Practical questions on DC heater circuit

[Luthierwnc]

Hi All,

I have a persistent heater-hum problem on an amp and am looking at putting in a DC supply for just the preamps. Lead-dress is good and everything is grounded where it is supposed to be (except maybe the current filament center-tap).

The power transformer has three 6.3 volt taps. One I use for the relay supply. That one has a voltage doubler. One is unused except for lighting the LED. That's the one I'd use for this project. The third is a heavier, center-tapped circuit that is currently running all filaments. I'm looking at elevating that with a hum-balance pot but it will be separate from the preamp heater circuit. All three are overbuilt for their applications.

The first question is whether I should use another doubler and go 12v to the pins or convert the 6.3 VAC to around 6.1 VDC. I have circuits for both on file and have used both 6 and 12 before (although the 12 came from a 12v tap). Real estate is something of an issue but I can cram it in there if it makes sense.

The second question is whether I should put the 12AT7 PI tube on the elevated circuit or the preamp circuit. I doubt it matters but you might have a stronger opinion.

That's about it. Thanks as always for looking, sh

[M Fowler]

I would try elevated dc on the filament first.

[Luthierwnc]

Thanks Mark -- that was quick!

I've seen component values all over the place. This is a compromise. The values are negotiable if you have a better idea. The amp is a D-style 2nd gen and has an internal loop so helping the cathode follower is also part of the thinking.

Skip

[Meat&Beer]

Those are the exact values I've used for about 55VDC with a B+ of 375 at the first (main) filter cap. Also with a pair of 100ohm precision R's. I'm not sure what your B+ is though, so it'll more than likely be a bit different.

What I'm wondering here, is that you mentioned space is a premium. You have that 22uf there "because you have one." Does that mean, because that's the value you have, of just because you have a cap you can put there? But, do you need that cap there? Are they usually needed for DC elevation like this?... (Actually a question, not aiming to spark philosophical thoughts here. I don't use one, so there's that.)

No cap takes up less space, is my point.

[roberto]

Yes, a small cap is needed.
I usually use 47u 200V, just because I have some and they are small.
It depends on the B+, but I usually go to around +45V with a 1M into 100k voltage divider.

[matt h]

(deleted)

[Luthierwnc]

B+2 (post choke) voltage is about 425 VDC.

I've got a bin full of caps. 22uf kept coming up in my searches but mine were either cathode bypass caps around 100v or filters at 400+ so I went with the latter. I've got a bunch of 47's but hadn't seen that value much. I Googled lots of 10's though and I've got those too.

I was more concerned about the resistor values on the VD. Those range from 1m/10k to combinations with about a 5:1 ratio (like mine). 470k/47k came up a couple times.

I'll punch a board tonight and give it a try.

Thanks all, sh

[martin manning]

You'd be fine with a 100V cap there.

[bruce egnater]

I have tested both the elevating and DC and find the DC is a 100% solution while the elevating is still somewhat affected by the filament balance (or lack of) in the tube itself. If you don't put the AC noise into the tube (DC filament) there is not chance of internal (or external) coupling. You can often apply the DC to just the first, most sensitive stage, and leave the AC on the others. Yes, you can add a hum balance control in addition to the elevating supply parts but at that point it seems to make sense to just use the DC supply.

[roberto]

I have to say I've never had problems elevating heaters by 40-50V through the central tap of the heaters' winding. This also helps with the cathode to heater maximum allowed voltage, while DC heaters doesn't.

Don't take this post against DC heaters, it's just MHE.

[trobbins]

sh, I don't think you have confirmed that the hum is generated only at the first input tube stage?

I note that you appear not to have at least tried a humdinger pot, and only used a fixed humdinger. You also haven't apparently tried different input valve samples. Those very simple efforts may not only give a solution, but will at least provide info on where the hum may be predominantly entering your circuit.

You should also attempt to clarify if the hum is fundamental mains, second harmonic, or predominantly higher frequency hash from the rectification of HT supply.

Heading off to a specific circuit modification is somewhat a knee-jerk reaction imho.

Ciao, Tim

[Luthierwnc]

I got the circuit installed and it did help but there is still an underlying hash (buzz) that is coming from the first gain stage. I guess there were at least two things wrong. I'll poke around and see if there is a lead-dress issue.

I've got a .01 630v cap on a wire with a grabber on the other end. Touching the treble, middle or bass caps kills about half the signal. Touching the .05 on the second half of the tube kills it clean. With the volume pot off or the guitar volume off, there is no noise. It ramps up a lot when the boost and OD are engaged but I'm pretty sure it is coming from V1a.

One of the more confusing aspects of this is that the waveform looks pretty good all the way through. I'm running a 1k sine and looking at on the scope. Other than some clipping, it's fairly wholesome.

Most of the noises I've dealt with in the past were hums. A buzz is something new. If anyone has a suggestion for uncovering the causes of them, my door is open.

Thanks all, Skip

[trobbins]

Hash can sometimes be lowered by a humdinger pot nulling some of the signal leaking in. That is a relatively simple test.

[Stevem]

Did you say that there is NO noise issue with the guitars volume off?
Here is the test to first off pin down if noise of any kind is coming from the amp,PLUG THE GUITAR IN, CRANK EVERYTHING FULL UP ON THE AMP AND THEN TURN THE GUITARS VOLUME OFF, ANY NOISE THAT IS LEFT IS FROM THE AMP!

[bruce egnater]

Of course you can elevate the heaters regardless of whether you use DC or AC.