DC heaters

[chromaticdeth87]

What is the advantage of these. I have someone that wants an amp built and they have requested DC heaters, what are the advantage of these? I am assuming you have to rectify the 6,3v filament wires to straight DC. would this require a half wave or full wave rectfier?

[Jana]

I would only do the dc for the preamp tubes, run the power tubes with the ac. You need to use a full wave bridge. Make sure your diodes can handle the current. If you have 6.3vac, then you will wind up with about 6.9 volts DC. You can either put another diode in line to drop it .7 volts or use a 5 watt resistor of about 0.22 to 0.47 ohms or so, depending on how many tubes are drawing current from it.

4700 to 10,000 uf should be enough filtering. It will still have quite a bit of ripple, but definitely not like the raw AC.

You will also need to reference to ground somehow. Don't use the center tap on the 6.3 if you have one. Use 100 ohm to create the usual faux center tap.

Having said that, it is a lot more hassle than it is worth. It is parts that can fail, more stuff to build, space to occupy. I have not found an amp yet that couldn't be made dead quiet from hum, including ones with 4 gain stage preamps, by using good layout, proper lead dress, attention to heater wires, proper grounding, etc.

Does the customer know why they want DC Heaters? Other than being a band-aid for sloppy work, there is no advantage to them.

[M Fowler]

DC filament is used in some class A amps to quiet them down since they are noiser than your pushpull Class A/B amps.

Mark

[funkmeblue]

....and on octals such as the 6sl7 abd 6sn7. The pics I've seen of george alessandros amps show a 11,000uf mallory, two diodes encased in shrink tubing, and are hooked to the yellow wires of the power trans which I always figured was the 5 volt. An orange wire from the power trans goes to the negative side of the cap. No resistors. How does he do this? Maybe it isn't a 5 volt tap?

[chromaticdeth87]

Thanks guys.

[jaysg]

I have someone that wants an amp built and they have requested DC heaters...

One point not mentioned is that for the typical long-tailed pair into a push-pull output design cancels common noise...so if you were build a Marshall/Fender sort of amp, you wouldn't bother with DC heaters on the output tubes or the phase inverter. Also, the most critical noise source is the input stage, so there's a certain amount of bang for the buck/effort if you use DC or elevated heaters on the first stage tubes, and diminishing returns moving towards the PI. One of the freebee possibilities is that if you have a 5V tap on your PT and you're not going to use a tube rectifier, you can full-wave rectify the 5V using Schottky diodes. This will create a supply very close to the 6.3v spec.

[chromaticdeth87]

thank you for the suggestion jaysg, so a proper use of shielded cable and dc heaters on the input stages and of the more extreme gain stages would probably remove at least some noise?

[jaysg]

thank you for the suggestion jaysg, so a proper use of shielded cable and dc heaters on the input stages and of the more extreme gain stages would probably remove at least some noise?

Yes...if you mean shielded heater lines, I would suspect you've been reading London Power books. Please forgive the analogy, but that's like waaring two condoms at once. It's probably all you can do to keep noise down, short of not building anything. Otoh, it may be overkill. If you're grearing up for a high gain amp, then studying what's been done by Soldano and others should point to how much effort is really necessary.

[bnwitt]

There are many high gain amplifiers out there that use DC heaters. Most of them will use DC on the early stage preamp tubes and not the latter stages. the Marshall JCM 2000 DSL series and the Peavey Triple X amps are just two that have come through my shop recently with DC heaters. Neither of these amps uses DC on the PI or power tubes. The Peavey amp not only uses DC heaters on V1 thru V3 but also elevates the ground reference of the heater supply.

[chromaticdeth87]

haha. hey if one breaks.... yeah I am building a high gainer I know that shielding and proper layout are really important. great info. bnwitt planning on ripping out the guts of my triple xxx here soon to build something else. The amp was pretty reasonably priced, yet, it's really like any kind of dynamics and life.

[drz400]

What is the advantage of these. I have someone that wants an amp built and they have requested DC heaters, what are the advantage of these? I am assuming you have to rectify the 6,3v filament wires to straight DC. would this require a half wave or full wave rectfier?

Switching power supply is the easy solution silence is golden.
AC in DC out and you are done, you must reference the DC out ground to chassis or you would still have buzz

output is adjustable, easy as pie, very little heat.
$19 no transformer, easily mounts to the inside side wall of most amps any AC from 100V to 240V, run them at 12~12.6VDC no reason for the extra current of 6.3

You could even do the power tubes with a larger unit but I would not bother.
For lets say 6 preamp tubes @150ma each 900ma total you can use a Condor GECA20-12 12VDC@1.67A
http://search.digikey.com/scripts/DkSea ... 71-2496-ND

Unregulated DC can sometimes be noisier than AC due to the shape of the signal. It winds up being a buzz instead of a hum. The Switching supply does it all for you.

[jaysg]

Switching power supply is the easy solution silence is golden.

Have you done this? I've never tried but often thought about it -- Condor medical grade switchers grow on trees where I work. Some of the new Crates have switchers for B+ and heaters, so it must not add noise. There's always the mojo issue and guys who can hear a mosquito fart at 200 yards.

[chromaticdeth87]

dr.z you lurk here too! hehe.

[stoo]

Have you tried riasing the ac heaters to a positive DC offset via the centertap?
Stew

[drz400]

Have you tried riasing the ac heaters to a positive DC offset via the centertap?
Stew

Yes I have usually to 70 or so VDC. many times and is an improvement most of the times but DC heaters is much better except it buys you nothing in helping the max heater to cathode limits.

Have I tried the Switching supply? Hell yes ! it works amazing.
Just make sure you ground the - output to your circuit ground or you would still have a buzz.