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Saurav 9th September 2003 04:04 AM

When using a DC filament supply on a DHT...
... should the cathode bias resistor (and bypass cap) be connected to the higher voltage end of the filament, or the lower voltage end? Does it make any difference? Any pros/cons for either approach?

SHiFTY 9th September 2003 04:46 AM

Best way is to use a couple of resistors, about 25-47 ohm in series between the filament ends. Then connect the cathode resistor to the junction of the two.

Sch3mat1c 9th September 2003 05:12 AM

It makes the difference of [heater voltage] in bias. I don't remember which end it is, but tube data for all DHTs, unless noted otherwise, is actually spec'd for DC fil. with one end grounded.


Saurav 9th September 2003 06:40 AM

2 Attachment(s)
Well, this has pretty much been one big fat failure so far :( I got the regulated DC filaments working, but my noise/hum level at the speakers is still the same as before. Pretty ironic, huh. I knew my layout was iffy, but I put the regulator board in the only place I could fit in.

The photo below shows what I have. The bottom left has the filament transformer, I tried covering the leads with copper tape (that's the brown/orange stuff you see) but it didn't make any difference. The regulator board had to be put between the 2A3 and the 6SL7 sockets. So, it's either radiating stuff that's being picked up by the input stage, or it's the fact that my AC filament wires are sitting right next to the OPT/speaker terminals.

It's starting to get pretty crowded in there. And even with the heatsinks, these are getting hot. Especially the brown tie-wrapped sense resistor, I'm not sure if the plastic tie-wraps are a good idea.

Anyway... I'm going to bed now, and hopefully I won't feel so discouraged tomorrow :) Any troubleshooting advice would be most appreciated.

Paul Barker 9th September 2003 08:34 AM

I can't solve your hum problem, but I just wanted to say that I think it is in the rca 211 data sheet we read that the ov of a dc heater supply is grounded. This results in a bias voltage of -5v in the case of the 211 (1/ the 10 volt plus whatever other bias we adopt.

In dc heating some retatain the hum dinger seen on most ac filaments. Using this would mean the bias voltage is unaltered. This is a refinement of the two resistor method.

I use the two resistor method for ac heating of indirectly heated grounds, but for my dht's I ground the 0v as per RCA. I have no idea if there is any efficacy in this, just follwing rca like a sheep!

Paul Barker 9th September 2003 08:35 AM

addendum, should read HALF the 10 volt. Somehow only 1/ and not 1/2 came out of my clumsy one fat finger typing!

SHiFTY 9th September 2003 09:48 AM

I have no hum on my SE 6B4G amp. However I did, before I used star grounding, the best hum reduction method ever...

Basically, get some single core copper wire, and an earth busbar, and run one wire from the busbar to all power supply grounds, another wire to all cathode bypass grounds, another wire to all signal grounds (ie input sockets and volume pot) another to output transformer secondary, and another to chassis, and another to AC mains ground. And so on....

If you still have hum after doing that, then you may need to play around with transformer and choke placement...

I built a single ended amp using ECL82, that was much, much more crowded than your amp, and it still had no hum. It must be possible! Check your solder joints, and make sure your preamp tubes are referenced to ground somehow...

Saurav 9th September 2003 02:12 PM

I'll go over my solder joints again. I also realized last night that the LT1085 datasheet said I should put a 10uF cap after the regulator, I haven't put that in there. Maybe that could make a difference. I also have about 2mV of AC ripple in my filament supply, and the cathode bias resistor is at one end of the filament (I think the high end) - could that be causing it? Grounding - that hasn't changed from what it was before. I had 2mV hum with AC heaters, and I should be able to bring that down with DC heaters, darn it :)


Saurav 9th September 2003 04:17 PM

OK, things are looking a little better this morning (I'm late for work because I was experimenting with the amp as soon as I woke up :)). I might have been chasing the hum in the wrong place. I put in the "Tubecad hum cancellation cap" (for want of a better term) between B+ and the cathode, and the hum went down noticably. It shows 1mV on my DMM now, which is probably the limit of its resolution, so I'll check with the scope tonight.

I had tried this cap earlier but hadn't heard much of a difference, this time the difference was quite noticable. So, I guess my filament hum is lower than it used to be, which is why I can hear the effect of changes in the B+ hum. Maybe I should try those IXYS regulators in the 2A3 B+. More hot stuff under the chassis...

... which brings me to power/heat dissipation questions. How hot is too hot on a heatsink? It's probably not clear from the photo, but I used smallish heatsinks on the Schottkys, and a larger heatsink on the regulator. I've run the amp for about 5-minute stretches so far, and the Schottkys' heatsinks get a little warm, and the regulator heatsink gets hot enough to be uncomfortable to touch. I've used mica insulators and thermal paste on all heatsinks. I think a bigger issue is airflow, some of the heatsinks are right up under the chassis.

Also, the 1 ohm sense resistor for the regulator is getting pretty hot, this is one of those cylindrical ceramic types. I have it tie wrapped to the chassis for now. Maybe I should look for metal-clad 1 ohm resistors and try and bolt them to the chassis.

Gabevee 9th September 2003 08:55 PM

Hi Suarav,

I had the same troubles once upon a time when I made my first DHT amp. Too much hum.

Here is what my situation was: I was using lots of filter for B+ rectification (330 F after 1.5H choke). I was using AC filament. I was using a hum pot for the DHT filament, with cathode resistor/capacitor at the center, but at best I got about 70-100mV hum.

My solution to the problem was to use DC at the filament (bridge rectifier and 10,000F cap) but still use the hum pot. It still had some hum at either end of the hum pot, but when set properly hum was totally gone. I mean, noise (hiss) is now louder than hum.

Why? I had been thinking about this since working on another amp with AC on the filaments and virtually no hum.

I use lots of filtering to reduce ripple on DC to almost nothing. I mean, the B+ has about 0.5mV of ripple. Therein I believe lies the problem. I tried to do this with the other DHT amp with AC filaments and actually got more hum! So, my thought is that the hum from B+ is actually being cancelled out in the output transformer by a little hum introduced at the filaments! Hence the need for the hum pot.

Since I have virtually no hum at B+, I get hum from the filaments even with a hum pot, because there is no hum opposite in polarity at the output transformer to cancel out! So my amp needs DC at the filament, with just a little bit of hum pot adjustment to completely cancel out the remaining B+ hum.

Wild and fun stuff!:nod:


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