DC Filament Supply

[stokes]

Trying to rectify my 6.3 filament to dc.What is the best way to accomplish this.I have no ct on the tranny for the 6.3 tap.Can I just use 2-100ohm resistors for a ground reference?Anybody have experience with doing this?Thanks in advance for any suggestions.This is going to be used in a stand alone reverb which is kind of a cross between a Fender type and the Ampeg.

[Ray Ivers]

This circuit works great IME, as-is - "improve" it at your own risk:

http://www.schematicheaven.com/marsh...5_15w_4001.pdf

Ray

[Tom Phillips]

BTW, are you planning on using DC on just the pre-amp tubes? I wouldn’t bother for the power tubes and PI.

I'd use a FW bridge rectifier on the 6.3 Vac winding and filter with a big cap such as 3300 to 4700uF / 10 V. Problem is that you will end up with almost 9 Vdc which you will need to reduce before it hits the tube heaters.
To reduce the voltage you could:
1) Add series resistance to drop the excess voltage. PRO = easy & straight foreward, CON: voltage rises if you change the load by pulling a tube. This could damage the remaining tube(s).
2) Add series diodes to drop the excess voltage
3) Add a regulator

If you have an un-used 5V winding you’d be better off to use that. Your filtered DC voltage would be just over 7 V and could be dropped to safe range with the resistor method. Then power the non-critical tubes off the 6.3 Vac winding.

For the balance circuit you can use the two 100 Ohm resistors on the DC side or you could reference the DC heaters to ground by grounding the negative side to the chassis.
Note: If you use the series diode dropping technique then split the diodes equally in each leg (facing the correct way) to keep the circuit balanced.

Hope that helps. Food for thought anyway.
If you have TUT 1 there are a couple of pages on DC filament operation.

Regards,
Tom

[Tom Phillips]

Ray,
I see that you posted a neat circuit reference wail I was writing. I question how the designer figures that the DC voltage is only 2*3.25V = 6.5V. Seems like it would be much higher. Maybe no one checked and it didn't turn out to be a problem.
Tom

[Ray Ivers]

Tom,

I wondered the same thing the first time I saw it, but it turns out that insignificant-at-HV 1.2V diode drop in a FWB - combined with diode efficiency, PT regulation, etc. - all become quite an issue at these low voltages. I think I recall measuring about 6.4VDC in my 4001, with fairly substantial - but balanced - ripple, all of which apparently cancelled in the 12AX7 heater as it was really very quiet. I added the "at your own risk" thing in my original reply as this circuit just begs to be changed - artificial-CT elevation! more filtering! - both of which I tried, and both of which failed, at least for me.

Ray

[Tom Phillips]

Ray,
Ah yes...we are used to ignoring the diode efficiency when working with the HV supply.
Tom

[stokes]

Thanks for that schem,Ray.It worked fine.I got 7vdc with no load it came down to 6.5 w/load of 1-6CG7 and a 12AX7.Got rid of the hum from this rev.It's gonna make de-bugging this thing that much easier.BTW I am using 4x1n4007 diodes with a 1000uf cap.

[carl gigun]

In a reverb unit I question the need for a DC filament supply in the first place. High gain amps might be a different story, but in something like this it's easy to build an amp with no audible hum at all with normal AC heating.

[Ray Ivers]

OK, great! For such a simple supply, it really works pretty well - and as you pointed out, now there's one less thing to worry about hum-wise.

Ray

[stokes]

Carl,this thing was humming like crazy before the dc mod.I am using an extra reverb tank I had from an old Ampeg.So I am trying to make a stand alone reverb that has a circuit similar to the Ampeg-no rev.trans.I have done some mods to the circuit to make it usable in front of the amp.So far the input is similar to the Ampeg input and the out put is closer to the Fender stand alone unit,still trying to tame the out put some,so getting rid of the 60 cycle hum is a great aid-one less noise to worry about.For what it costs to make a dc fil.supply I think it would be beneficial in any effect unit you are going to plug into a tube amp.The slightest amount of hum in the effect is going to be amplified when it hits the amp,so it is a lot like making the first stage of a high gain pre-amp dc.

[carl gigun]

I get your point about the value of a quiet stage at the front of the amp, I just haven't had that problem. Did you try anything else before deciding on DC heaters?

[stokes]

I guess when I said hum like crazy it was a bit of an exageration,the thing has a lot of gain so I am tweaking it to get that down,having so much gain the hum seemed worse,with the dc heaters the hum is gone completely.This is my first tube effect build so it is mainly just an experiment of sorts.I've built a number of amps over the years but never got into effects till now,got no room for more amps so I figured I'd try some effects.For what it costs I think dc heaters are well worth it in an effect.

[yankeerob]

BTW, are you planning on using DC on just the pre-amp tubes? I wouldn’t bother for the power tubes and PI.

In my case I'm building an overdrive head with a 12AX7 first and second stage preamp tube and a 12AT7 that has the first triode being used as a gain stage and the second triode as a split load PI - so I'm kind of leaning towards wanting that to be DC heated... if not for any other reason but keeping that first triode quiet - this is good thinking, no?

I'd use a FW bridge rectifier on the 6.3 Vac winding and filter with a big cap such as 3300 to 4700uF / 10 V. Problem is that you will end up with almost 9 Vdc which you will need to reduce before it hits the tube heaters.
To reduce the voltage you could:
1) Add series resistance to drop the excess voltage. PRO = easy & straight foreward, CON: voltage rises if you change the load by pulling a tube. This could damage the remaining tube(s).
2) Add series diodes to drop the excess voltage
3) Add a regulator

Anybody know of a suitable regulator? I have found SMC packages to do the job but can't find anything else and don't want the hassle of building a PCB to mount it on so the safest available method is gonna be series diodes - is there a particular diode anyone would suggest?

If you have an un-used 5V winding you’d be better off to use that. Your filtered DC voltage would be just over 7 V and could be dropped to safe range with the resistor method. Then power the non-critical tubes off the 6.3 Vac winding.

I'm going with a 5U4 rectifier that's consuming 3 of an available 3.5A - I'm guessing that using the 5V supply for anything else but the rectifier tube is out of the question?

For the balance circuit you can use the two 100 Ohm resistors on the DC side or you could reference the DC heaters to ground by grounding the negative side to the chassis.
Note: If you use the series diode dropping technique then split the diodes equally in each leg (facing the correct way) to keep the circuit balanced.

I'm gonna have a couple KT66's consuming 2.6A of the 5 available on the 6.3V supply before rectification - I need to heat the aforementioned 12AX7 and 12AT7 and possibly another 12AT7 for an all tube FX loop as well - that's .9A for all three - any ideas of what I'm gonna need to balance this thing out?

Best regards in advance,

Rob

[Enzo]

If you have a 5U4, then the 5v winding will be floating at B+, so DO NOT use it for anything else. If you had used a SS rectifier, then the unused 5v winding would be freee for any purpose.

[yankeerob]

DC Filament Supply

Thanks Enzo! I've decided to go with a Weber copper cap on the 375-0-375 (220mA) for the B+ (cheaper than glass in the long run and the right one will allow me to use EL34's, KT66's or 6L6's), 6.3V on the power tubes and a dedicated DC supply from the 5V (3.5A) for a 12AX7 and two 12AT7's... now all I have to do is figure out how to derive that - any suggestions are more than welcome - thanks for posting!!!

Best regards

Rob