Raising heaters

[daveyajd]

Hello,
So the PT I just got for my Dirty Shirley build does not have a center tap on the 6.3v. My plan was to raise the heaters by connecting the center tap to a 16uF cap. My question is whether I can now create an artificial center tap with a couple 100 ohms and ground those at said capacitor? Is it really that easy and can I expect the same results?


Aaron

[PRR]

> center tap with a couple 100 ohms

Yes.

[2deaf]

Hello,
So the PT I just got for my Dirty Shirley build does not have a center tap on the 6.3v. My plan was to raise the heaters by connecting the center tap to a 16uF cap. My question is whether I can now create an artificial center tap with a couple 100 ohms and ground those at said capacitor? Is it really that easy and can I expect the same results?

If the 16uF cap is a filter cap in the power supply, I don't think you want to raise the heaters that high.  It might be better for the cathode follower, but it will exceed the maximum heater to cathode voltage on all of the other triodes.

[Latole]

> center tap with a couple 100 ohms

Yes.

This is the best thing to do. 2 X 100 ohms 1/2 watts to ground.
I used that very often over years

[kagliostro]

My plan was to raise the heaters by connecting the center tap to a 16uF cap

I don't understand if you consider to connect the virtual CT only to a 16uF cap or to the 16uF cap present on the PS of your amp

A pair of examples of what you can do
(note that you can connect the CT or Virtual CT to the cathode of the Power Tube/s only if the amp is Cathode Bias, NOT Fixed Bias)





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You can use 100R resistors or a bit larger as 220R ... 330R

Franco

[daveyajd]

The amp I’m building is a modified JTM 45 of sorts. The real amp uses a 16/16 can cap with one side connecting to the PI and the other side raising the heaters. I will try to post a pic but the file is to large right now.


Aaron

[kagliostro]

Ciao Aaron

OK, I've understand, no necessity to post a photo

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BTW, a way to publish a large image is to use an image host

I use IMGUR (https://imgur.com), it is free, didn't require registration and they don't delete images when a few time is past

when you put the image on the host, copy the link and use the image tag on the post on the forum to attach it to your post

Franco

[daveyajd]

I’ve always run my HT center tap to ground. But in looking at some pics of a Dirty Shirley gut shot it looks like they reference that CT to pin 2 of the rect tube. Does that sound reasonable and is that done just to help with noise issues like raising heaters?


Aaron

[sluckey]

That does not sound reasonable.

[daveyajd]

Hahaha! I always love the succinctness! I *think* what I’m seeing is the CT connecting to the fuse holder above. Just looks like one big solder connection from the angle in the pic.


https://imgur.com/a/BktLr3l

Aaron

[2deaf]

I see the heater CT connected to one of the capacitors in a can and a 470K resistor between that capacitor and B+.  The other half of the voltage divider is not visible.

[daveyajd]

The other half of the voltage divider is not visible.

This would be the 8.2k resistor on the other end of that blue wire, right?


Aaron

[2deaf]

This would be the 8.2k resistor on the other end of that blue wire, right?

I don't see an 8.2K resistor in the picture.  This is how I think it goes:

[2deaf]

The 8.2K is probably a dropping resistor in the power supply.

[daveyajd]

Yeah you’re right I don’t see that 68-100k resistor in that picture. I need to get a different angle from my friend. Thanks for the knowledge!


Aaron

[tubeswell]

Ideally, you want the voltage reference point for the heater winding elevation to be as steady as possible. Running the 2 x 100R to a cap probably isn't as steady as running them to a fixed DC elevation point that has a cap to shunt any AC to ground. - like a voltage divider output that has a bypass cap attached, or (a) bypassed cathode(s) for the power tube(s).


(Edit: oops - I see Franco already said as much  )

[daveyajd]

I’m on the homestretch with my build and I’m using this cap to raise the heaters. Purple arrow will be my PI node. Green arrow will be my heater side. The amp I am cloning uses a 470k between the sides and an 82k to ground on the heater side. My voltages will likely vary so my question is this. Should I just ground my artificial CT at ground, fire my amp up to see what voltage I have at PI then decide what values I will need? Are those values pretty typical? What, if anything am I missing in this setup because I have never referenced heaters to anything but ground.


Aaron

[PRR]

The obvious path is to reference heaters to ground (it does work, and you have done it), get voltages, hear how it sounds, *then* plan your next move.

[daveyajd]

Ok I have finished my build. I will post gut shots in the forum soon. Attached is my voltage chart. It looks like I have about 360 on that PI node. How do I figure my values for the voltage divider? What voltage should I aim for on the heater side?


Aaron

[shooter]

get voltages, hear how it sounds, *then* plan your next move.

you skipped step 2 above 

[daveyajd]

No I haven’t. It sounds like I may want to raise the heaters.   There is an issue with my input jacks I will need to remedy but it sounds pretty good.



Aaron

[2deaf]

How do I figure my values for the voltage divider? What voltage should I aim for on the heater side?

50V is a nice round number to start with.  40V to 60V is the range I would use.

 

[daveyajd]

Alright awesome! So by that formula using an 82k for R2 should give me about 53v. Thank you!


Aaron