DC for tube heaters

[supro66]

I am reading a book
Designing Power Supplies for Tube Amplifiers
By Merlin Blencowe


He says this
The audio input stage is invariably the most sensitive to pick up hum.Power valves are very insensitive to hum so these almost never need DC supply, witch is a great relief considering how much more current they need. Therefore we can get away with only supplying only the input valve heater with DC while the rest can receive only AC. The most straightforward of doing this is simply to rectify the existing heater supply, leaving most of the valves operating on AC and the most sensitive valve on DC as shown in fig 9.9

[sluckey]

I am reading a book

That's good for the soul, but it can also add wrinkles to your brain! 

[kagliostro]

Ciao Supro66

What is odd for you about the text you read from Merlin's book ?

Why do you report it here ?

Franco

[supro66]

Has anybody tried this DC not AC on the heaters
and is it worth the effort

[sluckey]

Has anybody tried this DC not AC on the heaters
and is it worth the effort

It's probably more useful in a high gain amp where you are trying everything possible to reduce noise. I think everybody (except me) has done it. Even the lowly Epiphone Valve Junior...

[Willabe]

I've never tried DC heaters, but I do like using a DC stand off.

I'm setting up my bread board and I'm going to try adding a hum dinger heater balance pot to see/hear how I like it.

Here's a drawing. To adjust the DC stand off voltage leave the 100K R alone and play with the 330K R or use a pot.


              Brad   

[kagliostro]

Many guys at DIYItalia uses DC heaters for preamp with success

---

I think it is a must on a PCB Board Amp where you can't twist the wires, but also useful in some stubborn cases

---

As documentation about Mesa Boogie Heaters give a look to this old tread

http://el34world.com/Forum/index.php?topic=14449.msg136955#msg136955

Franco

[supro66]

Has anybody tried this DC not AC on the heaters
and is it worth the effort

It's probably more useful in a high gain amp where you are trying everything possible to reduce noise. I think everybody (except me) has done it. Even the lowly Epiphone Valve Junior...

I repaired my first Supro 6420[ filter caps and diods ] and made two more and a Fender 5e3 Delux and never had any noise that was bad


I like the idea of only doing the first tube

[Colas LeGrippa]

 I WOULDN'T SPEND TIME AND $ in DC heaters. There are many ways to have a a/c hum free amplifier : good lead dress, hum balance, elevated heaters......I've never had a problem with a/c heaters, neither had Fender.  Anyway, you're going to play louder than the hum and won't hear it.....

Colas

[trobbins]

It can end up being 'one step forwards, two steps back' if you aren't careful.  If you don't use a separate heater transformer, and attempt to rectify to DC for a whole batch of heaters, then the very high transient currents in the heater winding can couple noise in to the rest of the amp if not careful.  Also, you need to check the ripple current rating of the capacitor and then check what is actually the ripple current in circuit - you could end up blowing a capacitor.

To be sure, even with all the careful lead dress etc, there is still a very low level of hum coupling to the input circuit with AC heaters, so if you have used all other techniques to reduce hum, and have the spectrum analyser and test tools and high gain circuitry to confirm that it is from that residual ac heater on the input valve, then it may be worthwhile.  Preferably use a separate heater winding just to get DC for the first input valve only, and use another heater winding for all other valves.

In the old days with a cathode biased output stage, the input valve heater was part of the common cathode resistor - that effectively gave a DC heater for free.

[sluckey]

... then the very high transient currents in the heater winding can couple noise in to the rest of the amp if not careful.

What transient currents are you talking about?

[trobbins]

Rectifying the heater winding with lots of filter capacitance would cause very high crest factor charging pulses in the winding.  In addition, high current diodes may not be nice schottkys, and if pn junction then most likely the diode does not exhibit a benign level of reverse recovery.  Both mechanisms raising the liklihood of coupling rectification noise in to other PT windings, or even nearby circuitry.  Some diy attempts at adding in large electrolytics and high current diodes, in order to get perceived AC ripple on DC heaters down to uV level, are not great examples of good noise management.

[HotBluePlates]

Has anybody tried this DC not AC on the heaters
and is it worth the effort

I have a pair of tube microphone preamps. The power supply is housed in a separate chassis to minimize the possibility of noise injected into the low-level preamp circuitry (the signal levels for some mics could be well below that of a typical guitar pickup).

The preamp maker used d.c. to heat the tubes, again due to the low level mic signals and the use of input transformers inside the preamp chassis. They chose to use a pre-made 12.6v d.c. supply from Condor, which has a much more elaborate power supply and regulator than you're likely to find in home-brew d.c. heater circuits. As a result, the supply and the preamp perform very well.

In general, all this isn't necessary in a guitar amp: signal levels are higher and there are almost never any input transformers which can pick up and amplify hum due to their step-up turns ratio.