRR]

The wire from standby to first filter cap is "dirty".

You have it run around the left of the cap board. This is probably near your PI/driver. Not a real sensitive stage. But why run a sewer that way if you aren't forced to?

If you run it around the right, essentially back-tracking itself, it lays with the dirty-wires around the rectifier.

Me, _I_ would bring the PT HV CT lead directly TO the first filter cap. Then a jumper to the chassis ground. That's another dirty wire and the route you show injects buzz to the ground. Yes, many many amps have been built your way and essentially buzz-free. It is more an issue in HIGH current transistor amps than anything with a vacuum in it. Your choice.

[Mike_J]

The wire from standby to first filter cap is "dirty".

You have it run around the left of the cap board. This is probably near your PI/driver. Not a real sensitive stage. But why run a sewer that way if you aren't forced to?

If you run it around the right, essentially back-tracking itself, it lays with the dirty-wires around the rectifier.

Me, _I_ would bring the PT HV CT lead directly TO the first filter cap. Then a jumper to the chassis ground. That's another dirty wire and the route you show injects buzz to the ground. Yes, many many amps have been built your way and essentially buzz-free. It is more an issue in HIGH current transistor amps than anything with a vacuum in it. Your choice.

Thank PRR


Made the changes you suggested and have attached the modified layout. Any opinions as to whether it is better to run the DC for the heaters from B+ or down the line? Thinking about putting .1 @ 630V caps across the electrolytics. Jojokeo said to use ceramics. Any reason why ceramics would work better? If I had them on hand I would certainly use the ceramic but I don't have any on hand. Need to find out if I need to order them for this build or can use Xicon PP with outside foil to ground?


Thanks
Mike

[Willabe]

On the dcv stand off, that 22K to ground, there I use a 100K so it saves a few mA's from bleeding off to ground.

I go with the 100K and then for the other R of the voltage divider, I find the value R I need to get the dcv I want.

 

[Mike_J]

On the dcv stand off, that 22K to ground, there I use a 100K so it saves a few mA's from bleeding off to ground.

I go with the 100K and then for the other R of the voltage divider, I find the value R I need to get the dcv I want.

Thanks Willabe


Do you think it is otherwise wired properly in accordance with the way you wire yours? Seems like every schematic I have seen regarding drawing DC for the heater supply takes voltage from the B+ tap. However, it makes sense that you could take it from anywhere along the power string. Read somewhere that you are limited as to the amount of filtering you can place on your B+ according to the vacuum tube you are using which is a given. However, if you use a choke of  4H or more then you can use large values in the screen or preamp areas without damaging the rectifier tube. Taking the voltage from after the B+ is consistent with that as there should be no need to be concerned about having an additional 47uF cap in line with the power supply. Don't know if there is any actual logic to what I am saying but it makes me feel like I am making a better amp by taking the voltage after the B+.


Thanks
Mike

[ac427v]

I notice that the circuit for floating the heater ground comes downstream from the standby switch. Will that cause any problems when the switch is used?
Craig

[Mike_J]

I notice that the circuit for floating the heater ground comes downstream from the standby switch. Will that cause any problems when the switch is used?
Craig

Thank you for the reply Craig. I don't think it will matter because the B+ is also switched by the same standby switch. Only difference I see is the power downstream is a little lower and cleaner. May be a problem with the ground the way it is shown in the layout. Probably should have the B+ cap ground go to chassis ground and run a wire from the ground for the two resistors and cap feeding the heaters DC to the cap feeding power to it. That way there is no risk of the grounds having to cross over each other.


Attached is an updated layout diagram reflecting a change to the ground placement on the power board.


Thanks
Mike

[Mike_J]

Cleaned up the board. Improved layout for the bias supply secondary wires so all secondary wires come out on the right side of the transformer (farthest away from the preamp circuitry). Improved labeling and increase font size of letters so they could be more easily read.


Think this is the way it will be made.


Thanks
Mike

[Mike_J]

Here are pictures of the power supply portion of the build.

[Willabe]

Do you think it is otherwise wired properly in accordance with the way you wire yours?

Yes, that's how I would wire up. (I got it Kevin O'Connor's TUT books, IIRC, Kevin taps the dcv from the screen B+ node?)   

Seems like every schematic I have seen regarding drawing DC for the heater supply takes voltage from the B+ tap. However, it makes sense that you could take it from anywhere along the power string.

I don't understand that. Why would we want to inject whatever noise is found/present at the 1st B+ filter node into the heaters? Maybe it doesn't matter? Merlin doesn't think it matters.

Read the link on heaters, if you haven't yet, below from Merlin;

http://www.valvewizard.co.uk/heater.html

Taking the voltage from after the B+ is consistent with that as there should be no need to be concerned about having an additional 47uF cap in line with the power supply.

The stand off dcv doesn't draw any current itself, the only current that's drawn by the dcv stand off circuit is the charging current for 1/only filter cap it has and the dcv tail R in the voltage divider. After that cap is charged up in the 1st few seconds there's no current being pulled to it/from it except what ever is bleed off to ground through the voltage divider tail R. So it doesn't keep cycling through big charging cycles like with the power tube B+ supply node; charging up/current drawn from it/charging up/........., it has a small steady current draw limited by the dcv stand off voltage divider tail R and what ever very small leakage current the cap has.

[Willabe]

Where are the 6V6 grid leak R's going to be where you inject your -grid bias dcv?

Is the blue wire going to the right from the -bias dcv supply board the -grid bias dcv wire?

 

[Willabe]

You might have problems with having your B+ filter cap board right in front of your power tubes.

And you have the power tube grid wires running directly right under that board.

[John]

Looking at your pics, and just FWIW, I've had to cram things in small enclosures. I have found in my limited experience that keeping wires as short as possible, and arranging things to the "flow" of the current helps. I'd have the ground side of your filter cap board pointing towards that double pot, for instance.

[Mike_J]

Where are the 6V6 grid leak R's going to be where you inject your -grid bias dcv?

Is the blue wire going to the right from the -bias dcv supply board the -grid bias dcv wire?

The grid leak resistors are part of the dual pot. The dual pot is a Lar-Mar PPIMV. There is a two-conductor cable that goes from the wipers of the dual pot to the 1.5K resistors before the grid. One if the wires is red the other is blue in the layout diagram they are white with bands on them in reality. Otherwise the only blue wire is coming from the DC power supply that provides DC for the heaters. This amp is based on the Hoffman AB763 one-channel design.


Thanks
Mike

[Mike_J]

You might have problems with having your B+ filter cap board right in front of your power tubes.

And you have the power tube grid wires running directly right under that board.

I hope there won't be a problem with the filter cap board. The ground side of the caps are over an inch away from the tube socket. Hopefully the shielded wires will prevent interference from the cap board.


Thanks
Mike

[Mike_J]

Looking at your pics, and just FWIW, I've had to cram things in small enclosures. I have found in my limited experience that keeping wires as short as possible, and arranging things to the "flow" of the current helps. I'd have the ground side of your filter cap board pointing towards that double pot, for instance.

This is a pretty big chassis. It is 25-1/2" wide. Agree with keeping wires short. The double pot is quite a few inches from the dual pot. Don't think the cap board will cause a problem. Biggest concern is how long the wires going from the dual pot to the power tube grids are. These are supposed to be as short as possible in order to avoid oscillation problems. However, in order to have the master volume control on the front of the amp there was no other choice. I am hoping the shielded cable will eliminated the oscillation problem as well as interference between the grid wires and the power cap board.


Thanks
Mike

[Mike_J]

Here is a picture showing the master volume pot in relationship to the power cap board. The board to the right of the pot is the bias circuitry.


Thanks
Mike