Circuit for changing 6.3 VAC Heaters to DC?

[Decko]

Hi All,

Can anyone direct me to a schematic or resource about how to change the heater voltage from AC to DC? I would like to try running tube filaments off of DC voltage.

I have my own notion to use a couple diodes to rectify the AC then smooth it out with some filtering caps but I would like to get some ideas from the community.

Please advise...

Thanks,
Decko

[paulster]

Take a read of this thread if you've got the time as it describes a few different ways of getting DC heaters throughout it, with the associated pros and cons of each:

https://tubeamparchive.com/viewtopic.php?t=8236

That should hopefully give you some ideas of how you might want to approach it and then we can all help you some more.

[Phil_S]

It must be a full moon or something. I've been on 3 other boards today and each one of them had a DC heaters question, pretty much like this one.

So, I'll say it here, too. Why do you want to do this? IMHO, there is very little to be gained and it isn't worth the work. Most folks say something about hum. If hum is the problem, proper lead dress is almost always the answer. I'm not as accomplished as many, but I've built about 10 amps. All of them have AC heaters and none of them hum. Some of the earlier ones may even suffer from mediocre lead dress.

OTOH, if you just want to do it because you can, then have at it!

[stoo]

Maybe try the ol' Dc offset deally.
Stew

[David Root]

I use DC heaters a lot. It doesn't cost much either 6V or 12V and the higher the circuit gain stages are the more sensitive they become to hum.
If you only build tweeds no you don't need it.
If you use any kind of octal, or noval preamp tubes that don't have humbucking heater wiring, you will be well advised to use DC heaters, especially if they are hi-gain eg 6SL7GT or a pentode set up for hi gain.
Use a regulator chip after the rectifier like a fixed output LT1086 or even a 7806 or 7812 and you have 75 dB ripple rejection too.

If the Ancients had had 1N4007 diodes, cheap 25V 1000 uF caps and 7806/7812 regulators, they would have used them.

[David Root]

I use DC heaters a lot. It doesn't cost much either 6V or 12V and the higher the circuit gain stages are the more sensitive they become to hum.
If you only build tweeds no you don't need it.
If you use any kind of octal, or noval preamp tubes that don't have humbucking heater wiring, you will be well advised to use DC heaters, especially if they are hi-gain eg 6SL7GT or a pentode set up for hi gain.
Use a regulator chip after the rectifier like a fixed output LT1086 or even a 7806 or 7812 and you have 75 dB ripple rejection too.

If the Ancients had had 1N4007 diodes, cheap 25V 1000 uF caps and 7806/7812 regulators, they would have used them.

[Ripthorn]

I did DC heaters on my first (and only so far) build and it works great. I had to put about 3300uF worth of smoothing caps to make sure there was no hum, but with 5 gain stages, you have to be a little more careful. It was easy enough to do.

[Decko]

Why do I want to look at DC for the heaters?

Well I am on my 3rd build. After hours of cleaning up the lead dress, rewiringing and resoldering all the solder joints, elevating the heaters, dividing voltages, and providing a virtual ground reference via qnty 2 100R resistors I AM UNABLE TO GET RID OF THE HUM!

Finally, I disconnected the green 6.3 VAC leads from the filaments and connected a Duracell 6 volt lantern battery. The hum disappeared.

Paulster pointed me to a great link. Thanks!!!

I hear that DC voltages help the tube filaments last longer! Thats a plus especially for NOS tubes.

Any pointers would be helpful.

My amp is a high gain two watter.

Thak!
Decko

[Structo]

Are you using a center tapped heater supply or are you using a virtual center tap with two resistors to ground?

[Decko]

Hi Tom,

I am using a virtual center tap with two resistors to ground.

Thanks!
Danny

[paulster]

So, I'll say it here, too. Why do you want to do this? IMHO, there is very little to be gained and it isn't worth the work. Most folks say something about hum. If hum is the problem, proper lead dress is almost always the answer. I'm not as accomplished as many, but I've built about 10 amps. All of them have AC heaters and none of them hum. Some of the earlier ones may even suffer from mediocre lead dress.

Take a new tube like the Tung-Sol reissue 12AX7. Great sounding tube, but the heater design of the tube itself means it often hums. Not necessarily immediately out of the box, but it can happen after a week, a month, etc. and you've then got to write off an otherwise good sounding tube.

Not an issue with DC heaters, and lead dress has nothing to do with it, because it's a construction issue with the tube.

If you ever use any octal preamps then you're pretty well obliged to use DC because these tubes were not designed for AC heaters and will hum correspondingly unless you use DC.

[Andy Le Blanc]

I like the old danelectro trick, with two 6l6 or four 6v6, dress the two or first
two 12ax7 series and implement them as the cathode bias resistor. The
most gain sensitive stage has its heater on the ground side. It cuts dress time
to next to nothing, one wire for the heaters in the pre, its quiet, you can
drop a cathode resister from you BOM, and you can drop the stand by,
a 12ax7 was designed with a controlled heater warm up time and its a soft
start every time, DC heaters with less parts.

[Phil_S]

Take a new tube like the Tung-Sol reissue 12AX7. Great sounding tube, but the heater design of the tube itself means it often hums. Not necessarily immediately out of the box, but it can happen after a week, a month, etc. and you've then got to write off an otherwise good sounding tube.

Not an issue with DC heaters, and lead dress has nothing to do with it, because it's a construction issue with the tube.

If you ever use any octal preamps then you're pretty well obliged to use DC because these tubes were not designed for AC heaters and will hum correspondingly unless you use DC.

Hello Paulster,
I'm not meaning to be argumentative here and I fully appreciate your point about DC. Indeed, it sounds like good solid advice. I have no experience with the new TS 12AX7.

However, in my ignorace (is ignorance bliss?), I recently built a couple of all octal amps. One uses 6SL7>6AQ7>pair 6SN7 in parallel PP; pretty much a Fender type front end into a concertina driving the four triode sections. The other, basically a 5F2 Fender Princeton, uses 6AQ7>6AQ7>6V6. No hum on either. Maybe it is because these are relatively low gain amps? All of the tubes are old production (likely NOS, but who really knows?) Sylvania, GE, etc. My limited experience seems to run counter to what you say and I was hoping someone could shed some light.

For those who are unfamiliar, a 6AQ7 is nearly identical to 6SL7, but only one triode (and dirt cheap, too).

Phil

[Ripthorn]

Why do I want to look at DC for the heaters?

Well I am on my 3rd build. After hours of cleaning up the lead dress, rewiringing and resoldering all the solder joints, elevating the heaters, dividing voltages, and providing a virtual ground reference via qnty 2 100R resistors I AM UNABLE TO GET RID OF THE HUM!

Finally, I disconnected the green 6.3 VAC leads from the filaments and connected a Duracell 6 volt lantern battery. The hum disappeared.

Paulster pointed me to a great link. Thanks!!!

I hear that DC voltages help the tube filaments last longer! Thats a plus especially for NOS tubes.

Any pointers would be helpful.

My amp is a high gain two watter.

Thak!
Decko

Mine is a high gain 2W as well. What I did was rectify the AC heater voltage with a high current (4A) bridge rectifier (to be safe), then smoothe with about 3300uF worth of 25V caps. After that I ran it through voltage regulators (1A capacity each) and then put a small smoothing cap on the other side and that's it. Total cost of maybe $1-2 for the extra parts.

[paulster]

I'm not meaning to be argumentative here and I fully appreciate your point about DC. Indeed, it sounds like good solid advice. I have no experience with the new TS 12AX7.

However, in my ignorace (is ignorance bliss?), I recently built a couple of all octal amps. One uses 6SL7>6AQ7>pair 6SN7 in parallel PP; pretty much a Fender type front end into a concertina driving the four triode sections. The other, basically a 5F2 Fender Princeton, uses 6AQ7>6AQ7>6V6. No hum on either. Maybe it is because these are relatively low gain amps? All of the tubes are old production (likely NOS, but who really knows?) Sylvania, GE, etc. My limited experience seems to run counter to what you say and I was hoping someone could shed some light.

Phil

That's not what I'd call argumentative!

I probably generalised a bit on the octals because they were originally designed for DC (battery) supplies and therefore didn't have heaters that were optimised for AC use. Depending on the age, tube type and manufacturer it may well be that they started using better-designed heaters to allow them to be practically used with AC at some point in time.

It is a well-documented problem though, as evidenced by dehughes here in the Constellation-derived amp he built, and in the Constellation itself which uses DC heaters for the octal preamp tube.

That said, gain is another big factor. If you've got a fairly low gain amp then it's less critical because you don't get a large amplification factor that will push the noise floor right up. As soon as you get cascaded stages you find that the noise from the first stage can get significantly amplified by the second, third, etc. and can therefore be quite significant.

At the end of the day it comes down to a preference thing really. I probably wouldn't drop a DC regulator board into every amp I build had I not had PCBs made to make it a no-brainer for me. You could argue that it's moot in something like a 5E3 Deluxe, but for the sake of the 4 holes I have to drill for the mounting posts it's worth it to me, knowing that I'm not going to have any residual hum whatsoever, regardless of the tubes I throw at it.

Paul