DC heating

david_djd

Hi..
I've decided to put regulated DC heating in my madamp.. so, i've builded the rectifier from the schem existing here in the gallery.. but after wiring everything.. the tubes don't fire up.. the wiring of the rectifier is correct cause when i umplug it of the tube heaters i have 5,9v.. but when i plug it to the heaters the readed value (at the out of the rectifier) is 0,0..
with de AC heating the tubes fires up normally..

I have some questions:
- tubes have a positive and negative (heaters)??
- all the tubes have to be wiring the same way? red to a PIN and white to another (and exactly the same in all the tubes)?

Another question:
- If i change R35 (470k) to a smaller (330k) can i get a little more volume on channel 2? (to be more or less the same volume of the channel 1) the amp is an A15

Thanks for the anwsers

futurama

Re: DC heating

Zitat von david_djd

I have some questions:
- tubes have a positive and negative (heaters)??

No. if they had, it'd be impossible to heat'em up using AC.

Zitat von david_djd

- all the tubes have to be wiring the same way? red to a PIN and white to another (and exactly the same in all the tubes)?

Neither. You can compare the filaments to simple glow-bulbs wired in pa-
rallel. They only have two terminals, and doesnt matter how you connect
them to the 2-rail AC-power supply, they will glow.

5.9V unloaded may be too low. 0.0V isnt a good sign for sure. Are you
sure none of the components is faulty ? Is your power-supply-capability
big enough (the unloaded-loaded voltage-difference may vary SIGNIFI-
CANTLY, depending on the components used, the "weaker" the supply,
the bigger the drop) ? Means, the tubes may draw much more current,
than your power-supply can deliever.

Hope you will be able to solve the problem soon,
wr, futurama

eBernd

Re: DC heating

Hi,

if you tell the source of the circuit more precisely the answer would be als more precise

I assume that the circuit uses a voltage regulator. This would be a problem, because it has a current foldback protection circuit wich will shutdown the regulator. The (cold) tube heaters have a very low resistance wich demands in the moment of firing up a lot of current...

5,9 V is too low for the heaters, you will affect the function of the tubes.

Bernd

futurama

Re: DC heating

Hi,

An Addition: there is actually one reason to use a DC-heating in a high-gain,
high-power-amp. This reason is to get rid of excessive amount of hum. And
it is usually used for some of the preamp-tubes, less often for the whole
preamp, and alsomt never (in a typical guitar amp) for all tubes.

For that it may have some sense to provide DC-Heating to the first stage
of the high-gain-channel only. That will drop the current-demands and do,
what it is conceptually intended to do.

But...an amp like A15 should be quiet like a mice without any DC-heating,
i am sure, MAD knew what he was doing, when designing it. And i am pretty
sure myself, that there wont be any significant benefit of using DC in an A15,
provided the proper layout/assembly of the amp.

If you really have amounts of hum you cant live with, you may also consider
"upgrading" your amp with a simple choke. That may be cheaper and even-
tually more effective.

wr, futurama

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