This circuit works great IME, as-is - "improve" it at your own risk:
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.
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.
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.
Last edited by Tom Phillips; 09-04-2006 at 06:19 AM.
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.
Ah yes...we are used to ignoring the diode efficiency when working with the HV supply.
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.
Last edited by stokes; 09-04-2006 at 05:18 AM. Reason: Left out part of the reply
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.
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.
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.
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?
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.
Best regards in advance,
Last edited by yankeerob; 01-06-2008 at 04:25 PM.
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.
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!!!
And for what it's worth, DC filaments in a guitar preamp is so unnecessary that it astounds me that people are still trying to figure out how to do it using the 5v winding with a FWB and cap filter.
IMHO... a tremendous waste of time and energy.
Denver, CO. 80022
"And for what it's worth, DC filaments in a guitar preamp is so unnecessary that it astounds me that people are still trying to figure out how to do it using the 5v winding with a FWB and cap filter.
IMHO... a tremendous waste of time and energy."...Amen to that.
+1! You only need DC filaments if you're trying to make a 5150 gain monster. And even then, I've made gain monsters with AC filaments and the hum ended up below the noise floor with careful tweaking of a hum balance pot.
Also, if the first gain stage of an overdrive amp is sharing a bottle with anything other than the following gain stage, you're asking for oscillation trouble due to capacitive coupling between the plates.
And yes, the Copper Cap is just two diodes and a resistor in a purty looking copper can. Great for filling the rectifier socket of an existing amp with something that looks presentable, but doesn't make much sense in a scratch build.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
Firstly let me thank you all for posting - I welcome all the comments so far - there are - however - some factors that need clarifying...
But the amp in question (which I might add is still on the drawing board) is a higher gain Orange overdrive clone - and has zero negative feedback so that's why I'd prefer - by design - to use DC heaters on the pre-amp tubes as I do the occasional session and hate modellers with a passion - there's just something big and ballsy about a 4 x 12 and a good mic - especially if you have the dynamic range of an overdrive head with ZNF... and can if you desire - incorporate bias range switching so you can safely switch the sag resistor in or out.
As an aside - DC heaters are common practice in many Matamps and similar derivatives (including Orange) that have no feedback loop - I also have access to some info on how they do it/done it in the past from the 6.3V winding but it involves a regulator that I - so far - have found difficulty in sourcing - though the man with the aforementioned info reckons he can get them - that remains to be seen... but I digress..
My thinking was rather than go with a 5U4 (and the expense and reliability issues with another piece of glass in my amp which I plan to gig with too) - why not go for what seemed like a compromise between a FWBR and a tube - ie a copper cap - but if you're telling me that all a copper cap is is a FWBR with a 'sag resistor' then that's the route I shall take when designing the power supply. Perhaps now you get a better idea of why I - having the luxury on this one-off occasion of an unused 5V winding - was thinking of using it for the DC filament supply - as it would seem - from all I've read so far - that this would be an easier route than tapping off the 6.3V supply...
Finally - I really appreciate the posts - and understand why you've said what you've said!
I use a twin dc regulator for the pre-amp and power tube filaments. I do to ensure a consistant 6.3 volts regardless of how much the input power voltage varies. Even on my bench without regulation, I've seen the filament voltage swing as low as 5.5 volts up to over 7.0 volts. I do this to ensure long tube life and consistant tone.
Wow, with 120v mains, meaning a 20 to 1 voltage ratio on the 6v winding, that represents a 15-20v swing either way on your mains.
In my lab, all the power is off a 10 amp breaker, and it has other loads on it beside just my bench. This much of a voltage swing may be more common than what you would want to believe....
I wonder, why not a zener diode after your bridge or half-wave? This assumes you need to do this at all.
After much experimentation in the past, I have deemed the DC heater scheme to be a waste of time, components and chassis real estate. A good, balanced and tightly-wound twisted pair is really all that is needed for hum reduction, and regulation is highly overrated. Most heaters are within tolerance at rated line voltage. Also, heaters just do not last as long on DC because they are always at peak voltage. AC heaters reach peak voltage 120x per second (at positive and negative swing) and get to rest at the zero crossing at the rate of 60Hz. It's just another high-tech approach for a low-tech "problem".
John R. Frondelli
dBm Pro Audio Services, New York, NY
"Mediocre is the new 'Good' "
hi ray,one question to you:I see in the schematics the 2 100ohm resistors..what if you have center tap on the transformer,do you simply omit the two resistor??I'd like to do this mod to an express trainwreck clone,which is too noisy!!
Yes, if you have a heater winding without a center tap, omit the 100 ohm resistors. They are to provide a "virtual" center tap for a winding that does not have one.
Humor is the best alternative to serial killing. - Chuck H
Theres a big advantage in using DC heaters with a high gain 0 feedback design. Not just the hum reduction either. You can wire up your build with a lot less hassle/time. Also you don't need regulators, the loaded voltage should be pretty much the same as with AC. If it's a little higher on your bench wait till it gets plugged into the circuit at the bar with the SVT, lights, PA and refrigerator. Where do you think the filament vlotage will be sitting then?
I'm building mostly high gain amps and I'm always using regulated DC heaters on all preamp tubes. I know it's an overkill but for few bucks in parts why not run them all? On top of that my heaters are elevated to 80-90V because some brands of tubes started failing when used as CF.
I've been working on a 4 gain stage amp for a while now and found that it is dead quiet, aside from high gain hiss, with elevated AC heaters. There was noise with elevated DC heaters with ~10,000 uf of filtering.
Humor is the best alternative to serial killing. - Chuck H
I know the tubes that fail but you can't build and amp for somebody and tell them not to use this or that - it's better to play it safe.You may want to make sure youre using ECC preamp tubes for cathode follower stages. 12A series tubes aren't rated as high for cathode to filament breakdown.
ECC83 for instance isn't a tube brand, it's a tube type. They were rated for series string filament operation so the cathode to filament breakdown voltage is much higher than a 12AX7. It has nothing to do with the brand of tube used.
And when the customer walks into the store for a new ECC83 and the salesman assures him it is the same thing as a 12AX7?
Education is what you're left with after you have forgotten what you have learned.
There are currently 1 users browsing this thread. (0 members and 1 guests)